Comparison of Starting Methods for Polyphase A.C. Motors
Starting
Method
Acrossthe-line
Part
winding
Primary
resistance
reduced
voltage
Auto-Xfmr
reduced
voltage
Wye-delta
Operation
Connects motor
directly across
the power lines.
Starts motor with
only part of
windings connected,
then adds remainder
for running.
Inserts resistance
in series with motor
for start.
Uses autotransformer to
reduce voltage,
usually in 3 steps.
Starts motor
connected wye,
then reconnects
delta for running.
LRA = Locked Rotor Amperage
LRT = Locked Rotor Torque
Xfmr = Transformer
% of
LRA
100
% of
LRT
100
Advantages
Disadvantages
Lowest cost; highest starting
torque; least maintenance; used
with any standard motor.
Low cost; low inrush current for
increment starting; popular
method for low starting torque
applications; low maintenance.
High starting current; high
starting torque; may damage
driven machine.
Not good for frequent starts;
can be noisy; may require
specially wound motor; low pullup torque; may not come up to
speed on first step with load.
High power loss because of I2R
in resistors; heat must be
dissipated; low torque per amp
input; high maintenance cost.
60-80
35-60
60-80
35-50
25
42
64
25
42
64
Medium cost up to size 3;
smoothest start; least shock to
driven machine; highest starting
PF; uses any standard motor;
most flexible in application.
Medium cost; best for hard start
loads; adjustable in field; uses
any standard motor.
33
33
Low cost; low starting current.
Not smooth starting; may shock
driven machine.
Lowest starting torque; requires
delta-wound motor; not adjustable in field.