Starters used in motors

If a motor has to start at no-load conditions, a small torque is enough to overcome starting inertia. In a poly-phase squirrel cage motor, during starting, motor is directly connected to the main supply through a starter. During starting of motor, the high inrush current helps to develop high starting torque. This accelerates the motor to achieve greater speeds. This method can be used only in small motors with a power up to 5 hp. For higher capacity motors, reduced voltage starters are used. There exist 6 types in this category. They are resistance reduced voltage starter, increment resistance starter, auto transformer reduced voltage starter, reactance reduced voltage starter, part winding reduced voltage starter and star delta starter. In a resistance reduced voltage starter, a line resistance is inserted in the motor circuit when motor is first started. Here three varieties exist. They are manual primary resistor type starter, semi-automatic stepless resistance starter and automatic primary resister type starter. In increment resistance starter, the resistance connected in the motor circuit is cut off in steps. In auto-transformer reduced voltage starters, an auto-transformer is used between the motor and the supply lines to reduce starting current of the motor. A line-reactor reduced voltage starter is used for high voltage or high current installations where bulky resistors create heat dissipation complexities. Part winding motor starters are used with squirrel cage motors which have two separate and parallel stator windings. The most commonly used reduced voltage starting method is star delta starting where the stator windings of the motor are first connected in star and full voltage is connected across its terminals.