In machine tool drive application, the speed of separately

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In machine tool drive application, the speed of separately excited dc motor is required to be controlled both below and above the rated speed of the motor. Which one of the following methods is best suited for this purpose ?
(V_a = armature voltage, l_f = field current and I_{f ,rated} = field current at rated speed)

1. I_f = If, rated, Va variable
2. V_a fixed, I_f variable
3. V_a and I_f variable, with I_f ≤ I_{f ,rated}
4. V_a and I_f variable, with I_f ≥ I_{f ,rated}

Correct Option: C

With I_f > I _{f rated} , speed is reduced and with I_f < I _{f rated}, the speed is increased above normal. Also, with V_a greater than V_{a . rated}; speed falls, and with Va greater than V_{a rated} ; speed rises above normal. To avoid overloading of either the field finding; or the armature winding, though little expensive; hence choice (c) will be most suitable.