# Ripple Current in Aged Brushed DC Motors

In summary, the author is puzzled by why motor ripple seems to increase over the lifetime of a motor even though the magnets should lose their magnetism. It is not clear how this could happen.
I work as an engineer and I've been looking for an explanation for something that I have been puzzled by.

Physics background of problem: In brushed DC motors, electrical coils (poles) rotate as the motor moves. A change in inductance as they move makes small variations in motor current arise.

It would be intuitive for these ripples to decrease in magnitude over the lifetime of a motor but this doesn't seem to be the case based on data from a motor manufacturer and ripple images found in "Brush wear detection by continuous wavelet transform." (The paper however does not focus on magnitude so it can't be conclusive evidence for this) I would expect the magnets in the motor to lose their magnetism as the motor heats and cools during its life cycle. This change should be small with modern magnets but it should still decrease the back emf constant of the motor and decrease the ripple magnitude.

How could motor ripple actually increase over the lifetime of a motor?

That paper describes ripple arising from brush to commutator contact which is not variations in inductance. The author was working on a technique to infer brush condition.
Brushes are usually some graphite bearing substance and they wear away. If the dust is allowed to accumulate it can jam the brush in its holder and it loses contact with commutator. Maintenance on large DC machines includes verifying the brushes are free in their holder so the spring keeps them snug against commutator, and replacing brushes as they wear away.

How could motor ripple actually increase over the lifetime of a motor?
Look in the back of your electric drill or circular saw and switch it on, observe the sparks. They will be small so long as the motor is healthy. A dirty commutator will spark more as the brushes make and break contact.

Dang, I learn something useful every single day here at the PF!

Thanks Jim!

## 1. What is ripple current in aged brushed DC motors?

Ripple current refers to the fluctuating or pulsating current that flows through a DC motor due to the commutation process. In aged brushed DC motors, this ripple current can increase due to wear and tear of the brushes and commutator.

## 2. How does ripple current affect the performance of aged brushed DC motors?

High levels of ripple current can cause increased heating and stress on the motor components, leading to reduced efficiency and potential damage to the motor. It can also result in increased electromagnetic interference, affecting the operation of other electronic devices in the vicinity.

## 3. What factors can contribute to increased ripple current in aged brushed DC motors?

In addition to wear and tear of the brushes and commutator, factors such as high motor load, high operating speeds, and poor maintenance can contribute to increased ripple current in aged brushed DC motors. It can also be caused by a mismatch between the motor and its load.

## 4. How can ripple current be reduced in aged brushed DC motors?

Regular maintenance and replacement of worn out brushes and commutator can help reduce ripple current. Other measures include using higher quality brushes and commutator materials, proper alignment of the brushes, and operating the motor within its recommended load and speed limits.

## 5. Are there any safety concerns associated with high ripple current in aged brushed DC motors?

Yes, high ripple current can lead to increased heat and potential damage to the motor, which can be a safety hazard. It can also cause electromagnetic interference, affecting the operation of other electronic devices and potentially causing malfunctions. Therefore, it is important to monitor and control ripple current in aged brushed DC motors to ensure safe and efficient operation.

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