Three-Point Starter:
Construction:
The three-point starter with its electrical connections and protective devices (no-volt release and over-load release) is illustrated in fig.
It consists of a series starting resistance divided into several sections and connected to brass studs, brass arc by which the connection to shunt field is made, no-volt release, and over-load release.
Since only three terminals (L,F, and A) are available from the starter, it is called a three-point starter.
The last stud of the starting resistance is connected to terminal A to which one terminal of the armature is connected.
The + ve supply line is connected to the line terminal L through the main switch. From line terminal L, the supply is connected to the starter arm through over-load release.
A spiral spring S is placed over the lever to bring the starter arm to the OFF position in case of failure of supply.
A soft iron keeper is attached to the starter arm which is pulled by the holding coil or no-volt release under normal running conditions.
The far end of the brass arc is connected to the terminal F through the no-volt release and to the terminal F, one terminal of the field is connected.
The -ve supply line is connected directly to the remaining ends of the armature and field winding of the dc shunt motor.
Operation:
When the motor is at rest, the starter arm is in the OFF position due to the action of strong spiral spring S.
For starting the motor the dc supply is switched on by closing the main switch keeping the starter arm in the OFF position.
The starter arm (or handle) is then turned clockwise to the first stud and brass arc (or strip). As soon as it comes in contact with the first stud, the whole of the starting resistance R is inserted in series with the armature, the field winding is directly connected across the supply through the brass arc and the holding coil is also energized.
As the starter arm is turned further the starting resistance is cut out of the armature circuit in steps and finally, the entire starting resistance R is cut out of the armature circuit.
When the starter arm reaches the ON position, it is held against the action of spiral spring S by the force of attraction between the holding coil magnet and soft iron keeper attached to the starter arm.
The starter arm should not be held for an unduly long time in an intermediate position as it is likely to burn out the starting resistor.
No-Volt Release:
When the starter arm reaches the “ON” position, the resistance is completely cut off and the motor starts running at normal speed.
If the supply gets interrupted or disconnected, the starting arm will remain in the same position i.e. "ON" position and when the supply is switched on or gets restored, no back emf will be acting in the circuit, the armature being directly across the supply mains, and resistance of armature being low, the motor will draw excessive current and will get damaged.
Hence for the protection of the motor, some device must be provided, so that the starter arm may reach the “OFF” position automatically as soon as the supply is cut off or disconnected or fails and for this purpose "no-volt" release coil is provided.
No volt release coil consists of an electro-magnet connected in series with shunt field which holds the arm in the “ON” position.
Now when the supply fails or gets disconnected the electromagnet demagnetizes and so releases the starting arm A, which goes back to OFF position due to the spring attached to it and gets disconnected from the supply mains.
The other important advantage of connecting the no-volt release in series with the shunt field winding is that it prevents the motor from running away owing to an open shunt field because open-circuited shunt field will demagnetize the electro-magnet and release the starter arm A and, thus the starter arm will go back to its OFF position and the supply will be disconnected.
Over-Load Release Coil:
This coil is provided for the protection of the motor against the flow of excessive current due to over-load.
This coil is connected in series with a motor so carries a full-load current. When the motor is overloaded, it draws heavy current, which also flows through this coil and magnetizes it to such an extent, that it pulls its armature upwards and so short-circuits the no-volt release coil, as shown in fig. 9.22.
The no-volt release coil, being short-circuited, demagnetizes and releases the starting arm, which goes back to the "OFF" position with the action of spring attached to it and the motor is automatically disconnected from the supply mains.
Thus the motor is disconnected from the supply and is protected against over-loading. The starters for motors up to 15 kW are provided with over-load release to disconnect the motor from the supply mains in the event of an overload. Larger motors are provided with separate automatic circuit breakers,