**Important** reality check on motor, voltage, current etc.

[John in CR]

The instance where my 100v100a Methods controller failed was while going up a hill that was gradually increasing in grade, but traffic was slowing me to less than full throttle. With PWM in play, I guess the phase current skyrocketed with the high power demand of the hill, but at less than full duty. If that is indeed where the risk lies then I can make a point to avoid it. If I'm on the right track here, I can't really think of another time I'd be at high load and less than full duty unless I tried cruising along at a 60-70 mph that was somehow partial throttle using a single motor. With the dual motor I don't think the load would be high enough, and the only way I'd get the dual motor under high load will be a top speed attempt, so WOT.

John

As the physical load increased, i.e., the hill, your phase current would've sky-rocketed whether you were going full throttle or partial throttle. If you're at full throttle, the phase current will be even more than at partial throttle BUT the mosfets endure switching losses while PWMing during partial throttle, so if you're not at low enough partial throttle, the mosfets will endure more instantaneous heat than at full throttle.

If your phase currents are high enough, your controller will blow even at full throttle. There's a current limit to mosfets and full throttle doesn't protect you from that. This is why it's pretty wise to limit the phase current if your setup is at risk of sustaining high phase currents. On a hub motor, most of this extra "phase current" is just heat for the motor since the motor's magnets can start to saturate at high phase currents.

Sorry, but LFP is right, you don't understand the issue.

 

[liveforphysics]

How can you first say this sort of nonsense:

If the average phase voltage drops, how does the average phase current increase?

Ignoring BEMF calculations (It doesn't affect the general principle), it's simple Ohm's law. If you reduce voltage to a resistor (The phase resistance), you decrease current. What would make a motor the exception?

And then later say this (which is correct, and the whole point):

[ it's *gasp* equal to the power of the input battery. I_1*V_1 = I_2*V_2. when V_2 goes down (The motor's average voltage), I_2 goes up to compensate.

 

[swbluto]

Sorry, but LFP is right, you don't understand the issue.

Maybe I don't understand *your* issue, but I certainly understand motor dynamics. If that is LFP's assertion, LFP's assertion of that is most certainly wrong.

And then later say this (which is correct, and the whole point):

[ it's *gasp* equal to the power of the input battery. I_1*V_1 = I_2*V_2. when V_2 goes down (The motor's average voltage), I_2 goes up to compensate.

When you get to my level of understanding, you understand what you can brush aside and remain to the core of the subject. Apparently not all were at that level, so it was my mistake for not spelling out every single letter at the beginning.

 

[rhitee05]

Maximum acceleration, whether full throttle or partial throttle is used, must be full duty.

That's not necessarily true if the controller does phase current limiting. At low speeds with low BEMF, the controller may limit itself to a relatively low duty cycle to keep the phase current down. If this is the case, any throttle setting above the limit value would feel the same. If the controller is limiting itself to 1/3 duty cycle, everything above 1/3 throttle would cause the same acceleration.

As speed and BEMF increases, the phase current drops, the limited duty cycle increases, and the throttle feels like it has more dynamic range. At some point limiting is no longer required and the duty cycle is only determined by the throttle again.

If the setup was such that limiting was not required (high-resistance motor, beefy controller, etc), your point would be correct and duty cycle would be determined only by throttle under all conditions. This could result in really high currents. Starting from a dead stop, BEMF is zero and the commutation period is long enough that the motor inductance doesn't come into play. Current is limited only by battery voltage and the phase resistance. When the controller imposes a limit to duty cycle <100%, the inductance matters again and helps the controller limit the current to less than full I=V/R.

 

[swbluto]

Don,

That doesn't seem correct, because that's the same as saying the duty cycle is determined only by throttle position. I believe the controllers are smarter than that. While accelerating there are lots of points in throttle position that result in the same acceleration as WOT, at least until you approach the speed related to the lower throttle position where acceleration decreases.

Oh yes, controller's can do whatever they want with the throttle signal. I'm not arguing that. Personally, I don't know what controller you're using and with what settings, so I can't make a reasonable assumption about *your* controller's behavior. But, all controllers do have a duty cycle, and throttle often corresponds to that duty cycle in a speed-based controller. This isn't necessarily true when current limiting is in affect (And is what sucks about speed-based controllers with a current limit), where your throttle's requested duty cycle may be higher than the duty cycle necessary to achieve the battery current limit.

If your controller blew due to overload, though, I can guarantee that's a function of your phase currents and your controller's electric-thermal characteristics of some manner. TO-220 legs act like fuses at around 75 amps, and current distribution in a controller's mosfets aren't necessarily equal (Though good design should get close to that ideal.).

 

[donob08]

..Maybe this is a way to think about “current multiplication”. We say the motor’s “Voltage” is Battery voltage multiplied by the % duty cycle. In fact if we apply 36 Volts to the motor’s inductance at the beginning of a PWM cycle, the inductance will generate 36 volts when we try to stop the current ie. FET shuts off. The voltage will be opposite in polarity and push current through the “return” on the controller. The current will continue at the same level until the stored energy is depleted.

..So the voltage at the terminals of the motor is really constant in magnitude at “battery voltage” and the current is constant. It is true that the average power from the battery is (VBatt * I) multiplied by % duty cycle. (It's really the average current that is cut) The motor and the controller have a different exchange. The power exchanged between the controller and the motor is absolute value VBatt * Constant Current (sometimes the power is going one way, sometimes the other, current is unchanged) . Since with PWM, we have defined the motor voltage to be VBatt * % duty cycle, we need to define the motor current as (Constant Current * 1/%duty cycle) in order to get the magnitude right. So we have sort of defined our way into current multiplication.

..If VBatt = 36 V, current = 20 Amps and duty cycle = 25%, we DEFINE V Motor to be 9 volts. We then have to DEFINE “I” Motor to be 80 Amps to get the power correct. That’s because the voltage at the motor terminals stays near 36 V in absolute value and the current stays at 20 Amps.

..The battery in this case is producing 36 * 20 * 0.25 = 180 watts (36V * 5A) for a 200 microsec period by LFP’s example, or 36 MilliWattseconds The exchange between the motor and the controller is 36 * 20 * 0.25 * 0.2 toward the motor or 36 MilliWattseconds and then 36 * 20 * 0.75 *0.2 or 108 MilliWattseconds pushed back at the controller by the motor. This energy coming back is what makes heat and destroys controllers.

 

[Doctorbass]

I perfectly understand that current multiplication that act by balancing the phase current vs the opposite of the duty cycle to get that same power in and power out..

but wait a minute... since power in and power out is the same... why we are talking about phase current multiplication?... I guess ONLY if we want to calculate our controller to not overheat.. but otherwise we dont really need to care with that...

I agree also with donob08:

LFP

..A friend came up with a good explanation:

..If you are traveling at your systems design speed V = Battery Voltage / KV. You are not using a lot of battery current. You are voltage limited, NOT power limited. That battery would supply more but it hasn't the push. V Batt - BEMF = little bit.

..So if you go from 4 turns to 2 the BEMF goes to 1/2, the current goes to 2 times a little bit and you are back in business and push toward 2X speed. There is then enough voltage to oppose BEMF. Right before and right after the change in turns the PWM cycle will be 100%.

The two turn or 4 turn is similar to a transmission on a car ... the car stop accelerating when the motor rpm match with the car speed. The gas pedal is like the duty cycle and when we act on it, that make the rpm of the motor to try to increase.. same as increasing duty cycle.

if we change the gear ratio( with that transmission) it's like changing the KV of the motor... same as switching from 4 turn motor to 2 turn motor....

We can have 400 amp of phase current into a little 18 gauge wire... if it's when the duty cycle is 10%... it's teh same heat generated in that wire than having 40A at 100% duty cycle... 400A at 10% or 40A at 100% is the same power lost in the wire.. but just not i the mosfet of the controller.

Once we know and understand that overall ebike need like 500W to drive at 32kmh or 2000W to drive at 60 and can estimate the rest, we can decide wich battery voltage and current , wich controller voltage and current and wich motor winding we need to acheive the desired speed...

One think i know... Crazy speed need not just voltage or current.. BUT BOTH OF THEM!!

And since we are a great majority of overclocked ebike that use 100% throttle ( WOT) most of the time.. the duty cycle is near 100% so the motor current and battery current are not so different.. power in = (power out - heat lost...)

I might appear sometimes to confuse about motor phase current and battery current but it's because i'm always calculating everything for WOT... so near 100% duty cycle = minimal current multiplication :wink:

Doc

 

[donob08]

..I think this is the crux of the matter for the use of PWM. You can think of PWM as dividing the power (I * V) involved into (100/%duty cycle) shares. 20% makes 5 shares, 25% makes 4 shares etc. The important point is that only one of these shares of power pass from the battery to the motor to make it go. The other (100/%duty cycle) - 1 shares are energy that passes from the moving motor into the controller to be dissipated as heat and possibly do damage. The idea of current multiplication may be best forgotten. It is just a way to think about the way energy is shared.

..The idea of thinking of "Motor Voltage" as VBatt * %duty cycle has some use, as in saying the speed would be KV * VBatt * %duty cycle. KV is an approximate quantity, using it with defined "Motor Voltage" is a reasonable approximation. That's the only use I can think of.

.. So now it's easy to see that with the 25% duty cycle I wrote about before, one share will be useful power, three shares will be potentially damaging heat. So at 25% duty cycle there is 3 times as much wasted heat energy as there is useful energy. I think this is the important message about PWM. We need PWM, it saves batteries, but it is not free. Here's to WOT! And to systems where the design doesn't cause the controller to do current limiting. A hard shut down from the battery BMS is healthier for the controller than it doing current limiting. I think that says we'd be better off with Battery Voltage limited systems, not Power Limited systems with out of sight voltages. Of course, I'm talking from a point of making controllers last, not a thirst for speed.

 

[liveforphysics]

but wait a minute... since power in and power out is the same... why we are talking about phase current multiplication?... I guess ONLY if we want to calculate our controller to not overheat.. but otherwise we dont really need to care with that...

You only care about phase currents if you happen to care about the torque the motor makes, and the heat the controller makes. If you don't care about either of those things, then I guess we don't need to care about that.

The two turn or 4 turn is similar to a transmission on a car ... the car stop accelerating when the motor rpm match with the car speed. The gas pedal is like the duty cycle and when we act on it, that make the rpm of the motor to try to increase.. same as increasing duty cycle.

if we change the gear ratio( with that transmission) it's like changing the KV of the motor... same as switching from 4 turn motor to 2 turn motor....

This is a huge fallacy. It's not like a transmission at all. A transmission multiplies torque while keeping a power level constant. The motor is able to make the same torque, and the same continous torque rating etc etc if it's wound to be 20kv or 200kv, or 1-turn or 10-turn etc. The speed range changes, meaning the power capability for a given input voltage changes as the motor wind changes, but torque potential for any given input power is identical.

It functions more like the opposite of a transmission, in that one changes torque keeping power fixed, and one keeps torque fixed while making power potential (through RPM range) change.

And since we are a great majority of overclocked ebike that use 100% throttle ( WOT) most of the time.. the duty cycle is near 100% so the motor current and battery current are not so different.. power in = (power out - heat lost...)

Doc

If you've got lots of turns on the motor, and you're running low voltage, then it doesn't matter much. For a bike setup like John's running, he pulls a current limited 80amps from the battery both at 4mph, and the same current limited 80amps from the battery at 40mph. This means most all of his riding is happening during a point of phase current multiplication.

Even if you're always holding the throttle WOT, you don't stop multiplying current to the motor until the moment you see battery current begin to fall off, and at that moment, battery current and phase current will be equal.

 

[John in CR]

We can have 400 amp of phase current into a little 18 gauge wire... if it's when the duty cycle is 10%... it's teh same heat generated in that wire than having 40A at 100% duty cycle... 400A at 10% or 40A at 100% is the same power lost in the wire.. but just not i the mosfet of the controller.

One think i know... Crazy speed need not just voltage or current.. BUT BOTH OF THEM!!

And since we are a great majority of overclocked ebike that use 100% throttle ( WOT) most of the time.. the duty cycle is near 100% so the motor current and battery current are not so different.. power in = (power out - heat lost...)

Doc

Doc,

I disagree with these 3 statements above.

1. Loss to heat goes up by the square of current, so at a resistance of 1ohm you get 1,600w of heat loss at 100% duty at 40a and 16,000w of heat loss at 10% duty at 400a.

2. I've been as high as 97kph (61mph) at only 74v nominal and 40a (approx).

3. I rarely use WOT other than during acceleration, because it's too fast for most of my riding.

John

 

[John in CR]

LFP,

I'm sorry if I'm part of derailing your central point of the thread by people trying to explain how they understand current multiplication.

I still have some question about whether or not in real use the controllers allow the behavior you outlined in the original post, or, in the case of the Infineon based controllers at least, do the programmable settings of both battery side current limit and phase current limit permit us to mitigate the potential ill effects of low turn count motors used with high voltage controllers?

John

 

[adrian_sm]

Even if you're always holding the throttle WOT, you don't stop multiplying current to the motor until the moment you see battery current begin to fall off, and at that moment, battery current and phase current will be equal.

Just to be clear, all the current multiplication you are talking about is during the current limiting of the controller at WOT, not when you induce PWM by rolling off the throttle? Could you explain the difference. This would really help me understand.

Also keen to know if the infineons that have a programmable phase current limit can minimise the damage to the controller.

Cheers, Adrian.

 

[donob08]

This is a huge fallacy. It's not like a transmission at all. A transmission multiplies torque while keeping a power level constant. The motor is able to make the same torque, and the same continous torque rating etc etc if it's wound to be 20kv or 200kv, or 1-turn or 10-turn etc. The speed range changes, meaning the power capability for a given input voltage changes as the motor wind changes, but torque potential for any given input power is identical.

It functions more like the opposite of a transmission, in that one changes torque keeping power fixed, and one keeps torque fixed while making power potential (through RPM range) change.

LFP I think this is true but it's an incomplete the statement "The motor is able to make the same torque " is true. If there is enough voltage to drive current w/ 4 turns you'll get the same torque. If the system is voltage limited, the BEMF prevents passing enough current to get the same torque. With 2 turns the same voltage will drive current. Yes, equal power in you get equal torque, but with low current you get low torque because there is low power in. Current makes torque, right? A lower turn count allows the battery to send out more power.

And since we are a great majority of overclocked ebike that use 100% throttle ( WOT) most of the time.. the duty cycle is near 100% so the motor current and battery current are not so different.. power in = (power out - heat lost...)

Doc

If you've got lots of turns on the motor, and you're running low voltage, then it doesn't matter much. For a bike setup like John's running, he pulls a current limited 80amps from the battery both at 4mph, and the same current limited 80amps from the battery at 40mph. This means most all of his riding is happening during a point of phase current multiplication.

Even if you're always holding the throttle WOT, you don't stop multiplying current to the motor until the moment you see battery current begin to fall off, and at that moment, battery current and phase current will be equal.

I think you and I are thinking alike, at least partially. I'm not sure what "If you've got lots of turns on the motor, and you're running low voltage, then it doesn't matter much." means. People with low battery voltage are not current limited. WOT means 100% duty cycle. Those people will benefit by reducing turn count. They'll get more current and thus more torque and get up to a higher speed. There may be more people like me than there are people like John.

Don

 

[donob08]

A tidy up:

1) I should have said Energy Sharing not Power Sharing since time is involved here.

2) The voltage in the "OFF' part of the PWM cycle is not going to be exactly the battery voltage as I said as an approximation. The voltage will be IR. Where I is the current that was running during the ON time (constant current, from an inductor) and R is the resistance of the controllers "return path". This resistance will go up with temp and this current is driving the temp up. The Power will be IsquaredR. Again the currents in the ON cycle and in the OFF cycle are identical. That's what inductors do. They keep the flow constant.

3) For LFP's example (5% duty cycle) the time of the return path current is 19 times the length of the ON cycle. That's from number of energy shares = (100/%duty cycle) = 20 and shares for return path are = (100/%duty cycle) - 1= 19. So the energy fed back to the controller is:
Energy = Power * Time = I squared R * 19 relative to the Energy delivered to the motor = IV. I'm taking the On time as "Unit time". The ratio of useful energy transmitted to waste energy would be:

..... V battery to 19 (IR). You can see that the R of the return path is REAL important. For low duty cycle the useful energy is a small part of the energy flow the controller handles. I find this ratio of energies a better way of describing things than "Current multiplication". Current IS NOT multiplied. It is basically the time factor that is the killer.

I guess we could call it "Energy Multiplication"

 

[liveforphysics]

This is a huge fallacy. It's not like a transmission at all. A transmission multiplies torque while keeping a power level constant. The motor is able to make the same torque, and the same continous torque rating etc etc if it's wound to be 20kv or 200kv, or 1-turn or 10-turn etc. The speed range changes, meaning the power capability for a given input voltage changes as the motor wind changes, but torque potential for any given input power is identical.

It functions more like the opposite of a transmission, in that one changes torque keeping power fixed, and one keeps torque fixed while making power potential (through RPM range) change.

LFP I think this is true but it's an incomplete the statement "The motor is able to make the same torque " is true. If there is enough voltage to drive current w/ 4 turns you'll get the same torque. If the system is voltage limited, the BEMF prevents passing enough current to get the same torque. With 2 turns the same voltage will drive current. Yes, equal power in you get equal torque, but with low current you get low torque because there is low power in. Current makes torque, right?

Yes, I forgot to qualify the statement only applies for any point during battery current limiting. Once you're out of the battery current limiting stage, then the motors behave more intuitively.

And since we are a great majority of overclocked ebike that use 100% throttle ( WOT) most of the time.. the duty cycle is near 100% so the motor current and battery current are not so different.. power in = (power out - heat lost...)

Doc

If you've got lots of turns on the motor, and you're running low voltage, then it doesn't matter much. For a bike setup like John's running, he pulls a current limited 80amps from the battery both at 4mph, and the same current limited 80amps from the battery at 40mph. This means most all of his riding is happening during a point of phase current multiplication.

Even if you're always holding the throttle WOT, you don't stop multiplying current to the motor until the moment you see battery current begin to fall off, and at that moment, battery current and phase current will be equal.

I think you and I are thinking alike, at least partially. I'm not sure what "If you've got lots of turns on the motor, and you're running low voltage, then it doesn't matter much." means. People with low battery voltage are not current limited.

Every motor out there that I just finished looking at, even just a humble 9C 9x7 running at 36v with a 35amp controller is going to be running battery current limiting to 22km/hr. Same setup with a 20amp controller stays in current limiting to 28km/hr. It's something that applies to all of our setups, but the battery current limiting ends sooner as the number of turns on the motor go up, or the voltage goes down.

WOT means 100% duty cycle. Those people will benefit by reducing turn count. They'll get more current and thus more torque and get up to a higher speed. There may be more people like me than there are people like John.

Don

WOT does not equal 100% duty cycle. WOT equals having your hand move the throttle at max. For the first 1/3 to 3/4th of the entire speed range of the bike, WOT means some PWM setting that limits the battery current from exceeding a set value (and hence phase current multiplication). Once you reach the point at which the BEMF has grown enough to cause battery current limiting to stop, then and only then does WOT mean 100% duty. For bikes running high voltage with low turn-count motors, it's very possible to never have 100% duty cycle occur, because the bike runs out of power to overcome aero drag before the motor reaches a speed that the battery current limiting stops happening. In this, the controller never gets to quit phase current multiplication.

 

[Jeremy Harris]

Interesting thread.

Things get more complicated than they need be when we don't all share a common level of understanding, or even a common language, come to that.

There's been some good stuff here, though its been a bit derailed in parts by some misapprehensions about the way stuff works. Someone once said that you can simplify this stuff right down to a basic level by looking at the key characteristics of the three elements in the chain, the battery, controller and the motor.

Motors don't care what they are attached to and will (please accept the simplification that ignores the extremely low motor resistance) draw as much current as you can give them until they burn out.

Controllers have two jobs; the first is to control the speed that the motor runs and the second is to protect stuff from burning out (by some form of current limiting).

Batteries only have one job; to provide as much current as the controller needs without allowing the voltage to drop too much.

Note that this is all focussed on current. Motors don't care about system voltage and controllers and batteries can be built to run at any voltage you want.

As others have pointed out, the power being supplied from the battery equals the power being consumed by the system. This is a basic law of physics and can't be changed. Let's assume that the battery is able to supply any current demanded without the voltage dropping (in other words, its a perfect battery). Let's also assume that the controller is perfect and has no losses and that the motor is perfect and has no resistance (this is close enough for an example, as batteries, controllers and motors should all have very low resistance).

We need to define what we mean by "throttle". With an internal combustion engine (or an external combustion engine, come to that), the throttle controls the volume of gas in the cylinder. As the torque the ICE produces is directly proportional to this volume, IC engines have a throttle that tends to be a torque control, not an rpm control. It's important to recognise that the controllers we use don't normally work this way, the "throttle" control on an electric motor controller often works as a motor rpm control and doesn't control torque. The throttle on a typical ebike controller changes the mark/space ratio of the pulse width modulated signal to the motor phase wires, in effect it changes the voltage that the motor sees. Most of you will have already noticed this and Luke's already given a good explanation.

OK, so we now have a system where the motor rpm is controlled by the throttle position, with a linear relationship between % throttle and % motor rpm (assuming no over-current limiting and perfect components) and the power coming from the battery equals the power going to the motor. The battery voltage is constant, so the only thing that can change with changes in power demand is the current the battery supplies. Bear in mind that the relationship between power and speed for a vehicle is approximately a cube law; doubling the speed needs approximately 8 times the power. Let's look at some worked examples for the same bike (but please bear in mind they assume NO losses and are for a perfect electrical drive system):

Example 1: The throttle is at 100%, the motor see the full voltage from the battery and the power it needs to maintain a speed of 30mph on the level is 800 watts. The battery voltage is 50 volts, so the battery current is 16 amps (800W / 50V). Because its at full throttle the motor current is also 16 amps.

Example 2: The throttle is at 50%, the motor sees 50% of the voltage from the battery (the PWM duty cycle will be 50%) and the power it needs to maintain 50% of full speed (15mph, remember the rough cube law speed to power relationship) is about 100 watts. The battery voltage is still 50 volts, but because the power is now only 100 watts the current from the battery has dropped to 2 amps (100W / 50V). BUT, the motor is seeing a voltage that is 50% of the battery voltage, 25 volts, so the current flowing through it to deliver the needed 100 watts must be 4 amps, double the current being supplied by the battery.

Example 3: The throttle is at 25%, the motor sees 25% of the voltage from the battery and the power it needs to maintain 25% of full speed (7.5mph) is about 12.5 watts. The battery voltage is still 50 volts, but the battery current has now dropped right down to just 0.25 amps. The motor is seeing a voltage of 25% of the battery voltage, so the current going through it will be 1 amp.

Obviously this is a made-up case using perfect components and making some bold assumptions about power demand, but it neatly illustrates that at full throttle the motor current and battery current are the same, at half throttle (for this particular case) the motor current is double the battery current and that at quarter throttle the motor current is four times the battery current.

In reality the power needed at low speeds will be a greater, because of the fixed losses in the whole system and the motor, controller and battery won't be perfect and will all have losses to some degree. The core principle holds though.

As I mentioned right at the start of the rambling diatribe, the controller is there to stop things burning out. The only way it can do this is to restrict power and the only way it has to do this gracefully (i.e. without just switching everything off) is to cut back the "throttle", so that the voltage the motor sees reduces. This does increase the motor current if there is a heavy load on it, but by maintaining a safe ratio between battery current (measured by the current limit circuit in the controller) and pulse width applied to the motor the controller is able to restrict motor power to the point where the phase current is relatively safely controlled. It helps to think of the current limiter on one of these controllers as a throttle limiter, because that's what its really doing in an overload condition.

Hope this doesn't add to much fuel to the debate..............

Jeremy

 

[donob08]

Jeremy

That is a great and consistent explanation. I see that you think of the “Current Multiplication” as the Motor Current being a multiple of the Battery Current. I’m not sure that others had the same meaning. From your description, the motor current falls with decreasing throttle position or lower PWM. I don’t think that is what some mean by current multiplication. I think some believe that current increases as PWM duty cycle becomes smaller..
There are, for sure, issues around saying at 50% PWM the motor sees 50% voltage. Yes, that’s the average voltage, but the voltage is really full voltage 50% of the time and 0 for 50%. I think it is these changing states that have people worried about what they describe as Current Multiplication that contributes to controller heating at reduced duty cycle.
I, for one, don’t think current in an inductor changes quickly. I do think trying to turn off the current to an inductive load is going to cause problems. I think the problem is that the controller must “sink” the inductive current during the off cycle. While the current is just the same as it was during the ON cycle, for low duty cycles this “sunk” current exists more of the time than the productive current. That leads to more heat than light. I mean more heat than propulsion.

I think you like debate.

Don

 

[donob08]

LFP

.. Interesting that you quoted a part of my statement. The entire thought was:

"People with low battery voltage are not current limited. WOT means 100% duty cycle."

On my little down home system I can watch on my Watt's UP and see the current, at start up, go right to 30 Amps, the 2C limit my Ping BMS applies. It may be a coincidence that Ping and whoever chose the same current limit.

More important, " (and hence phase current multiplication)" again refers to an increase in current that your original drawing for this thread didn't include. It sounds like you are saying that when the duty cycle goes from 100% to 50% the "phase current" will double. What is driving that? Do you believe there is an inertia affecting power. The motor will demand more current to get back to the same power level? Does that imply that at 50% duty cycle the motor has twice the torque? And this occurs even though the battery is supplying only half the former power since average current is cut in half. Where is this power coming from.

I think the real issue here is Energy Multiplication. The energy fed back to the controller during the off cycle is inversely proportional to duty cycle. When PWM shuts down, the voltage at the motor terminal reverses and the current stays the same, Energy is forced into the controller from the motor. This Energy causes heat. The shorter the duty cycle, the longer the "current to controller" cycle, the more energy.

I think your drawing also should show the voltage going negative during the OFF cycle, not staying at zero.

Don

WOT means 100% duty cycle. Those people will benefit by reducing turn count. They'll get more current and thus more torque and get up to a higher speed. There may be more people like me than there are people like John.

Don

WOT does not equal 100% duty cycle. WOT equals having your hand move the throttle at max. For the first 1/3 to 3/4th of the entire speed range of the bike, WOT means some PWM setting that limits the battery current from exceeding a set value (and hence phase current multiplication). Once you reach the point at which the BEMF has grown enough to cause battery current limiting to stop, then and only then does WOT mean 100% duty. For bikes running high voltage with low turn-count motors, it's very possible to never have 100% duty cycle occur, because the bike runs out of power to overcome aero drag before the motor reaches a speed that the battery current limiting stops happening. In this, the controller never gets to quit phase current multiplication.

 

[liveforphysics]

Jeremy- I loved your post, and fully agreed with everything you said there, as I think I've found myself doing with every post you've ever written.

You did an excellent job explaining how the battery current can be doubled by the controller when running at 50% duty cycle, even with no current limiting occuring.

I noticed you intentionally stayed away from any situation where the controller is in battery current limiting though, a constant power-in for the controller. That period is kinda the whole emphasis of the point I was trying to make with the current multiplication.

Could we burden you for perhaps a couple similar examples, but while the controller is in a state of actively limiting battery current?

 

[ZapPat]

Very interesting discussion - What's cool about this is that just one year ago this same discussion would have mostly been not understood/ignored. We learn fast as a group! Thanks for starting this thread, LFP.

Jeremy posted above a good overview of how the controller works, but there's one point I would like to recall once again that some people have not seemed to grasp: Full throttle / WOT does not mean there's no PWM going on in your controller. The higher the battery voltage and/or the lower the speed and/or the lower the current limit(s) you have, the more chance there is that the controller has to perform PWM to limit either battery or phase current, no matter what throttle setting we are at.


Now...

..I think this is the crux of the matter for the use of PWM. You can think of PWM as dividing the power (I * V) involved into (100/%duty cycle) shares. 20% makes 5 shares, 25% makes 4 shares etc. The important point is that only one of these shares of power pass from the battery to the motor to make it go. The other (100/%duty cycle) - 1 shares are energy that passes from the moving motor into the controller to be dissipated as heat and possibly do damage. The idea of current multiplication may be best forgotten. It is just a way to think about the way energy is shared.

I, for one, don’t think current in an inductor changes quickly. I do think trying to turn off the current to an inductive load is going to cause problems. I think the problem is that the controller must “sink” the inductive current during the off cycle. While the current is just the same as it was during the ON cycle, for low duty cycles this “sunk” current exists more of the time than the productive current. That leads to more heat than light. I mean more heat than propulsion.
I think you like debate.

Don - You are right about the motor/phase current not stopping right away at each PWM cycle... but you are very wrong about what happens to this current during the PWM off time (when the current is flowing through the bottom FET, and the top FET is off). This energy is not just burnt off by the controller, it just continues circulating through the low side FET and slowly drops until the next PWM on time when the top FET switches on again. If controllers worked by just dissipating in themselves all the off-time current while doing PWM, they would be burning up all the time or at the very least they would be burning hot unless always used at full throttle and at cruise speeds where current limiting rarely happens. They do create more heat when doing PWM, but not that much more! I think that Jeremy wants to clear things up, not debate.

Pat

 

[donob08]

Jeremy

On further reading I see an issue. In example 2, you say the battery current will be 2 amps because (100W/ 50W ) = 2. But in Example 1 the current was 16 Amps and in example 2 the duty cycle is 50%. I would think at 50% duty cycle the average current would be 50% of the 100% duty current or 8 amps. How did the battery know it should limit itself to 2 amps?

If we say the battery is putting out 400 Watts: 8 Amps at 50 Volts and the motor is only needing 100 Watts to do half speed we are going to be going a lot faster than half speed.

I guess there will be debates. It's been quiet here anyhow.

Don

Interesting thread.

Example 1: The throttle is at 100%, the motor see the full voltage from the battery and the power it needs to maintain a speed of 30mph on the level is 800 watts. The battery voltage is 50 volts, so the battery current is 16 amps (800W / 50V). Because its at full throttle the motor current is also 16 amps.

Example 2: The throttle is at 50%, the motor sees 50% of the voltage from the battery (the PWM duty cycle will be 50%) and the power it needs to maintain 50% of full speed (15mph, remember the rough cube law speed to power relationship) is about 100 watts. The battery voltage is still 50 volts, but because the power is now only 100 watts the current from the battery has dropped to 2 amps (100W / 50V). BUT, the motor is seeing a voltage that is 50% of the battery voltage, 25 volts, so the current flowing through it to deliver the needed 100 watts must be 4 amps, double the current being supplied by the battery.



Jeremy

 

[Jeremy Harris]

Jeremy

That is a great and consistent explanation. I see that you think of the “Current Multiplication” as the Motor Current being a multiple of the Battery Current. I’m not sure that others had the same meaning. From your description, the motor current falls with decreasing throttle position or lower PWM. I don’t think that is what some mean by current multiplication. I think some believe that current increases as PWM duty cycle becomes smaller..
There are, for sure, issues around saying at 50% PWM the motor sees 50% voltage. Yes, that’s the average voltage, but the voltage is really full voltage 50% of the time and 0 for 50%. I think it is these changing states that have people worried about what they describe as Current Multiplication that contributes to controller heating at reduced duty cycle.
I, for one, don’t think current in an inductor changes quickly. I do think trying to turn off the current to an inductive load is going to cause problems. I think the problem is that the controller must “sink” the inductive current during the off cycle. While the current is just the same as it was during the ON cycle, for low duty cycles this “sunk” current exists more of the time than the productive current. That leads to more heat than light. I mean more heat than propulsion.

I think you like debate.

Don

I was deliberately trying to keep things simple by only considering a single condition, a bike at a steady speed on level ground. If, for example 3 above, the bike was going up a hill, facing a head wind or accelerating then the current for 25% throttle would be higher, in fact as high as the motor wants in order to try and keep the motor at the speed that the throttle setting was commanding. Its quite probable that, for a system like that above that's capable of delivering 800 watts, the motor could try and deliver 800 watts at 25% throttle, if the torque demand from the load was high enough. The battery current would still be 16 amps, but the motor current would have to be 64 amps to deliver the needed torque.

The key thing here is that, for the non-current limiting case, there is a tight relationship between battery current and motor current for a given power and throttle setting. At 25% throttle (so 25% duty cycle on the PWM, for the non-current limiting case) for a given power the motor current will be four times the battery current. For half throttle at the same power it will be twice the battery current, etc, etc. As soon as the current limit kicks in this relationship breaks down, as the controller will reduce the throttle setting (in effect) to limit power. As before, this is heavily caveated by my assumption that, to simplify things, we're dealing with perfect parts, but the core principle still applies with real components.

This relationship between throttle setting and current ratio is important. I'm sure people here have read of non-current limited RC controllers popping at low speed, part throttle. If you try and baby one of these off the line, with just a tiny throttle setting, then what you're really doing is forcing the controller to deliver very high peak current pulses to the motor. AFAIK, these things don't have any form of current limiting, so if, for example, you had a 50V system, decided to only open the throttle 10% at start up and had a start up power requirement of 800 watts to get the bike accelerating, then although the battery current would be a nice, safe, 16 amps (at 800 watts) the motor current would be ten times this, 160 amps. It may be that just whacking the throttle open wide is the best way to try and prevent a non-current limiting RC controller blowing up, as at least this will keep the current multiplication under control. Its all just theory, though, and may produce other, more spectacular side effects....................

Jeremy

 

[Jeremy Harris]

Jeremy

On further reading I see an issue. In example 2, you say the battery current will be 2 amps because (100W/ 50W ) = 2. But in Example 1 the current was 16 Amps and in example 2 the duty cycle is 50%. I would think at 50% duty cycle the average current would be 50% of the 100% duty current or 8 amps. How did the battery know it should limit itself to 2 amps?

I guess there will be debates. It's been quiet here anyhow.

Don

Don,

If you look closely, you'll see that I gave the power requirement for each throttle setting. Because the throttle directly controls motor speed (in a no-loss system with perfect parts), 50% throttle = 50% speed. Because power is roughly proportional to the cube of speed, halving the speed means about 1/8th of the power. Because the battery voltage is fixed, 1/8th of the power means 1/8th of the battery current. Because the motor voltage is 50% of the battery voltage (due to the 50% throttle setting) the motor current has to be double the battery current to maintain the "equal power both sides of the controller" law.

PWM doesn't directly control motor current. As Luke explained in the first post, PWM only directly controls motor voltage. 50% PWM = 50% voltage, but the current can be whatever the motor decides it needs (see the first comment about motors - they're greedy things with an insatiable appetite for current).

Hope this makes it a bit clearer, but please bear in mind that I've simplified things by making some bold assumptions about perfect components!

Jeremy

 

[donob08]

Jeremy

I understand that you gave us the requirements for each speed as far as the motor is concerned, but who gave them to the battery? We only told it we want average current to be half of what it was. It doesn't know about 100 Watts / 50 Volts = 2 Amps. It only knows 50% duty cycle means 8 Amps.

If we say the battery is putting out 400 Watts: 8 Amps at 50 Volts and the motor is only needing 100 Watts to do half speed we are going to be going a lot faster than half speed.

Am I missing something? Is the motor current going to be 1/2 of battery current and motor voltage 1/2 of battery voltage to get the 400 Watts down to 100. How is the Current Division done?

Don

 

[John in CR]

Thanks Jeremy,

It hadn't soaked in that the only tool the controller has to control anything is by chopping up the pulses. Am I correct in saying that the peak phase current of any pulse is going to be higher than battery current, even at full duty because it is being switched. If so, is there a fixed minimum ratio of phase current to battery current?

Along the vein that a controller is there to prevent a motor from burning up, how do we prevent the controller from killing itself in the process while attempting to maximize performance? Case in point, the motors I have are for small emotos in China. They come stock with a 15fet Infineon controller rated at 60v30a that has proven to be indestructible in 18 months of daily use despite, halving the shunt resistance and running a battery that is 81v off the charger, though it does run notably warm even right out in the open with great airflow on all sides. This combo pushes 110kg+ me and my 50kg+ bike to 60mph, but it takes quite a while to get up there. I want better acceleration.

Enter the 100v100a super controller with 18 4110's that I'm warned to be careful, because it could easily melt a motor. I use it with the same battery, bike and motor. Performance is great, though both the controller and motor run hotter than I'm comfortable with, and the super duper motor melter gives first and pops a phase bank of fets.

If controllers can only chop the pulses with PWM, how is the lesser controller able to restrict current more than the super controller? There has to be some other tool in their arsenal to control current. I have another even more super of these controllers, but I've been hesitant to use it until I understand the issues better, so I can program settings that protect the controller while maximizing performance. The motor is now ventilated, so it can handle whatever the controller can deliver as long as I don't bog it down to crawling speed on a steep hill.

How do we protect controllers from themselves? I know how to protect a motor from itself by avoiding low speed with high load for significant durations. Is there a way to extract the settings from the stock controller, because I believe there's something to be learned from it? That 15fet Infineon board is only slightly different from the common ones discussed, and it has the XC846 MCU.

John