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How to Check Brushes on a DC Motor

May 25, 2021

Small DC motors may be more common than you think. You'll find them in your garage, underneath the bathroom sink, in your storage closet, and even in the kitchen; in your power tools, hairdryers, vacuum cleaners, blenders, mixers, and more. Most appliances have small, brushed DC motors, and these brushes are the most common source of failure for the motor and device. Learning to replace worn-out carbon brushes will help you save tons of money on appliance replacements over your lifetime.

Why DC Motors Need Carbon Brushes

Inside the protective body of your DC motor, the carbon brushes make contact with the commutator on the spinning armature to complete the circuit that conducts electricity to the motor. Constant contact between the brushes and the commutator is required for the motor to operate. So, the brushes are made of a softer, more wearable material (carbon) than the commutator (copper). This helps avoid causing damage to the critical conducting components, the commutator segments, which are not interchangeable.

Since they are made with a softer material, carbon brushes can wear down enough to no longer contact the commutator segments, breaking the circuit that allows the motor to run. Learning to inspect, maintain, and replace the carbon brushes will ensure a long lifespan for your small DC motor. In turn, keeping your appliances running longer and save you on expensive appliance replacement costs.

How to Inspect Carbon Brushes

While not all carbon brushes look the same or are accessed in the same ways, this explanation should assist you no matter what DC motor you're working with. For reference, here are a couple of examples of carbon brushes:

To locate your motor's carbon brushes, consult your motor's manual or wiring diagram.

Once you've located your motor's carbon brushes, you'll want to complete a visual inspection. Look for a shiny, smooth surface that is free from chips and cracks. Ensure that the brushes make complete contact with the armature surface. Ensure that the brush pigtails are in good condition and adequately fastened to the brush and the holding clip. If even one of these conditions is not met, it's time for a replacement.

Also, compare the brush length to a new one. If it is worn to a quarter of its original size, then it needs to be replaced.

How to Inspect and Test the Commutator

Visual Inspection

The commutator is the split copper ring found on the end of the spinning armature coil. When inspecting the commutator, there are a few common issues to look for:

  • Threading – is when fine lines show up on the surface of the commutator. Caused by installing incorrect brushes, brushes with low pressure, or contamination of the brush. Copper from the commutator gets embedded in the brush; this, in turn, scratches the commutator surface.
  • Grooving – is when small, slotted areas form on the commutator. Caused by using the incorrect brush grade or a contaminated brush.
  • Copper Drag – is when copper particles from the commutator's surface are dragged to the edge of the segment. Caused by too little brush tension, excessive vibration, or an abrasive brush. This can cause a short circuit in the windings of the armature and needs to be fixed immediately.
  • Flashover – is a short circuit between the carbon brushes. Caused by a buildup of dirt, debris, and copper between the commutator segments.

If you find any of this damage on the commutator, it needs to be machined and undercut. Regular inspection and maintenance of your DC motor will help to avoid these failures in the future.

Electrical Test

Another way to find if there has been a failure with the commutator is to perform a simple electrical test. Using a multimeter, check the resistance between the commutator sections and compare readings to the manufacturer ratings. If the ratings are outside of the manufacturer's recommendations, it may be time to replace the motor.

Ordering New Carbon Brushes

If it's time to order new carbon brushes for your DC motor, you'll be able to find important information like dimension, type, and grade the brushes in your motor's user manual. If you no longer have a copy, you should be able to find it by searching on the manufacturer's website. This will give you a part number that you can search out online.

Alternatively, you can also search your appliance online to find part numbers if the motor is not marked with a part or catalogue number. Or you can call a local appliance or electric motor repair shop. Provide them with your appliance information or the motor catalogue number, and they should be able to source the carbon brushes for you.

Replacing DC Motor Carbon Brushes

  1. Remove the appliance you're working on from its power source.
  2. Detach all electrical components from the motor.
  3. Remove the motor from the appliance. You may need to remove hold-down bolts or clips.
  4. Locate the carbon brushes. This may be as simple as locating two clips on the outside of the motor, or you may need to remove the enclosure to find them.
  5. Remove the electrical connections to the carbon brushes.
  6. Remove the small brass clips holding the brushes in place.
  7. Pull the carbon brush out, making sure to note the bevelled edge's direction as the new brush needs to be installed in the same way. While removing the brush, you can keep track of the bevelled edge by drawing an arrow on the motor.
  8. Compare the old brush to the new one to ensure you have the correct parts.
  9. Install the new brush into place and fasten with the brass clip.
  10. Reattach electrical connection to the brush.
  11. Repeat on the other side.
  12. Replace the motor in the appliance. Bolt down and reattach electrical connections.
  13. Run appliance to test.

Summary

As carbon brushes are intentionally wearable, they are often the first component to cause a DC electric motor to quit running. By learning to inspect, maintain, and replace this element, you will ensure a long life for your motor and avoid the cost of replacing the motor or the appliance as a whole.

Have you replaced the brushes in your motor, and it still won't work? It might be time for a total motor replacement. Our technical staff is available by phone or 24/7 chat to help you find the replacement you need.

Have questions? Get in touch with our experts.

Connect with our expert team via email or phone.

1-800-890-7593
customerservice@emotorsdirect.ca

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