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the answer to this question is increases, though it would be surprising information to many but actually thing are different what common students think.
Here is the explanation.
whan a dc shunt motor is applied voltage to armature and field the backing EMF produced is depandant on field pole magnetic strength and the rotation of the rotor ,both combined together produced a backing EMF which stabilizes when the backing EMF equall to the applied voltage,
NOW when the field winding voltage reduced the Field's magnetic strength reduced which in turn reduced the backing EMF at same time the rotor current increases because of tne reduced magnetic field pole intensity thus rotor starts rotating faster till it develops same Backing EMF which should be equal to applied voltage . This is the reason the DC shunt Motor speed increases as the field voltage is decreased this has been experimented in the college as i was conducting experiment on EEE Emlab ||
speed is proportional to the field voltage therefore when field voltage voltage is reduced then DC shunt motor speed decreases.
