Driving a low voltage motor with high voltage PWM

[paulr]

Hi.

Are there any disadvantages to driving motor with a PWM signal at a much higher voltage than it is designed for? For example, driving a 3v motor with 12v, using a duty cycle of 0 - 25% to avoid overloading the motor.

I'm curious about the effect on:

- Noise, both electromagnetic and audible
- Efficiency
- ...and other consequences I haven't thought of.

Thanks,

Paul

 

[rjvh]

puls with modulation is a form of controling the energy in the motor while the voltage is constant

so if you have a 12 Volt PWM control than the voltage over the 3V motor will be 12V (4X more as it is designed for)

it'just another way of blowing the 3V motor winding

Robert-Jan

 

[paulr]

Thanks for your reply rjvh. That's why I'd use a duty cycle of 0-25%. Or probably less because of the nonlinear relationship between voltage and power. I believe most motors can withstand over-voltage of up to 10x for short periods, because motor windings are usually blown by overheating, not over-voltage.

As long as the duty cycle is throttled back to a safe level to avoid overheating, I see no reason why the motor would be harmed.

 

[dknguyen]

THe disadvantage is although your average voltage is the same, the average voltage is being achieved by higher voltage pulses of a shorter duration. If the motor can't actually handle these peak voltages...well...the winding insulation breaks down- there is more to motor life than just heat (not too much, but it comes into play when you start pushing the motor).

For example, not knowing anything about motors and just with common sense and intuition, does this sound reasonable? I have a 10V motor but a 1000V power supply. Does common sense say that I could run PWM through it at a 0.01% duty cycle? THere are real world limits, and it depends on how overdesigned the motor is.

Usually PWM is used to make a motor be able to operate at at variable speeds at it's rated voltage supply- not to run a motor off a much higher voltage supply.

It might be wiser to put a simple buck converter between the motor and power supply and PWM the buck converter instead. I've heard of overvolting motors by 2x, but not 4x. Really, all it would take is an additional capacitor and inductor along with the transistor and PWM source which both should already be there. Trick is finding an inductor able to handle the current. It might be enough just to put a large capacitor in parallel with the motor to smooth the voltage ripple out (the inductance in the motor itself will take care of the current ripple, but you knew that).

 

[rjvh]

Thanks for your reply rjvh. That's why I'd use a duty cycle of 0-25%. Or probably less because of the nonlinear relationship between voltage and power. I believe most motors can withstand over-voltage of up to 10x for short periods, because motor windings are usually blown by overheating, not over-voltage.

As long as the duty cycle is throttled back to a safe level to avoid overheating, I see no reason why the motor would be harmed.

I depends on the motor (how it's constructed) the thing is that every time the motor gets a puls it will create heat energy

you're right that if the motor can withstand the voltage for a short period than how long is that period and is it usable for the aplication

just spining a motor for nothing (no load) is In my opinion a waste of energy

Robert-Jan

 

[paulr]

Thanks dknguyen, that's very helpful.

The application I have in mind is to drive a 3v motor with 12v, at very low power, well within the motor's capability. But I'm trying to figure out the principles, so to take your extreme example of a 10v motor and 1000v supply, no that does not sound reasonable, but I want to understand all the factors that should be taken into account when making such a decision.

So I'll start a list:

- PWM duty cycle must be low enough to avoid overheating motor
- Voltage applied to motor must not be enough to break down winding insulation

Are there any more factors? Particularly regarding noise and efficiency?

Also, on the subject of adding a buck converter, I'm trying to keep the part count of my circuit as low as possible, so I want to look at ways to exploit the inductance of the motor to smooth current flow. But I want bi-directional control, so I need an H-bridge, which complicates things a little. I might have an idea about that though, which I'll post later if I have time. For now I want to complete my list!

 

[Ubergeek63]

a current mode pwm chip to run at 100KHz.

audible noise should not be a problem.

the enamel on the windings should be fine for 500V and at 12V you should have no problem catching it before the motor saturates. there are THREE ways to blow the motor: arcing over the windings or bearings, burning out the windings, and demagging it.

Dan