THREE-PHASE INDUCTION MOTOR CHAPTER 7 THREE-PHASE INDUCTION MOTOR Electrical Machines

It can be considered to be the cheapest motor. Three-phase induction motors are the most common and frequently encountered machines in industry It can be considered to be the cheapest motor. It is rugged and requires less maintenance. It is simple in design. It gives reliable operation. Its efficiency is very high. It is easy to control It runs at constant speed from zero to full load Electrical Machines

CLASSIFICATION OF INDUCTION MOTOR Depending on the rotor construction, induction motor can be classified into two categories: Squirrel-cage induction motor. Slip-ring induction motor or wound rotor induction motor. Depending on the number of phases it can be classified as: Single-phase induction motor Three-phase induction motor Electrical Machines

CONSTRUCTION The three basic parts of an AC motor are the rotor, stator, and enclosure. A stationary stator consisting of a steel frame that supports a hollow, cylindrical core core, constructed from stacked laminations, having a number of evenly spaced slots, providing the space for the stator winding Electrical Machines

CONSTRUCTION A revolving rotor composed of punched laminations, stacked to create a series of rotor slots, providing space for the rotor winding one of two types of rotor windings conventional 3-phase windings made of insulated wire 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. Electrical Machines

CONSTRUCTION Squirrel cage type: Wound rotor type: Rotor winding is composed of copper bars embedded in the rotor slots and shorted at both end by end rings Simple, low cost, robust, low maintenance Wound rotor type: Rotor winding is wound by wires. The winding terminals can be connected to external circuits through slip rings and brushes. Easy to control speed, more expensive. Electrical Machines

PRINCIPLE OF OPERATION When a 3 phase stator winding is connected to a 3 phase voltage supply, 3 phase current will flow in the windings, which also will induced 3 phase flux in the stator. These flux will rotate at a speed called a Synchronous Speed, ns. The flux is called as Rotating magnetic Field. Synchronous speed is given by the expression Where p = is the number of poles, and f = the frequency of supply Electrical Machines

PRINCIPLE OF OPERATION This rotating magnetic field cuts the rotor windings and produces an induced voltage in the rotor windings Due to the fact that the rotor windings are short circuited, for both squirrel cage and wound-rotor, and induced current flows in the rotor windings The rotor current produces another magnetic field A torque is produced as a result of the interaction of those two magnetic fields Electrical Machines

SLIP The rotor speed of an Induction machine is different from the speed of Rotating magnetic field. The % difference of the speed is called slip. Where; ns = synchronous speed (rpm) nr = mechanical speed of rotor (rpm) If the rotor runs at synchronous speed, s = 0. If the rotor is stationary, s = 1 Electrical Machines

f is the supply frequency, E2 is the standstill e.m.f. Effect of slip on rotor frequency Effect of slip on rotor induced e.m.f. Effect of slip on rotor resistance, reactance and impedance Where fr is the rotor frequency, Er2 is the rotor emf, R2, X2 and Z2 is the rotor resistance, reactance and impedance respectively. f is the supply frequency, E2 is the standstill e.m.f. Electrical Machines

TORQUE EQUATION OF A THREE-PHASE INDUCTION MOTOR The torque of a three-phase induction motor depends on the following factors: Rotor current (I2) Power factor of the rotor circuit Flux which links with the rotor (φ) or Electrical Machines

Torque Equation for Induction Motor in Running Condition Where, Electrical Machines

TORQUE SLIP CHARACTERISTICS When slip s = 0, Nr = Ns. Under this condition, the motor stops. So the torque (T) at this value of s is zero. This shows that the torque slip characteristics starts from the origin. For smaller values of slip, the torque is directly proportional to the slip. For larger values of slip, the torque is inversely proportional to slip. Electrical Machines

TORQUE SLIP CHARACTERISTICS Electrical Machines

RATIO BETWEEN FULL-LOAD TORQUE AND MAXIMUM TORQUE Where Tfl is the full load torque Tmax is the maximum torque sfl is the slip at full load and sm is the slip at maximum torque. Electrical Machines

RATIO BETWEEN STARTING TORQUE AND MAXIMUM TORQUE Where Tst is the starting torque Tmax is the maximum torque, sm is the slip at maximum torque. Electrical Machines

EFFECT OF ROTOR RESISTANCE ON TORQUE–SLIP CHARACTERISTIC Electrical Machines

Equivalent Circuit of Induction Motor Equivalent circuit with Rotor open circuited Electrical Machines

Equivalent Circuit of Induction Motor Equivalent circuit with Rotor short circuited Electrical Machines

Equivalent Circuit of Induction Motor Equivalent circuit with electrical equivalent of mechanical load Electrical Machines

No-load Test Electrical Machines

No-load Test The motor is allowed to run freely. This test determines the rotational and core loss of the motor . Electrical Machines

Block Rotor Test Electrical Machines

Block Rotor Test In this test, the rotor is blocked from rotating. This test gives the total winding resistance and leakage reactance of the motor. Electrical Machines

STARTING OF THREE-PHASE INDUCTION MOTOR There are several methods of starting squirrel-cage induction motors, which include: i) Direct on-line starting. ii) Autotransformer starting iii) Star–delta starting Electrical Machines

Direct on-line (DOL) starter Electrical Machines

Autotransformer starter Electrical Machines

Star-delta starter Electrical Machines

SPEED CONTROL From the stator side, the speed of an induction motor can be controlled by the following methods: i) V/f method of controlling the speed of induction motor ii) Speed control by changing the supply voltage. iii) Speed control by changing the number of poles. iv) Speed control by changing the rheostat connected with the stator terminals. From the rotor side the speed of an induction motor can be controlled by the following methods: i) Speed control by changing the rheostat connected with the rotor terminals. ii) Cascade control Electrical Machines

BRAKING The braking methods for induction motor are as follows: i) Regenerative braking of induction motor ii) Braking by plugging iii) Dynamic braking Electrical Machines