Friday, April 3, 2009

Running 3-phase motors with 1-phase

Three-phase motors will run on single phase as readily as single
phase motors. The only problem for either motor is starting.
Sometimes 3-phase motors are purchased for use on single phase
if three-phase provisioning is anticipated. The power rating needs
to be 50% larger than for a comparable single phase motor to
make up for one unused winding
. Single phase is applied to a pair
of windings simultanous with a start capacitor in series with the
third winding. The start switch is opened in Figure
below upon
motor start. Sometimes a smaller capacitor than the start capacitor
is retained while running.

Starting a three-phase motor on single phase.

The circuit in Figure
above for running a three-phase motor on
single phase is known as a static phase converter if the motor
shaft is not loaded. Moreover, the motor acts as a 3-phase
generator. Three phase power may be tapped off from the three
stator windings for powering other 3-phase equipment. The
capacitor supplies a synthetic phase approximately midway 90o
between the 180o single phase power source terminals for
starting. While running, the motor generates approximately
standard 3-φ, as shown in Figure
above. Matt Isserstedt shows
a complete design for powering a home machine shop.

Self-starting static phase converter. Run capacitor = 25-30µF per HP.
Adapted from Figure 7, Hanrahan


Relate Posts