DC motor running slow. How to ID concerns

[flyinhigh2]

I have a couple small DC gear reduction motors running in a machine and they are running a bit slower than they should Still working and all but not snappy like they should be.

I know the motors are being current limited but curious and brushes and such causing these problems? I removed one brush and it looks just fine. Some powder on it but don't they all...?

I am just wondering if there is a specific part that should be inspected that would cause reduced power? When testing the current on these motors, would I look for reduced amperage?

Reason I ask is from my experience with automotive starters, you can have a full amp load or normal anyway and still not carry the load due to brush damage BUT my brushes look ok so far.. These motors are a pain to take apart so I would like to be sure of the problem before opening.

 

[waross]

Are these series, compound or shunt motors?
How are they controlled?
Have you checked for mechanical drag in the system?
Your supply voltage may be low. That would make most DC motors run slower.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DickDV]

These may well be permanent magnet motors and the magnets are loosing there intensity.

You will need to do a little testing to confirm this. Place a known torque load on the motor and provide a known voltage on the armature. If the current is high compared to spec, the magnets are getting weak. The motor cannot be saved.

Other than age, PM motors can loose there magnets do to misapplication. They are not suitable for plug reversing or rapid forward/reverse operation. Also, not suitable for any form of dynamic braking.

 

[dArsonval]

The title of the post, “DC motor running slow…” implies only one motor, and the body of the post indicates a problem with two motors.
If indeed two motors have lost their “snappy like” behavior, waross is correct in checking supply voltage.

Small permanent magnet motors (or actually most all kinds of electric motors) typically don’t have any progression downward
in performance like in the case of an internal combustion engine. Mechanical drag as mentioned already is likely hampering movement.

As far as brush appearance? In addition to looking at remaining [working] brush length; look for any sign of heat damage to the brush springs,
and whether the shunt wires have not been broken. Utilize a flashlight and look into the motor through the brush opening and inspect the
commutator as the motor shaft is rotated (if you even can). It should have a consistent chocolate brown appearance all the way around.
Any indication of a bar being discolored is a sign the armature is “shot”, and reluctantly… the whole thing is likely a candidate for replacement.
“But” small specialized D.C. motor armatures can, and do get rewound.

It really reads like there’s some kind of mechanical problem though.

John

 

[jimmy3456]

The OP mentions current limiting. Is it possible that the current limit is set too low or can be adjusted? Will the voltage have to most bearing on speed rather than current?

 

[bigbillnky]

Do these motors have a wound field? If so, the field current could be the cause of the speed variation issue.

I would test the field current with the motor running and make sure that it matches the motor nameplate.

Typically, diode failure in a controller that has a seperate power supply serving the field winding causes a higher than normal field current, which in turn limits the armature speed. If you are interested, you can Google "field weakening DC motors" and you will be able to find relevant information that explains how the field current and armature current interact.

Also, check the field winding resistance. Use ohms law to calculate correct winding resistance based on the nameplate field voltage and current.

Other than that, I would check the bearings. Sometimes a thrust washer will allow a motor to turn freely when not energized, but when power is applied and the armature shifts to its "magnetic" center, it binds.

Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[waross]

A weaker field means less EMF which tends to cause an increase in armature current. The increased armature current will cause an increase in speed until the back EMF rises and finds a balance at a higher RPM. (In a perfect world)
With small motors on small controllers a few other factors come into play.
With the field weakened, the motor may not be able to develop enough torque to drive the load at full speed and the motor will slow down.A small motor may be fed from a small controller which can not supply much more than rated current to the armature and so the motor slows down.


Bill
--------------------
"Why not the best?"
Jimmy Carter