Transformers Flashcards
How many turns does the secondary winding of a 2:1 step-up transformer have?
More than primary.
To reduce eddy currents in a transformer you would?
Reduce the thickness of laminations in the magnetic core.
A transformer has an input of 400V and a ratio of 2:1. If the transformer is delta/star wound, what will the line voltage output be?
346V.
A Transformer with 115V primary voltage and a ratio of 5:1 is supplying a landing light (load 24V 45 amps), what is the current drawn?
9 amps.
A Transformer has 4500 secondary turns and 750 primary turns. Its turns ratio is?
6:1.
The point of coercivity occurs when the?
Magnetic flux is zero even though a magnetizing force is being applied.
The primary winding of a 3-phase transformer?
Could be either delta or star wound.
In a transformer, if the rate of change of current increases, the mutual inductance will?
Remain the same.
A 4:1 step down transformer draws 115 V and 1 A. The output power will be?
28.5 V at 4 A.
The secondary coil of a transformer has 1500 turns and 10 Ω resistance. The primary coil has 1kΩ resistance. How many turns does the primary coil have?
15,000.
In a star wound primary transformer, how could you wind the secondary winding?
Either delta or star.
A step up transformer has?
Two windings.
In transformer core loss is 200 W and copper loss is 220W at no load. What is the core loss at full load?
200 W.
The secondary winding of a transformer has an impedance of 10 ohms and 1500 turns. For the primary winding to have an impedance of 1 kilohm, how many turns must it have?
15,000.
Eddy currents in a transformer core are reduced by?
Decreasing the thickness of the laminates.
Transformer copper loss on full load is 220 Watts. On half load the loss will be?
55 Watts.
How can it be determined if a transformer winding has some of its turns shorted together?
The transformer will get hot in normal operation.
Eddy currents in a transformer can be reduced by?
Laminations.
What is the ratio of turns between the primary coil winding and the secondary coil winding of a transformer designed to triple its input voltage?
Primary will have one-third as many turns as its secondary.
A transformer should always have its?
Secondary open circuit or on load.
Which of the following would be used to calculate transformer turns ratio?
Secondary turns ÷ primary turns.
The voltage in a transformer secondary coil that contains twice as many loops as the primary coil will be?
Greater and the amperage greater than in the primary coil.
The area of a hysteresis loop is proportional to the?
Energy dissipated in putting the material through a complete cycle of magnetization and demagnetization.
In transformer operation, hysteresis losses are?
Energy wasted in reversing core magnetization.
What causes a transformer to be noisy?
An air gap in the core.
A transformer with a 5:1 ratio has a 24V output. What is the input?
120 V AC.
Copper losses in a transformer are caused by?
The resistance in the windings.
The copper loss on a single phase transformer is 25 Watts on full load. What will it be on half load?
6.25 Watts.
A transformer is designed to operate at 60 Hz. It is connected across a 50 Hz supply. The primary windings would?
Draw more current.
A current transformer, when disconnected from a circuit must be?
Short circuited.
What is the reflected impedance in the primary of the transformer circuit shown?
500 ohms.
A transformer rated at a frequency of 200 Hz?
May only be used at the rated frequency or above.
The dots on the transformer symbol shown are called, and indicate?
Phasing dots, point that have the same polarity at the same moment in time.
A transformer is built with a material having a?
Narrow hysteresis loop.
A transformer is built with a material having a
narrow hysteresis loop.
The area of the hysteresis loop is the power lost in repeatedly reversing the magnetism.
In a circuit which possesses mutual inductance, current is increased to the primary.
the induced EMF in the secondary produces a current in the secondary circuit which opposes the increase in flux due to the increase of current in the primary.
A transformer has 500 turns on the primary and 100 turns on the secondary. When connected to a 100 V supply, the output is.
20 V.
5:1 step down. 100/5 = 20V.
A transformer has 500 turns on the primary and 100 turns on the secondary. The secondary supplies 20V and is connected to a 10 ohm load. The primary current is.
0.4 amps.
Secondary current = V/R = 20/10 = 2A. Primary current = 2A/5 = 0.4A.
A transformer has 1200 turns primary and 4800 turns secondary. What value of load resistance would be required to give a reflected impedance of 1000 ohms?
16 kilohm.
Impedance matching formula, Turns Ratio = √(Impedance Ratio).
An autotransformer windings are.
connected.
What voltage can the neutral line carry in a star transformer?
Zero voltage.
The neutral line is grounded.
A transformer to supply a 24V load with a 5:1 turns ratio will have what primary supply?
120 V AC.
5:1 is step down, so the supply voltage must be 5 * 24 = 120 VAC (AC because transformers don’t work with DC).
A transformer has 2000V, 500 turns on the primary and 50 turns on the secondary. What would be the line voltage if the transformer were Delta/Star connected?
346V.
It is a 10:1 step down transformer. Phase voltage in secondary = 200V. With star wound secondary, line voltage = 1.73 * 200 = 346V.
The line voltage in a Wye wound transformer is.
root 3 * phase voltage.
Line voltage = root 3 * phase voltage in a Wye (star) wound transformer.
Eddy current losses in a transformer are.
magnetic flows induced into the core.
A 3-phase star-delta wound transformer gives.
30° phase shift.
A star-delta wound transformer gives a 30 degree phase shift.
In a 4:1 step up transformer, what would give a reflected impedance of 1Kilohm?
16k ohm.
Turns ratio = √(impedance ratio).
The input is 115V and is applied across the 300 turn portion of the autotransformer shown. What is the output?
460V.
Autotransformer 300:1200 ratio 1:4. Output = 4 * 115V.
A near perfect 4:1 step up transformer has its primary connected to 120VAC, if the secondary is connected to a load with 1600 ohms resistance, what is the current in the primary?
1.2 amps.
Secondary voltage = 480V, Secondary current = 480/1600 = 0.3A, Primary current = 4 * 0.3 = 1.2A.
Flux losses in a transformer can be prevented by.
winding the primary on top of the secondary.
To supply a 24volt output with a transformer with a 5:1 turns ratio, the input required is.
120V.
Assuming this is a step-down transformer (i.e 5:1 means Primary:Secondary).
What would you use for the core in a generator current transformer?
Air core.
A current transformer uses the generator output cable as its primary, so it must have a hollow (air) core.
An aircraft transformer is needed to step up a voltage. What sort would be used?
Laminated core.
Laminated core transformers are most efficient.
In a 4:1 step-up transformer, 120V is applied to the primary, and the load on the secondary is 1600 Ohms. What is the current in the primary?
1.2A.
Voltage in secondary = 4 * 120 = 480V. Current in Secondary = 480/1600 (Ohm’s Law) = 3/10A. Current in Primary = 4 * 3/10A = 12/10A = 1.2A.
A 10:1 step up power transformer has 120 V, and 3A flowing through the primary, and 0.285 A flowing through the secondary what is the efficiency?
95%.
Power in primary = 120 * 3 = 360W. Power in secondary = 1200 * 0.285 = 342W. 342/360 = 0.95.
Aircraft autotransformers are of what type?
Laminated core.
An autotransformer is.
primary and secondary coils on single bar.
The windings on a autotransformer are.
wound with no magnetic core.
How do you reduce Hysteresis loss of a transformer?
By laminating the core.
