Threequarter Horsepower Variac Motor Speed Control



Uses of Vaeiacs in Electrical Engineering Power

Laboratories 5

Western Instrument Company Offers Repair Service to West Coast Customers. . 7

third horsepower rating of insufficient for this duty, although it has proved ample for many special applications such as toroidal winding machines, resistance-card-winding machines, and for lathes used in finishing small parts. It has been apparent that a unit of about three-quarter horsepower rating

• THE VARIAC SPEED CONTROL provides adjustable-speed operation of d-c motors from a-c power lines. This control uses no electronic tubes, so is capable of instant starting, without warm-up. Its basic simplicity makes possible a compact, single-unit construction with the ruggedness and reliability needed in industrial applications.

Since the Variac Speed Control was first introduced, there has been a persistent demand for a unit large enough to handle light production machine work. The one-the Type 1700-B Variac Speed Control is

Figure 1 . Panel view of the Type 1702-A %-hp Variac Speed Control.

Figure 1 . Panel view of the Type 1702-A %-hp Variac Speed Control.

would greatly extend the field of application of controls of this type. The Type 1702 Variac Speed Control shown in Figure 1 is now offered to meet this need.


The circuitry, construction, and performance of the new three-quarter horsepower control are all practically identical with those of the one-third horsepower Type 1700-B Variac Speed Control recently announced.1

Figure 3 is a schematic circuit diagram. An adjustable armature voltage is supplied by a bridge-connected selenium rectifier fed by a Variac" auto-transformer. A fixed shunt-field voltage is supplied by a separate transformer and rectifier system. The base speed of the motor can be increased if desired by about 15 per cent by shifting the output lead of the field-supply transformer to the low-voltage tap provided.

Some of the important features of this control are:

(1) The use of selenium rectifiers eliminates tube replacement and the warm-up time delay when the unit is first turned on. The rectifiers adopted for these controls are of a type having long life even when operating in high ambient temperatures.

(2) The control is simple and rugged and is easy to install and maintain. A compact, single-unit construction is employed, including dynamic braking and a manually operated starting and reversing switch. The size of the three-quarter horsepower control is not proportionately larger than that of the one-third horsepower unit and is still small enough, 15 x 13 x 5Ke inches, so that it can be mounted beside a machine within convenient reach of the operator.

(3) An isolated field supply permits the use of compound-wound motors with their greatly improved starting characteristics. Since the control has very large short-period overload capacity, this means that heavy loads can be started or reversed quickly and often, without damage to the control system.

(4) The choke (Z>i in Figure 2) reduces the a-c ripple in the armature circuit, thus lowering motor losses and eliminating torque pulsation at the ripple fre-

1W. N. Tuttle, "An Improved Variac Speed Control," General Radio Experimenter, Vol. 25, May, 1951, pp. 1-5.

Figure 2. Typical installations of the 3A-hp Variac Speed Control, (left) on a lathe, and (right) on a toroidal winder.

Low Power Variac

quency. This feature is particularly important in precision grinding operations where other types of controls are unsatisfactory.

(5) The armature voltage source has a low internal impedance so that the inherently good regulation characteristics of shunt or compound-wound motors are largely preserved. The regulation with a standard compound-wound motor is about 24 per cent at base speed. Speed-torque curves are given in Figure 4.


Motors can be purchased directly from the motor manufacturer or from the General Radio Company. In the selection of motors, armature currents should be checked to determine that full-load ratings conform to the output rating of the Variac Speed Control as listed in the specifications, page 4.


Experience with the three-quarter horsepower controls in our own production work has shown that their characteristics are excellent for a wide range of applications. These include not only a large proportion of the cases where some sort of continuously adjustable

Figure 3. Schematic circuit diagram of the Type 1702-A Variac Speed Control.

Figure 3. Schematic circuit diagram of the Type 1702-A Variac Speed Control.

Variac Motor Controller
Figure 4. Typical speed-torque curves for the Type 1702-A Variac Speed Control operating a 3A-hp compound motor.

speed control is obviously required, but also many applications where conventional step-pulley drives were formerly employed and where increased production efficiency has been found to result from substitution of the Variac Speed Controls. The reaction of the operators has been that this drive is the best that they have ever used. A few situations in which the new controls have saved production time are the following:

1. Where several lathe operations are done on the same piece at different speeds, the Variac knob can readily be shifted from one setting to another without loss of time. An example is a shaft with shoulders of several diameters.

2. Where an operation must be gradually started or stopped, an adjustable control has a great advantage. An example is starting a large tap. In blind tapping, the speed control can be turned down as the end of the hole is approached so that the machine will stall at the finish without breaking the tap or spoiling the threads.

3. In facing, the speed can be changed as the diameter of the cut changes.

4. Since speed can be changed quickly and easily, certain operations such as withdrawing a tap are naturally speeded up to the limit, with appreciable saving in production time.

5. Some operations require rapidly repeated starts, stops, and reversals. Since an induction motor starts very inefficiently, such service may cause overheating and short motor life in conventional drives. A compound-wound motor, as used with the Variac Speed Controls, has much better starting characteristics, so starts faster and heats up less.

6. Since the optimum speed for a given operation can be quickly determined with an adjustable control, faster production is frequently obtained than with conventional drives which provide only three or four fixed speeds.

In many of the lathe installations made in our shops, the three-speed step-pulley drive of the original equipment has been retained, only the motor and control being changed. In these cases, it has been observed that the operator usually shifts the belt only when increased torque is required at low speeds. An installation has recently been made with the motor belted directly to a back-geared lathe at a fixed speed ratio. This has proved entirely satisfactory, there being plenty of overlap between the minimum useful speed with direct drive and the maximum speed using the back gears. This is a particularly simple and satisfactory installation.

This new three-quarter horsepower unit extends the application of the Variac Speed Control to light production operations, where its many advantages are even more evident than in the specialized jobs for which the lower-power models are suited.

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