Three Phase Two Speed Motor

 

Two speed (4 & 8 pole) constant torque motor

Arrangement windings.

                               Low Speed: 2 pole -- Series Delta

High speed: 4 Pole-- Double Star

 When pole changing is done, the voltage must very often be changed in order for the motor to operate properly. This is because a regrouping of the stator coils will change the flux strength, which in turn changes the required voltage.
Typically, if 2 pole motor is rated for 230 v, doubling the poles to eight will halve the rated voltage, making it 115 v.

CONSTANT HORSEPOWER
Another type of two speed motor that is utilized is the two speed, constant horsepower motor. In this case, the motor is designed so that the horsepower stays constant when the speed is reduced to the low value. In order to do this, it is necessary for the motorâs torque to double when it is operating in the low speed mode. The normal application for this type of motor is on metal working processes such as drill presses, lathes, milling machines, and other similar metal removing machines.


The requirement for constant horsepower can perhaps be best visualized when you consider the requirements of a simple machine like a drill press. In this case, when drilling a large hole with a large drill, the speed is low but the torque requirement is very high. Compare that to the opposite extreme of drilling a small hole when the drill speed must be high but the torque requirement is low. Thus, there is a requirement for torque to be high when speed is low and torque to be low when speed is high. This is the Constant Horsepower situation.

The Constant Horsepower motor is the most expensive two speed motor. Three phase, two speed motors are quite readily available in constant torque and variable torque. Two sp