Rockwell Automation 825 Smart Motor Manager User Manual - Manual (Page 17)

Introduction 1-3
Publication 825-UM001B-EN-P January 2001
Figure 1.2 Operating Characteristics of an AC Motor
Operating characteristics of an AC motor as a function of load. Between no load and half load, the losses increase only slightly
with rising load. Between half load and rated load, the change in efficiency is minimal, and the power factor approaches its
maximum. The losses increase approximately proportional to the load. Above rated load, the losses increase more rapidly than
the load.
Current and Temperature Curves
Power loss is approximately proportional to the square of the motor current. The potential
for motor hazards exists mainly during starting and in a locked rotor condition. When a
locked rotor condition exists, the maximum value of the starting current flows (approximately
4…8 times the rated service current I
e
), and all of the power absorbed is converted into heat.
As the motor speed increases, the power converted into heat decreases. But if the rotor
remains locked, the temperature of the stator and rotor windings rises considerably, caused by
the high losses and the short time that heat can flow into the laminated core. If the motor is
not switched off quickly, the stator or rotor winding can burn out.
After startup, the temperature of the winding rises according to the load and cooling of the
motor. In time, the winding reaches its steady-state value.
A high current results in a correspondingly high operating temperature.
I e n s
n
I
P v
cos ϕ
η
P e
P 
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.5
cos
ϕ, η,
P
ve
Pv 
I
e,
I 
n
e,
n 
25 50 75 100 125
[%]
P Power
Pe Rated operational power
Pv Power losses
Pve Power losses under rated conditions
, Operational current
,
e
Rated service current
n Speed
n
e
Rated operational speed
n
s
Synchronous speed
cos
ϕ
Power factor
η Efficiency