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A 4-pole 500V shunt motor has 720 wave connected conductors on its armature. The fullload armature current is 60A and flux per pole is 0.03 Wb. The armature resistance is 0.2Wand the contact drop per brush is 1V. What is the back emf ?
  • a)
    490V
  • b)
    460V
  • c)
    445V
  • d)
    486V
Correct answer is option 'D'. Can you explain this answer?
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A 4-pole 500V shunt motor has 720 wave connected conductors on its arm...
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A 4-pole 500V shunt motor has 720 wave connected conductors on its arm...
Given data:

Number of poles (P) = 4

Voltage rating (V) = 500V

Armature conductors (N) = 720

Full-load armature current (Ia) = 60A

Flux per pole (Φ) = 0.03 Wb

Armature resistance (Ra) = 0.2Ω

Contact drop per brush (Vc) = 1V

To find:

Back emf (Eb)

Solution:

1. Calculation of back emf (Eb)

The back emf of a DC shunt motor is given by the expression:

Eb = V - Ia(Ra + Rb)

where,

V is the applied voltage

Ia is the armature current

Ra is the armature resistance

Rb is the brush contact resistance

In this case, the brush contact resistance is not given, but we can assume it to be negligible compared to the armature resistance. Therefore,

Rb = 0

Substituting the given values,

Eb = V - Ia(Ra + Rb)

Eb = V - IaRa

Eb = ΦPN/60A

Eb = 0.03 × 4 × 720 / 60

Eb = 0.144V

Therefore, the back emf of the motor is 0.144V.

2. Conversion of back emf to actual voltage

The back emf is a theoretical value that is always less than the actual voltage of the motor. The actual voltage can be found by adding the voltage drop due to the armature resistance and the brush contact drop to the back emf.

Vactual = Eb + IaRa + 2Vc

Substituting the given values,

Vactual = 0.144 + (60 × 0.2) + 2

Vactual = 12.24V

Therefore, the actual voltage of the motor is 500 - 12.24 = 487.76V.

Rounding off to the nearest whole number, we get:

Vactual ≈ 488V

Hence, the correct answer is option 'D', 486V.
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A 4-pole 500V shunt motor has 720 wave connected conductors on its armature. The fullload armature current is 60A and flux per pole is 0.03 Wb. The armature resistance is 0.2Wand the contact drop per brush is 1V. What is the back emf ?a)490Vb)460Vc)445Vd)486VCorrect answer is option 'D'. Can you explain this answer?
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