## 28/03/2020

### Design of DC Machines Interview Question Answer - 10

 136 Whether the current density is uniform during commutation? Delay or retard commutation The current density is excessive during trailing edge of brush whereas it is small at the leading edge of brush. Accelerated commutation The current density is excessive during leading edge of brush whereas it is small during trailing edge of the brush. Sinusoidal commutation It is combination of retard and accelerated commutation.  The retard commutation is desirable during first stage whereas the accelerating commutation in the later stage. 137 Whether the value of inter poles mmf affects the commutation? Inter poles mmf The inter poles mmf must be equal to armature mmf in order to neutralize the effect of armature reaction. If the inter pole mmf is too weak, it leads to retard commutation. If the inter pole mmf is too strong, it leads to accelerating commutation. 138 Whether the diameter of commutator is kept equal to diameter of armature? The diameter of commutator is generally 0.6 to 0.8 times the diameter of armature. 139 What should be minimum commutator segment pitch? Commutator segment pitch The minimum thickness of commutator at the commutator surface should not be less than 3 mm. If the thickness of mica is 0.8 mm, the minimum commutator segment pitch is 0.4 mm. 140 Which type of losses occurs on the commutator surface? Losses on commutator surface Brush contact loss Brush friction loss 141 On which factors the brush friction loss depends in the DC machines? Brush friction loss The brush friction loss depends upon the peripheral speed of commutator, brush pressure and coefficient of friction between brush and commutator. 142 On which factor the coefficient of friction between commutator and brush depends? Coefficient of friction The coefficient of friction between commutator and brushes depends upon peripheral speed of commutator.  Higher the peripheral speed, lower the coefficient of friction. 143 Which are the factors affecting the friction loss in the DC machines? Factors affecting friction loss Coefficient of friction between bearing and shaft Peripheral speed of the shaft at the bearing Pressure on the bearing 144 Which are the factors affecting the windage loss in the DC machines? Windage loss Speed of machine Rotor diameter Length of core 145 Explain : Stray load loss. Which are the main reasons to occur it? Stray load loss The stray load loss occurs when the machine is actually loaded. The stray load losses occur due to following reasons. Large increase in iron loss due to distortion of main field caused by armature reaction. Eddy current in the current causes copper losses Circulating current during commutation causes losses. 146 State the condition for maximum efficiency in the DC machine. Maximum efficiency The condition for maximum efficiency in the DC machine is constant losses is equal to losses corresponding to square of current.

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