MODULE: MOTORS AND GENERATORS
Chapter 9: Electric motors (Questions, page 167)
1. Some AC motors rotate at the same frequency as the power supply which is precisely set at 50 Hz.
2. Universal motors are constructed like a DC motor, but the external magnetic field is provided by an electromagnet that is connected in series with the rotor. It therefore does not matter if a DC or AC current is used.
3. The stator consists of sets of field electromagnets connected to one or more phases of the AC power supply. This creates a magnetic field that rotates at 50 Hz. The rotor consists of a number of conducting bars made of either aluminium or copper. These are attached to two rings, known as end rings, at either end of the bars. There is no commutator or brushes.
4. (a) and (b) Label Part Function A Squirrel-cage rotor Rotates as the magnetic field rotates B Cooling fan Helps prevent overheating by driving air through the motor C Electromagnetic coil Sets up the rotating magnetic field D Laminated electromagnetic Intensifies the strength of the magnetic field core E Shaft Allows the rotor to rotate; connects to the working part of the machine
5. The rotating magnetic field is achieved by connecting each of the three pairs of the motor’s field coils to a different phase of the mains electrical supply. The coils that make a pair are located on opposite sides of the stator and they are linked electrically. The magnetic field inside the stator rotates at the same frequency as the mains supply, that is, at 50 Hz.
6. The squirrel-cage rotor consists of a number of conducting bars made of either aluminium or copper. These are attached to two rings, known as end rings, at either end of the bars.
7. As the magnetic field rotates in the stator, it passes over the bars of the cage. The relative movement of the bars through the magnetic field creates a current in the bars.
8. The end rings connect the bars and allow a current to flow through the bars.
1 Physics 2: HSC Course, 2nd edition (Andriessen et al, 2003), Chapter 9 9. (a) Slip is the slower movement of the rotor compared with the magnetic field. (b) If the rotor is moving at the same speed as the magnetic field, there would be no current induced in the rotor and no force on the rotor.
10. The ‘lost power’ of induction motors is consumed in magnetising the working parts of the motor and in creating induction currents in the rotor.
11. Electrical energy is transformed into rotational kinetic energy of the rotor and the beaters. Some electrical energy is also transformed into heat and sound.
2 Physics 2: HSC Course, 2nd edition (Andriessen et al, 2003), Chapter 9