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What can cause sparking at the brushes of a DC motor?
What can cause sparking at the brushes of a DC motor?
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?
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?
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?
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?
How is the armature winding connected in a DC shunt motor?
How is the armature winding connected in a DC shunt motor?
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In a 250 V DC series motor drawing 30 A, what is the resistance?
In a 250 V DC series motor drawing 30 A, what is the resistance?
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How will the induced EMF in the transformer secondary depend on?
How will the induced EMF in the transformer secondary depend on?
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What is the primary function of a starter in a DC motor?
What is the primary function of a starter in a DC motor?
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In a 220/110-V transformer with a secondary load impedance of 10 Ohms, what is the impedance referred to the primary side?
In a 220/110-V transformer with a secondary load impedance of 10 Ohms, what is the impedance referred to the primary side?
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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?
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?
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For a high efficiency in electrical machines, which condition should be met?
For a high efficiency in electrical machines, which condition should be met?
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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?
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?
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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?
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?
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What material are the laminations of a transformer core typically made from?
What material are the laminations of a transformer core typically made from?
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In an ideal transformer, what is true about losses and magnetic leakage?
In an ideal transformer, what is true about losses and magnetic leakage?
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Which type of steel sheets are NOT typically used for the laminations of a transformer core?
Which type of steel sheets are NOT typically used for the laminations of a transformer core?
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What is the armature resistance of a 200 V dc shunt motor taking 30 A current?
What is the armature resistance of a 200 V dc shunt motor taking 30 A current?
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What should be the equivalent complex impedance referred to the secondary?
What should be the equivalent complex impedance referred to the secondary?
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In a transformer, when the flux in the core is purely pulsating, what type of current is induced in the secondary winding?
In a transformer, when the flux in the core is purely pulsating, what type of current is induced in the secondary winding?
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What is the turns ratio of a transformer needed to match a 20 load to a 72,000 source?
What is the turns ratio of a transformer needed to match a 20 load to a 72,000 source?
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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?
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?
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In a transformer, what does it mean when the primary and secondary windings are coupled both electrically and magnetically?
In a transformer, what does it mean when the primary and secondary windings are coupled both electrically and magnetically?
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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?
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?
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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?
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?
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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?
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?
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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?
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?
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Which statement is NOT true for an ideal transformer?
Which statement is NOT true for an ideal transformer?
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If a 7:5 step-down transformer draws 2 A, what is the secondary current?
If a 7:5 step-down transformer draws 2 A, what is the secondary current?
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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?
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?
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Study Notes
Causes of Sparking at Brushes of DC Motors
- Poor contact between brushes and commutator can result in sparking.
- Worn or damaged brushes may create arc faults.
- Misalignment of brushes or commutator contributes to sparking.
- High armature voltage or load conditions can increase sparking occurrences.
Back EMF Calculation
- For a 110-V separately-excited DC motor drawing 50 A and running at 1000 rpm, calculate back EMF (EB):
- EB = V - (I × Ra) = 110 - (50 × 0.22) = 110 - 11 = 99 V.
Counter EMF in Series Motors
- Total resistance in series motor: 0.25 Ω + 0.150 Ω + 500 Ω = 500.4 Ω.
- Counter EMF (EC) can be calculated based on current and resistances, factoring the load conditions.
Armature Winding in DC Shunt Motors
- In a DC shunt motor, the armature winding is connected in parallel with the field winding.
Resistance in a Series DC Motor
- For a 250 V DC series motor drawing 30 A, the resistance (R) can be calculated using Ohm's law:
- R = V / I = 250 / 30 = 8.33 Ω.
Induced EMF in Transformers
- The induced EMF in the secondary of a transformer depends on:
- The number of turns in the primary and secondary coils.
- The rate of change of magnetic flux linked with the coils.
Function of a Starter in DC Motors
- A starter is used to limit the initial inrush current of a DC motor during startup.
- It helps prevent damage to the motor during the transition from stop to operating speed.
Impedance in Transformers
- In a 220/110-V transformer with a secondary load impedance of 10 Ω, the impedance referred to the primary side is:
- Zp = (Vs / Vp)² × Zs = (110 / 220)² × 10 = 2.5 Ω.
Starting Resistance for DC Shunt Motor
- To limit the starting current to 100 A for a 120-V DC shunt motor with an armature resistance of 0.2 Ω, calculate:
- Series resistance (Rs) = (V / I - Ra) = (120 / 100) - 0.2 = 1.2 Ω.
High Efficiency Condition in Electrical Machines
- For high efficiency in electrical machines, losses should be minimized, and the load should operate near rated capacity.
Number of Turns in Transformer Secondary Coil
- For a transformer with a primary of 1600 turns, a 6 A primary current, and a 48 A secondary current, calculate:
- Turns ratio = Iprimary / Isecondary = 48 / 6 = 8.
- Turns in secondary = Primary turns / Turns ratio = 1600 / 8 = 200 turns.
Output Power Calculation in DC Shunt Motor
- For a 200 V DC shunt motor drawing 30 A with armature resistance of 0.22 Ω and stray power loss of 600 W:
- Output Power = (V - I × Ra) × I - Stray Power Loss = (200 - (30 × 0.22)) × 30 - 600.
Transformer Core Laminations
- Laminations of a transformer core are typically made from silicon steel, which has low hysteresis loss.
- Non-oriented steel sheets are NOT typically used for laminations.
Ideal Transformer Characteristics
- In an ideal transformer, no losses occur, and magnetic leakage is absent.
- This means all input power is transferred to the output.
Transformers and Impedance Matching
- For a 220 Ω speaker and a 500 Ω amplifier, a matching transformer with a turns ratio of √(500/220) is required.
- In a transformer, primary and secondary windings being electrically and magnetically coupled implies efficient energy transfer.
Current Transformation in Step-Down Transformers
- For a 7:5 step-down transformer drawing 2 A, calculate the secondary current:
- Secondary current = (Primary current × Primary turns) / Secondary turns = (2 × 5) / 7 = 1.43 A.
Starting Resistance Calculation for DC Shunt Motor
- For a DC shunt motor with an armature resistance of 0.3 Ω operating at 120 V, to limit starting current to 10 A:
- Required resistance = V / I - Ra = 120 / 10 - 0.3 = 11.7 Ω.
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Description
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.
