Parallel Starting: Can I use one soft starter to control multiple motors?

Yes. The circuit configuration and soft starter selection depends on the application.

1. Each motor must have its own overload protection.
   
   
   
   
2. If the motors are the same size and are mechanically coupled, a constant current soft starter can be used.
   
3. If the motors are different sizes and/or the loads are not mechanically interlocked, a soft starter with a timed voltage ramp (TVR) start profile should be used.
   
4. The combined motor FLCs must not exceed the soft starter FLC.
   

Replacing Star/Delta Starters: Can I replace a star/delta starter with a soft starter?

Yes.

If the soft starter is capable of inside delta connection, simply connect it in place of the star/delta starter.

If the soft starter is not capable of inside delta connection, connect the delta connection to the output side of the soft starter.

Reversing: can soft starters be used to reverse motor direction?

On their own, soft starters cannot run motors in reverse direction at full speed. However, forward and reverse operation can be achieved by using a forward and reverse contactor arrangement.

Some soft starters also provide a part speed function that runs the motor at slow speed in either forward or reverse, without a reversing contactor. However, reverse operation is limited to short periods at a fixed slow speed.

Sequential Starting: Can I use one soft starter to separately control mutiple motors?

Yes, one soft starter can control two motors in sequence. However, the control and wiring is complex and expensive and any saving in soft starter cost is often outweighed by additional component and labour costs.

In order to use a soft starter in a sequential starting situation,

1. Each motor must have a separate:
   - main contactor
   - bypass contactor
   - overload protection
   
2. The soft starter must be suitably rated for the total start duty.

Slip-ring Motors: can they be started with a soft starter?

Yes, provided that the torque available from the motor under the new configuration is sufficient to accelerate the load. This may be difficult to determine and a trial may be required.

Soft starting is not suitable for applications where:
- the slip-ring motor was installed to deliver speed control.
- the load requires extreme start torque.

To develop starting torque, some resistance must remain in the rotor circuit during motor starting. This resistance must be bridged out using a contactor (AC2 rated for rotor current) once the motor is running close to full speed.

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
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Soft Braking: what is it and when would I use it?

Soft braking is one of two techniques used by soft starters to shorten motor stopping time. The other technique is DC braking.
Soft braking uses reversing contactors on the input of the soft starter. When the soft starter receives a stop command, it operates the reversing contactors and the motor is effectively soft started in the reverse direction. This applies a braking torque to the load.

Compared to DC braking, soft braking:
- causes less motor heating
- provides more braking torque for a given current
Soft braking is better for extremely high inertia loads.

Two-speed motors: how do they work and can I use a soft starter to control them?

Soft starters can be applied to the two most common types of two-speed motor. In both cases, separate motor protection must be provided for low and high speed operation.

Dahlander motors are special purpose motors often applied to two-speed compressor or fan applications. The motor windings are externally configured using contactors for high speed (dual star) and low speed (delta) operation.

Dual-winding motors have two separate pole configurations (e.g. 4 pole / 8 pole) on a common shaft. Each pole configuration (speed) is selected using an external AC3 rated contactor.