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22
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1
Experiment 22 The CapacitorStart Motor
OBJECTIVE
To measure the starting and operating characteristics of the capacitorstart motor.
To compare its starting and running performance with the splitphase motor.
DISCUSSION
When the splitphase rotating field was described, it was stated that the different resistance
reactance ratio of the two windings was designed to give the difference in time phase of the
currents in the windings necessary to produce a rotating magnetic field.
In twophase machines, where the windings are identical but displaced in space by 90°, the ideal
time phase displacement of the winding currents is 90°.
For both twophase and splitphase motors the toque developed at starting can be calculated
using the relationship:
ܶൌ݇ܫ
ଵ
ܫ
ଶ
sin ሺߙሻ
(
1
)
where k is a machine constant, I
1
and I
2
are the currents in the windings, and
α
is the angle
between the currents.
Because of the small magnitude of
α
in the splitphase machine the developed toque is relatively
low. It is possible to increase
α
by adding capacitance in series with the auxiliary winding. If too
much capacitance is added, the impedance of the winding is increased to the point that there is an
unacceptable reduction in the current which more than offsets the benefit gained from increasing
α
.
The optimum value of
α
is that where the product of the sine of
α
and the auxiliary winding
current is a maximum.
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The capacitor and the start winding are disconnected by a centrifugal switch, just as in the case
of the standard splitphase motor. Reversing the direction of rotation of a capacitor start motor is
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This note was uploaded on 02/11/2012 for the course EEE 360 taught by Professor Gorur during the Spring '08 term at ASU.
 Spring '08
 GORUR

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