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Name: Thomas
Status: Other
Grade: Other
Location: NJ
Country: United States
Date: Fall 2009


Question:
What is the effect on DC motor brushes if the motor is hooked to opposite polarity on power supply?



Replies:
Hi Thomas

I am not exactly sure what kind of effect you had in mind. If we are just looking at mechanical performance, reversing the direction of the motor will probably cause a different wear pattern on the brushes. This may result in some noise, both mechanical and electrical, until the wear pattern becomes stable.

Bob Froehlich


I do not think there is any detrimental effect, if that is what you are thinking about. At least with mechanical motor brushes I would expect none. Typically the positive and negative brushes are built exactly the same, graphite intercalated with copper or silver, spring-pressed against the smooth copper-alloy commutator contacts. There are two, one always gets (+) and the other always gets (-). So it would not have any particular effect to reverse the voltage.

It is possible, especially in the case of bare-wire brushes in tiny motors, that one brush erodes faster than the other because their disconnection arcs have differing polarity. But I do not think that one has been made larger than the other to compensate, so for wire brushes too you'd be free to reverse the voltage.

It is a time-honored thing to do with DC motors too, making them go backwards by reversing the voltage drive. I suspect motor manufacturers would be reluctant to make a motor that does not work well from reverse voltage, because there is usually a fair number of customers who will use the motor that way.

However:

Not so with brushless motors and electronic commutation, so far. Typically there are IC's and transistors which, if given reversed supply voltage, will not function and will almost certainly be damaged For these motors the rotation is reversed by telling the electronics to reverse the sequence of positive and negative voltages that they apply to the 3 or more motor-coil terminals. But the supply feeding the IC always keeps its original polarity. If you have a brushless DC motor with electronics built-in (fairly common for scooters), then there will be two big wires Red and Black for the motor power, and a few small control-circuit wires. One of those takes 0v or 5v signal and controls the direction of revolution, and another controls the motor-speed or how much current the motor is given.

Some small DC fans for computers have brushless DC motors with electronics inside. They have (+),(-), and maybe one more wire to control the speed. They never need to reverse, so the electronics were made without that ability. Reversing the voltage will only try to burn them out.

Jim Swenson


Hi Thomas,

The brushes of any DC motor will not themselves be affected, no matter which way the polarity of connected to the motor.

However, different types of DC motors will react differently depending on which way the polarity is connected. Here is a summary of what will occur:

1. Series-wound motors. These motors have the field coil connected internally in series with the armature. This means that current flows first through the armature (via the brushes), then through the field coil. With these series-wound motors, reversing the polarity will not make the motor reverse direction. The polarity can be connected either way and the motor will operate the same.

2. Shunt-wound motors. These motors have the field coil connected in parallel with the armature (that is, in parallel with the brushes). These motors also do not reverse direction when polarity is reversed.

3. Permanent magnet field motors. With a these motors, a permanent magnet is used to replace the field coil. DC power is connected via the brushes, to the armature only. Reversing the polarity, will cause the motor to reverse its direction of rotation. It is very common to use a switch to intentionally reverse to the polarity, to allow reversal of the motor if desired.

Hope this answers your question.

Regards,

Bob Wilson



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