Getting max power using the motor and battery amp limits

General topics and discussions about the VESC and its development.
Hummie
Posts: 105
Joined: 10 May 2016, 04:05

Getting max power using the motor and battery amp limits

will this method/formula get me safely to a determined max power?

:start with the question: "how many watts do I want available full throttle very close to 0 rpm?"

If the answer is: 1000 watts

Battery Voltage: 50V
Winding Resistance: 0.12566ohm

Ohm's Law:
(Calculate Effective Motor AC Volts & Motor AC Amps based on 1000 Watts & 0.12566 ohm Winding Resistance)

1000 Watts = 11.20982 "Effective" Volts Motor AC / 0.12566 ohm Winding Resistance = 89.20752 Motor Amps AC for 1000 watts full throttle very close to 0rpm

1000 Watts = 50V Battery Volts DC x 20 Battery Amps DC

22.41% Duty Cycle full throttle very close to 0rpm = (11.20982 "Effective" Volts Motor AC / 50V Battery Volts DC) x 100

For 1000 watts full throttle very close to 0rpm (assuming 50V DC & 0.12566 ohm winding resistance):

22.41% Duty Cycle @ 50V

20 Amps DC x 50V DC

89.20752 Motor Amps AC x 11.20982 "Effective" Volts Motor AC

Batt Max Setting: 20 amps Motor Max Setting: 89.20752 amps Absolute Max Setting: 89.20752 amps
---------------

Is it safe to put my motor amp limit at 10000 if my battery limit is 20 and running 50v assuming my motor resistance above?

artteth
Posts: 28
Joined: 22 Jul 2016, 00:28
Location: Belarus

Re: Getting max power using the motor and battery amp limits

in bldc tool 200 amp is max that you can type in
mosfet max amp is 240 (limited by package ) but can do more for small time period (if i understand all correct )
and vesc with 0.5 miliohm shunt i set 200 motor amps and all ok but in vesc with 1miliohm shunt i set 160 and drv chip is dead (will test more when spare drv chips will come)

arvidb
Posts: 205
Joined: 26 Dec 2015, 14:38
Location: Sweden, Stockholm

Re: Getting max power using the motor and battery amp limits

Hummie wrote:will this method/formula get me safely to a determined max power?

:start with the question: "how many watts do I want available full throttle very close to 0 rpm?"

If the answer is: 1000 watts

Do you really want to burn up 1000 W in the motor windings? (Very close to 0 rpm equals very close to 0 watts of output power. So efficiency will be almost zero.)

Hummie
Posts: 105
Joined: 10 May 2016, 04:05

Re: Getting max power using the motor and battery amp limits

Is that 240 amp limit continuous? that's huge. at very slow speeds there will be a low duty cycle and therefore whatever amps are being applied to the motor would be for a fraction of a second no?

I pull 1000 watts from the battery per motor and I cant see when it's happening but I thought it would be at the initial start up moments and that would be desirable.

pf26
Posts: 278
Joined: 28 Mar 2016, 14:37
Location: FR Valence

Re: Getting max power using the motor and battery amp limits

@Hummie, I think your calculation is rather correct (as for BLDC mode).

With 1000W dissipated internally, your motor windings will heat up quickly (and their internal resistance also will increase due to heat, which make it worse). Roughly speaking, it takes 0.38 Joules to increase the temperature of 1 gram of copper of 1 degree. 1000W during 1 second releases 1000 Joules. so this translate into 26° increase for a motor with 100g of copper windings..
A motor of a few 100's gram can only dissipate only a few 10s of watts continuously.
Also, beware of the max motor amps as for demagnetization (because you might deteriorate the magnets before you burn the windings).

Hummie
Posts: 105
Joined: 10 May 2016, 04:05

Re: Getting max power using the motor and battery amp limits

Was vedder's overlay video done in bldc or FOC? Do u think the equation will be different depending on the different program?
But why would u think 1000watts at close to zero rpm would be more of a problem than at a higher speed? Just a bit more wind but otherwise isn't it the same? I feel lthe more freedom to supply amps to the motor at any speed the better. True u damage the motor but that can happen equally at any speed

rew
Posts: 904
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Getting max power using the motor and battery amp limits

Hummie wrote:Is that 240 amp limit continuous? that's huge.
That is MAX continuous according to the manufacturer. This is determined by the marketing guys asking the engineers if they can't get a better value. The engineers end up putting big cooling devices on the device under test and they manage to squeeze out even more amps....

If your motor is specified at 50V an 1000W you cannot do 1000W at 0 RPM. Your numbers sound as if you have this kind of motor.

At 1000W output power, there is 20A running through the motor, and you're getting .125*20 = 2.5V across the motor. At 20A that means 50W in the motor. With 1000W of output, 95% efficient is respectable. However as the motor is specified at 1000W and not at 2000W,
the allowed current is 20A and not 40A. The motor can handle 50W of dissipated energy in the motor not 200W.

Now with your example, you're trying to put 1000W into the motor. That's about 20 times more than it can handle.

So, for longer times, your motor can simply handle 20A, not more. If you want a boost to get away from the traffic light, you can set the motor limit to say 40A, allowing you a higher current when moving slower. You're hoping that you don't need that for a longer time so that you're burning up the motor with the 200W of lost-power in the motor. This will probably work fine, until you need to drive up a mountain. You'll be using 40A for a longer time and burn up the motor....

Hummie
Posts: 105
Joined: 10 May 2016, 04:05

Re: Getting max power using the motor and battery amp limits

I'm set to 70/70. The motors can't do that continuously but it's great for blasting up hills as u say. I'd like to get as much power out for short bursts and keep pushing numbers higher. Can I do this safely since there's a auto shut-off if things overheat on the vesc? I think I will know before I heat the motor too much. At some point the amps will be so much as to over saturate the stator with magnetism and heat will build quickly I guess

Am I wrong in understanding that the max battery amp limit determines the ultimate power and the max motor amp limit determines the max power at low speed?

rew
Posts: 904
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Getting max power using the motor and battery amp limits

On my board, the measured temperature continues to rise even after a "too hot shut down". This is because there is some delay for heat reaching the sensor. So although the sensor is set to cut things off at a safe temperature, that only protects against slowly overheating due to long periods of "close to the limit". If you quickly reach the max temperature, the sensor may take a while to heat up and activate the shutdown.

Hummie, you propose putting 1000W into losses in your motor, and ask if that's ok. I then explain that I took a few guesses as to what your situation is, and explain that it probably isn't OK. You then totally ignore my answer and ask different questions. It would be polite for you to acknowledge my answer. "OK, you guessed completely right.", or "no those guesses are completely wrong".

Hummie
Posts: 105
Joined: 10 May 2016, 04:05

Re: Getting max power using the motor and battery amp limits

sorry if you were expecting the motor stated limits there aren't any. I've got temp limits of the magnets and wire enamel but that's it. the motors are 4725 stators and the amount of power they can take is unknown and I'm trying to find out the max power possible. I appreciate your knowledge and help not trying to piss you off.
maybe I should bring the temp limit lower when trying to figure the peak power out to account for the heat transfer delay to the sensor.