April 2002

ALL ABOUT ELECTRIC MOTORS (Almost)

Unfortunately, Meccano is not the only “Mechanics in miniature” hobby around (I would have more money to spend on Meccano then).

I am involved in Radio control car racing. It encompasses all the mechanics and dynamics of a real car, you guessed it, in miniature !

The tyres, chassis, shock absorbers, drive shafts, differentials, gearing, camber, toe in/out,  (Insert endless list here),  experience the same characteristics/dynamics of a “real” racing car.

Why I am writing this article is that both hobbies have one thing in common, Electric motors.

“Single, double, quintuple, google... There are lots of options to choose from when picking a wind.”

What are you going on about? , I hear you saying.

Yes, there is more to an electric motor than just the size and shape of it.

All hobbyists want the smallest, lightest motor with the highest torque, best efficiency and still want to get the highest RPM from it all !!

Unfortunately this is not all possible and I am going to explain why.

How do you make the same electric motor perform so differently?

The following extracts from http://www.rctek.com explains :

Turns and strands

Modified motors are rated by turns and strands. A 14-turn triple has three strands wrapped fourteen times around each pole of the armature.

Other things being equal a motor with fewer turns draws more current, has more no-load RPMs, and produces more power.

A motor with fewer strands has more "snap" -- it accelerates more quickly from low speeds.

A motor with the same number of turns and more strands has more power at the top end (Generates higher RPM).

Variables

Variables affecting performance of the motor  include the quality of laminations in the armature stack, the strength of the magnets, the wire size and winding pattern used, the diameter of the commutator, and the choice of brushes and springs.

Large commutator motors; these motors sacrifice some peak RPM and efficiency to gain a wider power band, and require less maintenance than conventional motors. 

big brushes; they deliver more power and last longer.

Brushes with a high silver content offer more conductivity = (power) but they wear down your commutator fairly quickly .

Timing

A tuning trick to get more power out of your motor is to change your timing.

Here is an practical example from http://bigjimracing.topcities.com/ on changing the timing of a motor :

Timing is the relationship between the brush center and the center of the magnets. When these two centers line up, that is called "zero" degrees of timing. Timing can either be "advanced" or "retarded" by rotating the endbell either clockwise or counter-clockwise to the can (On most endbell 540 type motors).

Electrical timing is another subject which we won't go into now.

The only timing we can change is the brush/comm relationship by turning the endbell on the can.

In this example we will assume the desired direction of armature rotation is CW from the top off the motor (endbell side).

Looking down at the endbell, rotating the endbell CCW from zero point will advance the timing. Rotating the endbell CW from the zero point will retard the timing.

Under no circumstances is the endbell to be turned past zero (CW) into the retarded position. This will cause the motor to arc excessively, run hot and everything else bad.

Now to setting the timing. Rotate your endbell CCW so that the it has moved about 10 degrees on the axis. This is a good place to start.

The amount of timing necessary for maximum performance is dependent on a lot of variables like current available, armature wind, gearing etc.

Run the motor, experiment by advancing the timing slowly until you get best results.

If you notice the motor arcing excessively, back off on the timing.

Play around up to 27 degrees. Anything more than 27 degrees up to 57 degrees results in high temperature generation and advanced wear. More than 57 degrees results in power dropoff , so be aware of the consequences you can expect with excessive timing.

The condition of the motor (comm, brushes, etc.) will determine how much timing you can run.

This motor is now tuned to run CW. Running this motor CCW will result in poor running and low efficiency. By deduction this is fine for the racer but will limit the practicality for the meccanoman.           

Confused?  So am I (but don’t tell anyone) !

Example

This graph comparing a 10 wind single and a 10 wind double might help put things into perspective.

Here is the same graph, with actual figures to give some perspective.  

Breaking in and maintaining an electric motor

The following modified extract is from http://www.rccaraction.com

“Here’s how to extract more power and keep your motor running fast.”

Step 1: Break-in

NB Only perform this step on a new motor or on a new set of brushes !!!

This should only be done on a “naked” motor i.e. It can’t be done on an MR motor.

Cricketball and sideplate owners should definitely skip this step as well ;-)

Sorry to carry on, but I DEFINITELY will not accept ANY responsibility or liability for any failed break-in attempts (water + electricity = BAD).

Attach a 4-cell (4.8V) battery pack to the positive and negative leads on the motor. Next, dip the motor (while it’s running) into a cup of water for 3 seconds. Actually count out “one thousand one, one thousand two, one thousand three,” then pull the motor out of the water. The water coursing between the commutator and the brushes will almost instantly shape the brush faces so they conform perfectly to the motor’s comm.

Steps 2,3, & 4 are general maintenance steps

Step 2: Drying the motor

After you remove the motor from the water, spray motor spray liberally through one of the cooling vents on the motor can; be sure to saturate the motor’s innards. Then place the motor on a rag so it dries, and any excess motor spray is absorbed. This will accomplish two things: first, the spray will clean the motor by removing all the carbon deposits left over from the break-in process, and second, the motor spray will displace excess water inside the motor.

Step 3: Lubing the bushings

After the motor has dried completely, add a couple of drops of bushing lube to the motor’s oilite bushings. Spin the armature to distribute the oil evenly on the bushings.

Step 4:
Applying comm drops
Comm drops can be applied to the commutator to increase conductivity between the brush face and commutator. Comm drops also lubricate the brushes and armature to reduce friction, and that translates into increased power and efficiency.

(A real plus for those cricketball and sideplate owners out there)

Motor spray, bushing lube and coms drops are all readily available from your local hobbies shops that cater for radio control enthusiasts.

PLEASE be aware that Comms drops have their pros and cons :

The cheaper,older makes actually advance wear on your commutator at a frightening rate.

The newer more expensive makes are more commutator friendly and don’t promote any unusual wear on your brushes and commutator.

Ask your hobby shop owner for his recommendation.

So WHY would ANYONE want to know all this !

Lets look at it from the perspective of the racer and the meccanoman.

The Racer :

If you pick a motor with the highest RPM, you are going to loose torque (Slower out the corners, Faster on the straight)

If you pick a motor with high torque, you are going to have to sacrifice some RPM (Faster out the corners, Slower on the straight)

Why would you want a slower spool-up time? It's all about traction. Low-traction conditions favor less torque, and a longer spool-up means less torque. So if traction is low, you can put down power more easily.

Other factors the racer looks at are efficiency of the motor. How long is the race ?

A High torque, high power 10 turn single wind will deplete a 1600Mah battery in half the time that a 27 turn triple wind will. Nobody’s a winner if they run out of “Fuel” 10 meters before the finish line, even if they were coming first !

The Meccanoman :

Hmmmmm

Why would a Meccanoman want a high power, low efficiency motor ?

Here is a good generalization to help you :

The less turns of wire on a motor (e.g. 11 turns compared to 15 turns), the faster it will generally be. However, it will also be generally tougher on batteries. Higher turn motors will be easier on batteries, and generate more torque - allowing you to gear higher.  

Ref : http://www.rccaraction.com

         http://www.rctek.com

         http://www.teamrcv.com

         http://bigjimracing.topcities.com/

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