A Primer on two Speed Motors

By Edward Cowern, P.E. – Baldor/ABB

There seems to be a lot of mystery involved in two speed motors but they are really quite simple. They can first be divided into two different winding types:

Two Speed, Two Winding

The two winding motor is made in such a manner that it is really two motors wound into one stator. One winding, when energized, gives one of the speeds. When the second winding is energized, the motor takes on the speed that is determined by the second winding. The two speed, two winding motor can be used to get virtually any combination of normal motor speeds and the two different speeds need not be related to each other by a 2:1 speed factor. Thus, a two speed motor requiring 1750 RPM and 1140 RPM would, of necessity, have to be a two winding motor.

Two Speed, One Winding

The second type of motor is the two speed, single winding motor. In this type of motor, a 2:1 relationship between the low and high speed must exist. Two speed, single winding motors are of the design that is called consequent pole. These motors are wound for one speed but when the winding is reconnected, the number of magnetic poles within the stator is doubled and the motor speed is reduced to one-half of the original speed. The two speed, one winding motor is, by nature, more economical to manufacture than the two speed, two winding motor. This is because the same winding is used for both speeds and the slots in which the conductors are placed within the motor do not have to be nearly as large as they would have to be to accommodate two separate windings that work independently. Thus, the frame size on the two speed, single winding motor can usually be smaller than on an equivalent two winding motor.

Load Classification

A second item that generates a good deal of confusion in selecting two speed motors is the load classification for which these motors are to be used. In this case, the type of load to be driven must be defined and the motor is selected to match the load requirement.

The three types that are available are: Constant Torque, Variable Torque, and Constant Horsepower.

Constant Torque

Constant torque loads are those types of loads where the torque requirement is independent of speed. this type of load is the normally occurring load on such things as conveyors, positive displacement pumps, extruders, hydraulic pumps, packaging machinery, and other similar types of loads.

Variable Torque

A second load type that is very different from Constant Torque is the kind of load presented to a motor by centrifugal pumps and blowers. In this case, the load torque requirement changes from a low value at low speed to a very high value at high speed. On a typical variable torque load, doubling the speed will increase the torque requirement by 4 times and the horsepower requirement by 8 times. Thus, on this type of load, brute force must be supplied at the high speed and much reduced levels of horsepower and torque are required at the low speed. A typical two speed, variable torque motor might have a rating of 1 HP at 1725 and .25 HP at 850 RPM.

The characteristics of many pumps, fans, and blowers are such that a speed reduction to one-half results in an output at the low speed which may be unacceptable. Thus, many two speed, variable torque motors are made with a speed combination of 1725/1140 RPM. This combination gives an output from the fan or pump of roughly one-half when the low speed is utilized.

Constant Horsepower

The final 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 motor’s 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 speed, constant horsepower motors are usually only available on a special order basis.

Two Speed, Single Phase Motors

Two speed, single phase motors for constant torque requirements are more difficult to supply since there is a problem of providing a starting switch that will operate at the proper time for both speeds. Thus, the normal two speed, single phase motor is offered as a variable torque motor in a permanent split capacitor configuration. The permanent split capacitor motor has very low starting torque but is suitable for use on small centrifugal pumps and fans.

Summary

The use of two speed motors in the future will grow quite rapidly as industrial motor users begin to realize the desirability of using this type of motor on exhaust fans and circulating pumps so that the air flow and water flow can be optimized to suit the conditions that exist in a plant or a process. Very dramatic savings in energy can be achieved by utilizing the two speed approach.