Product Design and Engineering Data

The following words or phrases are commonly used to describe characteristics of Staco Variable Transformers.

Input Voltage:  The supply voltage to which a Staco Variable Transformer is connected.

Frequency:  All units in this catalog operate in the range of 50 - 60 Hertz, unless otherwise noted.  These units may be used on higher frequencies within the limits shown in Figure B on page 6 of the Variable Transformer Voltage Control catalog.

Output Voltage: The range of voltage available at the output terminals.

Current Rating Terminology:

Constant Current Rating:  Output current that can be carried regardless of output voltage setting.  (Reduce for output above 125% of input voltage on voltage doubler connection.)

Constant Impedance Rating:  Output current that can be carried with loads such as incandescent lamps or resistance heaters in which the current drawn is approximately proportional to the applied voltage.  This rating applies only to applications where maximum voltage is limited to line voltage.

KVA Rating:  The maximum output current at maximum output line voltage, multiplied by that maximum voltage and divided by 1000 for single phase.  Divide by 577 for three phase.

Overvoltage Connection:  Output voltage from zero to 17% above line voltage (10% for 171 through 291 Series).

Line Voltage Connection:  Output voltage from zero to line voltage.

Voltage Doubler Connection:  Unit gives full overvoltage output with half normal input voltage.  Available on most 240- and 480-volt units.  Reduce output current when output voltage exceeds 125% of rated input voltage.

Driving Torque:  Torque required to turn the Staco Variable Transformer shaft.

Rotation:  Rotation of Staco Variable Transformer shaft gives an increase in output voltage (as viewed from referenced end).

Single Unit Tap and Terminal Diagram:  Input connection is shown for normal overvoltage output.  Optional line voltage and voltage doubler input are indicated.  Winding section voltages are for a normal overvoltage (or voltage doubler) connection at the input voltage shown.  Line voltage connections give 85% of these voltages (90% on 171 through 291 Series).  Cased units with line cord may omit several coil taps.

Connection - Single Phase Unit: Terminals are provided on most models for zero to 117% of rated input voltage with an overvoltage connection, and zero to 100% of rated input voltage with a line voltage connection.

Connection - Single Phase Parallel:  Up to nine units on the same shaft may be paralleled by using suitable chokes and circuits to multiply the current and KVA rating. Parallel operation of smaller ganged units is not recommended because it is usually more economical to handle rated loads with the capacity of larger single units.  Specification charts are included in the catalog section for each transformer series.

Connection - Single Phase Series: By jumpering the common connections, two equal single units (or two equal groups of paralleled units) driven by the same shaft may be used at double voltage (line-to-line) in single phase series with external connection only to the input and output terminals.  The load must be grounded.  If an input neutral is connected to the common, an output neutral may be used.  Transformer or loads need not be balanced to neutral.

Connection - Three Phase Open Delta:  By jumpering the common connections, two equal single units (or two equal groups of paralleled units) driven by the same shaft may be used at normal voltages (line-to-line) in three phase open delta.  One power line, identical on input and output connects to the common.  The other input lines connect to the two input terminals, and the two output terminals feed the other output lines.  (This connection is the same as single phase series with neutral, except here voltages on all input line pairs are equal and out of phase.)