Answer / vinodh kumar ks

A shunt motor speed is controlled by the field. Think of it
like this: The actual resistance of the armature is very
small so by Ohm's law the armature current (locked rotor)
will be very large. But as the motor turns it acts like a
generator and that generated voltage reduces the current
that the motor draws. Soon at no load the motor is turning
fast enough that very little current is required. if you
make the field stronger by increasing the field current the
generator action (called back EMF) is increased and the
motor doesn't have to turn as fast to balance the voltage
applied to the armature.
So the rule is reduce the field to increase the speed.

Answer / somasundaram t

ward leonard method is one of the dc motor speeed
control.this method requires combination of one motor
generator set for controlling the one DC motor.so this
method becomes expensive.and also requires high
maintanence.power consumption ,losses also more.this method
got too failure after the power semiconductor controllers
has been introduced.

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