Volts versus Amps

[Freddyflatfoot]

I came across a thread last night, and somewhere there was some discussion on the My1018 motor, and the power outputs that you can get from this little motor.
Ok, so (crudely speaking), power out is a linear relationship between volts, amps and efficiency. (volts x amps x % eff = power)
So if efficiency is relatively fixed, then I can change the other two variables, volts and amps, to increase the power output.
So my current setup, 24v, 15A, and ~80% eff = 288 watts.
I can increase my power by increasing the Amps by modding or using a different controller, say 20A, for 388 watts,
or I can increase the voltage to 36v, and say a 13 A controller for 375 watts.
So roughly the same power output, but in practical terms, how much differently does the motor behave?
I reckon Safe may have already tried this, and I do know that an increase in volts correlates to an increase in rpm, and an increase in amps correlates to an increase in torque. Although this is the standard way of thinking of this, I suspect that there may be an increase in both RPM's and torque for both changes, just one favouring torque, and the other favouring revs.
I also suspect that Safe will have a graph somewhere on this, showing the correlation.
But graphs don't show you how it relates to the bum in seat experience.
Can someone explain to me what would happen, in a practical sense, comparing the power increase in Amps, and then the same power increase in volts?
Thanks.

 

[Drunkskunk]

Well, to simplify things, ignore the efficancy for now. it actualy has no imediate bearing on the outcome of the voltage and amperage relationship, Also known as Ohm's law.

More voltage does equil more RPM, but it also means more torque.
RPM is almost exclusivly related to voltage. more voltage, more RPM.
However Torque is not related to amps in the same way. Its actualy Watts that effects Torque. More Watts, More torque. If you increase the amperage, while staying at the same voltage, your wattage increases and you get more torque.
So said another way,
Voltage = Speed
Wattage = Torque

Increase Voltage = Increase Speed and wattage, so = increase torque
Increase amperage = increase wattage and torque only

 

[safe]

Power = Torque * RPM

...so that's one way to think of it.

So as you overvolt you get more rpms and if the torque is left constant (same amps) then you get more power assuming that you are in the same spot (relatively speaking) in the powerband. (same efficiency)

But we've forgotten the BIGGEST factor for small motors...

Heat

As you increase the amps you get an increase in heat. So if you look at how the heat rises across the powerband (mostly at low rpm because of that PWM motor inductance effect that boosts the amps) you begin to realize how overvolting alone is not a very good idea unless you do something to dampen down that low end heat. This is where you think to yourself:

"Oh, I've got an idea ... why not use Armature Current Limiting to prevent that low end heat. "

...so it's a natural evolution of thought where you progress from one step to the next. (until TylerDurden steps in to remind you that "all change is bad" and you should never aspire to having more power :lol: )

 

[Dr. Shock]

I can't add much to what safe said, except to give you the practical example of power lines. To get power to your house, the electric company could try to send 120v all the way from their generator to your house, but the resistance in the wires would turn most or all of it to heat. What they do instead is jack the voltage up, sometimes to hundreds of thousands of volts, but not very many amps, and then bring it down to 120v (actually two lines of 120 in opposite electrical phases for most residences in North America) through a series of transformers once it gets close to your house.

That's because resistance is an effect on current, but not voltage (roughly speaking-- current and voltage are actually inter-related, but let's not go there.) So for a given electrical apparatus, it is generally less stress to raise the voltage than the amps, though as anyone who has run 100v through a 35v capacitor can tell you, there are limits.

 

[Freddyflatfoot]

Ok, I sorta knew all that. Was thinking about how it all relates to the riding experience, rather than electrical theory.
I already know that going to 36v will give me a very healthy boost in speed! .
However, I am uncertain what a corresponding increase in amps will do.
Another graph from Safe showing what happens to the motor performance, when amps only are increased?
Oh, BTW Safe, would be very handy if you label the axis on your graphs. Might make it a bit easier to understand.

I can only take from the comments, that if I increase amps only, then my bike will still have the same top speed, but will be able to maintain that speed better when climbing hills? (So still faster average?)
So, at this point, I have the choice of doing two things, one, increase the amps. Simple alteration to the shunt in the controller, no cost.
Second, add more batteries! Well, I have already tried an extra lead anchor, yep, extra speed, but hated the extra weight. (Unless I use smaller batteries, then my range would really suck!)
New batteries? Would be nice, but can't really afford more LiPo, unless I was gonna put it my trike!(Currently on a lead diet.)

Actually Safe, what I was really looking for, was two graphs, one showing the motor performance in relation to watts, power and speed, when volts only are increased.
And the second, showing the same motor performance when amps only are increased.

 

[safe]

Upload First...

 

[safe]

Voltage Constant, Variable Amps

First of all these three charts are of the same motor with the same voltage, but just different current limits. (amps) They all represent the same peak rpms (the bottom axis) since they all use the same voltage. This happens to be the 1016Z3 Unite motor.

Notice how a small increase in power is traded for a pretty big increase in heat in watts.

That's not good.

Efficiency expresses this loss in terms of heat... heat IS inefficiency and more current (amps) means more heat, so (logically) when you increase the amps you increase the heat and lower the efficiency.

Also notice that an increase in the current limit (amps) produces a LOWER power peak rpm. (the peak moves to the left in the power chart) This means that not only do you get more power, but it's also at a lower place in the powerband. This makes for a wider powerband. (this is a good thing)

 

[Freddyflatfoot]

Thanks Safe!
Much appreciated, I think I have a better understanding now.
Surprised to see a lower max rpm though. In practical terms, how much lower?
Still hard to see without any label/markings on the axis.
Ok, so I get a slower motor, create more heat, but get a wider power band?
Hmm, can see why guys just simply go for extra volts, just for the extra speed!
Extra torque is probably a bit harder to quantify.
Safe, in your opinion, if I were to stick with 24v (I have a 10 AHr LiFePo4 battery that I have invested in)
what would be the recommended amps that you would pump into the 1018 motor to get satisfactory performance?
Thanks

 

[safe]

Constant Amps and Variable Voltage

Before we jump ahead to the "finale" I want to talk about what the overvolting looks like. This chart has the motor rpm along the bottom (like all of them before) and at maximum rpm for the 24V curve you have 4400 rpm and at 36V it's 6600 rpm.

Notice how if you keep the current limit a constant (in this case 30 amps) that the power peak shifts to a location much higher in the rpm than before. This might mean that depending on your gearing you might be getting your peak power at 30 mph rather than 15 mph before. If you also look at the heat production you see that at the same lower rpm where the 24V motor ran very cool it's now roughly DOUBLE the heat. So it's a mixed picture... on the one hand your peak power is doubled, but it occurs later, and all the while the heat at low rpms is nearly doubled too.

How might this feel?

Well it's going to feel like you have more power than before, but it's also going to overheat your motor because you will be TEMPTED to hold the throttle open all the time because the power is so much fun. Again, this is my argument for something like Armature Current Limiting in that it eliminates the temptation to use too much throttle at low rpms. Hills are a very powerful source of temptation because it feels weird to have to intellectually think about backing off the throttle when your emotions are telling you to go faster.

The chart scale is in terms of "watts" on the y-axis and "rpm" on the x-axis.

 

[safe]

"Rated Load" Means "Rated Heat"

There's a subtle message when a manufacturer lists a "Rated Load" for their motor. What that really means in practical terms is that this is the load on which the motor is capable of running indefinitely WITHOUT OVERHEATING. I could imagine some more precise way of doing this that expressed the motor in terms of it's heat dissipation ability (maybe something like a heat dissipation gradient) or something like that but for now the way it's done is with a rated load. This allows the manufacurer some flexibility in what they "declare" as their official rated load, but suffice to say it's a GUIDELINE about how much abuse the motor can take.

So first let's look at a 250 watt (1016Z3 Unite) motor running at 24V and using a very conservative 10 amp current limit. It's hard to see on this chart but the "rated heat" line (magenta) has covered over the "average heat" (orange) line. This motor would run so cold that you are unlikely to be able to overheat it. It's efficient, but not powerful.

Okay, now we let the current limit rise to 20 amps and see that the "average heat" is about twice the "rated heat". This motor would overheat slightly if used constantly at full throttle, but considering that most batteries run empty in only about half an hour or so it's likely that the battery will run low before the heat gets to be too much. 20 amps gives a lot more power, double the heat, and lower efficiency. This is "okay" to use and is basically what the manufacturer is expecting to happen.

Finally we look at the motor with a 30 amp current limit. Now we have some SERIOUS heat to worry about and frankly at this point you need to either think about Armature Current Limiting or take a class in "Advanced Throttle Fiddling 101" from someone like Xyster. :wink: If you tried to hold this motor with this current limit at full throttle all the time you will end up with smoke and maybe fire. You get lot's of power, but it might not last long. Cooling techniques like forced air cooling or even ice or water cooling to the shell might help a lot to keep this motor alive. Gearing is also another big factor, if you have a low enough gearing then you can run this at high enough rpms to avoid heating problems. Too high of a gear ratio and you are toast.

Don't forget that the red line means heat...

 

[Freddyflatfoot]

Great stuff Safe!
I can see the gearing is no problem, just drop her down to a lower gear? Do that anyway when climbing. Well I always drop to the gear the motor feels comfortable at, and not losing speed. Or I pedal to assist. End result is the same! I do have 18 speeds to play with! Too many? Probably.But I use 9 for road use, and I have 28t granny chainring that would only get used off road. Guess what? Basically never used it!
So I think I have the options to reduce the heat.
My current overload protection, is a 20 amp breaker, so I'll crack open the controller and mod the shunt, and see if I can get near the 20 amp mark. (Last time I tried this just using solder, and gained a massive o.5A, )
Will let you know how it all goes.

 

[safe]

At only around 20 amps you should be fine...

In my project I'm planning to jump to 36 volts, INCREASE the current limit to a scary 40 amps and add Armature Current Limiting and wrap the motor shell in an Ice Cooling container.

This is either going to produce some "big time" power or a really exciting explosion and fire. :lol:

Notice that the vertical scale on this one goes all the way up to 1000 watts rather than the 600 watts on the others.

 

[Freddyflatfoot]

Ok Safe,
I added a length of resistor leg wire to the shunt, and soldered it along the length of the shunt, and tried it around my street, under load, up a slight hill.
The first thing I noticed, was that the motor seemed more responsive. I loaded up the motor as much as I could, and saw 19.7A on the Watts Up.
Plenty close to my target, so we'll see how we go.
Actually, have to go down town later on today, so I'll take the MTB. Should give me a pretty good idea, and then I'll report back.
I have to go to my LBS, as I went to change the rear tyre, and found 6, yes 6 broken spokes! Bugger, I hope this isn't going to be an ongoing issue. The trike has only done about 700 k's on the hub motor.
Anyway, I'll repost later today with how it all went.

 

[Freddyflatfoot]

UPDATE
I went for a shortish ride (13 k's), motor was definitely pulling stronger.
Can cruise at 30 - 33 k's no probs. Still slows down on hills, but that's to be expected.
Most of my running was at the amps I would normally run at, by keeping the rpm up. If I loaded the motor by going up a gear, could get the amps to rise, but not by a lot.
Most noticeable differences were in the gear changes, definitely a lot more clunky! Will have to learn to back off the power before I change a gear!
Also notice that i now have some throttle control, whereas before I was running flat out. I find I can hold my speed at about half throttle.
On way back to home, just around the corner, I deliberately loaded the motor up, Pulled really well as the revs dropped off......................... and then the motor cut out. Not sure if it was the controller, or the overload. Wasn't fatal though.
System reset after about 5 mins, so was probably the current overload that worked.So may need to get a higher rated fuse or current overload switch.
Anyway, will need to do some more testing at a later time, to see how the hill climbing ability has improved.
Oh, heat wasn't an issue, both controller and motor were both cool.
Max watt draw was around 450 watts, and I think I saw a peak in A of ~22. Lost all the data when the power cut-out. I think that means the motor system is now around 360 watts.
Probably wont mod the controller any further though.

 

[BiGH]

I have to go to my LBS, as I went to change the rear tyre, and found 6, yes 6 broken spokes! Bugger, I hope this isn't going to be an ongoing issue. The trike has only done about 700 k's on the hub motor.
Anyway, I'll repost later today with how it all went.

that can be a bit of a problem - welcome to my world. did 13 in total on the one wheel. I suggest you get some high quality spokes, a good book on wheelbuilding and build it yourself carefully. this should eliminate the problem. I found my motor came with some very loose spokes.

I'm thinking of investing in a spoke tensioner tool.

 

[Freddyflatfoot]

BigH,
I had my wheel laced by my LBS, shouldn't have been an issue. Unless its a combination of the extra weight and some rough roads?

 

[safe]

The secret on spoke breakage is to start with fresh new spokes and make up your wheel with high spoke tension. Over time the tension will loosen up and you need to go back and do a second round of tightening to get the stretched spokes to be tight again. Once you get the fully stretched and settled spokes tight enough then you will be okay. It's loose spokes that cause fatigue.

But if the spokes have already been fatigued then tightening them up makes them break... so it's a tricky balance... once the spokes start to absorb a lot of fatigue they are doomed to continue to break.

People tend to get the wheel tightened and trued at the beginning and then forget about them. When the spoke tension becomes less that's when the real damage is done.

Once those spokes go loose and they fatigue they are junk...