NEMA Constraints on Voltage and frequency
• A voltage variation of up to +/- 10% rated
voltage while operating at rated frequency
• A frequency variation of +/- 5% rated
frequency while operating at rated voltage
• A combined variation in voltage and
frequency, with the sum of the absolute
values of the respective variations not
exceeding 10% providing the frequency
does not exceed 5% of rated frequency.
Effect of Off-Rated Voltage and OffRated Frequency on Induction Motor
Performance
• Effect on running torque
21.12V  s
T 
f
R (120 )
p
V s
T 
f
2
D
s  0.3
2
2
D
s  0.3
Effect on Locked-Rotor Current
V
I 
(R  R )  (X  X ) 


s
V
V
I 

Z
(R  R )  ( X  X )
Z  "locked  rotor " impedance
2
2
2
2
1
1
2
s 1.0
2
lr
lr
2
1
2
2
1
2
Locked-Rotor Impedance (s=1)
  arct an(
z
X X
)
R R
1
1
2
2
If   75
z
Z X X
lr
I
2
s 1.0,z 75
1

2
V
V
V


X X
2 f (L  L )
f
1
2
1
2
Locked-Rotor Current
I I I
1
2
I  I
2
o
o
I I I
lr
I
lr
s 1.0,z  75
1
V

f
2
Effect on Locked-Rotor Torque
TD  Bstator I rotor
Bstator  I stator  TD  I stator I rotor
Tlr  I1 I 2
I lr  I1  I 2
V 
Tlr  I  Tlr   
f
2
lr
2
Operating 60-Hz Motors on a 50-Hz System
• Operation below rated frequency causes
– a significant decrease in magnetizing
reactance XM
– an increase in magnetization current
– resulting overheating
• To prevent overheating,
– reduce the applied voltage, keeping the ratio
of volts per hertz constant
Operating 60-Hz Motors on a 50-Hz System
• General-purpose 3-phase 60-Hz NEMAdesign induction motors with 2, 4,6, or 8
poles operate satisfactorily on 50-Hz
systems if the horsepower and voltage
ratings at 50-Hz are 5/6 of the
corresponding ratings at 60-Hz.
• The locked-rotor and breakdown torques
at 50-Hz will be essentially the same as for
60-Hz operation.