Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 23)

The operating characteristics ( Fig. 1.7-6 ) show that the asynchronous motor has a so-called
“hard” speed characteristic, i.e. the speed changes only slightly with a change in loading. At low
loading, the current consumption approaches the value of the idle running current, which is
basically the same as the magnetization current of the motor.
0.00
0.25
0.50
0.75
1.00
1.25
1.50
0.00 0.25 0.50 0.75 1.00 1.25
P
2
/P
e
P
1
/P
e
I /I
e
n /n
s
s
cos
η
φ
Fig. 1.7-6
Operating characteristics of an asynchronous motor as a function of load
n = speed
n
s
= synchronous speed
s = slip
P
1
= power intake
P
2
= power output
P
e
= rated operational power
η = efficiency
cos
φ = power factor
I = current consumption
I
e
= rated operational current
n The speed n only decreases slightly with increasing load. Normal squirrel-cage induction
motors thus have “hard” speed characteristics.
s The slip s increases roughly proportionally with increasing load.
cos φ The power factor cos φ is strongly dependent on the load and only reaches its highest
value in a state of overload. The power factor is relatively adverse in the part-load range,
as the magnetization is practically constant.
η The efficiency η remains relatively constant and in the upper half-load range remains
practically unchanged. It generally reaches its highest value below the rated operational
power P
e
.
I The current I increases proportionally from around the half-load mark. Below this point it
falls less strongly and then goes over to the idle running current I
0
(constant magnetiza-
tion)
P
1
Starting from the idle running consumption, the power intake P
1
increases roughly
proportionately to the load. In the overload range, its rate of increase is somewhat higher
as losses increase more strongly.
LVSAM-WP001A-EN-P - April 2009 1-13