When an integral HP DC Motor is suddenly connected to a supply source, it creates stresses on the armature and supply system. Since at standstill the armature is not developing any back e.m.f, the inrush current will be several times the rated current and this will cause damage to the motor.
A Motor Starter is used for the smooth starting of a motor. It consists mainly of a variable resistance placed in series with the armature. The purpose of this resistance is to restrict the armature current to a safe value for starting and accelerating.
The other uses of using a Motor Starter is
· To disconnect the motor in case of an overload
· To disconnect the motor when the supply voltage fails
· To disconnect the motor when the field current is reduced beyond limit.
Ramson & Sons manufactures Manual starters
and Automatic starters.
The Three common types of Starters are:
1. Two Point Starters used for Series Motors
2. Three Point Starters used for Shunt and Compound Motors
3. Four Point Starters used for Compound and Shunt Motors
|No||Type||3 Point||4 Point||2 Point|
|1||Shunt||For Speed of Based Speed||For Speed above base speed|
|2||Series||NO Volt Protection||NO Load protection|
|3||Compound||For Speed of Base Speed||For Speed above Base Speed||..|
3 POINT STARTERS
The three terminal starters have tapped resistors enclosed in a ventilated box. Contact buttons located on a hylam panel mounted on the front of the box are connected to the tapped resistors. A movable arm with a spring reset can be moved over the contact buttons to cut out the sections of the tapped resistors. As the motor speeds up, the handle is moved gradually towards contact B. The time required being dependent on the time needed for the machine to build up speed. At the Point B, the armature is connected directly across the source voltage and the magnetic coil M holds the handle in the full On position. A spring tends to return the handle to the Off position, if the shunt field current became greatly less. The motor would race, if the armature circuit remains connected. However this is prevented in keeping the holding coil in series with the shunt field. Reduced current in the holding coil lets the handle to fly back to the off position. Also when the line voltage is interrupted, the holding coil releases the handle requiring the motor to be started when the line voltage is restored.
The Starting resistance is in series with the shunt field when the arm is in run position at contact B. This additional resistance is so small when compared with the field resistance that it has practically no effect on field strength and speed.
Starters are designed to carry the starting current for only a short time and are not intended for speed control. Any attempt to obtain below normal speed by holding the arm on any intermediate contact is likely to burnout the starting resistors.
4 POINT STARTERS
The Three point starters are not suitable for use where it is desired to obtain above normal speed by use of a field rheostat, since reduced field current will release the handle and stop the motor. Hence, in cases where field control is required, a 4 Point Starter is used.
Functions of a 4 Point Starter
· To accelerate
the motor to the rated speed
· To limit the starting current in the armature to a safe value
· To disconnect the motor in case of a supply failure or overload
Here, the holding coil is not connected in series with the shunt field as it is in the case of a 3 Point Starter. The holding coil is in series with a resistor and is connected directly across the supply voltage. The holding coil current is independent of field current and still serves as a No Volt release. If line voltage drops, the attraction of the holding coil is decreased and the reset spring returns the movable arm to the Off position.
The 4 Point Starter is used when speed above base speed is required. It is also used as a 3 Point starter for series motor eliminating the field terminal.
2 POINT STARTER
In this starter, the holding coil carries the full armature current. When the load is reduced, the armature current is also reduced and as a result, the holding coils strength is reduced. The moving arm is released and the motor is disconnected.
Figure 1 Shunt Motor (3 Point)
This is a connection diagram of a 3 Point Starter with a shunt Motor. The overload relay will operate when the load current exceeds limit. There is an N O operating contact for the overload relay. The N O* contact is connected in parallel with the holding coil. When the overload trips the N O is made to N C* and holding coil terminals are shorted. This reduces the holding coil current and as a result the handle is released and the motor is disconnected.
* N O Contact (Normally open contact)
* N C Contact (Normally closed contact)
Figure 2 Compound Motor (3 Point)
This arrangement is same as in the case of the shunt motor except for the series field included in the armature circuit
Figure 3 Shunt Motor (4 Point)
This is a connection diagram of a 4 Point Starter with a shunt motor. The overload relay will operate when the armature current exceeds the limit. The N O of the relay is connected in parallel with the holding coil. When the overload operates, the holding coil terminals are shorted thereby reducing the holding coil current and the handle is released. The motor is disconnected.
Figure 4 (2 Point)
The connections are same as in figure 3 except that a series motor is connected instead of a shunt Motor