Download White paper on Soft Starter used with Slip ring motors

White paper on
Soft Starter used with
Slip ring motors
Soft Starters
A soft starter is another form of reduced voltage starter for A.C. induction
motors. The soft starter is similar to a primary resistance or primary reactance
starter in that it is in series with the supply to the motor. (Three wire or
standard connection) The current into the starter equals the current out. The
soft starter employs solid state devices to control the current flow and
therefore the voltage applied to the motor. In theory, soft starters can be
connected in series with the line voltage applied to the motor, or can be
connected inside the delta loop of a delta connected motor, controlling the
voltage applied to each winding. (Six wire or Inside Delta connection).
Slip Ring Motors with soft starter
Soft starters can be applied to many slip ring motors, however there are some
where the application of a soft starter will not give satisfactory results.
Slip ring motors are often employed for their ability to produce a very high
torque across the entire speed range. The slip ring motor is able to do this at a
very low start current. Another reason for the application of a slip ring motor is
that it is able to offer a high degree of control.
If the slip ring motor is employed to give a very high start torque across the
entire speed range, then the soft starter is not going to provide a satisfactory
solution. This is because the application of a soft starter or any other primary
starter, is going to reduce the torque available. Where the requirement is for a
gentle start at reduced torque, the soft starter is of benefit.
A common misconception is that the slip ring starter can be converted to a
cage type motor by shorting the slip rings and starting by the normal methods.
If the secondary winding is shorted, the slip ring motor will exhibit a very high
LRC (typically >1000%) and a very low LRT (typically < 100%). If a reduced
voltage starter is applied under these conditions, the start torque will be very
low and will not start a machine. To apply a reduced voltage starter to a slip
ring motor, first ascertain that a reduced torque is going to start the machine,
then fit resistors to the rotor circuit which will give curves similar to a high
start torque cage motor. These resistors must then be bridged once the
machine has reached full speed. The value of the resistance is dependent on
Are soft starters suitable for use with slip-ring motors?
Soft starters are suitable for use with slip-ring motors provided that the motor
can still deliver the torque required to accelerate the load. Soft starters are not
suitable if the load requires extremely high start torque, or if the slip-ring motor
is intended to provide speed control.
If a Soft Start Unit is to be used to start a Slip Ring Motor there are a number of
things to be considered. When considering a soft starter for slip-ring
applications, a trial should be conducted to verify the performance.
The resistors are to be removed and the Rotor shorted out, but engineers agree
that a small amount of resistance should be maintained. This is usually achieved
in the cables.
This has in theory turned the motor into a Squirrel cage motor with HIGH
TORQUE starting. To use a soft start on a high torque application the current
limit must be set to maximum this would mean the motor would be able to pull
high current to get it away. The average starting current for a Slip Ring Motor is
250% of its Full Load Current where as if using a Soft Start it would be 700% of
Full Load Current. Using a 55Kw motor as an example at 400V
SLIP RING FLC x 250 =
100 x 2.5 = 250A
FLC x 700 =
100 x 7 = 700A
SOFT START -
As can be seen from the above calculations all the wiring and circuit equipment
will have to be changed to be able to cope with the higher currents
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Rotor resistance (R) can be sized using the following formula
R(per phase) = 0.2 X
Where VR = Open circuit rotor voltage
IR = Full load rotor current
Power (per phase) = 20% X motor (kW) / 3
Points to consider when fitting a Soft Starter to a slip ring motor
1. Slip ring motors are normally fitted where low starting current or high starting torque
are required.
2. Soft Starters are suitable for replacing resistance and electrolytic starters where these
have been used for reduced current starting with reduced torque.
3. The normal solution when using a Soft Starter on a slip ring motor is to have 1 stage of
resistance that is in circuit during the soft start. The resistance is shorted out once the
Soft Starter has reached full voltage. In some cases two stages are required.
4. Fitting a Soft Starter with single stage resistance will give lower available starting
torque with the possibility of slightly higher starting currents and longer starts.
5. Check cabling, fusing and switchgear to ensure it can withstand the potentially longer
starts
6. Soft Starters are not suitable for loads that require the full Locked Rotor torque to
accelerate. Consider fitting a larger standard motor that has sufficient torque.
7. It is not possible to remove or short out the slip rings at standstill and achieve reliable
starting whatever the nature of the load.
8. If a soft stop time is used, the rotor resistance will produce an initial breaking effect
which may need to be considered.