|
Common confusing symptoms in troubleshooting
of DC drives and motors ... |
|
Here are a few common observations which make the basic
distinction between Motor or Drive difficult, as far as the cause of the
symptom is concerned. a) Motor speed
hunting b) Motor overheats c) Sparking on
commutator d) Motor shaft too
hard to turn by hand a) Motor speed
hunting: Motor speed, with belts or coupling to
the load removed, goes up and down in a rhythmic pattern. Gain adjustment of
the drive has no effect in the amplitude of oscillations. Armature Current
drawn by the motor also shoots up with the same rhythm as that of speed. This
is an indication of faulty armature winding, or shorted field winding. If
there one of the four poles in the field is short, the field distribution is
uneven causing such a symptom. Noting down the field resistance when things
are normal helps. For a four pole arrangement, if the field winding
resistance has changed from normal value of 20 Ohms to 15 Ohms it is clear
indication that one winding is short. When checked on oscilloscope, the
current waveform shows a large peak and big time gap with no current flowing
for quite some time. When the motor speed drops to a low value, again an
inrush of current takes place speeding up the motor with a current kick. This
continues in a cyclic pattern with a rhythm of typically 3 to 5 seconds. b) Motor overheats: It is possible that the field winding
is given a voltage in excess than the rated one. Or only four thyristors are
firing instead of six. Oscilloscope waveform of current reveals this. Drawing
a conclusion that the motor winding is at fault could be wrong in such cases.
c) Sparking on
commutator: Cause of Sparking occurring at
irregular intervals could be the electronic controller loosing uniform smooth
firing of thyristors, impressing a sudden high voltage across the armature
momentarily. Regular continuous sparking could be because of faulty inter
pole winding, if the motor is otherwise taking the load and speed as normal. d) Motor shaft
jammed: While trying out basic checks, one
suddenly realizes that the shaft of the motor is too hard to turn by hand,
even when it is decoupled from the machine. To add to confusion, it is free
in one direction and hard in other! The situation suddenly changes if the
electric supply to the control panel is switched off. The shaft becomes free
in both directions when the power to the panel is put off. The explaination
to these observations is as under: When the field winding is getting its excitation
voltage, turning the shaft by hand makes the motor to generate voltage across
the armature terminals. If the thyristor controller is of Semi-Controlled
type ( i.e. with three thyristors and four diodes. See this link here for
Drive Types ), the free wheeling
diode acts as load on the motor in one direction and creates an opposing
force. This makes the shaft hard to turn. In other direction, the diode does
not conduct and there is no load on the generator. This makes the shaft easy
to turn in the other direction. With short circuit in the armature winding,
it will be difficult to turn the shaft in both the directions.
|
|
|
|
In addition to the above, observations like the following one
make the decision to take the motor out for repairs difficult. Motor windings
show low insulation resistance values
on a megger: If the motor is not in use for a few days or a few weeks, the
carbon accumulated around the brush holders absorbs moisture from the
atmosphere. This can lead to low megger values as low as 1 meg Ohm. In such
cases also, the motor can run safely without any problems. Cleaning the area
around the brush holders and keeping two or three 100 watts incandescent
bulbs around the brush holders for few hours, will improve the megger value.
|
|
|
|
|
|
Motor okay? Check
Electronic parts
|
|
|
|
About the author
|
|
|