Single Phase Induction Motor Dr. Sanjay Jain Department Of EE/EX
Application :- The single-phase induction machine is the most frequently used motor for refrigerators, washing machines, clocks, drills, compressors, pumps, and etc.
The single-phase motor stator has a laminated iron core with two windings arranged perpendicularly. 1. One is the main and 2. The other is the auxiliary winding or starting winding
The single-phase induction motor operation can be described by two methods: Double revolving field theory; and Cross-field theory. Double revolving theory is perhaps the easier of the two explanations to understand Learn the double revolving theory only
Double revolving field theory A single-phase ac current supplies the main winding that produces a pulsating magnetic field. Mathematically, the pulsating field could be divided into two fields, which are rotating in opposite directions. The interaction between the fields and the current induced in the rotor bars generates opposing torque
Double revolving field theory Each of the rotating fields induces a voltage in the rotor, which drives current and produces torque. An equivalent circuit, similar to the equivalent circuit of a three phase motor, can represent each field The parameters of the two circuits are the same with the exception of the slip.
Double revolving field theory The two equivalent circuits are connected in series. Fig. shows the equivalent circuit of a singlephase motor in running condition. The current, power and torque can be calculated from the combined equivalent circuit using the Ohm Law The calculations are demonstrated on a numerical example
AC Motor::: An AC motor is an electric motor driven by an alternating current (AC).
Types of AC Motor::: [depending on the type of rotor used] 1. The first type is the induction motor or asynchronous motor: This type relies on a small difference in speed between the rotating magnetic field and the rotor to induce rotor current. 2.The second type is the synchronous motor: This type does not rely on induction and as a result can rotate exactly at the supply frequency or a sub-multiple of the supply frequency.
Types of Single Phase Motors Are::: [construction & method of starting] These motors are designed to operate from single phase supply, are manufactured in large no. of types for the use in home, offices, factories etc. small motors come in Kilo Watt power ratings. Induction Motors Repulsion Motors A.C. Series Motors Un-exited Synchronous Motors
Double Revolving Field Theory::: 1. If the rotor is given an initial rotation in either direction, the torque due to the rotating field acting in the direction of initial rotation coil will be more than that due to the other rotating field and the motor will develop a net positive torque in the same direction as the initial rotation. 2. Thus the motor will keep running in the direction of initial rotation. 3. According to this theory, any alternating quantity can be resolved into two rotating components which rotate in opposite directions and each having magnitude as half of the maximum magnitude of the alternating quantity.
3.In case of single phase induction motors, the stator winding produces an alternating magnetic field having maximum magnitude of Φ 1m. 4.According to double revolving field theory, consider the two components of the stator flux, each having magnitude half of maximum magnitude of stator flux i.e. (Φ 1m /2). 5. Both these components are rotating in opposite directions at the synchronous speed N s which is dependent on frequency and stator poles.
Let Φ f is forward component rotating in anticlockwise direction while Φ b is the backward component rotating in clockwise direction. The resultant of these two components at any instant gives the instantaneous value of the stator flux at the instant. So resultant of these two is the original stator flux.
fig shows the stator flux and its two components Φ f and Φ b. At start both the components are shown opposite to each other in the Fig.1(a). Thus the resultant Φ R = 0. This is nothing but the instantaneous value of the stator flux at start. After 90 o, as shown in the Fig. 1(b), the two components are rotated in such a way that both are pointing in the same direction. Hence the resultant Φ R is the algebraic sum of the magnitudes of the two components. So Φ R = (Φ 1m /2) + (Φ 1m /2) =Φ 1m. This is nothing but the instantaneous value of the stator flux at θ = 90 o as shown in the Fig 1(c). Thus continuous rotation of the two components gives the original alternating stator flux.
Both the components are rotating and hence get cut by the motor conductors. Due to cutting of flux, e.m.f. gets induced in rotor which circulates rotor current. The rotor current produces rotor flux. This flux interacts with forward component Φ f to produce a torque in one particular direction say anticlockwise direction. While rotor flux interacts with backward component Φ b to produce a torque in the clockwise direction. So if anticlockwise torque is positive then clockwise torque is negative.
At start these two torque are equal in magnitude but opposite in direction. Each torque tries to rotate the rotor in its own direction. Thus net torque experienced by the rotor is zero at start. And hence the single phase induction motors are not self starting.
Torque speed characteristics::: The two oppositely directed torques and the resultant torque can be shown effectively with the help of torque-speed characteristics.
Starting methods::: o single-phase capacitor-start motors o capacitor-run motors osplit-phase motors o shaded-pole motors o small poly phase induction motors.
Single Phase Induction Motor The single-phase motor starting torque is zero because of the pulsating single-phase magnetic flux. The starting of the motor requires the generation of a rotating magnetic flux similar to the rotating flux in a three-phase motor. Two perpendicular coils that have currents 90 outofphase can generate the necessary rotating magnetic fields which start the motor. Therefore, single-phase motors are built with two perpendicular windings.
The phase shift is achieved by connecting a resistance, an inductance, or a capacitance in series with the starting winding. Most frequently used is a capacitor to generate the starting torque.
Capacitor start induction motor:::
Capacitor run motors:::
Split phase motor:::
Shaded pole motors:::
Blocked Rotor test for Induction Motor::: Blocked rotor test is conducted on an induction motor. It is also known as short circuit test or locked rotor test or stalled torque test. From this test short circuit current at normal voltage, power factor on short circuit, total leakage reactance, starting torque of the motor can be found. The test is conducted at low voltage because if the applied voltage was normal voltage then the current flowing through the stator windings were high enough to over heat the winding and damage them. The blocked rotor torque test is not performed on a wound rotor motors because the starting torque can be varied as desired. However, blocked rotor current test is conducted on squirrel cage rotor motors. 0-10A 0-300V {some one can hold this shaft to block the rotation}
Method::: In the blocked rotor test, the rotor is locked A low voltage is applied on the stator terminals so that full load current flows in the stator winding. The current, voltage and power input are measured at this point. When the rotor is stationary the slip,s=1. The test is conducted at 1/4 th the rated frequency as recommended by IEEE. This is because the rotor's effective resistance at low frequency may differ at high frequency. The test can be repeated for different values of voltage to ensure the values obtained are consistent. As the current flowing through the stator may exceed the rated current, the test should be conducted quickly. By using the parameters found by this test, the motor circle diagram can be constructed.
No Load Test of Induction Motor::: By the name no load test it means that there is no load- that is load is zero. But it is exact opposite. No load means infinite load test. It is because in no load there is NO load, and no load means it is open circuit. Open circuit means infinite resistance. But that was the case in transformers, where you can open circuit the low voltage of transformer and obtain readings. But how will you do that in Induction motor? How can you make infinite resistance at load side? If slip=0, then Rl' will be infinite. And you can make slip zero by making synchronous speed Ns equal to actual speed N. So slip will be zero. Load resistance will be infinite. 0-10A 0-300V No load at output of motor
Method::: Connect the circuit. Supply the rated voltage to induction motor, keep it running. The current drawn by motor is quit low Take care of the voltmeter should be of voltage ratings of induction motor & the ratings of ammeter should be low because the current drawn by motor is very small. Take the readings of voltmeter, ammeter and wattmeter.