maxing out phase amps

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flippy   1 MW

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maxing out phase amps

Post by flippy » Aug 05 2016 5:57am

currently i am running with a "120A" kelly controller and i am limiting the battery current to 50% in order to keep the motor from releasing the magic smoke. (2.5kW 10" hub motor)

i have the phase amps at around 50% and have some questions about the actual limits of the motor.

if i increased the phase amps to 100% would that cause problems? the battery current would stay below 60A (the battery is capable of MUCH more) but how would the motor respond to such treatment? would it simply burn out because of the high phase amps or would it just get hotter?

or would inceasing the battery amps and lovering the phase amps be better for the motor?

from reading around i cannot get a decent answer to this. the relationship between phase amps and battery amps are still a bit of black magic topic it seems.
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Punx0r   100 GW

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Re: maxing out phase amps

Post by Punx0r » Aug 05 2016 8:29am

The motor only sees and cares about phase amps. The more you put in (up to a point), the more torque and heat the motor will make. The battery amps obviously affects your battery and BMS (these need to be rated accordingly).

The ratio of battery:phase amps is the concern of the controller, my understanding is that the greater the ratio the harder it is on the controller. IIRC it also reduces top speed a little as the controller must buck extra voltage to current.

However, your Kelly controller is (unlike most controllers), rated for phase, not battery, amps. As long as you go no higher than 120A phase you should be fine. If your motor is 2.5kW rated it will handle this easily (maybe not continuously). A battery:phase ratio of 2:1 (60:120A) is pretty good.

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Alan B   100 GW

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Re: maxing out phase amps

Post by Alan B » Aug 05 2016 10:10am

Phase current (Motor current) makes torque, though magnetic saturation changes the ratio as the current gets higher. Phase current squared makes heat, so raising current quickly makes thermal problems. Battery current is power into the controller. Voltage on the battery side is pretty simply the battery voltage, but voltage on the motor side is a combination of back EMF plus motor current times resistance. The controller can be viewed as a voltage converter, down converting the battery voltage to the motor voltage. There is a power balance so battery current times battery voltage equals motor current times (back EMF plus motor current times motor and wiring resistance)(minus the small losses in the controller itself). The ratio of motor current to battery current is therefore determined by these physical things, it is not controlled. Motor current is almost always larger than battery current, at low speeds it is many times larger, at high speed it gets closer to equal. At full throttle and high enough speed it may become the same.

Kelly is right to rate controllers in phase current. That's what really heats the FETs, especially at low to mid speed. The battery current isn't as much of an issue.

The motor doesn't really care about battery current.

The two problems when increasing phase (motor) current are heat and torque production. Beyond the linear region the torque produced by additional current drops, so the return on investment so to speak is poor. The heating goes up with current squared, so quickly the heat production exceeds the capability of the motor to shed heat. At some point a different motor is needed, additional current will only produce molten motors. See various flaming motor videos...

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flippy   1 MW

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Re: maxing out phase amps

Post by flippy » Aug 06 2016 4:42am

thank you for making it a bit more clear.

Q: would pushing 120A peak not instantly cook the windings? how do you figure out what the actual safe limit is of the motor?
if i get it right is basically like this:
for example, my 48v 2.5kW rated motor should basically max out at 58A (2500W / 48V).
but running at 120A phase and limit the battery current to 60A would simply cut the voltage in half that is fed to the motor in order to keep within that 2.5kW limit set on the controller side but the increase in current would still melt the windings as the motor does not care about voltage but only current.

ps: i am not running the motor on a bike, but on a 35mph/50kph scooter (no rim) and trying to tweak everything so i can blow petrol mopeds/scooters away at the lights but still have a reliable motor that can handle city traffic stops and starts constantly.
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Punx0r   100 GW

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Re: maxing out phase amps

Post by Punx0r » Aug 07 2016 2:44pm

58A is the "nominal" phase current for your motor. It's what you can run it at continuously without ever having any problems. As a crude guide, most motors will take around 5-10 times this rated power for short bursts, with the limiting factor being over-heating. If you can keep it cool, you can keep overloading it. Magnetic saturation does impose an upper limit, though.

It may reassure you that I have run 120A phase (4000W) into a cheap 1000W rated (in reality I believe the original design is actually rated for 500W) Goldenmotor hub. I worked up from 80A to 100A to 120A and found an increase in torque at each step (although I suspect 120A is at or close to the saturation point). The catch is that with no additional cooling it will overheat (stator iron at >110°C) from around 10 miles of running at 2000W.

Your 2.5kW motor should be just fine ;)

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Re: maxing out phase amps

Post by Alan B » Aug 08 2016 1:22am

The throttle determines the voltage the controller puts out - more throttle, more voltage through higher PWM duty cycle. The motor current that flows depends on the back EMF and the resistance. This resistance is pretty low, so at low speeds where back EMF is low the voltage available to force current through the motor resistance is high and extremely high currents are possible. The controller will sense the battery current and calculate (or measure) the motor current and limit the PWM to stay within limits.

Setting the battery and motor current limits to particular values doesn't change the ratio or set the voltage directly, it just causes the controller to limit the PWM when those values are reached. So it effectively limits the voltage the controller can send to the motor, but even this limit will change with speed as the back EMF changes. It is simple, but the results are not.

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Re: maxing out phase amps

Post by Hummina Shadeeba » Aug 21 2016 7:18pm

So the controller can convert the voltage to amps for the motor right? So u could have 20 amps from the battery yet many more going to the motor especially at low speeds?

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Re: maxing out phase amps

Post by Alan B » Aug 21 2016 7:25pm

Hummina Shadeeba wrote:So the controller can convert the voltage to amps for the motor right? So u could have 20 amps from the battery yet many more going to the motor especially at low speeds?
Yes.

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Re: maxing out phase amps

Post by Bullfrog » Feb 22 2020 11:27am

Question on Phase/Battery Amperage ratios and settings for a 12T MAC specifically....I have an 18 FET 4110 controller, 14s5p battery with HG2 cells and my BMS is rated for 50A continuous. Total weight for me and the bike is about 260 pounds and I am running my 12T MAC in a 26" rim with a 686mm OD Maxxis Hookworm tire.

Question #1
If I ignore what my freewheel clutch might be able to handle :lol: and I set my controller for a battery amperage maximum to 50A to protect my BMS, how high can I set my phase amperage or more accurately...how high "should" I set my phase amperage to maximize acceleration without turning my stator into a 58.5 volt toaster oven?

Question #2
What determines how and when magnetic saturation occurs...I realize "the design of the motor" is the real answer but is/are there one or two things that dominate/govern the phenomena at relatively low rpms? My guess is resistance of the windings and number of poles but I am no expert :wink: .

Comment...Alan B., your explanations on the effect of phase and battery amperage are awesome and easy to understand :D . Thank You!

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Re: maxing out phase amps

Post by Hummina Shadeeba » Feb 22 2020 3:55pm

1. I think would be hard to find what amount of current causes saturation and it happen by small degrees. I like 100 amp phase current and likely am saturating for moment with that setting. But ur motor is bigger.

2 the high current tries to create a magnetic field strength beyond what the stator is capable of. Only can get so much magnetic field strength and torque from so much magnetized iron


Does voltage matter with saturation?

An old thread. Thanks all for your help understanding

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Re: maxing out phase amps

Post by Bullfrog » Feb 23 2020 8:16am

Yes old thread but a lot of good information :wink: .

IMO, Yes voltage will affect saturation but only how quickly saturation occurs for a given motor. Voltage or Electromotive Force (EMF) is how hard/a measure of the force being exerted on the "amperage" being pushed through a circuit. And the amperage is a measure of volume...to make a "Mechanical" correlation I can understand :lol: . You can have high amperage with low voltage and saturation will take longer than high amperage AND high voltage. For real world motor behavior, I may be splitting hairs...I am just applying basic V=IR and the time constants may be so close that in the real world, going from 52 to 72 volts may not make a difference you can actually feel. I don't know for sure but I'd guess saturation occurs 1.4 times as quickly with a 72 volt battery as it does with a 52 volt battery i.e. the ratio of 72/52. Assuming all other things were equal...which is a stretch but since I am dealing with "theory", I can set the parameters :lol: .

I'd welcome an opinion from someone with more "electrical" expertise than me :D .
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Hummina Shadeeba   1 MW

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Re: maxing out phase amps

Post by Hummina Shadeeba » Feb 24 2020 12:57pm

Is voltage only relevant in that it is the source of the current that causes saturation or is there something about voltage exclusively that causes saturation? I thought was just current related

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Re: maxing out phase amps

Post by Bullfrog » Feb 24 2020 4:09pm

Everything is interrelated...Voltage, Current, Resistance, Magnetic Field, Electric Field...impossible to have one without the others in a completed circuit.

The faster the motor spins the faster losses build up...and by losses, I mean HEAT. Laminations, Iron poles, each little wire in the windings...anything conductive, each and every time one of those little magnets passes by a current is induced and with current the temperature rises (I squared R) and the resistance increases which means it heats up even more when the next magnet passes by.

So at some point, no matter how much you increase the voltage and amperage, all you are doing is producing more heat and very little increase in usable work from the motor.

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Re: maxing out phase amps

Post by Hummina Shadeeba » Feb 24 2020 4:50pm

Thought we were talking saturation which is a loss due to the magnetic field strength being at the limit of what the stator can produce no? Sure voltage is necessary to produce a current but I believe the current, amp turns and produced inductance, and stator size and shape and material alone determine if will saturate

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Re: maxing out phase amps

Post by Bullfrog » Feb 24 2020 9:41pm

We are but anything that inhibits current flow will in turn inhibit the ability of the motor to react to the permanent magnets magnetic field.

Found some good info at this web site that might help: https://www.quora.com/What-is-the-effec ... on-a-motor

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Re: maxing out phase amps

Post by Hummina Shadeeba » Feb 25 2020 12:54am

Bullfrog wrote:
Feb 24 2020 9:41pm
We are but anything that inhibits current flow will in turn inhibit the ability of the motor to react to the permanent magnets magnetic field.

neither of which has to do with saturating motors in my mind. what are you talking about.

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Re: maxing out phase amps

Post by John in CR » Feb 25 2020 2:19am

The electromagnets (the teeth of the stator around which the copper is wound) have a limit for how strong the magnetic force can become. More current generally means more force. Saturation is that limit, while the absolute maximum is a finite number, it's a gradual transition. You want to stay well away from saturation with any hubmotor, because they're so poor at dispersing heat. As current goes up, the heat due to resistance in the copper goes up by the square of current.

50A seems like way too much for that little motor to me. On flat easy terrain it might be fine, since peak current will be just for a few seconds early in acceleration, but get on a hill and it's likely to quickly fry the stator. If you really want to get an idea of what current it can handle, then measure the phase-to-phase resistance and you can easily calculate heat in the motor. Geared hubs are especially bad a shedding heat, so 200-300W of heat would be about the most I'd roast one with. If you added an oil bath with 100ml or so of transmission fluid, then you might be able to double that.

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Re: maxing out phase amps

Post by Hummina Shadeeba » Feb 25 2020 3:11am

John in CR wrote:
Feb 25 2020 2:19am
The electromagnets (the teeth of the stator around which the copper is wound) have a limit for how strong the magnetic force can become. More current generally means more force. Saturation is that limit, while the absolute maximum is a finite number, it's a gradual transition. You want to stay well away from saturation with any hubmotor, because they're so poor at dispersing heat. As current goes up, the heat due to resistance in the copper goes up by the square of current.

50A seems like way too much for that little motor to me. On flat easy terrain it might be fine, since peak current will be just for a few seconds early in acceleration, but get on a hill and it's likely to quickly fry the stator. If you really want to get an idea of what current it can handle, then measure the phase-to-phase resistance and you can easily calculate heat in the motor. Geared hubs are especially bad a shedding heat, so 200-300W of heat would be about the most I'd roast one with. If you added an oil bath with 100ml or so of transmission fluid, then you might be able to double that.
why you guys keep talking about the resistance of the winding related to saturation I don't know.

When at the point of saturation it has the effect of increasing ur kv, so same winding resistance but less torque per amp, coupled with greater hysteresis.


you can saturate any size stator with 1 amp if it has enough amp turns.

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Re: maxing out phase amps

Post by John in CR » Feb 25 2020 10:09am

Hummina Shadeeba wrote:
Feb 25 2020 3:11am
John in CR wrote:
Feb 25 2020 2:19am
The electromagnets (the teeth of the stator around which the copper is wound) have a limit for how strong the magnetic force can become. More current generally means more force. Saturation is that limit, while the absolute maximum is a finite number, it's a gradual transition. You want to stay well away from saturation with any hubmotor, because they're so poor at dispersing heat. As current goes up, the heat due to resistance in the copper goes up by the square of current.

50A seems like way too much for that little motor to me. On flat easy terrain it might be fine, since peak current will be just for a few seconds early in acceleration, but get on a hill and it's likely to quickly fry the stator. If you really want to get an idea of what current it can handle, then measure the phase-to-phase resistance and you can easily calculate heat in the motor. Geared hubs are especially bad a shedding heat, so 200-300W of heat would be about the most I'd roast one with. If you added an oil bath with 100ml or so of transmission fluid, then you might be able to double that.
why you guys keep talking about the resistance of the winding related to saturation I don't know.

When at the point of saturation it has the effect of increasing ur kv, so same winding resistance but less torque per amp, coupled with greater hysteresis.


you can saturate any size stator with 1 amp if it has enough amp turns.
Because it's pointless to worry about reaching saturation when you will burn the motor up due to resistance losses in the copper if you try to run the motor near saturation. Rovii can worry about saturation with his dragster emoto, since the motor will only run for about 10 seconds. Maybe with RC aircraft those outrunners can get enough forced air cooling to push near saturation without failure. On our ebikes you don't need to worry about stator saturation. You need to worry about heat in the copper, because it becomes a problem long before saturation.

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Re: maxing out phase amps

Post by Hummina Shadeeba » Feb 25 2020 11:48am

It takes a long time to get the motor windings hot while you can saturate a motor’s stator instantly.



Vedder’s new position control program he says will lose synch with 50 amps in a medium sized out runner due to saturation. I forget the exact size now but think was like 180kv. Makes me think when running other programs you wouldn’t know but I you’re slightly saturating. I have my little 4752 motor with 70kv set to 90 phase amps. I think thats at least half the kv he was using so I imagine will only need about half the current to start to saturate if were the same size. Even though It’s likely saturating for me with such high current on startup it still produces more torque and I get a smoother start off the line with no cogging. Less efficient but performs

Is there a good test to see at what current/field strength the motor is starting to saturate?

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Re: maxing out phase amps

Post by Alan B » Mar 07 2020 10:45pm

Justin has measured saturation on a number of motors, there's an old thread on the subject. http://www.endless-sphere.com/forums/vi ... =2&t=14494

It's hard to saturate a motor instantly due to the inductance of the winding, unless you have infinite voltage to drive it with. Justin found it difficult to measure the torque without overheating the motor near the saturation level. Even the short time required to measure the torque was enough to seriously heat the motor, and it took a long time for it to cool enough to make another measurement.

Justin's results indicated for real motors that the motor was overheating long before first saturation knee was reached. One data point I noted was for the 9C hubmotor this occurred at a torque level of 70 n-m developed from 468 amp-turns.

Beyond the first saturation knee the torque per amp dropped by 40%. Given the heat increases by the square of current this would suggest:

1) operate at the torque knee only for very brief periods of time (depends on thermal mass, but seconds, probably not minutes).

2) never operate beyond this first knee as the torque generation efficiency is dropping very fast.

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Re: maxing out phase amps

Post by Bullfrog » Mar 07 2020 10:55pm

Thanks Alan B...appreciate the reference for Justin's testing, he always has some thought provoking data :wink: .

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Re: maxing out phase amps

Post by Hummina Shadeeba » Mar 08 2020 11:16am

Alan B wrote:
Mar 07 2020 10:45pm
Justin has measured saturation on a number of motors, there's an old thread on the subject. http://www.endless-sphere.com/forums/vi ... =2&t=14494

It's hard to saturate a motor instantly due to the inductance of the winding, unless you have infinite voltage to drive it with. Justin found it difficult to measure the torque without overheating the motor near the saturation level. Even the short time required to measure the torque was enough to seriously heat the motor, and it took a long time for it to cool enough to make another measurement.

Justin's results indicated for real motors that the motor was overheating long before first saturation knee was reached. One data point I noted was for the 9C hubmotor this occurred at a torque level of 70 n-m developed from 468 amp-turns.

Beyond the first saturation knee the torque per amp dropped by 40%. Given the heat increases by the square of current this would suggest:

1) operate at the torque knee only for very brief periods of time (depends on thermal mass, but seconds, probably not minutes).

2) never operate beyond this first knee as the torque generation efficiency is dropping very fast.
Thanks for the link. Using the search function here rarely gets to the goal I found

As I understand the two tests by Justin the first doesn’t show saturation as the 38 amps was inadequate. The second test does show saturation but also with the repeated testing and small phase wires the phase wire insulation started to melt. It still seems u can instantly saturate a stator before heat becomes a problem and in the second test if the dc current were 70 amps from the start when cold there will be the same saturation, no?

inductance increases slightly with heat so with a hot winding saturation would come at the slightest bit lower current but still the same torque producible as determined by the stator material and shape. I think.

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