Driving DC motor rated at 24VDC@2A Max with 50VDC@10A 40% duty cycle possible?

[sghosh]

Upcoming project I need to speed control DC motor rated at 24VDC@2A Max.

Lab has 12VDC@2A and 50VDC@10A PSU only.

I thinking why not try run DC motor rated at 24VDC@2A Max with 50VDC@10A PSU if I keep 40% or less duty cycle?

PWM frequency is be 20kHz or more.

Is this good idea without burning or damaging that motor?

If this possible, what maximum voltage I can use (after adjust duty cycle properly).

For example, will it be good to try run this same motor off 100V at 20% or less duty cycle?

 

[magnatro]

i think its not a good idea. the motor ratings is very low compared to input voltage. i think in practical use, owning to the back emf, stray capacitance etc in the circuit, f there is an integration of the signal, the motor will get messed up.

 

[crutschow]

It should work ok if there are no electronics in the motor (e.g. it is not a brushless type). The motor inductance will average the PWM signal to give an average voltage equal to the PWM duty cycle percentage times the peak voltage.

 

[DemonicSandwich]

While a PWM would average voltage and current in the motor, it would cause brushes to wear out quicker because the higher current arcing in the commutator.

 

[BrownOut]

How did you come up with 40%? Doubling the voltage quadruples the power, so I'd say it should be more like 25% duty cycle.

 

[crutschow]

How did you come up with 40%? Doubling the voltage quadruples the power, so I'd say it should be more like 25% duty cycle.

Since the motor inductance averages the voltage, it's the average that we need. V squared doesn't enter into the calculation. He wants 24V average from a 50V supply. The average voltage is equal to the peak voltage times the PWM duty cycle so he needs a 24/50 = 48% duty cycle.

 

[BrownOut]

Thanks Carl.

 

[misterT]

Upcoming project I need to speed control DC motor rated at 24VDC@2A Max.

Lab has 12VDC@2A and 50VDC@10A PSU only.

I thinking why not try run DC motor rated at 24VDC@2A Max with 50VDC@10A PSU if I keep 40% or less duty cycle?

PWM frequency is be 20kHz or more.

Is this good idea without burning or damaging that motor?

If this possible, what maximum voltage I can use (after adjust duty cycle properly).

For example, will it be good to try run this same motor off 100V at 20% or less duty cycle?

That is ok if you can keep the current ripple small enough.
Put some inductance in series with the motor and keep PWM frequency as high as possible. 20 kHz is not high enough in many cases.

 

[4pyros]

Lab has 12VDC@2A and 50VDC@10A PSU only.

Does your lab have more than one 12 volt supply? Hook two in series for 24 volt.
Why a 50 volt supply thats odd, is it not adjustible? You should try and reduce your 50 volts to 24 or 25 volts for your motor. Cut the voltage in half, maybe change transformer taps.
Andy

 

[sghosh]

Since the motor inductance averages the voltage, it's the average that we need. V squared doesn't enter into the calculation. He wants 24V average from a 50V supply. The average voltage is equal to the peak voltage times the PWM duty cycle so he needs a 24/50 = 48% duty cycle.

Thanks you for confirming my thinking!

That is ok if you can keep the current ripple small enough.
Put some inductance in series with the motor and keep PWM frequency as high as possible. 20 kHz is not high enough in many cases.

I not understanding this portion.

Although 50V PSU is regulated, ripple will appear I think on loading?

Can you please explaining what ripple has to do with this?

Also, why put more inductance in series with the motor?

If so, what value? 2A inductor costly enough to buy fresh 12V@2A PSU!

Also, why >20Khz? I asking as higher freq => more heat dissipation from HBridge and possibly blow it off :-(

Question for all of you:

Would be positive temperature coefficient thermistor (PTC) be fast to save motor from damaging if PWM circuit or HBridge fail (so motor exposed directly to full PSU voltage and current).

Is polyfuse and PTC same thing or polyfuse faster than PTC (asking as polyfuse uses in computers and saves ports like USB from overcurrent very fast)

 

[misterT]

Although 50V PSU is regulated, ripple will appear I think on loading?
Can you please explaining what ripple has to do with this?
Also, why put more inductance in series with the motor?

PWM causes current ripple in the motor windings. If the inductance is too small the ripple will be very large and it will burn the motor. Solution to this is to add more inductance in series (to smooth out current ripple). You can also increase the PWM frequency to reduce current ripple.

You can estimate the amount of current ripple with this equation:

Iripple = Vcc / (2 * Fpwm * (Lm + Lc))

Where:
Iripple = ripple current amplitude.
Vcc = supply voltage.
Fpwm = pwm frequency.
Lm = motor inductance.
Lc = choke inductance.

So, with 20kHz and 50V.. and assuming (relatively large) inductance of 1 mH the ripple is:
50 / (2*20*1) = 1.25 A

And that ripple appears even with steady-state no-load condition.. all it does is heat the motor.

If so, what value? 2A inductor costly enough to buy fresh 12V@2A PSU!

If you don't want to add inductance, increase the frequency. Use the equation above to estimate the ripple current.

Also, why >20Khz? I asking as higher freq => more heat dissipation from HBridge and possibly blow it off :-(

Less frequency and more heat dissipation at the motor. That is the trade-off.

 

[4pyros]

Would be positive temperature coefficient thermistor (PTC) be fast to save motor from damaging if PWM circuit or HBridge fail (so motor exposed directly to full PSU voltage and current).

Is polyfuse and PTC same thing or polyfuse faster than PTC (asking as polyfuse uses in computers and saves ports like USB from overcurrent very fast)

You will need a slow blow to handle the inrush current.