sparks on commutators when the dc motor speeds up

[is_razi]

hi every body,
i have problem with a dc motor drive .the motor works well without no sparks until about 1000rpm but in higher rpm's large sparks are produced on the commutator.
is there anybody who had faced a problem like this if so please share your experience with me.
thanks in advance.

 

[dick_freebird]

The commutator ring may be worn or out-of-round, causing some brush
float / skip. This can be self-reinforcing as the spark chows out material.
Some arcing is inevitable when you're interrupting significant current.
But higher brush pressure (worn brushes relax the spring) and a smooth
commutator surface help.

 

[kak111]

The most common causes of dc motor arcing in commutator
-rotor winding fault
-commutator surface is not smooth
-commutator is not perfectly round
-commutator short-circuit between the lamellas (dirty)
-worn carbon brushes
-too small spring pressure to brushes
( brushes vibrations )
( in bigger motors drive fault, field current controller ,rotor current controller)

Some general guidelines...................

**broken link removed**

Maintenacne World: Maintenance and Troubleshooting of Electric Motors

**broken link removed**


How to measure rotor winding & commutator............

The commutator has several segments, and there are several
armature windings, each one connected between 2 commutator
segments.

The simplest way to test it is to put the meter across the mains
plug pins (NOT plugged in),( or across brushes) and turn the tool round very slowly so
you see the R of each section of the motor armature.
Analogue meters are much easier to use for this.

Comparing Rs wil pick up some faults, though not all. If this
doesnt work you'll need to compare L values rather than R, as L
nosedives with a shorted turn or 2, whereas R can stay almost the
same. Its easiest to do this by connecting the tool in series
with a lightbulb, and putting the pair across a low voltage ac
supply, eg 12v. Measuring the voltage across the tool will show
up a bad winding section as the motor is turned. The lightbulb's
non-linearity accentuates the fault. Your psu v needs to be low
enough that the tool doesnt spin.

 

[Sergio16463]

If the commutator is not aligned as it should be with the field/excitation the commutation will not be performed at the time when there is a less chance of sparks (lower field value) - it is worth a look.

 

[Raza]

You said :

the motor works well without no sparks until about 1000rpm but in higher rpm's large sparks are produced on the commutator.

Is this happening with or without load (not specified in your post) ?
What are the motor specifications ?
The designed to run up to RPM ?

 

[is_razi]

thanks for your responces, but the problem seems to be because of driver
can somebody tell me what is the field weakining for? is it only for speeding up the rotor when the maximum armature voltage has reached or it is also for reducing the sparks ,
in my case there is no field reduction in higher speeds, can this be the problem??
(there is no load when i am testing as a responce to raza's question.)
thanks in advance.

 

[kak111]

Reducing the shunt field voltage decreases the field current, which in turn reduces the field flux allowing the speed of the
motor to increase. Increasing the field voltage to obtain a speed below base speed cannot be used as the field will overheat
at higher than rated current. DC motors operated at constant armature voltage and with field weakening have a constant
horsepower capacity over their speed range. Field control speed values range from 1:1 to 6:1.

Utilizing both methods of speed control will give wide speed ranges. Armature voltage control is used for speeds below base
speed, resulting in a constant torque capacity. Shunt field control is used to obtain speeds above base speeds resulting in
a constant horsepower capacity.

---------------------------------------------

Is it possible , that angle between magnet field and brushes is changed. ( for ex. Brush frames taken out and back for repair )
Older commutator motors sometimes had all brushes mounted on movable frames so that the position of the brushes in relation to the magnetic fields of the stator poles could be adjusted manually.

Look The Commutating Plane
Commutator (electric) - Wikipedia, the free encyclopedia