Electric Machine Induction Motor By Dr. Shorouk Ossama

Introduction In a DC motor this power is supplied to the armature directly from a DC source. In an induction motor this power is induced in the rotating device. An Induction Motor (Synchronous Motor) is a type of alternating current motor.

An induction motor is sometimes called a rotating transformer because the stator (stationary part) is the primary side of the transformer and the rotor (rotating part) is the secondary side. No external electrical connections to the rotor. Its name is derived that the current in the rotor is induced by the magnetic field

Three-phase induction motors are the most common machines in industry because: Simple design, rugged, low-price, easy maintenance. Wide range of power ratings: fractional horsepower to 10 MW. Run as constant speed from no-load to full load Its speed depends on the frequency of the power source.

Types of Induction Motor The 3-phase AC induction motors are classified either as squirrel cage or wound- rotor motors. 3-phase motors are self-starting or use no capacitor, start winding or other starting device but the single phase induction motor is required to have a starting mechanism.

Production of a Rotating Magnetic Field When the three-phase supply is connected to symmetrical three-phase windings, the current flowing in the windings produce a magnetic field. This magnetic field is constant in magnitude and rotates at constant speed as shown below, and is called the Synchronous Speed.

Balanced three phase windings, i. e Balanced three phase windings, i.e. mechanically displaced 120 degrees from each other, fed by balanced three phase source. A rotating magnetic field with constant magnitude is produced, rotating with a speed.

Construction of a Three-Phase Induction Motor

Stator: (Stationary Part) is constituted by silicon steel alloy or by steel laminations, insulated one from the other.

Rotor: (Rotating part) Conventional 3-phase windings made of insulated wire (wound-rotor) similar to the winding on the stator Aluminum bus bars shorted together at the ends by two aluminum rings, forming a squirrel-cage shaped circuit (squirrel-cage)

Squirrel cage rotor Wound rotor Notice the slip rings

Faults in Induction Motor Induction motors and drive systems are subject to many different types of faults such as: over-current and over-voltage. The need of induction motors condition monitoring has increased recently. Early detection of incipient faults and correct diagnosis result in fast unscheduled maintenance and short downtime for the motor drive system.

Mechanical Faults in Induction Motor Bearing Failure:

Eccentricity: When eccentricity becomes large, the resulting unbalanced radial forces can cause stator to rotor rub, and this can result in damage of the stator and rotor.

The symptoms caused by eccentricity in induction motor can be summarized as following:  (1) Mechanical Vibration; (2) Asymmetry and deviation of air gap flux, voltages and line currents; (3) Increasing torque and speed variations; (4) Decreasing average torque; (5) Increasing losses and decreasing efficiency; (6) Rising temperature.

Principle of Operation of a Three-Phase Induction Motor When a three-phase supply is connected to the stator windings. A rotating magnetic field is produced. As the magnetic flux cuts a bar on the rotor, an e.m.f. is induced in it. The magnetic field associated with this current flowing in the bars interacts with the rotating magnetic field and a force is produced.

Slip Speed = ns ― nr ( rpm or rev/s ) The force exerted by the rotor bars causes the rotor to turn in the direction of the rotating magnetic field. The difference between the rotor speed, nr, and the synchronous speed, ns, is called the Slip Speed. Slip Speed = ns ― nr ( rpm or rev/s )

The ratio (ns ― nr)/ ns is called the Fractional Slip or just the Slip, s, and is usually expressed as a percentage. Thus Slip , S = (ns ― nr)/ ns Typical values of slip between no load and full load are about 4 to 5 per cent for small motors and 1.5 to2 per cent for large motors.

Rotor E.M.F. and Other Parameters When an induction motor is stationary, the stator and rotor windings form the equivalent of a transformer. when running, rotor e.m.f. per phase = Er = SE2 rotor e.m.f. per phase

Fr = (ns ― nr) p =( ns ― nr) p × (ns / ns) Rotor Frequency The rotor e.m.f. is induced by an alternating flux and the rate at which the flux passes the conductors is the slip speed. Thus the frequency of the rotor e.m.f. is given by: Fr = (ns ― nr) p =( ns ― nr) p × (ns / ns) However (ns ― nr)/ns is the slip S and (ns p) is the supply frequency f , Hence, fr = S f

Rotor Resistance The rotor resistance R2 is unaffected by frequency or slip, and hence remains constant. Xr = 2π fr L = 2π (S f ) L = S (2π f L) Xr = S X2

Rotor Impedance Rotor impedance per phase, when running: Z r = At standstill, slip S = 1, then: Z r = Rotor Current At standstill (S=1), starting current: I2 = E2 / Z2 Rotor current, when running: Ir = Er / Zr

Losses and Power Stages the various losses during the energy conversion are: 1. Fixed Losses Stator iron loss (ii) Friction and windage loss 2.Variable Losses (i) Stator copper loss (ii) Rotor copper loss

Pout = Pm ­ Friction and windage loss Stator Input, Pi = P1 = Stator output + stator losses Rotor Input, P2 = Pr = Stator output Mechanical Power, Pm = Pr ­ Rotor Cu loss Mechanical Power at Shaft, Pout = Pm ­ Friction and windage loss

Example 1 The power supplied to a three-phase induction motor is 32 kW and the stator losses are 1200W. If the slip is 5 per cent, determine: The rotor copper loss, The total mechanical power developed by the rotor, The output power of the motor if friction and windages losses are 750W,

Input power to rotor = = Stator input power - Stator losses = 32kW ‒ 1.2 Kw = 30.8 Kw So, Rotor copper losses = S P2 = 0.005 × 30.8 kW= 1.54kW b) Total mechanical power developed by the rotor = rotor input power ‒ rotor losses = 30.8 ‒ 1.54 = 29.26 kW c) Output power of motor = power developed by the rotor ‒friction and windage losses = 29.26 ‒ 0.75 = 28.51kW (d) Efficiency of induction motor,

Speed Control of 3-phase Induction Motor N = (1 ‒ s) N s = (1‒ s)   The speed N of an induction motor can be varied by changing: Supply frequency f Number of poles P on the stator Slip (S)

Starting of 3-phase Induction Motor At starting, the voltage induced in the induction motor is maximum. Since the rotor impedance is low, the rotor current is large. This large rotor current is reflected in the stator because of transformer action. This results in high starting current (4 to 10 times the full-load current)

Therefore, it is necessary to reduce the magnitude of stator current at starting and several methods are depending upon the size of the motor and the type of the motor. The common methods used to start induction motors: Direct-on-line starting Stator resistance starting Autotransformer starting Star-delta starting Rotor resistance starting  

Induction Motor Ratings The nameplate of a 3-phase induction motor provides the following information: - Horsepower - Line voltage - Line current - Speed - Frequency - Temperature rise

Thanks