Question

6. Explain why high speed motors have a higher power to weight ratio than low speed motors. Explain the reason for applying a reduced voltage to d.c. motors at start up. Describe how to change the speed and direction of rotation of a separately excited dc motor from 2000 rpm forward to 2000 rpm reverse direction.

Answer 1

6) For power equipment, the capacity is roughly proportional to volume based on using conventional materials that are conductors, insulators and to make magnetic circuits. All power equipment is basically limited by heat transfer and temperature rise. The physical size will also increase when the voltage rating increases due to greater insulation requirements.

So, yes, in general, a larger electrical motor will have a greater capacity than a smaller one provided both use similar material technology, use the same heat transfer methods, have similar temperature rise ratings and operate at approximately the same voltage level.

7) At startup reduced voltage is applied using a starter. Whenever a DC MOTOR starts it requires a current that is equal to 5-6 times its load current to start.Thus it is required to reduce the voltage when the motor starts.This job is done with a starter.In its simplest form, the starter of a dc motor works like a variable resistance in series with the armature circui.Its work is to reduce the starting voltage upto such a value so that the increased current does not burn the armature windings. As the rotating armature of dc motor picks up speed, the starter resistance is gradually reduced to almost zero. At full speed the motor starts running normally, i.e. the job of starter finishes here.
In other words, the starter offers resistance during starting of dc motor only,to provide large current. At full speed,the starter is electrically out of armature circuit of the motor