28 Questions
What can cause sparking at the brushes of a DC motor?
Worn bearings
In a 110-V separately-excited DC motor drawing 50 A and running at 1000 rpm with an armature resistance of 0.22, what is the back EMF?
104 V
During normal operation of a series motor, if the field rim cuddank resistances are 0.25 Ω, 0.150 Ω, and 500 Ω, what is the counter EMF?
106 V
How is the armature winding connected in a DC shunt motor?
Parallel
In a 250 V DC series motor drawing 30 A, what is the resistance?
0.25 Ω
How will the induced EMF in the transformer secondary depend on?
Maximum flux in the core and frequency only
What is the primary function of a starter in a DC motor?
Limit starting current
In a 220/110-V transformer with a secondary load impedance of 10 Ohms, what is the impedance referred to the primary side?
20 Ohms
What is the resistance needed to be connected in series with a 120-V DC shunt motor's armature to limit the starting current to 100 A if the armature resistance is 0.2 Ohms?
0.25 Ohms
For a high efficiency in electrical machines, which condition should be met?
Its losses are minimum
What is the number of turns in the secondary coil of a transformer with a primary coil of 1600 turns if the current in the primary coil is 6 A and in the secondary coil is 48 A?
12,800 turns
What is the output power of a 200 V dc shunt motor drawing 30 A, with armature resistance of 0.22 Ohms, shunt field resistance of 100 Ohms, and stray power loss of 600 W?
7.6 hp
What material are the laminations of a transformer core typically made from?
Silicon steel sheets
In an ideal transformer, what is true about losses and magnetic leakage?
No losses and magnetic leakage
Which type of steel sheets are NOT typically used for the laminations of a transformer core?
Chrome steel sheets
What is the armature resistance of a 200 V dc shunt motor taking 30 A current?
0.22 Ohms
What should be the equivalent complex impedance referred to the secondary?
0.0458 + j0.0778
In a transformer, when the flux in the core is purely pulsating, what type of current is induced in the secondary winding?
Alternating
What is the turns ratio of a transformer needed to match a 20 load to a 72,000 source?
60:1
What is the number of secondary turns required if a transformer with 100 primary turns is supplied with 200 V on the primary side and needs to provide 400 V on the secondary side?
50 turns
In a transformer, what does it mean when the primary and secondary windings are coupled both electrically and magnetically?
Electrically and magnetically coupled
For a 100 V DC series motor with specific resistances, if the constant losses amount to 500 W, what is the motor output at a specific load?
2320 W
What is the new KVA rating of a transformer that is originally rated 500 kVA at 200 Hz when the frequency changes to 50 Hz?
2000 KVA
In a transformer with 200 turns on the primary and 20 turns on the secondary, if the primary is connected to a 1500 V supply, what is the secondary voltage on open circuit?
1,500 V
What is the primary to secondary turns ratio requirement for a matching transformer to drive a speaker with an impedance of 200 Ohms, while the amplifier works best into an impedance of 500 Ohms?
1/5
Which statement is NOT true for an ideal transformer?
Zero permeability
If a 7:5 step-down transformer draws 2 A, what is the secondary current?
3.92 A
For a DC shunt wound motor with an armature resistance of 0.3 ohms operating on a 120-V circuit, what should be the starting resistance in series with the armature to limit the starting armature current to 10 A?
$9.2 \Omega$
Test your knowledge on DC motor troubleshooting and calculations with questions on sparking at brushes, back emf calculation, counter emf determination, and series motor operation. Determine the causes of sparking, back emf value, counter emf, and series motor resistances.
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