Autotransformers

Question 1

Autotransformer

Answer 1

Any transformer in which the primary and secondary circuits have a portion of their two winding in Common
-Higher Power rating than that of an equivalent two winding transformer.
-Limited by low impedance and lack of electrical isolation between the primary and secondary circuits.
-USED for reduced voltage motor starting.

Question 2

Auto-transformer Benefits

Answer 2

Can deliver more power to a load than when wired as a two winding transformer.
Also be smaller and less expensive

Question 3

Autotransformer Limitations

Answer 3

1 Low impedance
- Short circuits will have higher current flow
2 Lack of isolation between the primary and the secondary.
- The delta-wye zigzag winding of standard transformers are not present = not able to block the flow of transients and harmonics throughout a 3 phase power system.

Question 4

Autotransformer Applications

Answer 4

-Used in reduced voltage motor starting
-Also used a voltage regulator
-Variable transformer

Question 5

Autotransformer Reduced Voltage Starting

Answer 5

-Provides the highest possible starting torque per ampere of line current.
-Current to the motor may change because of the motor’s changing characteristics but the voltage to the motor remains constant.

Question 6

Starting Circuits applied motor voltages

Answer 6

50% 65% 80%

Question 7

Starting Connections

Answer 7

1 Oped Delta
- Used for small motors
- Require that the taps be changed on TWO coils only
2 Wye
- Used for large machines
-Gives better voltage balance than
the open delta
- Require that the taps be changed on THREE coils

Question 8

Autotransformer Voltage Regulators

Answer 8

-Can be used to slightly boost or buck the line voltage to correct for small voltage changes.
-Thus use allows changes in the voltage without interruption of the power to the loads.

Question 9

Variable Transformers

Answer 9

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.

The brush is always in contact with one or more wires and continuously taps off any desired fraction of the winding voltage

It is possible to remove the contact under load without interrupting the circuit.