PWM current multiplication effect Practical Branch

[xyster]

The PWM current multiplication thread is most engaging and theoretical, but I had a related, practical question to ask:

Knoxie's recently written here (somewhere... ) that his Puma geared hubmotor puts out lots of heat at 72v 35a; that he thinks 60v is a good sweet spot; but he'd like more power and so is going to try and get it without added heat by raising the voltage using a modded 150 volt crystalyte controller.

Because of the PWM Effect, is his approach likely to do anything but increase Amps to the motor, just as would increasing only the controller's current limit?

We know, minus controller losses, that
Volts In X Amps In = Average Volts Out X Amps Out

Let's say Knoxie's cruising uphill at 20 mph with 72v 35a (2520 watts) and the throttle half open such that the motor "sees" 36v 70a. Now let's say Knoxie uses his high voltage controller to double the input the power to 144v 35a (5040 watts). To go the same speed up the same hill, he needs to adjust the controller to 1/4, so that the motor "sees" the same 36v as before, correct?
But if he does that, the motor is seeing 36v 140amps, which is double the amps as before, correct?

In which case it seems the increased voltage bought only more amperage, the heat produced being no different than if he was running a 72 volts 70 amp controller.

But of course he only needs 2520 watts to go 20mph up this hill, so if the throttle is held steady the motor will draw exactly the same 36v 35a as before. So it's not until Knoxie cranks on the throttle that the 144v system will draw double power, accelerating up the hill faster...but when that happens, the heat will be exactly the same with 144v35a going in as at it would be with 72v70a going in. Correct?

If true, because of PWM, doubling the voltage makes no difference in cruising heat losses, and the same difference in full-power WOT heat losses that doubling the current limit does. Which means Knoxie can not make his motor any more efficient at high power by raising the voltage instead of the current limit.

Am I misunderstanding anything?

 

[Matt Gruber]

in practice all the direct hub motors are dogs at lower voltage.
they wake up at HIGHER V. not by building a million volt pack and giving it just enough throttle to go the same speed.
if knoxie's overheats at 72v, that is it, 60 is best. OK fine, end of story. a million volts won't make it cooler.

well he could measure the wheel up amps and look for a sweeter spot. maybe 63v? 66v? 68v?

 

[Leeps]

I would say you pretty much got it. If the load is the same then thats all there is to it when your cruising.
The only benefit i see as far as motor heating goes, and the math has to be done to see if there is a benefit, is to raise voltage and raise the gear ratio at the same time. This way there is a lower torque load seen at the motor and it will thus draw less amperage.

If acceleration is what he wants, then without changing the gearing, the only way to get it is to raise torque; torque = amps=heat. If he wants higher speed its the same deal your going to need more torque to fight wind resistance. Wanting higher power and a cooler motor is chasing your own tail, something has to give. 80% efficiency and 1000 watts is 200 watts of heat, 85% efficiency and 2000 watts is 300 watts of heat, see my point.

Joe

 

[Matt Gruber]

practical use:
say u have 36v 35a motor
but don't want to put 2 packs in parallel to get 35a
u can put them in series72v 18a, run at 1/2 throttle and the motor still gets 36v 35a while the packs are at 18a.
neat, huh?

and the big bonus is, with a tailwind u can zip along MUCH faster due to the light load

 

[Matt Gruber]

also
to get more range u only have to add cells in series
and let the controller divide things up.
of course u have to go the same speed as before, so u need an easy to work throttle or a pot in series.
lots of times there isn't room to double up the whole pack, or it is too costly or too heavy. so if u only need 10-30% more range, series can do the trick, if the controller can handle the extra volts.

 

[fechter]

Doubling the voltage will have nearly the same effect at low speeds as doubling the current limit. The motor will see about double the current.

You could reduce the current limit based on motor heating and run any voltage you want.

The maximum no load speed will be double with twice the voltage.

If you wanted to run Nimh batteries and didn't want to deal with the problems of running strings in parallel, I could see a benefit to running a higher voltage.

 

[xyster]

If you wanted to run Nimh batteries and didn't want to deal with the problems of running strings in parallel, I could see a benefit to running a higher voltage.

Wouldn't there also be a benefit in less resistance between the batteries and the controller?

 

[fechter]

Yes, that's also true. Less I2R losses in the batteries and battery to controller wiring.

 

[Meuh]

Current multiplication work specialy good on "long motor" (like HS crystalyte with 80-100v voltage).
With a current meter on Battery, you can view that on start :
- 20A battery = 50A on phases motor
- if motor are on flat, the current battery raise to 50A when current on phases motor stay at 50A during the start-up.

it's the current multiplication because on low RPM, the controller use a 12v or 24v PWM tension with a "multiplication" of current (restricted by program, mosfet conception or manufactured controller like Kelly).

on proper controller, at start-up, phase current can raise 350-600A with a steady 150A at battery.

 

[John in CR]

You guys are leaving out the part that with higher voltage acceleration is less efficient for the same rpm. The lower throttle position doesn't make up for it, so there's no way around more heat. That's part of the reason he experienced more motor heat problems at 72V vs 60V.

FWIW, using a high voltage controller on a geared hubbie would be a complete waste, because part of their design includes the motor itself operating at it's more optimum rpm. With direct drives you typically need to use a smaller wheel to get closer to their optimum rpm range.