Re: [CAD_CAM_EDM_DRO] Re: Phase Converter

dave_ace_me wrote:

> If you want to just get the motor spinning, remember that you only
> need a fraction of the amps to get 50% RPM on the motor. The soft
> start motor starters or very small VSD's can be 1/4 of the hp (or
> less) to get motor spin near 80% of speed.
>
> I'm not positive but I think the amp draw is a square root function,
> so 1/4 amps will spin it 50% and 1/16 amps will spin it 1/4 speed.
> We used to test large hp motors with small VSD's to verify operation,
> just keep under the VSD rate hp (amps) and you can get it to spin.
> Once it is moving that fast, the amp draw to get it spinning full
> speed is vastly reduced.

It really doesn't work like that at all. Squirrel cage induction motors are
VERY nonlinear devices when operated far from synchronous speed.
They draw HUGE currents at full voltage while accelerating. If you
don't supply full voltage but at nominal frequency, you may well get
nothing but humming, or get rotation at 5 RPM.

VFDs work by applying rising frequency in coordination with rising
voltage, to properly excite the rotor windings as the RPM builds
up. This is really hard to do with constant frequency.

There is a special type of motor that can operate far from sync. speed.
That is the wound rotor motor, often erroneously called "repulsion start".
These motors have a commutatior and brushes that look exactly like
DC and universal motors, but the brushes are just shorted together
wit a little wire. What is not so visible is a "bracelet" inside or adjacent
to the commutator that shorts all the commutator segments together
when the motor approaches synchronous speed. These motors are
also used in trolley cars, carousels, amusement park rides, and other
machines that take a really long time to accelerate. Instead of the
wire that shorts the brushes, they have huge drum controllers with
big resistors, and as the motor picks up speed, the handle is turned
to reduce the series resistance on the rotor windings, allowing the
rotor flux to build.

Jon