Battery Vehicle Society

Don't mind me, I talk to myself all the time

Anyway, I got impatient and wired up the tachometer the simplest way possible, with 12V supply to positive and negative terminals, and the reed switch connected between the coil terminal and the positive terminal. No luck, all I got was a little flicker from the needle when I brought the reed switch close to the magnets on the rotating shaft.

Nothing ventured, nothing gained, I added another battery so that the coil terminal was seeing 24V. By Jove it works! The reading is a steady 2200 rpm with the motor running clockwise at no load on 12V, and around 2050 rpm with the motor running anticlockwise. Not really sure what that tells me though. It suggests that the motor is advanced for clockwise running, but I've no idea by how much. I guess the only way to find out is to try rotating the commutator housing in increments to see what effect it has on rpm. Onwards...

Good luck with it!

Mary.

Managed to do some testing on my motors today. I took out the bolts from the end plate that carries the brushes and used a couple of sash cramps to clamp the brush plate at various angles to see what effect advancing and retarding the brush timing has on rpm.

To advance the timing you rotate the brush plate in the opposite direction to the rotation of the motor, and to retard it you rotate the brush plate in the same direction as the motor rotates. Advancing a motor increases the voltage it can be run at without severe arcing or flashover at the commutator. It also increases the rpm and reduces the torque slightly. To run these motors at 108 volts rather than the rated 36V, it's been recommended that I advance the brushes 8–10 degrees. (This is stuff I've gleaned from slightly obsessive reading of everything I can find online, so if anyone knows better please shout )

The resolution of my rev counter isn't great but it's good enough to show a trend in readings. I adjusted the brush plate angle in 2.5 degree steps to +10 degrees and then -10 degrees, and measured the rpm with the motor rotating clockwise and anticlockwise at 12V with no load. It turns out that the motors consistently rotate about 10% faster clockwise than anticlockwise at all timing angles. It also looks as if the motors were originally neutrally timed, as rpm increased gradually as the timing was advanced for both clockwise and anticlockwise rotation.

The difference in speed in either direction is a bit of a pain, as I want to run one motor clockwise and the other anticlockwise. I'm hoping though that this is just the result of small differences in brush pressure due to the angle that the brushes are mounted at. When the motors are under load this difference will hopefully become insignificant.

MalcolmB wrote:Don't mind me, I talk to myself all the time

Well I for one am quite happy reading you talking to yourself.

MalcolmB wrote:The difference in speed in either direction is a bit of a pain, as I want to run one motor clockwise and the other anticlockwise.

Don't get too obsesed with that. The issue would be any SIGNIFICANT torque difference. Also there will be times when you want them to run at different speeds, ie if you ever plan on turning a corner.

How have you decided to driving them? Series / parralel or more than one controler?
4qd have some interesting words on the subject.
http://www.4qd.co.uk/faq/bmnc2.html#diff
http://www.4qd.co.uk/faq/bmnc4.html#serpar

Thanks for the reassurance Jon.

Yes, it was the 4qd site that actually sparked my interest in this approach originally. I'm planning to use a single controller and run the motors in parallel. I did consider using series/parallel switching, but then realised that it would be a lot simpler, and probably no more expensive, to choose a controller that is rated for the maximum current of both motors. I'm leaning towards the Kelly controllers at the moment.