DC brushed motors are familiar in industrial machinery due to their advantages: high starting torque, low cost, and ease of maintenance and repair.
However, after a period of operation, motors may develop faults or fail and subsequently be rewound, but do not meet standards.
Here are some consequences that can occur if a DC motor is rewound improperly:
1. Reduced Operating Efficiency
High Power Loss: If the windings do not meet specifications or the brushes do not make good contact with the commutator, the motor will consume more power to produce the same output.
Reduced Torque: A weak magnetic field due to improperly wound coils or unstable commutator contact will reduce torque and performance.
2. Increased Temperature and Risk of Failure
High Heat Generation: Poor contact between the brushes and commutator or improperly repaired windings will generate heat, leading to overheating, insulation failure, and the risk of fire.
Damage to Other Components: High temperatures can damage bearings or other parts.
3. Increased Noise and Vibration
Abnormal Vibration: If the commutator or shaft is not properly aligned, the motor will vibrate heavily, affecting the transmission system.
High Noise: Uneven brush surfaces or a rough commutator will create uneven friction, causing loud noises during operation.
4. Reduced Lifespan
Rapid Brush Wear: Substandard or improperly installed brushes will wear faster, increasing maintenance costs.
Winding Damage: Overheating or unstable current will damage the insulation and windings, reducing the motor’s lifespan.
5. Sparking
Poor contact between the brushes and commutator will create sparking, which not only wears down the commutator but also increases the risk of fire, explosion, and damage to control equipment.
So How to Check a DC Brushed Motor?
1. Visual Inspection
Observe Brushes and Commutator:
Open the motor cover or protective part and observe inside. DC brushed motors have brushes that make direct contact with the commutator.
Brushes: Are carbon or graphite bars, usually located on springs that push against the commutator.
Commutator: Is a cylindrical part made of copper, divided into insulated segments.
2. Check Electrical Structure and Design
Field Winding
Measure the resistance on the field wires (including any wires between the controller and motor). Disconnect the field wires from the motor controller before performing this measurement. Compare the specifications on the motor label for field resistance. (If the specification is not on the motor label, use Ohm’s law to calculate field resistance, R = rated field voltage / rated field current)
Use a Megger to check the field to ground and field to armature for insulation faults that can lead to current leakage.
Fault warnings with the Parker DC590 drive due to field winding issues include: FIELD FAIL, FIELD OVER I, STALL TRIP, SPEED FEEDBACK, OVERSPEED
Armature Winding
Inspect the commutator ring for even spacing between bars and the absence of grooves.
Visually inspect the brushes and brush holders – securely fastened, not touching the armature, brushes not too short, not accumulated too much carbon dust. Also, ensure that you are using brushes recommended for your specific motor.
Use a Megger to test the armature leads to ground and to the field for insulation faults that can lead to current leakage (including any associated cabling). Disconnect the motor leads from the motor controller before taking this measurement.
Possible warnings that may occur with the 590+ controller due to armature failure include: OVER VOLTS (VA), OVER I TRIP, STALL TRIP, SPEED FEEDBACK, OVERSPEED.
3. Check During Operation
Observe the Operation of the Brushes:
When power is applied and operating, you may see:
Sparking from the commutator (characteristic of brushed motors).
Small mechanical noise due to brushes contacting the commutator.
Check the Direction of Rotation:
DC brushed motors allow easy reversal of rotation by reversing the polarity of the power supply.
