Re: SRSG Phase Control

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: FutureT-at-aol-dot-com 

In a message dated 11/4/03 6:06:54 PM Eastern Standard Time, 
tesla-at-pupman-dot-com writes:

>I have read John Freau's web featuring a description of a electrical method
>for implementing phase control of a synchronous motor using a variac,
>resistor, and capacitor.  If John Freau is reading the list, or if someone
>else has sucessfully used this phase control method, I request assistance
>in implementing it myself.  I am currently using a modified-for-sync 3600
>cap start motor (1/2 HP).  In particular, I do not understand how the
>described procedure of adjusting the variac, while monitoring the voltage
>across the motor for resonant rise, translates to a suitable capacitor value.
>
>Per John's site, use of a 1/2 HP motor might make this effort an
>non-starter, but I thought I'd ask, anyway :)
>
>Aric
>


Aric,

The phase shifter will work fine for a 1/2HP motor.  First select
an estimated capacitor value to use, for example 180 uF, and
install it into the circuit.  Next attach a voltmeter across the
two motor input wires.  Then, apply power with the variac set at
zero inductance.  The motor should start up and lock into synchronous
operation.  Gradually rotate the variac knob while watching the
voltmeter.  See if the voltage eventually rises or falls.  If the
voltage drops lower as the variac knob is turned, then the capacitor
is probably too low of a value.  If the voltage rises higher than
120 volts (or whatever your input voltage is), then measure just
how much higher it rises.  For example let's say the highest voltage
reached is 125 volts at some point of the knob's rotation.  That
means there's a 5 volt resonant rise across the motor which is
just perfect.  This means the capacitor value is the correct value.
Now suppose the voltage rises to 140 volts instead of 125.  This
means the capacitor is of too large a value.  A lower capacitor
value should be selected to give 125 volts (or a 5 volt rise over
the 120 volt input voltage.  Now, despite what I just said, some
motors will remain locked throughout the variac's range even
though no resonant rise occurs.  If you have motor is of this type,
then that's fine too.  There doesn't *have* to be a resonant rise,
but usually it's needed to keep the motor locked throughout the
variac's range.  Also, it's not really necessary to keep the motor
locked throughout the variac's range.  But in this case you'd need
to put a stop on the variac to prevent it from being turned to the
point where the motor drops out of sync-lock.  (limit the knob's
rotation range.)  Some motors may require a 10 volt resonant
rise to keep the motor locked thoughout the variac's range.
Each situation is a little different, but I tried to give rules that
apply to the most common situations.

John