Resistance-Start Split-Phase Motor
R = Rext
Tlr  ksp I mw I aw sin 
Tlr  I aw sin 
ECE 441
1
Graphical Analysis
Iaux decreases with increasing Rext
angle α increases with increasing Rext
Locked-rotor Torque “peaks” for an
“optimal” value of Rext . Phase
displacement angle α is between
25° and 30°.
ECE 441
2
Practical Resistance-Start Motor
“Centrifugal” switch
or TRIAC
Closed (shorted)
when the motor is at
rest
Opens when motor
speed is 75% – 85%
of synchronous
speed
ECE 441
3
Practical Resistance-Start Motor
Phasor Diagram at start-up
ECE 441
4
Torque-Speed Characteristic
ECE 441
5
Cutaway view of a Split-Phase Motor
ECE 441
6
Capacitor-Start Split-Phase Motor
Develop a larger value of
Iaw sinα, and, hence, a
larger locked-rotor torque
Phase-displacement angle
between 75° and 85°
ECE 441
7
Capacitor-Start Motor
Phasor Diagram at start-up
ECE 441
8
Torque-Speed Characteristic
Higher Starting Torque
Same Running
Torque as before
ECE 441
9
Permanent-Split Capacitor Motor
• Uses a permanently-connected auxiliary
circuit containing a capacitor.
• Smoother and quieter operation than
resistor or capacitor starting motor
• Speed control by autotransformer across
the line, or external resistor or reactor
(inductor) in series with the main or
auxiliary winding (or both).
ECE 441
10
Permanent-Split Capacitor Motor
“Permanent”
Capacitor
Speed control by
autotransformer
ECE 441
11
Two-Value Capacitor Motor
main
Small capacitor
for running
auxiliary
Large capacitor
for starting
Centrifugal
switch
ECE 441
12
Example 6-2
• Using the motor from Example 6-1,
determine the capacitance required in
series with the auxiliary winding in order to
obtain a 90° phase displacement between
the current in the main winding and the
current in the auxiliary winding at lockedrotor and the locked-rotor torque in terms
of the machine constant.
ECE 441
13
Example 6-2 continued
• From Example 6-1
Z mw  2.00  j 3.50  4.0311 60.2551
Z aw  9.15  j8.40  12.4211 42.5530
120 0
I mw 
 29.7688  60.2551 A
4.0311 60.2551
120 0
I aw 
 9.6610  42.5530 A
12.4211 42.5530
ECE 441
14
Phasor Diagram
i',aw  90  60.26  29.74
ECE 441
15
Modified Circuit
Z
'
aw
Z
'
aw

'
z , aw
VT 0
 '
I aw 29.74
 z' ,aw  29.74
ECE 441
16
Impedance Diagram for Auxiliary Winding
tan(
'
z , aw
X aw  X C
'
)
 X C  X aw  Raw tan( z ,aw )
Raw
ECE 441
17
Calculation of Capacitance
X C  8.40  9.15 tan( 29.74)  13.628
1
1
C

 194.6  F
2 fX C 2 (60)(13.628)
ECE 441
18
Locked-rotor Torque
Tlr  k sp I mw I aw sin 
120 0
I 
 11.387 29.74
9.15  j8.40  j13.628
Tlr  k sp (29.7688)(11.387) sin 90  338.9 k sp
'
aw
338.9  107.1
%increase 
(100%)  216%
107.1
ECE 441
19
Graphical Analysis
Auxiliary winding current
increases then decreases with
increasing capacitive reactance
(why?)
Angle α increases with
increasing capacitive reactance
ECE 441
Locked-rotor torque “peaks” for
the optimal value of capacitive
reactance. The resulting phase
displacement angle is
approximately 75°
20