DC motor (brushed) rating help

[wakoko79]

I have a DC motor that I need to run using PWM signals, (chopper).
All I know for this motor is the following:
power rating: 30W
voltage rating: 24V

30/24= 1.25 A


Remember, I'm using PWM. So is the 1.25A limit an absolute limit?
Or should I compute for Average/ RMS current (since I'm driving it with PWM)?

 

[Boncuk]

A brushed DC-motor requires four to six times the nominal current for start.

Using PWM that current won't be much less, since PWM can take care of a soft start. However the motor will require at least 3/4 of the nominal current to start rotating.

If start takes too much time the motor will hum and the brushes stuck on the commutator will take care of excessive wear and tear, burning off the copper of the commutator.

I suggest to select components for at least four times the nominal current.

A 24V/30W motor points pretty much to a toy motor.

Boncuk

 

[cachehiker]

Don't forget the flywheel diode. The inductance of the motor will filter the current to nearly DC so average and RMS will be essentially the same. That is not the case for the voltage though.

Assuming the rating is for continuous duty, 1.25 Amps should be the limit under normal conditions. Sudden application of 24V will result in a 5-10 Amp spike and is hard on the brushes as well as everything mechanically coupled to the shaft. Using a 2.0-2.5 Amp constant current or current limited mode to ramp the speed up to its target would probably be OK for a fairly quick start without much of a spike as long as the load drops back to 1.25 Amps or less once the target speed is reached.

 

[wakoko79]

Don't forget the flywheel diode. The inductance of the motor will filter the current to nearly DC so average and RMS will be essentially the same. That is not the case for the voltage though.

I also assumed this at first, but this is not the case: see **broken link removed** (from another of my posts)

That is taken by using a 2kHz 15% duty cycle PWM signal. That is the current across the current sensing resistor (1ohm 5Watts). BTW, this waveform was taken by using an L298 driver. clearly, it is nowhere near DC.

As I understand it, nameplate power ratings of DC motors refers to temperature rating meaning a if it indicates 30W, and if you put a DC voltage of 15V across its pins, the current should not exceed 2A right? but what if instead of a DC (100%), I use 50%? 30? or any other duty cycle? is the current limit should still be 2A(I mean the peak, like the peak in the picture I included)?

 

[cachehiker]

The inductance of that motor must be pretty low relative to its resistance. Assuming a DCΩ of 24V / 8A = 3Ω, the inductance would have to be in the range of 50 or 100-ish µH. I'm accustomed to PWM motor control implying frequencies above the audio range. At 16 or 20KHz and a millihenry with a flywheel, my DC and rms values don't differ by enough to worry me much. At 2KHz, the motors I work with would emit a very annoying midrange whine and some frame members and covers tend to resonate around 2KHz as well.

Use the rms current to calculate power being dissipated by the motor. Unless there are cooling fans involved, the heating of the motor will be proportional to I * R squared where R is the DC resistance of the motor. Lowering the voltage will not allow you to increase the amperage. Lowering the voltage merely lowers the rating. 30W at 24V implies 15W at 12V. You're going to be stuck at 1.25 Amps no matter what. At 30W, I would also assume the rating is for power consumed and not power delivered. 30W input at 80% efficiency = 24W output.

 

[cachehiker]

The current limit, if you implement one, is usually set at about half again or double the rated current. It limits the acceleration and provides some protection in the event of a stall condition. You couldn't leave it stalled at its current limit for hours unattended but you would have anywhere from 10 minutes to an hour or so to get it shut down before the motor ends up roached. With no current limit, 8 Amps locked rotor current will fry that motor in a minute or two.