Rockwell Automation 825 Smart Motor Manager User Manual - Manual (Page 18)

1-4 Introduction
Publication 825-UM001B-EN-P January 2001
Figure 1.3 AC Current Profile of a Motor Starting Direct-on-Line
Oscillogram of switching on a squirrel-cage induction motor by direct-on-line starting. The high motor starting current ,
A
flows
during the starting time (t
A
). If this is less than the limit specified by the manufacturer (usually 10 s), the starting current does not
cause an excessive temperature rise. The brief, asymmetrical peak when switching on can be ignored.
Motors are not thermally homogeneous. The winding, stator iron, and rotor have different
heat capacities and conductivities. Following unduly heavy loads, e.g., during starting,
temperature equalization occurs between the various parts of the machine (heat flows from
the warmer winding into the cooler iron until the temperature difference is minimal).
Figure 1.4 Temperature Rise Characteristics of Motor Windings
Temperature rise in a motor winding. During the starting time (t
A
), the temperature of the winding rises very rapidly; at the end of
startup, the temperature drops temporarily because heat is transferred to the laminated core.
Limiting Temperatures, Insulation Classes
The permissible temperature limit for a winding — and thus the load-bearing capacity — of
the motor is primarily a function of the motor's insulation. Applicable standards (UL, CSA,
IEC, and NEMA) distinguish between different classes of insulation and corresponding
temperature limits.
2 2
I e
1.6 2
I A
2 2
I A
t A
I ,
A
Starting current
t
A
Starting time
,
e
Rated service
current
t Time
ϑ
G
ϑ
e
ϑ
s
ϑ
K
ϑ
t B t A
t
0
ϑ
G
Temperature limit of the insulation
ϑ
K
Coolant temperature
ϑ
s
Temperature rise at start
ϑ
e
Temperature rise when operated
continuously at rated current
t
A
Starting time
t
B
Permitted stalling time