A Variable-Speed Power Drive

The problem of providing variable-speed power drives is of considerable interest, not only because it affects an ever-increasing number of manufacturing processes, but because the solution involves a choice between many methods, both mechanical and electrical.

Amongst the more common electrical methods of obtaining a variable-speed drive are change pole induction motors, Ward-Leonard sets, AC. commutator motors of various kinds, and, where a DC. supply is available, the shunt-controlled DC. motor. Because of its excellent speed characteristics, the shunt-wound DC. motor commends itself to many engineers, but its use nowadays tends to be restricted because of the modern preference for individual drives in production machines, coupled with the adoption of standard three-phase AC. supply throughout this country.

Particular interest therefore attaches to the "Varispeed" drive developed and manufactured by the Electric Construction Company, Limited, Bushbury Engineering Works, Wolverhampton, using a standard DC. motor supplied from AC. mains through the medium of a mercury arc rectifier. The rectifier supplies DC. separately to the armature and field system of the motor, the two supplies being independently variable. By this arrangement the speed can be varied over a very wide range, as much as 1000 to 1, providing constant torque throughout this speed range.

A simple single-phase circuit, illustrating the principle of the system, is shown in the accompanying diagram, in which the AC. supply is connected to the primary of a transformer having two secondary windings. The main secondary winding supplies the two main anodes of a mercury arc rectifier, which converts AC. to DC. for supplying the motor armature. The auxiliary secondary winding is connected to two auxiliary anodes to provide DC. for the motor field.

A maximum speed range of about 4 to 1 can be obtained from a DC. motor of normal design by varying the shunt field current while keeping the applied armature voltage constant. Under these conditions the output horsepower remains approximately constant throughout the speed range, for the torque falls as the speed increases, that is, as the field current is reduced.

If, on the other hand, the shunt field current is kept constant while the armature voltage is varied, the speed of the motor varies approximately as the applied armature voltage. By varying the voltage supplied to the armature from zero to maximum, an infinitely great speed range can be obtained with constant maximum torque from the motor, since the field current remains at its maximum at all speeds. Maximum horsepower is therefore developed at maximum speed.

Since the majority of drives require maximum horsepower at maximum speed, with the torque remaining constant at its maximum value throughout the speed range, it is usual to use armature control, which considerably reduces the overall size of the motor. A further reduction in cost results from the fact that armature control normally eliminates the necessity for separate starting gear. The speed control gear can be used to start the motor smoothly from rest. A combination of armature voltage and shunt field control can be applied with advantage in many instances where maximum torque is required at some speed less than full speed, or where some compromise between constant torque and constant horsepower is required.

Assuming, however, that the field is maintained constant, the motor speed will depend upon the armature voltage supplied from the rectifier valve. This voltage can be varied by anyone of several methods, depending entirely upon the kind of drive and the speed range required. Since the ratio of the DC. voltage to the AC. voltage of the rectifier is fixed, a variable DC. voltage can only be obtained by applying a variable AC. voltage to the rectifier. In general, there are three methods whereby the AC. and therefore the DC. voltage can be controlled.

An E.C.C. 'Varispeed' drive for a draw bench.

The first method requires an on-load tap-changing switch to vary the main secondary voltage from the transformer in a predetermined number of fixed steps. The second method employs an induction regulator to vary the main secondary voltage smoothly over a range designed to suit the speed range of the motor.

The third method makes use of grid control between the anodes and cathode of the rectifier, giving smooth control over the DC. output voltage and the speed of the motor. Normally the first method is used for small machines, where a number of fixed speeds is acceptable. With larger machines, when smooth speed variation is important, either of the remaining two methods can be used.

There are therefore three essential components in the equipment; the transformer and controller, the rectifier cubicle, and the motor itself. All of these components can be mounted separately, which allows considerable flexibility in layout. Wiring is reduced to a minimum, since there are only three wires between the rectifier cubicle and the motor (without any intervening starter), and the fine wiring between the rectifier cubicle and the controller.

A typical "Varispeed" installation is depicted in the accompanying engraving, which shows a 267 hp. shunt-wound DC. motor driving an automatic draw bench. The rectifier cubicle and the transformer can be seen behind the main motor. This equipment, which is entirely automatic, gives the motor a speed range of 0 to 1200 rpm. providing constant torque throughout this range. An interesting point is the use of a forced ventilating system which accounts, in part, for the small size of the variable speed motor.

Since acceleration is completely smooth throughout the speed range, the accelerating time can be reduced to a minimum. The system lends itself satisfactorily to automatic control, reversing duty, dynamic braking, and rapidly variable acceleration. Throughout the speed range the efficiency and power factor of the system are high. It will be noted that the rectifier has no excitation losses, since the excitation is used to supply the field of the motor, resulting in an appreciable gain in efficiency.