Chapter 18 Single-Phase Induction Motors

1. Chapter 18 Single-Phase Induction Motors Electro Mechanical System

2. Construction of a single-phase induction motor • Single-phase induction motors are very similar to 3-phase induction motors. They are composed of a squirrel-cage rotor (identical to that in a 3-phase motor) and a stator. • The stator carries a main winding, which creates a set of N, S poles. It also carries a smaller auxiliary winding that only operates during the brief period when the motor starts up. • The auxiliary winding has the same number of poles as the main winding has. Electro Mechanical System

3. Construction of a single-phase induction motor • The steps in winding a 4-pole, 36-slots stator. Starting with the laminated iron stator, paper insulators or slot liners inserted. • Main winding is laid in the slots; Next, the auxiliary winding is embedded so that its poles straddle those of the main winding. Electro Mechanical System

4. Construction • Each pole of the main winding consists of a group of four concentric coils, connected in series as shown below. • Adjacent poles are connected so as to produce alternate N, S polarities. The empty slot in the center of each pole and the partially filled slots are used for auxiliary winding. Electro Mechanical System

5. Magnetomotive force distribution • In order to optimize the efficiency, the magnetomotive force produced by each stator pole must be distributed sinusoidally. • That is the reason for using the special number of turns (l0, 20, 25, and 30) on the four concentric coils. • Let us examine the mmf created by one of the four poles when the concentric coils carry a peak current of, say, 2 amperes. • For example, the 25-turn coil in slots 2 and 8, produces an mmf of 25 X 2 = 50 amperes between these slots. • the 10-turn coil in slots 4 and 6 produces between these slots an mmf of 20 A. Electro Mechanical System

6. Magnetomotive force distribution Following table shows mmf distribution. • The distribution of these mmfs is illustrated. Total mmf produced in the middle of the pole is 60 + 50 + 40 + 20 = 170A & it drops off in steps on either side of center. • A smooth mmf having a perfectly sinusoidal distribution is shown. • Unlike 3-phase, the mmf of single phase does not rotate and remains fixed, but amplitude varies sinusoidally in time. Electro Mechanical System

7. Torque-speed characteristics • Suppose the rotor is locked in a 2-pole single-phase induction motor. If an ac voltage is applied to the stator. • The resulting current Is produces an ac flux s. The flux alternates back and forth but, unlike the flux in a 3-phase stator, no revolving field is produced. • The flux induces an ac voltage in the stationary rotor which, in turn, creates large ac rotor currents. • In effect, the rotor behaves like the short-circuited secondary of a transformer; consequently, the motor has no tendency to start by itself. Electro Mechanical System

8. Torque-speed characteristics • However, if we spin the rotor in one direction or the other, it will continue to rotate in the direction of spin. • As a matter of fact, the rotor quickly accelerates until it reaches a speed slightly below synchronous speed. • The acceleration indicates that the motor develops a positive torque as soon as it begins to turn. • Following diagram shows the typical torque-speed curve when the main winding is excited. Although the starting torque is zero, the motor develops a powerful torque as it approaches synchronous speed. Electro Mechanical System

9. Principle of operation • The principle of operation of a single-phase induction motor is complex, and may be explained by the cross-field theory. • As soon as the rotor begins to turn, a speed emf E is induced in the rotor conductors as they cut the stator flux s. • This voltage increases as the rotor speed increases. It causes currents Ir. to flow in the rotor bars facing the stator poles. These currents produce an ac flux r. Which acts at right angles to the stator flux s. Electro Mechanical System

10. Principle of operation • r does not reach its maximum value at the same time as s. In effect, r lags 90° behind s, due to the inductance of the rotor. • Combined action of s and r produces a revolving magnetic field, similar to a 3-phase motor. • The value of r increases with increasing speed, almost equal to s at synchronous speed. Electro Mechanical System

11. Principle of operation • The diagrams gives a snap­shot of the currents and fluxes created respectively by the rotor and stator, at successive intervals of time. • We assume that the motor is running far below synchronous speed, and so r is much smaller than s. By observing the flux in the successive pictures, it is obvious that the combination of s and r produces a revolving field. • The flux is strong horizontally and relatively weak vertically. Thus, the field strength at low speed follows the elliptic pattern shown . Electro Mechanical System

12. Synchronous speed • Like 3-phase motor; synchronous speed of single phase motor is given by: • ns = 120 f / p • where: • ns = synchronous speed [rpm] • f = frequency of the source [Hz] • p = number of poles • Motor turns at slightly less than synchronous speed • At full load slip is 3% to 5% for fractional horse power motor • Example: • Calculate the speed of 4-pole single phase motor. If slip at full load is 3.4% . Line frequency is 60Hz • ns = 120 f / p = (120 x 60)/4 = 1800 rpm • Speed n is given by: s = (ns – n)/ns • 0.034 = (1800 – n)/1800 • n = 1793 rpm Electro Mechanical System

13. Making a Motor • http://sci-toys.com/scitoys/scitoys/electro/electro.html#single • http://sci-toys.com/scitoys/scitoys/electro/electro2.html#double • http://sci-toys.com/scitoys/scitoys/electro/electro3.html#two_coil Electro Mechanical System