# Battery to motor voltage

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fazekas
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Battery to motor voltage

Dear all,

allthough being a newbie to electric drives, I did study EE but am not best informed on BLDC. My collegue and I have the honor to build a car prototype which drives only with 6-10 km/h and we have a 24 V LiIon battery (7S8P). Now I have the feeling that our little motor, which is a 15 W, 24V motor rated for the 8S7P will juts not have enough power. So i think of changing the motor but have the difficulty finding one, which is rated for 7S8P. (24V).

So my question is: How does the battery voltage relate to the rated motor voltage? If e.g. I would choose a great Hacker motor which is only rated for 8.4 V, can I use it with my 24V battery on the vesc and what is the consequence? Trivially, i would say that the max available power of the motor will only be 1/3 of the one given at the datasheet, because I limit the current in vesc tool but the voltage is only 1/3. But is this true? And why do so many good motors have no real voltage rating? (https://www.hacker-motor-shop.com/Elektromotoren/Hacker-Innenlaeufer/Hac...

What if I use a higher voltage battery, than the motor likes?

As a follow-up question I would also ask: what does this mean in theory? We use FOC controlling, which also generates some kind of pwm signal for the motor. Is the voltage "height" of the pwm signal equal to the battery voltage? so if the battery delivers 24V, and the battery is only rated for 8.4V, than will this fry the motor due to overvoltage?

I am totally confused, althouth I do understand the relation between amps from battery and to motor (basically keeping the input/output power equal).

Thank you very much for your help!

danilolattaro
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Joined: 2021-01-17 23:26
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In general, motor rated voltages are irrelevant on BLDC or FOC applications with the VESC limiting the current.

Voltage is just an issue if you go beyond the dielectric constant of the enamel used on the copper wires
which should already be about 200V minimum.
These motor coils have like 0.1ohms resistance. If you ignore the inductance, 10Vdc through it would already produce 100A. So in really simple terms (disregarding the inductance factor on the PMW signal), anything above 10V all the way to 200V would be the same for the motor, if limited to 100A (for example).
The VESC will do that by limiting the duty cycle of the FOC PWM.

On your specific application, as long as you limit the Motor current to safe levels and limit also the motor max speed, you could just as well use battery packs within the limit of what the VESC hardware can take (13s for VESC 4 and 6).
Yes, the FOC works with PWM, and for every cycle the modulated voltage is the battery voltage. However it gets "smoothed" by the extremely short pulses (20~25kHz PWM normally) and the coil inductance, and what the motor "sees" is actually lower. It would only see the full voltage at full throttle unloaded, which is when the back EMF from the motor (kv * rpm) reaches the same value as the battery voltage.

If you overspeed your motor it could be dangerous. So limit your max current, limit your max power, limit your max erpm and correctly set the number of magnets.

fazekas
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Hi,

thank you for the valuable insight!

So then the battery is mostly irrelevant (given the well defined current limits). Would it be safe to say, that the motor max power is also irrelevant, as long as the rpm at max duty cycle is the same? So if i'd take a max 100W motor and a max 400W motor, they would  draw similarly little amount of current (and power) for the same vehicle speed as long as they are both rated at the same max rpm!? I actually do not think that we will need more than 40W peak power. Weight of the motor is not an issue in this case! Would there be any good reason to not take an overpowered motor besides weight and price?

I would even expect that I can even take a "faster" motor with gearing and still use the same amount of power for low speeds. (although probably gearing reduces efficiency a little)

Thanks again!