"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.