Transcript Document
Induction Machines • • • • • • • • Simple! Stator winding is similar to that of synchronous machine! Mostly used as motors. No separate excitation is required for the rotor. The rotor typically consists of one of two arrangements: – Squirrel cage. – Wound rotor. Induction motor operates by virtue of currents induced from the stator field in the rotor. Speed of rotation n is below the synchronous speed ns. See eq. 17.73 for the slip. Solve Ex 17.12; 17.13; 17.14 ns n s ns 0 Squirrel cage induction motor; (b) conductors in rotor; (c) photograph of squirrel cage induction motor; (d) views of Smokin’ Buckey motor: rotor, stator, and cross section of stator (Courtesy: David H. Koether Photography) Figure 17.37 1 Circuit model for induction machine Figure 17.38 2 Equivalent circuit of an induction machine Figure 17.40 3 Performance curve for induction motor Figure 17.42 4 General configuration of adjustable-frequency drive Figure 17.45 5 Single-Phase Motors • Single-phase motors are used mostly to operate home appliances such as air conditioners, refrigerators, pumps, and fans. • They are designed to operate on 120 V or 240 V. • They range in capacity from fractional horsepower to several horsepower depending on the application. • Voltage is induced in the rotor as a result of magnetic induction, and a magnetic field is produced around the rotor. This field will always be in opposition to the stator field (according to Lenz’s law). 6 Split-Phase Motors Split-phase motors use inductance, capacitance, or resistance to develop a starting torque. These motors split the current flow through two separate windings to simulate a two-phase power system. A rotating magnetic field can be produced with a twophase system. • There are three types of split-phase motors depending on the means of starting: – The resistance-start induction-run motor. – The capacitor-start induction-run motor. 7 The Resistance-Start Induction-Run Motor • The out-of-phase condition between start and run winding current is caused by the start winding having more resistance than the run winding. • The amount of starting torque produced is determined by: – The strength of the magnetic field of the stator. – The strength of the magnetic field of the rotor. – The phase angle difference between current in the start winding and current in the run winding (maximum torque is achieved when these two currents are 90o out of phase with each other). 8 The main winding has a high inductance and a low resistance. The current lags the voltage by a large angle. The starting winding have a low inductance and a high resistance. The current lags the voltage by a smaller angle. Suppose the current in the main winding lags the voltage by 80°. The current in the auxiliary winding lags the voltage by 40°. The currents are, hence, out of phase by 40°, which is enough to generate rotating field. Main Winding, L and R Starting Winding Rotor Main Supply R 9 The Capacitor-Start Induction-Run Motor • The stator consists of the main winding and a starting winding. The starting winding is connected in parallel with the main winding and is placed at right angles to it. • A 90-degree electrical phase difference between the two windings is obtained by connecting the auxiliary winding in series with a capacitor and a starting switch. When the motor is energized, the starting switch is closed. This places the capacitor in series with the auxiliary winding. • Now we will have an RC circuit (Starting winding and the capacitor) and an RL circuit (main winding). The currents in each winding are therefore 90° out of phase, so are the magnetic fields that are generated. 10 When nearly full speed is obtained, a centrifugal device (the switch) cuts out the starting winding. The motor then runs as a single-phase induction motor. Main Winding Rotor Power Supply Switch Starting Winding Capacitor 11