Types of Dc Machine Starter

Types of Dc Machine Starter Points : Types of Dc Machine Starter, Three point starter, Four point starter, Two point starter (for series motor) There are three standard types of starter:
(A) Three point starter
(B) Four point starter
(C) Two point starter (for series motor) a. Three Point Starter A three-point starter with its protective features. Since three terminals L, A and F are available from starter called three point starter. The three terminals L (Line), A (armature) and F (Field) must be connected respectively to the supply terminal, motor armature and shunt field terminal.

When the motor is at test, starter handle H is kept in the “off position” by strong spiral spring. Stud 1 indicates off position. Starting resistance is connected between contact with stud 1, 2 6. For starting the motor, handle H is rotated manually to come in contact with stud 1, the shunt field and holding coil (HC), get connected in series across the supply. Whereas armature gets connected in series with entire starting resistance.

Since current begins to flow in both field winding and armature winding hence motor start rotating After motor pick up sufficient speed, the handle H is moved to stud 2, thereby cutting out the resistance between 1 and 2. Movement of handle is continued slowly till the soft iron keeper touches the holding coil. In this way all the additional resistance will be cut out from circuit.

The field current flowing through H.C. produces the force of attraction which will be greater than force of pull of spiral sprig. In view of this holding coil, keeps the handle H in “On” position. The holding coil is also called as “No you release”, It has the following functions:

1. In case of power electromagnet (UC) gets demagnetize and due to force of spiral spring handle H comes to the Off position. If during the power failure, the handle fails to return to off position, the motor might be damage in case of power is resumed.

2. If shunt field becomes open circuited accidentally, “H.C.” gets demagnetized hence starter handle returns to off position:

Another protective device OR” known as overload release is provided in series with armature circuit. This OR is a small electromagnet. In case, the armature current exceeds the present value due to overload, OR becomes more stronger and attracts the moveable soft iron M. As the soft iron M is attracted by OR, two terminals a and b of holding coil gets short circuited and in this manner HC gets demagnetized and handle H comes to off position.

b. Four Point Starter In four-point starter, holding coil is connected in series with an additional resistance R. The function of resistance R is to prevent the short circuit of supply mains, in case the overload release OR operates. When JIC gets short circuited by OR the current through R is limited by its own resistance and starting resistance.

The other components of four-point starter same as 3-point starter. The four point starter permits the change of field current by field rheostat, without effecting the holding coil current. Therefore, the pull of holding coil is unaffected and remains more than spring pull.

c. Two Point Starter This type of starter is used in series motors because in case of series motor field winding and armature winding both are connected in series. Two points are available from two-point starter. The construction of two-point starter and three-point starter are same. The starter is provided with a protective device called H.C. (No volt release). This coil consists of few turns of thick wire, because this coil will be connected in series with armature and field winding of motor. In case of light load or at no load the current through holding coil will be less. The coil becomes weak due to low current. Hence handle H returns to off position due to spring force. This is the important function of two-point starter because we know series motor cannot be run at no load.