3.15: Transformers Flashcards
(a) Primary Turns / Secondary Turns
- A current transformer should always have its
(a) primary shorted or off load.
(b) secondary open circuit or on load.
(c) secondary shorted or on load.
(c) secondary shorted or on load.
- In transformer operation, hysteresis losses are:
(a) current overcoming coil resistance,
(b) energy wasted in reversing core magnetisation,
(c) induced currents in the iron core.
(b) energy wasted in reversing core magnetisation,
- A step up autotransformer has:
(a) two windings.
(b) three windings,
(c) one winding
(c) one winding
- In transformer operations, Hysteresis Losses are caused by:
(a) current overcoming the coils resistance
(b) energy wasted in reversing the core’s magnetization
(c) induced currents in the iron core
(b) energy wasted in reversing the core’s magnetization
- In transformer operations, Eddy Current Losses are caused by:
(a) current overcoming the coils resistance
(b) energy wasted in reversing the core’s magnetization
(c) induced currents in the iron core
(c) induced currents in the iron core
- In transformer operations, Copper Losses are caused by:
(a) current overcoming the coils resistance
(b) energy wasted in reversing the core’s magnetization
(c) induced currents in the iron core
(a) current overcoming the coils resistance
- A transformer has 250 turns on the primary and 25 turns on the secondary. If the
primary is connected to a supply of 115 volts a.c. what will be the secondary voltage?
(a) 11.5 volts.
(b) 1.150 volts.
(c) 23 volts.
(a) 11.5 volts.
- The purpose of a center tap in a transformer is to provide
(a) two separate dc voltages to a rectifier
(b) two equal voltages from one Transformer
(c) a step-down voltage to a rectifier
(b) two equal voltages from one Transformer
- In transformers overheating and insulation failure can be caused by
(a) higher frequencies than normal.
(b) lower frequencies than normal.
(c) higher or lower frequencies than normal.
(b) lower frequencies than normal.
- A step-up transformer has a turns-ratio of 10. If the output current is 20A, the
input current is :
(a) 2A
(b) 200A
(c) 20A
(b) 200A
- The core of a transformer is laminated to :
(a) prevent eddy currents from occurring
(b) reduce the effects of eddy current loss
(c) limit hysteresis loss
(b) reduce the effects of eddy current loss
- A transformer has a step-up ratio of 5 to 1 and a primary source voltage of
24VAC. The secondary current is 0.20 Amps. What is the primary current drawn?
(a) 0.40 Amps
(b) 1 Amp
(c) 4.8 Amps
(b) 1 Amp
- An advantage of an auto-transformer is that :
(a) copper loss is reduced
(b) it gives a high step-up ratio
(c) iron losses are reduced
(a) copper loss is reduced
Auto Transformer has one winding only so copper loss is reduced (i,.e same winding for both primary and secondary is used)
- Transformers are treated as being ideal because:
(a) they have no moving parts
(b) they have iron cores
(c) they are 98% efficient
(c) they are 98% efficient
- Eddy currents can be reduced by:
(a) laminations in the transformer core.
(b) using ionized metallic material.
(c) using a single solid piece of brass
(a) laminations in the transformer core.
- Eddy currents can be reduced by:
(a) Laminations
(b) Using special non-metallic materials
(c) Using a single solid piece of meta
(a) Laminations
- The core in a high frequency transformer which provides for the magnetic flux is
made from:
(a) A non-ferrous material
(b) Iron
(c) Silicon
(a) A non-ferrous material
Iron-core transformer for low frequency
Air core (non ferrous) for high frequency
- A power supply transformer has :
(a) single secondary winding.
(b) Multi secondary windings.
(c) No secondary windings.
(a) single secondary winding.
- If the power consumed by the secondary circuit of a transformer is 52 watts, what
will be the primary current if the primary voltage is 115 volts?
(a) 2 Amps.
(b) 1.5 Amps
(c) 0.45 Amps
(c) 0.45 Amps
'’Power in the primary = power in the secondary’’
Therefore, Power = V*I
52 watts = 115 volts * I
I = 52/115, Current = 0.45 amps
- Eddy currents in a transformer core are reduced by
(a) decreasing the thickness of the laminates
(b) increasing the thickness of the laminates
(c) making the core from a solid block
(a) decreasing the thickness of the laminates
- Transformer copper loss on full load is 220 Watts. On half load the loss will be
(a) 110 Watts
(b) 440 Watts
(c) 55 Watts
(c) 55 Watts
Half load = 1/2 current losses are I squared R
so 1/4
- 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 k ohm, how many turns
must it have?
(a) 15,000
(b) 150,000
(c) 1,500,000
(a) 15,000
- In a transformer core loss is 200 W and copper loss is 220W at no load.
(a) What is the core loss at full load
(b) 200 W
(c) 220 W
(d) 0 W
(b) 200 W
Core loss is not changed by changes in load current, they are frequency dependent.
I2R losses copper losses are impacted by il changes
- A step up transformer has
(a) one winding
(b) two windings
(c) three windings
(b) two windings
- How can it be determined if a transformer winding has some of its turns shorted
together?
(a) measure the input voltage with an ohmmeter
(b) the transformer will get hot in normal operation
(c) the output voltage will be high
(b) the transformer will get hot in normal operation
Shorted turns is the same as reducing the number of turns, which results in higher currents and more I2R heating
- A transformer is built with a material having a
(a) narrow hysteresis loop
(b) wide hysteresis loop
(c) high coercive force
(a) narrow hysteresis loop
- A current transformer, when disconnected from a circuit must be
(a) left open circuit
(b) short circuited
(c) have a resistor connected across the terminals
(b) short circuited
Must be shorted or left on load
- A transformer rated at a frequency of 200 Hz
(a) may be used at any other frequency
(b) may only be used at the rated frequency or above
(c) may only be used at the rated frequency or below
(b) may only be used at the rated frequency or above
- The copper loss on a single phase transformer is 25 Watts on full load. What will
it be on half load?
(a) 5 Watts
(b) 12.5 Watts
(c) 6.25 Watts
(c) 6.25 Watts
1/2 current flowing. I^2R loss will be 1/4
- A transformer is designed to operate at 60 Hz. It is connected across a 50 Hz
supply. The primary windings would
(a) draw more current
(b) draw less current
(c) draw the same current
(a) draw more current
- Copper losses in a transformer are caused by
(a) circulating currents through the transformer
(b) the resistance in the windings
(c) wasting of the copper wires due to friction
(b) the resistance in the windings
- A transformer with a 5:1 ratio has a 24V output. What is the input
(a) 120 V DC
(b) 4.8 V AC
(c) 120 V AC
(c) 120 V AC
- A transformer to supply a 24V load with a 5:1 turns ratio will have what primary
supply?
(a) 120 VDC
(b) 48 VAC
(c) 120 VAC
(c) 120 VAC
- 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?
(a) 200V
(b) 346V
(c) 450V
(b) 346V
Delta Star is step up 1: SQRT 3
Star delta is step down
Vs = Vp / Turns ratio
Vs = 2000/10 * SQRT 3 .
Vs = 200 * 1.73 = 346V
- 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
(a) 1 kohm
(b) 4 kohm
(c) 16 kohm
(c) 16 kohm
1200/4800 = 1/4 = SQRT 1000/Zs square both sides 1/16 = 1000/Zp
Hency Zp = 16k
- 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
(a) 1 kohm
(b) 4 kohm
(c) 16 kohm
(c) 16 kohm
1200/4800 = 1/4 = SQRT 1000/Zs square both sides 1/16 = 1000/Zp
Hency Zp = 16k
- An autotransformer windings are
(a) on separate formers
(b) connected
(c) wired in parallel
(b) connected
-All one winding - Auto transformer
- A transformer has 500 turns on the primary and 100 turns on the secondary.
When connected to a 100 V supply, the output is
(a) 500 V
(b) 20 V
(c) 25 V
(b) 20 V
(a) 10 amps
Step down 5:1 So secondary voltage is 20 V with a 10 Ohm load, I = 2A
Current (I) is stepped up = 10 A
- A Current Transformer (CT) is used:
(a) to step up/down currents
(b) for impedance matching
(c) to measure large currents with conventional ammeters
(c) to measure large currents with conventional ammeters
(a) 100 mA
What should be noted about a current transformer secondary coil?
What are hysteresis losses?
How many windings does a step-up autotransformer have?
What causes Eddy current losses?
What causes copper losses?
What does a center tap do?
What causes overheating or insulation failure in a transformer (in terms of Z and I)?
How would you reduce Eddy current losses?
What is the advantage of an Auto-transformer?
Why are transformers ideal?
What core would you use for a high freq transformer core?
High frequency transformer core = Made from non-ferrous (or Air)
How many secondary windings in a power supply transformer?
What type of losses in a transformer?
What is the formula that relates N Number of turns and Z impedance (primary and secondary)
How many windings does a step up transformer have?
Transformer material should give a ____ hysteresis loop
What is the formula for line voltage?
Delta star =
Star delta =
Autotransformer winds have ?
A current transformer is used to?
