I have two identical PMDC motors. I ran each of them on a dyno at the same voltage and the same load. They both consumed the same amount of current, yet one of them runs 300 RPM slower.

The one that is running slower was actually under test for several hundred hours, while the faster one is brand new. Wondering if there's something going on with the brush/com, I put each motor in a motoring dyno to check their BEMF (both motors were driven with the same speed). Not sure what I was looking for, just hoping to see a difference. The graph is below. The blue trace is from the motor that runs slower. There are obvious voltage spikes that the other motor doesn't have. Is this the reason for the motor running slow? If so, why?

enter image description here

  • \$\begingroup\$ To my eyes, both have spikes. The blue ones are just more pronounced. \$\endgroup\$ – JRE Feb 28 '18 at 20:14
  • 2
    \$\begingroup\$ "... one of them runs 300 RPM slower." Than what? Is that 20% or 1% of the higher speed? \$\endgroup\$ – Transistor Feb 28 '18 at 20:23
  • \$\begingroup\$ @Transistor I was thinking the same thing. \$\endgroup\$ – Trevor_G Feb 28 '18 at 20:28
  • \$\begingroup\$ One runs at 5700 RPM , while the other runs at 5400 RPM. For the BEMF test, both were driven at 5400 RPM. \$\endgroup\$ – gtetil Feb 28 '18 at 20:35
  • \$\begingroup\$ @gtetil run it again at 5700, the blue trace should be quite illuminating. \$\endgroup\$ – Trevor_G Feb 28 '18 at 20:41

The spikes are caused by the commutator. As it makes and breaks contact with the coils, the inductance of those coils causes kickback voltages.

enter image description here

That is why snubbers are important on these motors.

As brushes wear the nature of those spikes will change.

Looking at your curve though, notice that the cyan trace comes up to voltage (current) and "flattens" out within the commutation period while the blue one is still rising when it switches. That suggests the blue motor has a longer coil time constant so I would expect it to give less torque and run slower.

The reason for that could again be brush related or it could also be something in the magnetics. For instance, if the motor suffered a period of over-heating or over-current.

BTW: There is no such thing as "two identical PMDC motors".

  • \$\begingroup\$ Could this have any impact on the speed issue I'm seeing? \$\endgroup\$ – gtetil Feb 28 '18 at 20:20
  • \$\begingroup\$ @gtetil see edit \$\endgroup\$ – Trevor_G Feb 28 '18 at 20:25
  • \$\begingroup\$ Instead of a longer coil time constant it could be a slightly different brush orientation - this is adjustable on better motors for efficient commutation and minimizing sparking. One thing to check : is the poorer motor being driven backwards with the brushes set for forward running? That would do it... \$\endgroup\$ – user_1818839 Feb 28 '18 at 21:00
  • \$\begingroup\$ @BrianDrummond true, that's why I termed it "brush related". That's a whole chapter all on it's own ;) Either way, it still affects the time constant, just exact cause can vary. \$\endgroup\$ – Trevor_G Feb 28 '18 at 21:01

The spikes indicate commutation arcs perhaps from a rough copper commutator, debris or weak brush spring or brush side stiction.

When the contacts open the current drops as fast as the voltage rises. V=LdI/dt which then causes an arc until the next insulated contact is reached by the spring loaded carbon brush.

The blue vs pink envelope show the dramatic difference in commutation voltage.

A visual inspection and a DC ammeter with slow hand turning of the commutator to measure erratic current and contact resistance may indicate maintenance is required on the rotor contacts or brushes. Rotate in the correct direction.

enter image description here Loading the output will reveal even more characteristics.

The motor running slower indicates the armature brush conduction losses and voltage drop that reduces RPM. Measuring Coil + contact reistance variations slowly turning with an analog current meter, might be useful. Very Fine sandpaper might remove burrs to the armature contacts.

Search for motor maintenance tips on Youtube.


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