So you think you’ve got what it takes to rebuild your broken DC servo motor. Just a rotor with a couple bearings right? Wrong. There are many factors to consider before disassembling your own DC servo motor.
|Substandard insulation, exposed copper||Electricity will go to ground|
|Scratched insulation||Corrosion of affection area|
|Improper winding configuration||Commutator worn out, repair or replacement necessary|
|Nicked or damaged wires in windings||Malfunctioning or burned up motor|
|Armature removed from wrong type of field frame||Loss of magnet strength, demagnetization, servo failure|
|Damage to tachometer armatures or windings||Replacements necessary|
|Poor field frame positioning relative to brush rigging||Torque loss, over current, motor failure|
You Must Be Able to:
Test the Armature Insulation Correctly
There are special testers used to test the insulation that protects the winding from grounding out to the iron on the armature. It is very important that the insulation be up to standard. When the motor is being run under load there is potential for electricity to go to ground.
If there is any exposed copper on the winding this would be a high potential for grounding out during operation. Any area were the insulation is scratched will have a high potential to corrode over time and fail.
Test the Armature Circuit Correctly
There is also special testing equipment to test each circuit of the armature insuring that the windings are going to work to specification. Special attention needs to be paid to the commutator. If the commutator is worn or burned it will need to be turned in a lathe or replaced.
Avoid Damage to the Electrical Windings as You Separate the Armature from the Field Frame
Removing the armature is a delicate process, which is compounded by the magnetic force of the field frame. The exposed wires, and commutator are extremely prone to getting nicked or damaged during the disassembly process. Even minor physical damage to the circuit of the armature can cause a motor to malfunction and burn up.
Separate the Armature from Its Magnetic Field without Causing Demagnetization
There are many types of magnets. The most popular types used in servo motors are Neodymium Iron Boron (NdFeB or NIB), Samarium Cobalt (SmCo), Alnico, Ceramic, Ferrite and Rare Earth. Certain magnet types require that their fields not be broken. If a motor or tachometer armature is removed from the wrong type of field frame, the magnets will loose their strength effectively ruining the servo. The only way to rectify demagnetized field frames on a disassembled servo motor is to recharge the magnets. Otherwise the servo will never run properly.
Avoid Damage to the Tachometer Armature as you Remove It from the Motor Armature
Often, as you try to take the Tachometer off of the motor shaft it can be very hard to remove because of corrosion and tight tolerances. We have made several special removal tools to take the tachometer armatures off to prevent destroying them. The windings on a tachometer armature are smaller than the hairs on your head, and much less resilient to forces applied to them.
Assemble the Field Frame on the Brush Rigging Correctly
The relative position of the field frame to the brush rigging needs to be correct. If this relationship is not correct the motor will not commutate correctly resulting in torque loss, over current, and motor failure.
These are a few of the important things to know before you start to take apart your own DC servo motor. If you decided that you want your motor professionally repaired then send it to Servotech Inc. We have all the necessary knowledge and equipment to rebuild you DC servo motor.
Keep Coolant and Oil Out
Replace Shaft Seal When the Motor Is Worked on
Routine maintenance? Replace the seal. Changing bearings? Replace the seal. It is common for maintenance personal to reuse old seals versus replacing with new ones due to time constraints, budget, or laziness.
Even Environmentally Sealed Plugs Leak over Time
Replace worn plugs especially if there are loose pins that liquid could get around.
Servo Motor Connectors Need to be Mated
Even sealed connectors need to be mated with matching sealed cable connectors to ensure proper protection. Unsealed cable connectors will allow contaminants into gap between motor and cable connectors. As no seal is perfect, this unnecessary exposure to the plug surface will lessen the time it takes for contaminants to infiltrate the connector.
Stacked Laminations Allow Liquid to Leak through
AC servo motors have a stator that is made up of stacked laminations which will allow liquid to slowly leak through them. Some brands, such as Yaskawa, put a thin metal case around the stator to prevent liquid intrusion. Many other manufacturers such as Fanuc do not and just paint over the laminations. We have found that sealing the laminations with a two part epoxy paint significantly reduces the chance of contamination through the laminations.
Avoid Direct Spray on Servo by Oil or Coolant
Make sure that your servo is not being directly sprayed by oil or coolant. If it is not possible, there are many custom solutions out there to modify the motor or cabling to limit exposure.
These are some general tips on preventing liquid intrusion into your servo motor. If your servo motor is flooded and you need help send it in to us and we will repair your servo and work with you to find a way to prevent flooding in the future.