Single Phase Induction Motor By Nafees Ahamad Asstt
Single Phase Induction Motor By: Nafees Ahamad, Asstt. Prof. , EE Deptt, DIT University, Dehradun © Nafees Ahamad
Actual 1 -phase induction motor Ceiling fans © Nafees Ahamad
Actual 1 -phase induction motor Ceiling fans: Internal structure Cage rotor © Nafees Ahamad
Actual 1 -phase induction motor Other single phase induction motor: © Nafees Ahamad
Actual 1 -phase induction motor Some other single phase induction motor: © Nafees Ahamad
Construction is similar as that of 3 -phase squirrel induction motor A single phase induction motor has two main parts 1. Stator § Similar to 3 -phase induction motor § Consisting of a steel frame that supports a hollow, Laminated cylindrical core § Core, constructed from stacked laminations (why? ), having a number of evenly spaced slots, providing the space for the stator winding. © Nafees Ahamad
Construction… L N Frame Or Yoke 1 phase Stator winding © Nafees Ahamad Stator Teeth Core (Laminated) Schematic Diagram of stator
Construction… 2. Rotor § Squirrel cage rotor is used § Aluminum bus bars shorted together at the ends by two aluminum rings. © Nafees Ahamad
Construction… End Ring Al or Cu bars End Ring Squirrel cage rotor © Nafees Ahamad
Construction… Constructional representation of rotor and stator: © Nafees Ahamad
Construction 3. Shaft: § Mild steel shaft with maximum breaking strength. 4. Bearings: § Ball bearing or roller bearing. It is used in vacuum cleaner, fans, washing machine, centrifugal pump, blowers, washing machine, small toys etc. © Nafees Ahamad
Principle When stator winding is fed from a single phase supply, an alternating (pulsating) field is produced. Due to this alternating flux an alternating EMF (current) is induced in the rotor conductors Now when a current carrying conductor is place in magnetic filed it experiences a force. But after each half cycle the direction of induced current is change and hence the direction of force (torque) is changed after each half cycle. © Nafees Ahamad
Principle… So pulsating flux acting on stationary squirrel cage rotor can’t produce rotation and therefore 1 -phase induction motor is not self starting*. However, if the rotor in such machine is given initial start by hand or otherwise in either direction, then motor starts rotating in that direction. To make it self-starting, it can be temporarily converted into a two-phase motor while starting. This can be achieved by introducing an additional 'starting winding' also called as auxiliary winding. © Nafees Ahamad *Above peculiar behavior can also be explained by ‘Double filed revolving theory’.
Principle… Hence, stator of a single phase motor has two windings: (i) Main winding and (ii) Starting winding (auxiliary winding). These two windings are connected in parallel across a single phase supply and are spaced 90 electrical degrees apart. Phase difference of 90 degree can be achieved by connecting a capacitor in series with the starting winding. © Nafees Ahamad
Principle… Hence the motor behaves like a two-phase motor and the stator produces revolving magnetic field which causes rotor to run. Once motor gathers speed, say upto 80 or 90% of its normal speed, the starting winding gets disconnected form the circuit by means of a centrifugal switch, and the motor runs only on main winding. © Nafees Ahamad
Principle. . © Nafees Ahamad
Double Revolving Field Theory This theory is based on the idea that pulsating field (induced in 1 -phase IM) can be resolved into two components of half its amplitudes & rotating in opposite direction with synchronous speed (This is called as Ferrari’s principle). This is shown by a diagram on next slide © Nafees Ahamad
Double Revolving Field Theory … 1 -pase supply L N Main switch Stator field winding N Forward flux © Nafees Ahamad Rotor Backward flux
Double Revolving Field Theory … © Nafees Ahamad
Double Revolving Field Theory … © Nafees Ahamad
Double Revolving Field Theory … So now torque-slip characteristics of 1 -phase induction motor will be Torque s=2 Torque due to forward flux © Nafees Ahamad Ne t To rqu e Torque due to forward flux s=1 s=0
Double Revolving Field Theory … It is clear from the previous figure that net torque is zero at time of starting ( at s = 1). Hence 1 -phase induction motor will not be self starting. If a push is given in either direction, it will results in net torque and motor will rotating in that direction. © Nafees Ahamad
Type of single phase induction motors The single phase induction motors are made self starting by providing an additional flux by some additional means( Additional winding ). Now depending upon these additional means the single phase induction motors are classified as: 1. Resistance start induction motor 2. Capacitor start induction motor 3. Capacitor start capacitor run induction motor. 4. Permanent capacitor motor 5. Shaded pole induction motor. © Nafees Ahamad
Thanks © Nafees Ahamad
- Slides: 24