Running DC motors at higher speed

[jcmassey@netspace.net.au]

Hi all

This is really for our motor guru's, I have been plotting over a couple of
motors that I have available to me, but I would like some clarity to help
make a decision.

If a motor has a rating of so many horsepower at such a voltage and RPM,
what is the effects of increasing voltage?

I'll make an example using simple numbers: A motor that is capable of 10kW
continuous at 100V at 3000RPM. Since it is 10kW we would know it is drawing
100A. Since (correct me if I'm wrong) the heating of a motor is purely from
amps, we can increase the voltage, but to keep the amps unchanged, we need
to run at a higher speed.

If we increase the voltage to 150V, we would now have 15kW at 100Amps,
instead of 10kW, but instead of being 3000RPM, we would now be at
(4500RPM?)? What is the magnitude of RPM changes with system voltage
change? (for the same current).

I am trying to clarify the overall effects of changing the system voltage.
Part one of this is that the vehicle system needs to be designed to allow
the motor to run at higher RPMs during 'normal' operation. 

Part two is the motor itself - being operated at higher RPMs means
increased risk of 'thrown' comm bars, and potentially rotor windings. I
have seen several mentions of thrown comm bars, but no mention of rotor
windings letting go. 

Has anyone experienced a 'blown' rotor winding (without thrown comm bars)?
If so, do they know why?

What have people tried to keep comm bars in place at higher-than-designed
RPM? I noted where someone recently said that they had (IIRC) a carbon
fibre wrap at each end of the commutator. This was what started this train
of thought. What has been tried and failed, what has been tried and been
(so far) succesful?

With the recent lowering of suburban speed limit here to 50km/h (around
30MPH) I'm keener than ever to get my budget EV conversion finished (since
if it is slower than I'd like no-one will notice...). 

All comments appreciated.

James Massey