Re: Running DC motors at higher speed

["1sclunn" <1sclunn@msn.com>]

It would seem from this  that if you put a volt meter across say a 9" motor
your could measure rpm by the voltage load or no load .  what would be the
rpm of a 11" kostof when 120v appears across it and a adc 9" at 120 ?
----- Original Message -----
From: "Lee Hart" <leeahart@earthlink.net>
To: <ev@listproc.sjsu.edu>
Sent: Thursday, May 30, 2002 2:24 PM
Subject: Re: Running DC motors at higher speed


> 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
>
>