Medium Voltage Application Guide

03986.C
SOFT STARTERS
AuCom slip-ring motor control sub-states
1
t1 t2 t3
2
V1
V2
3
S1
S2 S3 S4
P1 P2 P3
1 Sub-states 3 States
t1 Main contactor close time S1 Ready
t2 Rotor resistance contactor close time S2 Pre-start tests
t3 Bypass contactor close time S3 Starting
2 Output voltage S4 Running
V1 100% voltage 4 Phases of operation
V2 Slip-ring retard voltage P1 Start command
P2 Rotor resistance current ramp
P3 Shorted rotor current ramp
Rotor resistance sizing
When using a soft starter for slip-ring motor starting, a single stage, three phase resistance bank must be used.
For an existing installation with a multi-stage resistance bank, the existing final stage resistance can normally be used.
To specify a new single-stage resistance bank, use the following guideline:
Slip-ring rotor resistance sizing formula:
Ur
Rp
= 0.
2×
3×
I
Pm
Pp
= 0.
2×
3
Slip-ring synchronous motors
r
Where:
R p = rotor resistance per phase ()
U r = open circuit rotor voltage (V)
I r = rotor current (A)
P p = power rating of rotor resistance per phase (kW)
= motor shaft power (kW)
P m
Although they are rare, there are some older synchronous motors which use a special double winding rotor. One
winding set is used for standard slip-ring rotor starting. The other winding set is a DC excitation winding used for
synchronous speed running.
These motors can be started using a soft starter with final stage resistance, but the control system must include a
synchronisation package. This package is supplied separately to the soft starter and must be integrated into the entire
system. This control method becomes complex and expensive and in most cases an upgrade will involve a complete
replacement of the synchronous motor for a standard squirrel cage induction motor.
710-12280-00A Medium Voltage Application Guide Page 67