# Precise influence of the advance of brushes



## Dalardan (Jul 4, 2008)

Hi everyone,

Right now I'm looking into finding a subject for my "End of studying project" (don't know exactly how to call it in english...) and I'm thinking about making a servo actuated adjustable brush holder for a series wound motor.

First of all, I plan on coupling this motor to a dynometer than, for a given speed and voltage, plotting the torque and amp rating thus finding the best efficiency. By doing this on the whole rpm range of the motor, I should get a advance degree/rpm plot that I can then program in the servo actuated brush holder to get optimal efficiency. This way I should be able to get reverse ability and high voltage capability. Am I right?

Just to think, does a similar system already exists? I've found none. Also, is the efficiency/rpm curve dependant of the voltage? I think this can be interesting to looks into.

Waiting for feedback!

Dalardan


----------



## JRP3 (Mar 7, 2008)

I think someone on the EVDL did something with this concept, you might want to search or inquire there.


----------



## judebert (Apr 16, 2008)

You're pretty close to right.

Hi-Torque Electric makes adjustable brush rings for Advanced DC motors, and possibly for others as well. You'll definitely want to check out his website.

I was running my brushes with opposite advancement, and it caused all kinds of trouble. Ugly noises, nasty smells... I don't know how bad the efficiency was, but the probability of failure due to arcing was high.

As you probably know, but I will point out for those who are not familiar with brush advancement, the point of advancing the brushes is to avoid arcing. As the speed of the motor increases, the change of any one commutator bar from positive to negative is delayed. If the voltage between one commutator bar and the next gets too high (especially with high currents), the air may act as a conductor, causing a spark to travel between the two bars (which is known as "arcing"). Obviously this can cause damage, and is not the most efficient way for electricity to move the motor.

So we move the brushes to a location that allows enough time for the commutator bars to switch polarity at high speeds. This eliminates the arcing, which solves the big problem. But the trade-off is that the switch comes too soon at lower speeds, adversely effecting efficiency. Worse yet, the brushes are exactly backwards for regeneration, where voltage and current are likely to be high. This is what makes most series motors unsuitable for regen.

The optimal placement for the brushes at any point is wherever the voltage between one commutator bar and the next is minimal (in an ideal world, 0V). 

Therefore the optimal system would measure the voltage between two adjacent bars and move the brushes accordingly. We can estimate that based on the motor's speed and direction, but perhaps there's a better way: add an extra brush one comm bar away from one of the conducting brushes. Measure the voltage between the two brushes and adjust accordingly. Since we've already got voltage there, perhaps using it to run a small motor that screwed the brushes into place would be plausible. Heck, if the motor reversed according to the polarity, it wouldn't need any electronics at all!

Just an empirical alternative for you to consider.


----------



## major (Apr 4, 2008)

Dalardan said:


> I'm thinking about making a servo actuated adjustable brush holder for a series wound motor.
> 
> First of all, I plan on coupling this motor to a dynometer than, for a given speed and voltage, plotting the torque and amp rating thus finding the best efficiency. By doing this on the whole rpm range of the motor, I should get a advance degree/rpm plot that I can then program in the servo actuated brush holder to get optimal efficiency. This way I should be able to get reverse ability and high voltage capability. Am I right?
> 
> ...


Hi Dalardan,

Brush advance is done to reduce arcing. Mild to moderate advance has little to do with efficiency. Advancing the brush position moves the armature to field alignment taking it out of quadrature, thus is a form of field weakening. So the resultant torque is lower and RPM higher for a given current. The lower torque per amp is the main downside to brush advance for drag racers.

Brush advance, as well as field weakening in general, does affect the motor efficiency. But this is not an across the board increase or decrease. It may be a few percent higher at light loads and a few percent lower at high loads, or visa versa.

Realize that the arcing, the reason to advance brushes, is dependent on several things, like the voltage, current and speed. It is difficult or impossible to set the brush advance to optimize commutation for all conditions. It is usually set for a compromise by a set-of-the-pants experience factor. To get an automated adjustable position system, it would be best to bring in a feedback from the parameter you want to minimize, ie. arcing. How about a light sensor and move the brushes to the darkest position?

I guess if one was so inclined, he could manually test and record commutation (arcing) for a given motor under varying loads, voltage and speeds. Then construct a look-up table, or formula, to run the position actuator.



> Just to think, does a similar system already exists?


I have heard of old motors, like the prop motors on a WWII German submarine, having brush rotators. These were manually adjusted under load by a sailor viewing the commutator.



> Also, is the efficiency/rpm curve dependant of the voltage?


Yes.

Interesting project, good luck.

major


----------



## Dalardan (Jul 4, 2008)

I'm more and more interested in this project so I'll see if I can get it credited as an end of baccalaureate project. 

Effectively, I was also thinking about building a 3D look-up table. I've done some during dyno tuning of the air fuel ratio of a Formula SAE car by varying the Throttle Pedal position and the RPM over the whole motor range. The worst is placing the setting for the first test. For the others, take a good coffee and do them. The tuning of the system should take no more then one day to do once everything is working very well. The worst of the whole project would be building the prototype and coding the algorhytm.

Just to know, for the same voltage and RPM rating, if the advance is changed, will the torque and amp ratings not vary proportionally, thus having a peak power advance setting and a peak efficiency advance setting that are not the same?

Also, even if I'll find the answer to this question during dyno testing, is the peak torque when at the limit to arc (whether with too much or too low advance) or somewhere between them? I know I've some peaky questions, but the question in itself may help building the file for the project to be accepted.

Thanks for your interest,

Dalardan


----------



## Mjolinor (Sep 15, 2008)

I am sure I have seen this done on the commutator with bob weights, same outcome but you move the other bit. It only applies to increased revs, it wouldn't help with regen.

I used to tune the motors on slot cars years ago by turning the commutator, you could get them flying when they went forward but they never ran backwards afterwards though you never needed them too anyway, with that, shimming the magnets and running them in with toothpaste on the gears nothing would beat you.


----------



## major (Apr 4, 2008)

Dalardan said:


> Just to know, for the same voltage and RPM rating, if the advance is changed, will the torque and amp ratings not vary proportionally, thus having a peak power advance setting and a peak efficiency advance setting that are not the same?
> 
> Thanks for your interest,
> 
> Dalardan


Hi again,

The above question does not make sense to me. Sorry. Not sure what ratings have to do with peak power or peak efficiency. But I think the answer to your question is "not necessarily".



> Also, even if I'll find the answer to this question during dyno testing, is the peak torque when at the limit to arc (whether with too much or too low advance) or somewhere between them?


Here again, a bit unclear as to what you mean. But, the torque at a given current will be the highest with brushes at mechanical neutral. There the field and armature are at quadrature. Torque is the cross product of the two, so a maximum when they are at 90 degrees (electrical).

Hope that helps,

major


----------



## Dalardan (Jul 4, 2008)

Well, for the peak torque and peak effiency, I'll manage to find if ever I start this projet. 

For the quadrature, I do agree that at 0 RPM if the motor is at mechanical neutral the amp and current will be at 90º of each other. Is this still true at high revving speeds, thus needing to advance the timing to compensate?

About the physical installation, I've access now to a 40 HP peak water dynamometer (Dynomite water pump). I do know that a warp 11" would simply overkill this little pump. Is a standard small series wound forklift motor too big for this setup? I sure need to get to a high voltage setup to experiment arcing (and thus need to advance the brush holder) but I need also to stay under this limit of 40 HP. If you've got ideas, my mind is open to you.

Also, what can be the maximal safe revving speed of a motor? I do know this depend of the quality of the rotor build, the speed limit of the bearings and the diameter of the motor. Are we saying 500 RPM or 10'000 RPM? That can have an influence, I need to know what's the top speed of the pump. Is the top speed of the motor simply not efficient because of arcing, but if the advance of the brushes would have been optimal for a very high top speed, the motor would not even start?

Well, I've gotten a lot of questions, I'll see what I can find.

Dalardan


----------



## dbonner (May 7, 2009)

Dalardan said:


> Well, for the peak torque and peak effiency, I'll manage to find if ever I start this projet.
> 
> For the quadrature, I do agree that at 0 RPM if the motor is at mechanical neutral the amp and current will be at 90º of each other. Is this still true at high revving speeds, thus needing to advance the timing to compensate?
> 
> ...


Hello, I am VERY interested in your findings if any. I have built a Brushed DC electric sportscar with a 1000 amp output controller. The motor is a netgain 7 inch built by Jim Husted, a drag race motor wizard who unfortunately no longer appears to be active today. The motor is balanced to 7000 RPM and has a mobile brush carrier and a powerful and fast servo to provide continuous servo-driven advance from 5 to 15 degrees. See photo of the servo advance on Electric Car Project (just need to scroll down a bit). So far I've been running it with a fixed plate replacing the servo, at 5 degrees advance. I did this because I wanted to take things one step at a time and the car runs very well with the fixed avance up to the speeds I have tested so far which are up to about 100 kph / 5000 RPM. 
Now that I have a baseline I am very keen on activating the variable advance feature. 
I want to create a 2 dimensional map that I can load into an arduino to control the servo. Inputs will be instantaneous motor amps and RPM, or motor amps and voltage. The output will be advance position for the servo. 
Do you have any insights for this? 
Thanks!


----------

