Re: Running DC motors at higher speed

[Lee Hart <leeahart@earthlink.net>]

jcmassey@netspace.net.au wrote:
> If a motor has a rating of so many horsepower at such a voltage and
> RPM, what is the effect of increasing voltage?

First, what type of motor are you talking about? I'll assume a
brush-type series DC motor, since that is the most common type. The
results would also apply to a shunt or PM DC motor if you don't change
the current.

> 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.

The main losses in a motor are resistance, friction, windage, and core.

Resistance is from the resistance of the windings and brushes. It is
proportional to current squared (Power = I^2R).

Friction is the mechanical friction in the brushes and bearings. It is
directly proportional to speed.

Windage is the wind resistance of the rotating parts, spinning in the
air. It is proportional to the speed squared. If the motor has an
internal fan, this loss will be very large at high speeds.

Core is the magnetic losses. There are several contributors to this one;
it is roughly proportional to both speed and current, but then goes up
dramatically if the iron approaches saturation.

> 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).

To a first approximation, speed is proportional to voltage. But in your
example, it would actually run at somewhat less than 4500 rpm at 150v
and draw slightly more than 100 amps because the other factors are
adding losses due to the higher speed.

> 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.

Yes. Just be aware that if you want 10kw, running a 10kw motor at its
rated voltage and speed is usually the most efficient. If you ran the
same motor at a higher voltage and lower current to get the same 10kw,
it would be less efficient, because the increased friction, windage, and
core losses go up faster than the resistive loss goes down.

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

Yes. It happens more with higher voltage motors because the smaller
gauge wire is weaker. If the wires are soldered to the commutator bars
(typical of very cheap motors), you can also overheat a commutator to
the point where the solder melts and the wire is thrown out.

Fixes include varnishing or epoxying the windings in place, "banding"
(wrapping glass or kevlar around the windings to hold them in place),
and brazing the wires to the commutator.

> What have people tried to keep comm bars in place at higher-than-designed RPM?

A lot has to do with how the commutator is built. Most inexpensive
motors have a molded plastic commutator, with the bars simply bonded in
place.  At high temperatures and high rpm, the plastic lets loose. Other
motors have a built-up commutator with mica insulators and metal end
rings; they can be a lot stronger.
-- 
Lee A. Hart                Ring the bells that still can ring
814 8th Ave. N.            Forget your perfect offering
Sartell, MN 56377 USA      There is a crack in everything
leeahart_at_earthlink.net  That's how the light gets in - Leonard Cohen