A-level Physics (Advancing Physics)/Motors/Worked Solutions

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1. How could you adapt the simple DC motor to use AC?

Connect the coil in the stator in series with the coil in the rotor via a slip-ring commutator. This must be in the correct direction to ensure that the fields created by the two parts of the coil are constantly repelling each other, rather than constantly attracting each other.

2. Why does a three-phase motor have a constant angular velocity?

The vectorial sum of the three currents is constant, so the vectorial sum of the forces on the rotor due to each of the three currents is constant.

3. What is the difference between a split-ring and a slip-ring commutator?

A split-ring commutator makes the current change direction every half-rotation, whereas a slip-ring commutator merely maintains a connection between the moving rotor and the stationary stator.

4. How could the angular velocity of a three-phase motor be increased?

Increase the frequency of the three-phase power.

5. A squirrel-cage motor relies on eddy currents running along the rotor to function. However, if eddy currents run across the rotor, then the force on the rotor is reduced. How may these eddy currents be reduced without reducing the desired eddy currents?

Laminate the plates which connect the rods to each other so that currents are restricted from running in the wrong direction.