68 CONTROL OF MACHINES

68 CONTROL OF MACHINES

68 CONTROL OF MACHINES

68 control of machines

68 control of machines

68 CONTROL OF MACHINES Fig . 3.4 Manual primary resistor type starter The motor starts with resistance in the circuit . When the motor picks up speed the master switch handle is brought to RUN-position by the operator , contacts R1, R2, R3 also close and they short circuit the line resistances and the motor gets directly connected to the supply . Master switch handle is held in the RUN-position by an electro-magnet shown in the diagram as under-voltage (latch) relay UV. This relay coil is connected through STOP-push button and over-load relay contact across two supply lines . Whenever the over-load relay trips or power fails , the coil is de-energised and the Master switch handle is released , which falls back to OFF-position . The switch is also released when voltage falls below a certain value thus giving under voltage protection to the motor . The operation of this starter depends upon the skill of the operator . The time gap between START and RUN-position may be varied by the operator to suit the particular load requirement of the motor . The motor current versus time characteristic during starting is shown in Fig . 3.5. The motor current versus time characteristic is shown by a dotted line when the motor is started direct ON-line . The starting current has been taken as six times the full load current of the motor . The motor current versus time characteristic with resistance in circuit has been shown through a continuous line as shown in Fig . 3.5. The current has been shown reduced to four times the full-load current . The motor current follows the path parallel to dotted line but at a reduced level . At time t1, when motor has reached near its normal speed , the resistance is cut off from the circuit and full supply voltage gets applied to the motor terminals . At this time there is sudden inrush of current as shown in the figure . Afterwards , the motor follows the upper curve and runs at its rated speed .

You may also like...