transformer 2 lesson 2

Transformers connected in a wye configuration require that the taps be changed on ? coil(s).

3

An autotransformer is used to reduce the voltage of a 100-kilovolt amp, 480-volt secondary of an isolated type transformer, to supply a 100-kilovolt amp load with 277 volts. What size autotransformer is needed? (Use the co-ratio to determine the size.)

42 Kva

Autotransformer reduced-voltage motor starting reduces the applied motor voltage to ? of the line voltage when starting.

50%, 65% or 80%

Different loads require different starting torque. For a load with a very high starting torque, the autotransformer can be tapped to deliver ? voltage.

80%

Autotransformers have high impedance compared to a conventional transformer.

false

In an autotransformer, the open delta configuration provides better voltage balance than the wye configuration.

false

In autotransformer starting, the motor terminal voltage depends on the load current.

false

In autotransformers used for motor starting, the wye configuration is used for small motors and the open delta configuration is used for large machines.

false

The impedance of an autotransformer is greater than the impedance of a two-winding transformer.

false

For very large autotransformers, the cost of ? increases faster than the reduction in copper reduces costs.

insulation

For voltage regulation, an autotransformer can be installed in combination with the ? to split the percentage of change.

tap changer

An autotransformer can be made from a two-winding transformer; by connecting the two windings, the two windings are on ? core(s) and connected in ? .

the same/series

An autotransformer has a higher power rating than an equivalent two-winding transformer.

true

Autotransformers are designed to be overloaded during motor starting. This allows a less expensive autotransformer design to be used.

true

Autotransformers are not able to block the flow of transients and harmonics throughout a 3-phase power system.

true

Autotransformers have a lack of isolation between the primary and the secondary.

true

Because the impedance of an autotransformer is lower than that of an equivalent two-winding transformer, higher current flows when a short circuit fault occurs.

true

For loads with a lower starting torque, the autotransformer can be tapped to deliver 50% voltage. The lower voltage start draws a smaller line current than the higher voltage.

true

In a variable transformer, it is possible to move the contact under load without interrupting the circuit.

true

In autotransformer starting, transformer motor current and line current are not equal.

true

Reduced voltage during motor starting results in reduced current and torque.

true

Since an autotransformer can be designed with only one winding, less copper can be used and costs may be lower than for an equivalent two-winding transformer.

true

A(n) ? transformer is a continuously adjustable autotransformer consisting of a single layer of wire wound on a toroidal core, and a carbon brush that traverses this winding.

variable

Transformers configured in the open delta configuration require that the taps be changed on ? coil(s).

2

When used during motor starting, an autotransformer must be allowed time to cool down before it is used again. A typical duty cycle is ? .

10 seconds ON and 10 minutes OFF

Which of the following is not an application for autotransformers?

Used in the switching circuitry for generator transfer switches

For a step-up autotransformer application, the source is applied ? .

across only part of the coil

For a step-down autotransformer application, the source is applied ? , which acts as the primary.

across the entire coil

A(n) ? is any transformer in which the primary and secondary circuits have a portion of their two windings in common.

autotransformer