"Why
does my servomotor or stepper motor have low
torque?" This is perhaps the most commonly
ask question of A-1-Servo Motor Repair's Technical
Support Staff. The answer is usually simple
- low magnetism!
Generally, low torque is a direct result of
low magnetism. servo motors and stepper motors
utilize very powerful permanent magnets to
create torque. If the magnets in the motor
are not fully charged it will be impossible
for the motor to generate full torque.
The term "permanent" magnet is, to some extent,
misleading. For a motor to develop full torque,
it is necessary for the magnets to be saturated,
or fully charged. While the permanent magnet
remains "permanent", the strength of the magnet
does not. In fact, there are a number of ways
a magnet's strength can be reduced. No matter
how a magnet's strength is lost, it's just
a matter of time before you'll notice the
warning signs and ultimately be forced to
resolve the problem or discard the motor.
So
how does a magnet loose its "full charge"?
Two of the biggest contributors are overloading
or disassembling a motor.
First, any time a motor is disassembled,
there is a high probability that the magnets
will be discharged to a point where the torque
will be diminished. The results vary and may
or may not initially be noticeable. With some
motors, the results will be immediate. In
fact, disassembly of a stepper motor will
typically demagnetize the magnets to a point
that the motor is useless until it has been
properly remagnetized. With a few motors,
the reduced energy will have little effect
until the motor is under full load. At that
time the drive / controller will indicate
an error.
Overloading
a motor is another way motors can be demagnetized.
This could be caused by many factors: excessive
feed/speed rates, crashes, mechanical binds,
parameters that are not set correctly, as
well as many other electrical or mechanical
problems.
Overloading a motor results in excessive
current. Excessive current demagnetizes
motor magnets regardless of the cause. What
makes things worse is that high current is
only required for a short period of time,
less than a second in some cases!
Typical scenario: A device is driven past
a limit switch and crashes into a stop causing
a momentary over current condition which causes
the motor to be partially demagnetized. The
motor now requires slightly more current to
operate normally. The drive provides the extra
current. However, more current equates to
more heat generation. Heat reduces the magnetism
even more. As the magnetism is decreased,
the process continues to deteriorate: more
heat = less magnetism = less torque = more
current for normal operations = more heat...
get the point? This goes on until the drive
reaches its maximum current limit and alarms
out, or the motor can't meet acceleration
demands, or the motor self-destructs, etc...
It is easy to see how this process occurs
over a period of time. By the time the motor
totally "fails", the original crash / overload
has often been forgotten.
So
what is the solution? First, avoid
overloading your servomotors. Pay particular
attention to the daily demands you place on
your system. You might be able to get parts
out more quickly by pushing your machines,
but that quickness comes at a cost. Second,
be observant to any potential mechanical binds
and follow your manufacturer's recommended
maintenance schedule. Finally, check your
parameters. Don't set the maximum current
limit too high. You will find other limits
related to current in the parameters. Check
these settings to be sure they are correct
for your specific motor. In many cases this
will prevent a motor from being damaged and
add years to its life.
Once
a motor is demagnetized the ONLY WAY it will
ever work properly again is to have it rebuilt.
The rebuild MUST include remagnetization by
a facility having the experience and the specialized
equipment necessary to do the job right.
If you have a motor rebuilt, ask the following
questions BEFORE you turn your motor over
to any repair shop: (1) Do they have a substantial
amount of servomotor repair experience? (2)
Do they have the ability and the equipment
to properly magnetize your motor and bring
it up to the original specifications? If you
don't get a strong, believable response, don't
let them touch your motor. Keep looking for
a shop you can trust.
Next, do not take your motors apart.
It is easy to cause a great deal of damage
by simply "looking" into a motor.
Here at A-1 Servomotor Repair we see thousands
of dollars in damage each month caused by
well meaning but uninformed customers. We
see even more damage caused by electric motor
repair shops that lack the knowledge, training
and equipment to work on servomotors. Often
inexperienced people damage or brake parts
that can not be replaced. This can cause a
motor to be non-repairable and convert an
otherwise good motor into scrap.
For more information contact A-1 Servomotor
Repair at (888) 336-8430 or visit our web
site www.servorepair.com.
