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The direction of rotation of D.C series motor can be changed by
Interchanging field terminal
Interchanging supply terminal
Both a & b
None of the above
Interchanging field terminal
Quick Summary: The direction of rotation of a DC series motor depends on the direction of current flow through both the armature and the field windings simultaneously. Reversing the supply terminals does not change the direction of rotation because the current in both the field and armature reverses at the same time, maintaining the polarity relationship required for torque in the same direction.
The direction of rotation of a DC series motor depends on the direction of current flow through both the armature and the field windings simultaneously. Reversing the supply terminals does not change the direction of rotation because the current in both the field and armature reverses at the same time, maintaining the polarity relationship required for torque in the same direction.
Te=ka⋅Φ⋅Ia — Electromagnetic torque equation
Φ∝If — Flux relationship in series motors
The torque T in a DC motor is proportional to the product of flux Φ and armature current Ia (T∝ΦIa). In a series motor, Φ∝Ia. If supply polarity is reversed, both Ia and Φ reverse direction, so their product remains positive ((−Ia)×(−Φ)=+T). By interchanging only the field terminal connections, the relative polarity between the field and the armature is altered, resulting in a reversal of the electromagnetic torque.
Changing the supply polarity reverses both field and armature current, keeping the direction of rotation the same.
Reversing the physical connections of either the field or the armature winding (but not both) reverses the rotation.
DC series motors are commonly used in traction where bidirectional operation is controlled by contactors reversing the field.
The net torque direction is governed by the relative orientation of the magnetic field and armature current.
Simple control circuitry
Effective torque reversal for traction applications
Requires mechanical access to field terminals
Risk of arcing during terminal reversal under load
Electric traction systems
Cranes and hoists
| Feature | Reversing Supply | Reversing Field/Armature |
|---|---|---|
Effect of reversing supply | Same direction | Reversed direction |
In a shunt motor, reversing the supply also keeps the rotation same, but reversing the field or armature changes it.
Option B is incorrect as it results in the motor maintaining its original rotation due to the simultaneous reversal of both flux and armature current.
A is correct — The direction of rotation of a DC series motor can only be changed by reversing the current in either the field winding or the armature winding, not both.
Always remember that for any DC machine, reversing both the armature and field terminals simultaneously results in the same direction of rotation; you must reverse only one to achieve reversal.