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I have a small, 4W, 24 VDC, PMDC gear motor. When feed from a 24 VDC regulated power supply the motor current is decreasing and speed of unloaded gear motor is increasing about 5% per hour. The starting motor temperature is 70°F. After an hour the motor temperature reaches 90°F.

Is there any way to stabilize the motor speed using electronic circuits rather than using a tachometer or an encoder as feedback? Will IR compensation work? Thanks.

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    $\begingroup$ Yes, motor speed can be sensed in the control circuit by monitoring the back-emf of the motor (e.g. during the off-phase of the drive circuit) ... these days you can buy cheap, tiny, efficient electronic speed controllers that do this ... there would still be some temperature effect $\endgroup$
    – Pete W
    Feb 2 at 15:42
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    $\begingroup$ Hmm, I wonder if the gearbox grease viscosity is reducing slowly with frictional heat? Bolting a 24 V, 1 W resistor to the gearbox and powering it when in standby might preheat the grease ... $\endgroup$
    – Transistor
    Feb 2 at 15:45
  • $\begingroup$ possibly related: electronics.stackexchange.com/questions/573838/… $\endgroup$
    – Pete W
    Feb 2 at 15:46
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    $\begingroup$ Sensorless DC brushless motors drivers routinely do this with their back EMF sensing which they need for commutation anyways. You did not say whether your motor was brushed or brushless. You can do BEMF sensing with a brushed motor too, but brushless is more accurate due to the discrete nature of the BEMF as it jumps between phases. Measuring how quickly this indicates RPM more accurately than relying on the BEMF voltage since you know exactly how many phases a motor has, while you knowing the exact number of turns and other winding parameters is tricky. $\endgroup$
    – DKNguyen
    Feb 2 at 19:59
  • $\begingroup$ Thanks a lot everyone! $\endgroup$
    – Star REB 7
    Feb 3 at 17:33

2 Answers 2

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You did not say whether your motor was brushed or brushless.

Sensorless DC brushless motors drivers routinely do this with their back EMF sensing which they need for commutation anyways. In brushless motors, you have independent access to the windings (you do need to commutate them electronically after all) and you time how fast the current jumps between the windings. Since you know the exact number of windings and phases you can accurately determine motor RPM.

You can do BEMF sensing with brushed motors too but it's more involved. Since you do not have independent access to the phases, you can only measure the BEMF voltage. This is not as accurate since you need to know the exact winding parameters of the motor to translate BEMF voltage to RPM. You also have to deal with noise involved in driving the motor and find a dead-time where the motor windings are not being energized to sample the BEMF voltage. You can technically do this with brushless motors too, but no one does because it's much more complicated and the timimg method is inherent to what is required to drive a brushless motor anyways.

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The old-school method is to realize that in a DC motor, current is proportional to torque and RPM is proportional to the back EMF. So you can monitor current through the motor and get a representation of torque, and also interrupt the drive voltage momentarily and then sense the back EMF to get a representation of RPM.

You then use the voltage and current as control system inputs.

BTW the product of torque and RPM is the instantaneous power being developed by the motor.

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  • $\begingroup$ Thanks a lot. My motor is Brush type 24 VDC motor. $\endgroup$
    – Star REB 7
    Feb 8 at 19:35

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