Transformers Nivel Dos: Revenge of the Autotransformers
Calculate the current flow for ILoad. Calculate to two decimal places. Note: The values calculated for this question will be used to answer additional questions.
208.33 A
Calculate the kilovolt amp rating of the transformer needed to supply the 50-kilvolt amp load using the VAS of the transformer.
30000 VA
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
Calculate the current flow for Iz-b. Calculate to two decimal places. Note: The values calculated for this question will be used to answer additional questions.
83.33 A
With a connected 50-kVA load, as shown in the diagram, calculate the current flow for Ia-x. Calculate to two decimal places. Note: The values calculated for this question will be used to answer additional questions.
83.33 A
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
Autotransformers have high impedance compared to a conventional transformer.
False Note: Autotransformers have low impedance compared to a conventional transformer.
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.
d.variable
Calculate the current flow for Iz-y. Calculate to two decimal places. Note: The values calculated for this question will be used to answer additional questions.
125 A
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 ? .
d.the same / series
An autotransformer has a higher power rating than an equivalent two-winding transformer.
True Note: The two windings of an autotransformer are wired in series. The current through the primary combines with the current induced in the secondary and flows through the load. Therefore, the load current is higher than the current in an equivalent two-winding transformer.
For a step-up autotransformer application, the source is applied ? .
a.across only part of the coil
A(n) ? is any transformer in which the primary and secondary circuits have a portion of their two windings in common.
a.autotransformer
Transformers configured in the open delta configuration require that the taps be changed on ? coil(s).
b. 2
For a step-down autotransformer application, the source is applied ? , which acts as the primary.
b. across the entire coil
When used during motor starting, an autotransformer must be allowed time to cool down before it is used again. A typical duty cycle is ? .
b.10 seconds ON and 10 minutes OFF
For very large autotransformers, the cost of ? increases faster than the reduction in copper reduces costs.
b.insulation
Transformers connected in a wye configuration require that the taps be changed on ? coil(s).
c. 3
Which of the following is not an application for autotransformers?
c.Used in the switching circuitry for generator transfer switches
The voltage across the load is ? .
d. 480 V
Autotransformer reduced-voltage motor starting reduces the applied motor voltage to ? of the line voltage when starting.
d. 50%, 65%, or 80%
For voltage regulation, an autotransformer can be installed in combination with the ? to split the percentage of change.
d. tap changer
The current through the load is ? .
d.50 A
Different loads require different starting torque. For a load with a very high starting torque, the autotransformer can be tapped to deliver ? voltage.
d.80%
