Setting Example - Thermal Protection - Siemens 7SR105 Rho User Manual

field created by the rotor. This induces double-frequency currents into the rotor which cause very large eddy
currents in the rotor body. The resulting heating of the rotor can be severe and is proportional to (I
Note that a 1% voltage unbalance typically translates into a 6% current unbalance. In order to prevent nuisance
trips the pick-up level should not be set too low but, as current unbalance can cause serious rotor overheating the
motor manufacturers recommendation as to the maximum allowable unbalance or negative sequence should be
set.
The NPS withstand figure quoted by the motor manufacturer shall be used where available. Magnitude difference
protection should be selected where harmonics are present.
2.2

Setting Example - Thermal Protection

The motor full load current (FLC) and start current can be calculated:-
Input power =
P.F.
Full load current
CT secondary FLC is:
MotorFLC
=
CT Ratio
Motor starting current can be taken to be the same as the locked rotor current, the motor start CT secondary
current is:
5 x 0.86 = 4.3A (= 4.3 x In)
©2018 Siemens Protection Devices
100/1A
Figure 2.2-1 Setting Example – Motor Circuit Data
Output 400
=
× efficiency 0.85 × 0.955
KVA 493
= = = 86.3
3 kV 3 × 3 3 .
86 3 . × 1
= . 0 86 A ( = . 0
100
7SR105 Rho and 7SR17 Rho Applications Guide
Rated output
Rated Voltage
Rated Frequency
Rated Power Factor
Rated Efficiency
Service Factor
Stall Withstand Time
Starting Method
Locked Rotor Current
Permitted starts cold/hot
Start Time @ 100% Voltage
Min. Time Between Starts
Control Device
= 493 KVA
Amps
86 × In )
2
) t.
2
MOTOR
Motor Rating Plate Data
400kW CMR
3.3kV
50Hz
0.85
95.5%
NA
11 secs (cold)
7 secs (hot)
Direct-On-Line
5 x Full Load
2 per hour (hot)
N.A. (cold)
4 sec
NA
Vacuum circuit breaker
Chapter 7 Page 13 of 31