A starter motor is typically wound in which configuration to achieve maximum torque at stall?

A starter motor is typically wound in which configuration to achieve maximum torque at stall?

• A. Series wound
• B. Shunt wound
• C. Compound wound
• D. Permanent magnet

Answer: (A)

The starting torque of a DC motor hinges on how the magnetic field responds to current. Torque is roughly the product of armature current and the magnetic field (flux). In a series-wound motor, the field winding is in line with the armature, so the same current flows through both. At startup, the rotor is stalled, so the current is only limited by the supply and winding resistances, which is a relatively large current. Because the field strength in a series motor grows with the current, the flux increases as the current rises, and the torque ends up proportional to the square of the current (torque ∝ Ia^2). That means the motor can develop very high torque right at stall, which is exactly what a starter motor needs. In contrast, shunt-wound motors keep the field current largely independent of the armature current, so the flux remains nearly constant. With a fixed flux, starting torque is limited to what a given current can produce, which is lower than in a series configuration. Permanent magnet motors have a fixed flux from the magnets, so starting torque is also limited by the available armature current and does not benefit from increasing flux with current. Compound-wound motors mix series and shunt elements, so they can have strong starting torque thanks to the series component, but the maximum stall torque is typically not as high as a pure series-wound design. So, the configuration that provides the maximum stall torque is the one where the field winding is in series with the armature.

The starting torque of a DC motor hinges on how the magnetic field responds to current. Torque is roughly the product of armature current and the magnetic field (flux). In a series-wound motor, the field winding is in line with the armature, so the same current flows through both. At startup, the rotor is stalled, so the current is only limited by the supply and winding resistances, which is a relatively large current. Because the field strength in a series motor grows with the current, the flux increases as the current rises, and the torque ends up proportional to the square of the current (torque ∝ Ia^2). That means the motor can develop very high torque right at stall, which is exactly what a starter motor needs.

In contrast, shunt-wound motors keep the field current largely independent of the armature current, so the flux remains nearly constant. With a fixed flux, starting torque is limited to what a given current can produce, which is lower than in a series configuration. Permanent magnet motors have a fixed flux from the magnets, so starting torque is also limited by the available armature current and does not benefit from increasing flux with current. Compound-wound motors mix series and shunt elements, so they can have strong starting torque thanks to the series component, but the maximum stall torque is typically not as high as a pure series-wound design.

So, the configuration that provides the maximum stall torque is the one where the field winding is in series with the armature.