DC motor- does the brush resistance vary during operation?

[MedievalMan]

My question is regarding the brush of a separately excited dc motor, given in the context of the standard DC motor control model.

I am investigating whether the effective armature resistance (that is, the resistance due to the armature windings as well as the brush) varies during motor operation.

I realize that overtime as the brush wears, the effective armature resistance increases, decreasing the efficiency of the motor (due to higher brush resistance / voltage drop). I’m trying to find out if there’s any significant variation in this parameter while the motor is running (perhaps due to commutation/brush effects.)

I can’t seem to find any literature on this issue, so perhaps I’m beating a dead horse, so to speak.

Thanks in advance for any insight,

Medieval

 

[Skogsgurra]

There are temperature effects. I think that looking at the Le Carbone and Morgan sites can give you some facts.

Also, I think that the opposite is true (resistance decreases, not increases, as brushes wear). The effect is not very pronounced - but if it is there it should be decreasing. There are two reasons for that: A the brushes do not have a very good contact with the commutator before they have been run in and B The "carbon path" decreases as the brush wears (gets shorter). The side walls of the brush holder may reduce that effect, but not very much.

There is also a mythical semi-conducting interface between brush and commutator. It is sometimes said to have a near constant voltage drop (negative resistance) and sometimes decreasing with current. Not so sure about that.

Gunnar Englund

http://www.gke.org

 

[sreid]

The brushes do change resistance but the brush-commutator resistance is always small relative to the winding resistance. The windings, of course, change resistance as they heat up during operation. This can be as much as a 50% increase.

 

[MedievalMan]

skogsgurra,

From what I've read the resistance increases as the brush wears, but given your explanation it makes sense that the resistance would decrease. Hmmmn.

Sreid,

Just to verify: You are saying the brushes do change resistance over a longer period of time due to wear,

or that there is a resistance variation during operation due to the brush-commutator interface, but that it is small relative to the winding resistance?

Thanks for the info guys.

Medieval.

 

[sreid]

MedievalMan,

That the resistance changes at the brush commutator interface during operation. I believe that there is a resistance change due to the speed of the motor (possibly due to brush lofting from air entrainment) and due to the motor load (Brush current density).

 

[powerjunx]

MedievalMan,
"I realize that overtime as the brush wears, the effective armature resistance increases, decreasing the efficiency of the motor (due to higher brush resistance / voltage drop). I’m trying to find out if there’s any significant variation in this parameter while the motor is running (perhaps due to commutation/brush effects.)"

I bet your interesting with your observation.

Well, Carbon brush and commutator contact has "voltage drop" which relatively and slightly affects the efficiency. This drop will eventually increases as brush wears increase over time as evidently sparking appears. Hence, resitance increases.

 

[BobM3]

In his book "Carbon Brushes and Electrical Machines", author P. Hunter-Brown presents the following data:

1) Contact drop increases from .6 volts to 2.4 volts as the current density increases from 1 amp/in^2 to 80 amp/in^2. It's not linear in the low range of current density. Most of the change occurs from 1-20 amps/in^2.

2) Contact drop increases from 1.4 volts to 2.7 volts as the speed increases from 2000 to 7500 peripheral ft/min.

3) Contact drop decreases from 2.4 volts to 1.0 volt as the pressure increases from .9 to 10 lb/in^2. Nonlinear with most of the change occuring at the low pressures.

 

[powerjunx]

bobM3,
i think you've pointed out the difference! These factors really affects the parameters as you've cited? You miss the Temperature! How about it?

i hardly recall, are these value under ANSI standard?

 

[aolalde]

Hi MedievalMan.

I am investigating whether the effective armature resistance (that is, the resistance due to the armature windings as well as the brush) varies during motor operation.

The answer is yes, the brush resistance changes during loaded motor operation (the armature winding changes too, due to temperature change).

But as Skogsgurra pointed, the brush resistance decreases not only because of wear but when the load increases. That is due to a negative temperature resistance coefficient (NRTC) for most of the graphite blends.

Voltage drop is another concept, since it means current times resistance. If we analyze the figures given by P Hunter-Brown for current densities changing from 1 to 80 Amp/sqin the voltage drop only changed from 0.6 to 1.4 Volts. If the brush resistance should be constant the voltage drop should increase proportional to the armature current, but it increases at a lower rate.

By the way, this NRTC could destroy some brushes in short time, since the current will be localized on the hottest spot (lower resistance spot ) to improve that performance the commutator surface has machined spiral grooves, these forcing a change of the electric contact brush-commutator, averaging the conducting contact surface.

 

[BobM3]

fbcybil -

P. Hunter-Brown tried to relate voltage drop to temperature but got inconsistent results. He found in most cases that the contact drop would initially go down as the temperature increased but would then go up with further increases in temperature. He attributed the increases to the build up of an oxide layer on the commutator.

I don't know if any of these test results are part of an ANSI standard.

I should have pointed out that the contact voltages I listed were the sum of the drops on both the positive and negative brushes.

 

[edison123]

aolalde,

"the commutator surface has machined spiral grooves".

That's interesting. I have seen sliprings with spiral grooves but never in a commutator.

Do you happen to have any pictures of such a commutator that you can post here ?

* If money can buy happiness, who is selling it ? *

 

[aolalde]

Hi Edison.

I will look in my files an if I find one picture I will post it.

Certainly a few commutators have spiral grooves, analyzing the reason I think the mica separators produce a similar effect as a spiral groove. The commutators I have seen with spiral grooves are for traction DC motors spinning at 3500 rpm may be the spiral grooves were added due to the high peripheral speed of those commutators.

 

[edison123]

Thx aolalde.

Would be great if you can find the picture.

* If money can buy happiness, who is selling it ? *

 

[oftenlost]

Some of the GE transportation brochures have pictures of the grooved commutators. It forms a checkered pattern with the mica separators. If no one has an electronic copy, I will scan one.

 

[MedievalMan]

A followup question/clarification:

I see the brush drop increases as motor speed/ armature current increase (from BobM3's post). I've seen some papers which model the brush (arc) drop as Vd=Const*Ia*Wm.

Many Electric machine books say the brush drop can be modeled as a voltage drop or included in the armature resistance. If you moved the voltage drop as a resistance, wouldn't the resistance increase as the voltage drop increase (Ohm's Law)?

Yet aolalde's post and others mention the negative temperature resistance coefficient (NRTC) for most of the graphite blends of the brushes, which leads to the brush resistance decreasing when worn / increased Ia.

Perhaps the voltage drop and resistance are two seperate issues?

Thanks again for the valuable posts,

Medieval

 

[BobM3]

Look at aolaolde's post from above (the one right after mine). As he points out, an increase in current can cause the voltage drop to go up even though the resistance goes down.

 

[max586]

We have used a 10 microF capacitor for a 230V motor instead of a required 12.5 microF one by mistake. What would the short, mid or long term consequences be if we don't replace it? Any feedback would be gratly appreciated.

Thanks,

Alfonso

 

[oftenlost]

Here is a link to a picture of a grooved commutator

http://www.jolietoffshore.com/jtmq14.htm

 

[edison123]

oftenlost,

Thx for the pic. I was looking for a spiral groove commutator. I have seen commutators with cylindrical grooves.

* Algebra - The weapon of math destruction *