The Variac® autotransformer is an efficient, trouble­free device for controlling ac voltage and any other quan­tities that derive from ac voltage: heat output, light in­tensity, motor speed, and the outputs of various power supplies. The name Variac comes from the unit's func­tion - "vary ac" - and is General Radio's registered name for its continuously adjustable autotransformer.

Unlike most transformers, the Variac has a transforma­tion ratio that can be smoothly and continuously changed so the output of the unit can be controlled from zero to line voltage.or even higher. Because it is a transformer, the Variac is:

• efficient transforms power more efficiently than rheo­stats
• durable because it runs cool
• overload-able withstands 1000% short-term overloads
• independent of load size or power factor voltage to the load changes little from full load to none
• quiet adds no noise or distortion to the line
• reliable exclusive Duratrak® contact surface prevents injurious high-temperature oxidation and resultant brush-track deterioration

In addition, the Variac is

• Easy to install. All mounting hardware is included; wiring diagram is on the terminal plate; conduit knockouts are included on all enclosed models.
• Available in hundreds of standard versions to satisfy line frequency, voltage, and phase requirements, load size, mounting demands (including portable and metered models). They can be supplied with motor drives, ball bearings, and in ganged assemblies to increase basic line-voltage and load-current ratings.
• Assured safe by Underwriters' Laboratory listing and Canadian Standards approval of many models
• Available in militarized models specifically designed for 400-Hz operation
  

Variac Selection Guide

How To Select A Variac

The Variac® adjustable autotransformers are grouped by line frequency, voltage, and phase, with brief specifications for each model.* Within each group, the units are listed in order of increasing load rating that can be expressed in either current (amperes) or power (kVA). To make the selection you must know the line and load characteristics for your application. A brief look at these quantities may help.

Line frequency Most Variac models in the "W" series are designated for 50-to-60 Hz operation ("L" models are for 60 Hz only). Some "W" models can be used, without being derated, up to at least 400 Hz, but the regulation will be greater than normal and the physical size and weight larger than necessary. Therefore, we offer the "M" series Variac that is designed for operation from 350 to 1200 Hz. The M-series units are smaller and have better regulation at the higher frequencies. When series connected or when ordered specially, these units will also operate from 240-V lines.

Phase Variac models are available for both single- and three-phase operation. In general, three-phase ratings are governed by the ratings of each individual transformer in the assembly. That is, the voltage applied to, or the current drawn from, each individual unit must not exceed that specified for its single-phase uses. Thus, the considerations discussed below for single-phase applications apply separately to each unit in a three-phase assembly. A more detailed discussion on three-phase ratings and how to calculate them is given later in this section.

Line voltage Single-phase lines are normally either 120-volt or 240-volt, and GR Variac models come in two basic families to match. Should your line voltage be less than nominal, a unit rated for the nominal value will operate perfectly with no derating in current. Line voltage up to 17% above the nominal can be applied if overvoltage output is not required. For example, up to 140 volts line voltage can be applied to nominal 120-volt models if the maximum output voltage required is no more than the line voltage applied.

For single-phase line voltages from 480 to 560 volts, two Variac units rated for 240-V operation must be used with their coils connected in series across the line and the load connected one side to each of the Variac outputs. For such use, the load cannot be grounded at any point.

Load rating The load capacity of GR Variac autotransformers is specified in three ways: maximum current, rated current in amperes, and power in kVA (kilovoltamperes). Although closely related, they are different and the differences are important to the proper selection of your Variac.

An autotransformer cannot supply as much current at midrange settings as it can at full-voltage setting without overheating. Yet some nonlinear loads, incandescent lights for example, may draw nearly as much current at.half voltage as they do at rated voltage, while other (linear) loads will draw current proportional to the applied voltage. As a general rule, if the load is nonlinear, or if the overvoltage connection is used to apply more than line voltage to the load, a Variac should be chosen that has a Rated Current adequate for the load. Otherwise, the larger Maximum Current is the load-rating limit. Special applications may permit higher current to be drawn; for a more complete discussion of ratings, see "Get More Out of Your Variac."

The Variac power rating in kVA is given as a convenience in matching the right Variac to the load. It is the product of the rated line voltage and the maximum current rating of the Variac. There is a risk of misinterpreting it and exceeding the limits mentioned above; the kVA rating can be used only if the load is linear and the overvoltage connection is not used. Otherwise, load current must be determined and a Variac selected that has adequate rating.

Power ratings in kVA are given for three-phase Variac applications and must be interpreted as described above.

Trade-offs While some trade-offs, like those mentioned above, are included in the selection tables, there are others you may wish to consider. The load-current capacity of the Variac is limited by temperature and life. Specified ratings assume a maximum ambient temperature of 50°C and a minimum life span of 7 years. If the expected ambient is lower or forced cooling is possible, the autotransformer can be uprated without affecting life. Also, if a shortened life is not a problem in your application, a further uprating can be realized.

Finally, if the load is expected to be switched on and off regularly (as with a thermostatically-controlled heater), the Variac can be uprated. In general, if the time for an on-off cycle is 2 hours or less and the off time is 10% or more of the total cycle time, some significant improvement in rating can be realized.

Selecting the proper Variac Autotransformer Knowledge of the line frequency, voltage, and phase of your application will lead you to one of several tables that follow. The considerations above will have helped you determine the current or power that the Variac must be capable of handling. Now, merely scan down the left columns in the table ("Rated Current," "Maximum Current," or "kVA") until you find an entry that equals or exceeds the value determined by your load. It may be rewarding to consider several models, including those with slightly higher ratings than necessary, as there is the possibility of saving money, space, or both. Some models (designated "L" as in W5L) offer higher ratings per dollar and have only the minor restrictions of 60-Hz operation only and no overvoltage connection.

Parallel connections In some instances, the selection tables will indicate that the ganged assembly you have chosen requires parallel connection of the individual units. Reference is made to a Type W50-P1 Choke, which must be used between the output connections of the individual units in the assembly to prevent one unit from forcing current into another, possibly causing excessive temperature and early failure. One choke is needed if two units are to be paralleled; three or more parallel units require one choke for each Variac. The chokes are not included with the ganged assembly and must be ordered separately (except for 9- and 12-gang W50 and W50H units which are shipped prewired with chokes).

Type Number Terminology

In their various combinations, type numbers for Variac autotransformers consist of letters and numerals that indicate exactly what elements are included in each assembly. The following examples show the various combinations:

Variac Applications

In most applications, a full turn of the Variac control shaft (320°) varies the output voltage, applied to the load, from zero to line voltage or 17% above if connected for "overvoltage" operation. Thus, the light or heat output or speed or torque of the load is varied from zero to rated or above. Some typical applications are shown below.

Voltage doubling If the available line voltage is only about half that required by the load, the Variac can double the voltage while providing full control of the output. Units designated by an "H" (W20H) are supplied with an input connection for this use; output current rating of the transformer is one-half its normal value in this case. On special order, similar connections for other multiplying ratios can be supplied.

Other applications The Variac autotransformer can also be used as a phase-shifter in three-phase circuits, as a color-temperature control, for calibrating voltmeters, ammeters, and wattmeters, and in many unique applications. It is the basis of a wide line of General Radio automatic line-voltage regulators and can be used in many similar custom applications.

Special models General Radio welcomes inquiries con­cerning special models. We can, for example, modify taps, include limit switches, change shaft length, add ball bearings, provide for 360° mechanical rotation, add one or more independently controlled brushes, treat the units with fungicide or otherwise prepare them for use in abnormal environments.