What is soft starting of an induction motor

Describe with a circuit using thyristor used for soft starting

Discuss its advantages and disadvantages.

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)

Soft starters use a combination of power electronics and electronic control circuitry to slow increase the voltage on the motor during starting; ensuing a smooth acceleration. 

This method of supplying a gradually increasing a.c. voltage during start-up generally refers to an efficient electronic switching technique

A basic method shown in Fig is to use back-to-back connected thyristors or triacs in the supply lines which are "gated" to delay "turn-on" within each a.c. half-cycle. This delayed switching applies a reduced average a.c. voltage to the motor.

The applied motor voltage is gradually ramped up by the starter software program until the full voltage level is reached. To achieve maximum efficiency, the electronic switching circuit can now be bypassed for normal running.

Soft starters use thyristors (Silicon Controlled Rectifier) to control the energy delivered to the motor.  A thyristor is a device which turns on when a pulse is applied to its gate and will continue to conduct until the current drops to zero (at which time it turns off).   In an AC sine wave current goes to zero each half cycle, allowing the current to be turned off and making it possible to use use thyristors to implement soft starting. 

Voltage Control by Thyristor Firing

If the thyristors are turned on at the start of each half cycle, the full voltage is applied to the motor.  If the thyristors are never turned on , then no voltage is applied.  If the thyristors are turned on part way through the half cycle only a proportion of the voltage will be applied to the motor.  By controlling the turning on (firing angle) of the thyristors the amount of voltage on the motor can be controlled.  Starting with a large firing delay, this is gradually reduced and the voltage on the motor will ramp-up during starting.

In addition to starting, soft start units can also be used for stopping the motor, by ramping the voltage down.  This is particularly useful where sudden loss of driving torque would create mechanical shock on the load.

Electronic soft starters contain the thyristors (power side) and necessary electronics to control the firing (via user settings).  Modern soft starters have a host of features; the most common being options to set varying start and stop ramps, setting of the initial starting voltage, current limiting control and thermal overload protection.

The easiest application of electronic soft starters is one unit per motor.  To reduce cost, sometimes more than one motor is connected in parallel to a soft starter, or motors are started in sequence by switching the soft start unit between motors.  While these methods can be used, care has to be taken to ensure that the units are adequately rated for the required duties.

As a cautionary note, firing of thyristors other than a zero voltage (current) will create a non-linear load characteristic,  generating transients and harmonics.  In general application (and given that the transients/harmonics are only present during starting) these are not a problem.  However, there could be instances and special situations where these do have an adverse effect on the power system. 

Power and Control Circuiting

Power Circuit

The figure illustrates possibly the simplest connection of an electronic soft starter.  In this the unit is simply connected into the circuit and carries out the necessary functions. 

While this is the simplest implementations, you will often find a bypass contactor (to switch-out the thyristors when the motor is up to speed) and a line contactor to switch the circuit in/out.  Other variations of control circuit  would include cascading start and control for both forward and reverse directions.  Invariably there would also be communications to either control or monitor the functioning of the motor.

Features and Application

Soft starters have many features not found in traditional starting methods and which can be of benefit in some situations.  These include:

Adaptive Acceleration Control - soft starter learns motor’s performance during start and stop, then adjusts control to optimize performance

Soft Stopping - slowly bring the motor to a stop

DC Braking - injecting DC to reduce motor stopping time

Soft Braking - changing contactors on starter input, reversing motor direction and applying braking

Current Limiting - limit starting current to a pre-defined value (may lower torque to much and motor may not accelerate)

Current Ramping - increasing the current over a given time to a specified limit

Jog - allows the motors to be run at reduced speed for some time

Kick Starting - using a brief high bust of current at the motor start to get it going, then reducing the current

Thermal Modeling - motors the motors thermal characteristic, allow performance optimization 
Communication - most industrial protocols will be supported, enabling enhanced control/monitoring

Line contactors can be used (but not strictly necessary) to isolate the soft start when not in use (should be rated AC3). If required, by-pass contactors can by used to minimize heat build up due the to SCR's (can rated AC1 as they do not carry starting currents).

Sequence starting - can be used to start several motors in sequence (or parallel). Starters needs to be rated for full start duty. Additional wiring, contactors and control relays may not make this an economic option. 

Power factor correction if required should only be installed on the line side and switched in when the motor is at full speed (AC6 contactors). Capacitors installed on the motor side can cause resonance, increased voltages and equipment failure.

Inside delta connection is a technique which can be used with six winding motors. In this configuration, only have the delta is completed in that starter, thus reducing the current the starter is required to carry. On some soft starters are able to be used in this type of application.

Characteristics:

Available starting current:   25 to 75%, adjustable

Peak starting current: 2 to 5 In, adjustable

Peak starting torque:  10 to 70%, adjustable

Advantages:

Soft start enhances motor start performance in many ways including:

➜ Smooth acceleration without the torque transients associated with electro-mechanical

reduced voltage starters.

➜ Voltage or current is applied gradually, without the voltage and current transients

associated with electro-mechanical reduced voltage starters.

➜ Lower start currents and/or shorter start times because constant current control gives

higher torque as motor speed increases.

➜ Easy adjustment of start performance to suit the specific motor and load.

➜ Precise control over the current limit.

➜ Consistent performance even with frequent starts.

➜ Reliable performance even if load characteristics vary between starts (eg loaded or

unloaded starts).

In addition to superior starting performance, soft starters also provide a range of features not

available from other reduced voltage starters. This includes areas such as:

➜ Soft stop (which helps eliminate water hammer) ➜ Metering and monitoring

➜ Braking ➜ Operating history and event logs

➜ Motor and system protection ➜ Communication network integration

Disadvantages:

More Expensive

Can Inject Transients into Supply