Francisco J. Perez-Pinal, Ph.D.

Profesor

Motores de CA

II

• A three phase induction machines have two parts – A stationary stator

– A rotating rotor

Stator consists of steel frame with a hollow cylindrical core •

made of laminated steel. There are slots made in the core, into which the windings are imbedded.

• The core of the rotor is also laminated and it also has winding imbedded into it. There are two types of windings used for rotors of induction motors

•

– Conventional winding made of insulated wire induction motors)

(wound rotor

– Squirrel cage winding (squirrel cage winding)

Wound Rotor Wound rotor has three phase windings, •

uniformly distributed around the rotor. The windings are usually connected in Y.

Squirrel Cage Motor The rotor winding consists of bare copper or •

aluminum bars slightly longer than the rotor that are pushed into the slots on the rotor

The bars are welded on each end to a copper ring, which creates a short circuit.

Principle of Operation Consider several parallel conductors connected by two conducting • bars . A permanent magnet moves over the conductors with a constant speed so conductors.

that a magnetic field B is swept over the Permanent Magnet

Parallel conductors

Conducting connecting bars

1. 2.

3.

A voltage Blv is induced in each conductor The voltage produces current I in the bar

The current in the conductor just below the magnet interacts with the magnetic field, which produces a mechanical force on the conductor.

4. The force pushes the conductor along with the magnetic field

Permanent Magnet

Parallel conductors

Conducting connecting bars

As the conductors increase their speed, they will move along with the magnet and the induced voltage in the conductors will decrease, which means that the current will also decrease and the force on the conductors will decrease.

Permanent Magnet

Parallel conductors

Conducting connecting bars

• In a squirrel cage motor the ladder is closed on itself and forms a “squirrel cage”. The magnetic field of a permanent magnet is replaced by the magnetic field created by the stator windings

•

• the result is that the rotor of the induction motor rotates field

at almost the same speed as the magnetic

The rotating field of the stator • Elementary stator

having terminals A, B, C connected to a 3-phase source (not shown). Currents flowing from line to neutral are

•

considered to positive.

be

• The windings are spaced 120° from each other. They are connected in Y and present a balanced load to a power supply.

• Flow of electric current through the coils creates magnetic and magnetic flux.

field

• The magnetic flux direction is given by a right hand rule

As time goes by, we can determine the instantaneous value of current in each coil and direction of magnetic flux created by the currents

Flux pattern at instant 1.

The six poles produce magnetic field with one broad magnetic north pole and one broad magnetic south pole

SOUTH

Flux pattern at instant 1. NORTH

Flux pattern at instant 2.

• If we follow this through we find that the field makes one turn during one cycle (or in one period).

If the frequency is 60 Hz, the period is 1/60 s and the field rotational speed in our case is 60 revolutions per second.

Because the speed is synchronized with the frequency, it is called synchronous speed.

The magnetic field in our example had two poles. The number of poles of a motor can be increased by increasing number of stator windings and connecting them so that several poles are created

•

•

•

• The synchronous speed is given by

Where ns = synchronous speed in revolutions/min f p

= frequency in Hz = number of poles

Summary of Operation of S.C.I.M

• When three phase induction motor, a

voltages are applied to a stator of an rotating magnetic field is set up

The interaction of the rotor currents and of the stator magnetic field creates a mechanical force which pulls the rotor along with the magnetic field of the stator.

The rotating field induces voltage in the rotor winding. The rotor windings are shorted out and therefore the

rotor currents are large.

Starting Characteristics of S.C.I.M

• The rotation of the magnetic field is created the moment the stator is connected to the power

As the rotor speeds up, the relative velocity between the rotor and the magnetic field decreases, the induced voltage in the rotor gets smaller and the rotor current decreases.

•

• The rotor speed never completely catches up to the

magnetic field

supply. The rotor starts rotating from a standstill.

• The rotor speed is always a bit less than that of the magnetic field; just sufficient to produce force to overcome the braking torque of the motor (braking torque is due to friction) The percent difference between the synchronous •

speed and the rotor speed is called slip (s)

• Slip is less than 0.1% if the motor runs at no load Slip is 1 if the rotor is locked

• Where

s = slip ns = synchronous speed

n = rotor speed

Motor Under Load •

•

Assume a motor is running with no load. If we apply mechanical load to the shaft, the motor will start slowing This will result in higher current in the rotor.

down higher induced voltage in rotor, rotor, and higher torque acting on

•

• This will continue until a new equilibrium is reached where the torque on the rotor equals torque of the load. At this moment, the speed will become constant

Under normal loads, motors run with slip less than

0.5%

•

•

Characteristics of Squirrel Cage Induction Motors

Motor at No Load •At no load, the current to the stator is between 0.3 and 0.5 p.u. of full load current

•the no-load current is similar to the exciting current of a transformer. It consists of –magnetizing current that creates the rotating magnetic field –current that supplies the real power losses of the motor

•real power losses are due to heating of the cores of the stator and rotor, friction, and windage

•(copper losses are negligible at no-load)

•most of the current is used to create the magnetic field, therefore the power factor is low (0.05 to 0.2 or 5% to 20%)

•since there is no output power, the efficiency is 0

Nameplate Data

Nameplate Data

•Rated Horsepower is the amount of power the motor can deliver at the shaft on continuous basis without exceeding its rated temperature.

•Rated Voltage for three phase motors is always given as line-to-line voltage. Rated voltage for induction motors is usually about 5% less than the nominal system voltage

•Rated Current is the line current the motor should draw when it operates at rated load, rated speed , and rated voltage

–rated current and rated voltage can be used to calculate the apparent power

•Rated RPM is the expected speed of the machine when operating at rated load and rated voltage and frequency

•Rated Frequency is the frequency at which the motor is designed to operate.

•Time rating: Standard motors are rated for continuous duty (24/7) at their rated load and maximum ambient temperature.

•Insulation class and rated ambient temperature: A critical factor in life of a motor is the maximum temperature that occurs at the hottest spot in the motor. The allowable maximum ambient temperature, usually 40°C (104°F), and the class of insulation used in the design of the motor. Available classes are B, F, and H.

•NEMA design letter NEMA defines five standard designs for three phase induction motors – each has a different characteristic. The five types are A, B, C, D, and E.

Active power flow in a 3-phase induction motor