# Advancing Brushes Explanation



## jackbauer (Jan 12, 2008)

I'm not a motor expert but i've had some first hand experience with lack of advance and the problems incurred. The reason can get hugely technical but here's my simple explanation. As a dc motor is used above its rated voltage and speed then the magnetic field generated by the field coils gets "pulled" out of position by the armature. The distorted field then causes the brushes to change bars on the com (called commutation) at a less than ideal point. Ideally , the motor should commutate when the armature bar that is in circuit is still under the influence of the field pole its just leaving. This reduces the inductive kick back as the current changes direction. Called armature reaction if i remember correctly.

So advancing the brushes give the motor a better chance to commutate successfully at higher voltages and speeds. I've a few videos on the subject that may help.

In this video the brushes are neutrally timed:
http://www.youtube.com/watch?v=AobcKoow8Z0

And here advanced about 12 degrees:
http://www.youtube.com/watch?v=V8fuaMJ8N1Q

hope this helps.


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## major (Apr 4, 2008)

Hi mech,

This has been explained many times. But WTH:



mechman600 said:


> -How high can you go in voltage before advancing the brushes becomes necessary?


It all depends on the motor and the application. Some guys will tell you some numbers, but they are shooting from the hip. Maybe it will work for you if you have the same motor, maybe if you have a different motor, maybe not.

I have seen unidirectional pump motors on forklift trucks with advanced brushes using 24 volt systems.



> -On a 36V or 48V forklift motor, is advancing necessary if I want to give it 72V?


Maybe  Again it depends on your application and the particular motor. If you have a unidirectional application for the motor, I'd advance the motor about 4 or 5 degrees. You lose a little torque, not much and overload sparking is reduced.



> -What happens if you do not advance the brushes?


Again it depends on your application and particular motor. Maybe it will work just fine, maybe it will zorch. Zorching (catastrophic commutation failure) gets more likely with higher motor voltage, higher motor current, and higher RPM.



> I understand the principle of timing advance in an ICE, but can someone explain why it is necessary in an electric motor?


Commutation is the process of switching current, more specifically in the DC motor, the process of reversing current. The comm and brushes do this mechanically. So as the armature coil connected comm segment passes under a brush, the current in that coil is forced to reverse direction. Current does not like to be changed if there is any inductance in the circuit in which it is flowing. All the armature coils have inductance. Changing current in inductive circuits will induce voltage. The faster the current change; the higher the voltage.

It is this voltage, and also the result of armature distortion of the magnetic field, which cause the voltage to become excessive as the brush and commutator segment separate. Too much voltage and she arcs. The bigger the arc, the more damage. Maybe just some degradation of brush life due to local heating. Maybe worse. If things really get out of hand, the arcing starts to load the local atmosphere with carbon and copper dust and it then becomes conductive and then you get a plasma event which is a ring of fire around the commutator. This plasma is just that. I have seen it actually sever metal parts like brush holders or other parts. Sometimes like chunks of the comm itself. It doesn't take long for the motor to become a rather large short circuit, or open circuit.

Now if you take a look at our 4 pole motors, they are arranged N, S, N, S. As a single armature coil rotates around under these 4 poles, it passes from N to S to N and so on. While that coil is under a North pole, it has a generated voltage in a positive polarity and positive current flow. Under the South pole, it has negative polarity and negative current. In between the N and S pole, there is a "neutral zone" where the generated voltage in the armature coil sees zero voltage. This is the ideal time to reverse the current, if there were no induced voltage on that coil from a change in current. So at no load (no armature current) that ideal point is midway in between N and S poles (physically).

Armature distortion (due to a field in the armature from armature current) moves this zero voltage crossing point from midway between N and S poles. Also, to counteract the induced voltage caused by the armature current and coil inductance, one can further reposition the point of commutation to purposely put that armature coil under a generated voltage of opposing polarity. Hence "advancing brushes" forces the comm bar to brush voltage close to zero reducing the likelihood of arcing.

But this ideal brush position is dependent on the magnitude of armature current, applied voltage and speed at which it is commutated (RPM) for each particular motor design. No one position is ideal for all conditions (loads, RPM and voltage) for all motor designs.

Add to this that the side affect of advancing brushes is field weakening which reduces torque for a given armature current. The amount of torque reduction is a complex function, but I have been previously forced to give an estimate. For a 4 pole motor, if you advance the brushes 90° you reverse direction of torque. 45° would be zero torque. So I approximate it at 2% torque loss for each degree of advance. 

I think a lot of folks go overboard advancing motors when it is really not needed for commutation. Just think, a 13° advance knocks you down 26% in torque. Could you have successful commutation with 26% less current and get the same torque? Kinda hard to tell especially for all motors and all conditions 

Sorry you asked 

major


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