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

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?

UPDATE:

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)

• Driving it from 50V with a 40% duty cycle would roughly double the motor's power. I doubt that would work well. You want about a 25% duty cycle to keep the power the same. P = V^2 / R – David Schwartz Sep 27 '11 at 9:06
• Hai David, the peak voltage is not appearing across real motor, only in maybe ideal motor without inductance – sekharan Sep 28 '11 at 15:50
• Inductance should slow the appearance of peak current, but the peak voltage should be reached. My point was more to ensure you start with a safe level and only raise the duty cycle when you confirm the motor is spinning at less than the correct speed. – David Schwartz Sep 28 '11 at 16:01

This will work with a brush motor BUT only because the motor is highly inductive.
Doing it with a resistive load may or may not work - in a significant % of cases it would damage the load - especially as Vin/Vrun ratio rises.

It will not work directly with a "brushless DC motor" and would probably cause damage and let out the magic smoke.

With a brushed motor it will work because the motor forms the inductive element in a buck converter - BUT you will need to provide a diode across the motor that is reverse biased when power is applied and which then provides a path for motor current when the PWM is off. The diode should be rated at a minimum to be able to handle the motor current at the PWM duty cycle (eg 50% on in your 1st example), but it makes sense for it to be able to handle motor current permanently. So use a 2A diode. Or 2 x common 1N400x diodes in parallel would work OK as long as PWM frequency was not too high.

Like this. Ignore component values. Diagram below from here

Dont try this at home - How **NOT to do it.**

Supply is labelled Vcc so power level may be low. FET will required to carry motor current by breaking down under inductive spike from motor when PWM turns off. In low energy circuits this may happen withou damage and without people being aware of it. In your circuit destruction would probably occur.

SUPERB RESOURCE

Many many many circuit ideas for PWM motor control courtesy Google. Links on Google page from photos are live.)

Google for eg PWM motor and then select images and you get -

and much more ... .

• I know ready to use L298 but prototyping it on breadboard difficult as it use breadboard-unfriendly pinout. Any suggestion of breadboard-friendly 24V+ 2A+ HBridge? – sekharan Sep 26 '11 at 1:14
• 24 VDC is OK on breadboard, 2A is "not so good". You can solder a wire to the driver pin. You can also bend the driver pin out sideways and connect to it with a soldered wire or a test clip or similar. – Russell McMahon Sep 26 '11 at 2:23
• Question updating – sekharan Sep 26 '11 at 20:46

Your idea of running it with PWM from the 50V supply is right on. You may want to add a current limit feature so that the PWM is shut down by some independent means if the maximum current is exceeded due to a firmware bug.

• Thanks you for always precise reply. Will be PTC work? However, I bit unsure about PTC as cutoff not precise for it and it itself has lot internal resistance :-( – sekharan Sep 25 '11 at 21:46
• Also - will it be good to try run this same motor off 100V at 20% or less duty cycles? – sekharan Sep 25 '11 at 21:49
• @SGhosh -- eventually you'll hit the breakdown voltage of the insulation on the windings, even if you have the duty cycle adjusted to keep the average voltage down to where the current is acceptable. – JustJeff Sep 26 '11 at 0:13
• @JustJeff, Olin: Yes, everyone needs to use a peak voltage less than the breakdown voltage of the insulation of the motor -- but what is that voltage? For stepper motors, the rule of thumb is "peak voltage no more than twenty times the rated DC voltage of the motor" -- is there a similar rule of thumb for DC motors? – davidcary Sep 26 '11 at 14:43
• @Davicary: I have never heard that rule of thumb and it sounds like a bad idea. I certainly wouldn't put 480 V on a 24 V motor without the manufacturer saying this was OK, via a datasheet or explicitly. – Olin Lathrop Sep 26 '11 at 15:13

Please check the motor specification first before applying any voltage.

I would suggest to use a voltage divider circuit and design a thevenin equivalent circuit which gives you 24V @ 2A max. This will give you some more work to do and learn. I would not over rate the voltage on the motor, although it may very well run OK.

Another thing is 20 kHz frequency for PWM? This is way too high. I would use something in the range of 200 Hz may be.

It may depend on the moter itself, but the motor speed does not have a linear relationship with PWM duty cycle. As long as it gets a high voltage even a small pulse, the motor speed is significant.

• The voltage divider is a bad idea. Think of how much power it would waste to achieve the 24V at 2A but not overvoltage the motor at lower current. Perhaps you wouldn't apply a higher than steady state allowed voltage, but you gave no reason not to. In practise this is commonly done. 20 kHz can be a reasonable PWM frequency if you want to keep audible whine down. Around 25 kHz is common for this. There may not be a direct mapping of voltage to speed, but from PWM duty cycle to voltage is pretty linear and predictable. – Olin Lathrop Sep 26 '11 at 19:50
• For lab work the power wastage is not an issue but learning is. Speed control with PWM from my exerirement was not very reliable, neverthelesss you get some speed control. It might depend on the type of motor. – Dandy Sep 26 '11 at 20:48