Transcript Useful Approximations for Normal and Overload Conditions of Squirrel - Cage Motors

Useful Approximations for Normal Running
and Overload Conditions of Squirrel-Cage
Motors
• “Normal” running conditions
– No load to 15% overload
– Rated voltage
– Rated frequency
• For these conditions, slip is usually <0.03
• Make some simple approximations
21.12I R
T 
lb  ft
sn
R
r
r
D
s
IT 
D
2
21.12V
2
2
s
V
(R  R ) R (X  X )  n
R  jX s   jX

s
2
2
2
2
1
1
1
2
1
V
I 
(R  R )  (X  X ) 


s
2
2
2
2
1
1
2
for very low values of slip
R
R
R  21.12
 (V
X X )
s
s
T 
2
2
1
D
s
2

1
R
2
2
2
2
2

(R  R )  (X  X ) 


s
2
2
2
1
1
R
( )
s
2
2
s 0.3
2
V
V s
I 

R
R
s
R
21.12V
21.12V  s
s
T 

R
Rn
( )n
s
I s
Rotor Current, Rotor Torque
2
s  0.3
2
2
2
2
2
D
s  0.3
2
2
2
s
s
2
s  0.3
T s
D
s  0.3
are both directly
proportional to the slip
Rotor Current and Developed Torque
as functions of slip
The motor is
loaded from it’s
running no-load
condition
Notice slip = 0 is here --->