Solid State Motor Starters

ElectronElectronics & ElectricalDigital Electronics

Basic Theory of Solid State Starters

A solid state starter (or soft starter) controls the starting torque and current of an AC motor electronically by controlling the applied voltage. A solid state motor starter is made of solid state devices like SCRs or triac, which allows the current flow when they are conducting and stops the flow of current when they are not conducting.

The solid state motor starters have the ability to control the starting characteristics to match the application requirements such as acceleration and deceleration time, starting and overload current and the motor toque etc.

The solid state motor starters control the voltage instead of current. The torque available varies proportionality with the square of the ratio of the reduced voltage to the normal line voltage, i.e.

$$\mathrm{Torque,\tau_{2}=\tau_{1}(\frac{I_{2}}{I_{1}})^{2}\approx\tau_{1}(\frac{V_{2}}{V_{1}})^{2}}$$

Where,

• τ2 is torque at reduced current or voltage,

• τ1 is torque at locked rotor current,

• I2 is reduced current,

• I1 is locked rotor current,

• V2 is reduced voltage, and

• V1 is full voltage.

When the motor is being started, the logic of solid state starter issues ON command to the power module, causing the SCRs to turn on and gradually increases the voltage across the motor terminals. When the SCRs are full on, the motor reaches full voltage.

Components of a Solid State Motor Starter

• Power Switches – These are the power semiconductor devices like SCRs or triac, which are to be phase controlled so that they are applied for each part of the voltage cycle. For a three phase motor, two SCRs are connected back to back for each phase. The rating of power switches should be at least three times of the line voltage.

• Control Logic – The control logic may be either a PID controller or microcontroller or any other type of controller. These are used to control the gate voltage of the SCRs, so that the firing angle of SCRs can be controlled according to requirement.

Block Diagram of Solid State Motor Starter

The block diagram shows a typical solid state motor starter. This solid state starter uses six full current rated SCRs as its power switches. The control logic circuit monitors the threephase input voltage, three-phase output voltage and the three output currents. From these inputs, it can provide starting current limitations, running over current protection and undervoltage protection.