DC Motor brush life 3

[Clyde38]

Why do some dc motor manufacturers say you shouldn't run a motor at a voltage lower that rated? These are small motors, less that 2" in diameter.
Example:
12 volt motor at 6 volts.
They state that they need to install different brushes.

 

[THooper]

There is a number of factors that will determine proper commutation and brush performance. Tension,film, type of brush and grade, brush seating, and the mechanical trueness of the rotating surface. One important characteristic of a brush is the current carrying capacity. Current carrying capacity is influenced by ventilation, duty, load, and speed. Capacity of a brush is the ability of the brush to carry the proper amount of current to operate the motor under normal conditions. If the brush is not the proper grade for the change of current, brush life will be shortened and commutation will be lost.

There is formulas for brush grades and adjustments to meet your applications as suggested from the manufacture.

Kind Regards
motorhead

 

[jbartos]

Suggestion: If the motor terminal voltage decreases, the motor current through the brushes increases, since HP is a constant. Therefore, the brushes would have to be adjusted to the higher commutation currents through the brushes.

 

[UKpete]

Carbon brushes have a volt drop that isn't linear with current, it is almost like a forward biased diode. So when you have a low voltage motor such as yours, the volt drop (which may be 1 or 2 volts for a graphite brush) becomes very significant.

Carbon brush manufacturers add metals such as copper to reduce the resistance and volt drop in these applications. You can sometimes see this on the brush face.

 

[jbartos]

Suggestion: In some cases, motor brushes tend to be oversized to reduce sparking and increase their life-cycle.

 

[Clyde38]

Thanks for all of your replies. I should clarify my question. The motors in question are permanent magnet DC motors. In theory, the speed vs. torque curve at the lower voltage would be parallel to the original at rated voltage. The stall torque/current in this size motor is generally limited by the armature resistance. The torque is proportional to the current and the speed (no load) is proportional to the applied voltage. As such, the output power capability will be reduced. In this application (reduced voltage), the current would not be increased. If anything it would be decreased (I=E/R were R stays the same and E is reduced). However, the voltage drop of the brush system will become a larger proportion of the terminal resistance. Perhaps the brush material is changed to lower the resistance for the lower voltage motor.

 

[jOmega]

JB:

Just out of curiousity, where did you come up with the notion that :[ul]"Suggestion: If the motor terminal voltage decreases,
the motor current through the brushes increases,
since HP is a constant."[/ul]

Didn't see anything in Clyde's original post that stated any facts to support your statement.... Nothing about application or operation that would lead one to conclude that the motor was being operated at constant HP.

Hmmmmmm.

 

[UKpete]

Clyde, it is not simply the resistance of the brush:
For example, for operation at 5A/cm2:

A typical electrographitic brush (i.e. no additives) will have a total contact drop for both brushes added of approx 2V.
A typical soft graphite will have a total drop of around 1.5V.
A metal-graphite grade may have a total drop of 0.5V or less.
- hence the metal-graphite grade is preferred.
Note these are CONTACT drops i.e. the voltage between the brush contact surface and the commutator, this doesn't include any voltage drop along the brush due to the bulk resistance of the material.

 

[jOmega]

Y'all might find these sites helpful in understanding dc brushes and their life ..

[li] DC Motor Brush Life

http://www.reliance.com/mtr/cmdcbrmn.htm

[/li]

[li]White Paper:
DC Motor Brush Life

http://www.reliance.com/prodserv/motgen/c7090/

[/li]

 

[jbartos]

Suggestion to jOmega (Electrical) Nov 3, 2003 marked ///\\JB:
Just out of curiousity, where did you come up with the notion that :
"Suggestion: If the motor terminal voltage decreases,
the motor current through the brushes increases,
since HP is a constant."
///Motor load is often considered with a constant shaft load (HP). Since HP~Watts=ExI, then the only way to preserve the relationship for E decreasing is by I increasing.\\
Didn't see anything in Clyde's original post that stated any facts to support your statement.... Nothing about application or operation that would lead one to conclude that the motor was being operated at constant HP.
///This is a very common and tacitly implied assumption. The electrical software imputers would be very puzzled by the level of this kind of comment.\\
Hmmmmmm.
///Shape up a little bit.\\

 

[jOmega]

jb:

So as not to obfuscate the Clyde38's original post any further.... (discussion of constant HP operation of dc shunt machines has nothing to do with his original post)
I have replied to your comments in a new thread:

DC Motors : Constant HP vs Constant Torque

thread237-77879