D.C. Series Motor Starter

D.C. Series Motor Starter

AU: Dec.-05

• Three point and four point starters are used for d.c. shunt motors. In case of series motors, field and armature are in series and hence starting resistance is inserted in series with the field and armature. Such a starter used to limit the starting current in case of d.c. series motors is called two point starter. The basic construction of two point starter is similar to that of three point starter except the fact that it has only two terminals namely Line (L) and Field (F). The F terminal is one end of the series combination of field and the armature winding. The starter is shown in the Fig. 4.18.1. 

•  The action of the starter is similar to that of three point starter. The handle of the starter is in OFF position. When it is moved to ON, motor gets the supply and the entire starting resistance is in series with the armature and field. It limits the starting current. The current through no volt coil energises it and when handle reaches to RUN position, the no volt coil holds the handle by attracting the soft iron piece on the handle. Hence the no volt coil is also called hold on coil.

• The main problem in case of d.c. series motor is its overspeeding action when the load is less. This can be prevented using two point starter. The no volt coil is designed in such a way that it hold the handle in RUN position only when it carries sufficient current, for which motor can run safely. If there is loss of load then current drawn by the motor decreases, due to which no volt coil looses its required magnetism and releases the handle. Under spring force, handle comes back to OFF position, protecting the motor from overspeeding. Similarly if there is any supply problem such that voltage decreases suddenly then also no volt coil releases the handle and protects the motor from verze supply conditions.

• The overload condition du be prevented using overload release. When motor draws excesivaly high current due to overload. then current through overlond magnet Incroaggs. Thus unergiaan ugnet upto such an extent do it attracts the lever below it. When lover is lifted upwards, the triangular piece attached to it touches the wa points, which are the two ends of no volt coll. Thus no volt coil geto shorted, losing its magnetism unl releasing the luulle back to OFF position This protects the motor fum uverinang conditions.

Key Point: The aurter has two points L connected to line i. supply and costel to surten field al the moter. I Tonce it is called two point started

Review Question

1. Dram the circuit of 2 pont starter and explain the principle of operation. AU Dec.-05, Marky 8