DC Motor Troubleshooting and Calculations Quiz

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.