# Electric Machine

August 5, 2024 | Author: Anonymous | Category: N/A

#### Description

Introduction • One of energy can be obtained from the other form with the help of converters. • Converters that are used to continuously translate electrical input to mechanical output or vice versa are called electric machines. • The process of translation is known as electromechanical energy conversion.

Electrical system emf, i

Electric Machine

Mechanical system F, n Motor

Energy flow Generator

• An electrical machine is link between an electrical system and a mechanical system. • Conversion from mechanical to electrical: generator • Conversion from electrical to mechanical: motor

Electrical Machines DC machine

AC MACHINE

Rotating machine Synchronous machine

Static m/c

Induction machine

• Machines are called AC machines (generators or motors) if the electrical system is AC. • DC machines (generators or motors) if the electrical system is DC.

Electrical system e, i

Coupling magnetic fields

Mechanical system T, n

Two electromagnetic phenomena in the electric machines: • When a conductor moves in a magnetic field, voltage is induced in the conductor. • When a current-carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force.

AC Rotating Machines

Electrical Machines DC machine

AC machine Synchronous machine

Induction machine

AC Motors

Basic Idea • A motor uses magnets to create motion. • The fundamental law of all magnets: – Opposites attract – Likes repel.

• Inside an electric motor, these attracting and repelling forces create rotational motion

Basic Idea Magnetic field of a straight conductor • The magnetic field lines around a long wire which carries an electric current form concentric circles around the wire. • Right hand rule-1

Basic Idea Magnetic field of a circular conductor • Right hand rule-1 gives the direction of the magnetic field inside and outside a current-carrying loop.

Basic Idea Magnetic field of a coil of wire • A solenoid is a long coil of wire • The field inside a solenoid can be very uniform and very strong. • The field is similar to that of a bar magnet.

Basic Idea • The use of soft metal increases the magnetic field strength • Use right hand rule-2 / eye rule to determine direction of magnetic field in a coil

Basic Idea • Fleming’s left hand rule for motors • Don’t be confused with Fleming’s right hand rule for generator

Working Principle Elementary AC motor • Consider a rotor → formed by permanent magnet. • Consider a stator → formed by coil of conductor to create AC electromagnetic field

Working Principle • An AC Current flowing through conductors energize the magnets and develop N and S poles. • The strength of electromagnets depends on current. • First half cycle current flows in one direction. • Second half cycle it flows in opposite direction.

Working Principle • Consider the AC voltage at 0 degrees, then, no current will flow, and there is no magnetism.

Supplied voltage Initial position of the rotor

Working Principle • As voltage increases, current starts to flow and electromagnets gain strength and North and South poles appear. • The rotor magnet is pushed CW, and the rotor and motor starts to rotate.

Working Principle • When voltage decreases, the current decreases also, the electromagnet loses the strength, and when V=0 there is no magnetism.

Working Principle • Now, AC voltage builds up as part of the negative cycle. • Then, current flows in opposite direction, and the magnets reverse polarity. • Therefore, the CW rotation continues.

AC Motor Rotation

Limitation of the Elementary Motor • The initial position of the rotor determines the direction of the motor rotation.

Practical AC Motor • By adding another pair of electromagnets the limitation mentioned before is removed. • Example: Two electromagnets = Vertical & Horizontal

• Three phase system has three electromagnets

Practical AC Motor

Practical AC Motor Counter clockwise rotation

Practical AC Motor • We can see that the poles rotate around the circumference of the motor. • The rotor, no matter how it is positioned at rest, will be locked-in with the magnetic field and will turn in one direction only. • (Same rotation as the poles).

Induction Motor • Most AC motors are induction motors • Induction motors are favored due to their ruggedness (no brush), simplicity and cheap. • 90% of industrial motors are induction motor. • Application – (1-phase): washing machines, refrigerators, blenders, juice mixers, stereo turntables, etc. – (2-phase) induction motors are used primarily as servomotors in a control system. – (3-phase): pumps, compressors, paper mills, textile mills, etc.

Induction Motor • The single-phase induction motor is the most frequently used motor in the world • Most appliances, such as washing machines and refrigerators, use a single-phase induction machine • Highly reliable and economical

Induction Motor • For industrial applications, the three-phase induction motor is used to drive machines • Large three-phase induction motor. (Courtesy Siemens). Housing

Motor

Construction of Induction Motor • An induction motor is composed of a rotor, (armature) • A stator containing windings connected to a poly-phase energy source • The pair of coils correspond to the phases of electrical energy available. • Each pair connected in series creating opposite poles: – 1 pole for North and 1 pole for South.

Induction Motor

Stator frame showing slots for windings.

Stator with (a) 2-phase and (b) 3-phase windings.

Induction Motor • It has a stator and a rotor like other type of motors. • 2 different type of rotors: – Squirrel-cage winding, – Wound-rotor

• Both three-phase and single-phase motors are widely used. • Majority of the motors used by industry are squirrel-cage induction motors • A typical motor consists of two parts: – An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, – An inside rotor attached to the output shaft that is given a torque by the rotating field.

Squirrel-cage Rotor • Rotor is from laminated iron core with slots. • Metal (Aluminum) bars are molded in the slots instead of a winding. • Two rings short circuits the bars.–Most of single phase induction motors have SquirrelCage rotor. • One or 2 fans are attached to the shaft in the sides of rotor to cool the circuit.

Wound Rotor • It is usually for large 3 phase induction motors. • Rotor has a winding the same as stator and the end of each phase is connected to a slip ring. • Three brushes contact the three slip-rings to three connected resistances (3-phase Y) for reduction of starting current and speed control.

• Compared to squirrel cage rotors, wound rotor motors are expensive and require maintenance of the slip rings and brushes, so it is not so common in industry applications • Wound rotor induction motor was the standard form for variable speed control before the advent of motor

Slips • It is virtually impossible for the rotor of an AC induction motor to turn at the same speed as that of the rotating magnetic field. • If the speed of the rotor were the same as that of the stator, no relative motion between them would exist, and there would be no induced EMF in the rotor. • Without this induced EMF, there would be no interaction of fields to produce motion. The rotor must, therefore, rotate at some speed less than that of the stator if relative motion is to exist between the two. • The percentage difference between the speed of the rotor and the speed of the rotating magnetic field is called slip. • The smaller the percentage, the closer the rotor speed is to the rotating magnetic field speed.

Slips NS  N R SLIP   100% NS where NS : synchronous speed or the rotating magnetic field (rpm) NR : rotor speed (rpm)

The synchronous speed (NS) of a motor is given by:

120 f NS  NP

where F : frequency of the rotor current (Hz) P : number of poles

Example Problem A two pole, 60 Hz AC induction motor has a full load speed of 3554 rpm. What is the percent slip at full load? NP

• • • • • •

120*60/p=3554 3554*P=7200 P=7200/3600 P=2 Ns=3600, Nr= 3554 Ns-Nr/Ns= (3600-3554)/3600=0.001

Torque • Torque is a rotational force. • The torque of an AC induction motor is dependent upon the strength of the interacting rotor and stator fields and the phase relationship between them.

T  K Φ I R cosθ R where T : torque K: constant Φ: stator magnetic flux (Wb) IR : rotor current (A) cos θR : power factor of rotor

Voltage and frequency induced in the rotor • The voltage and frequency induced in the rotor both depend on the slip. They are given by the following equation. • Ns-Nr • f/p-f’/p=s f2 = s f

E2 = s Eoc (approx.) f2 = frequency of the voltage and current in the rotor [Hz] f = frequency of the source connected to the stator [Hz] s = slip E2 = voltage induced in the rotor at the slip s Eoc = open-circuit voltage induced in the rotor when at rest [V]

• • • • • • •

(Ns-Nr)/Ns=S Ns-SNs=Nr (1-S)Ns=Nr Ns=120f/P For rotor F=NrP/120=(1-s)NsP/120=(1-s)P120f/p120 F=(1-s)f

• • • • • •

N=120f/p Nr=120F/p S=Ns-Nr/Ns F=sf F=(ns-nr/ns)f F= (

Active Power in a Induction Motor

Efficiency () =

Poutput Pinput

Example 1 • Calculate the synchronous speed of a 3-phase induction motor having 20 poles when it is connected to a 50 Hz source. Source frequency = 50 Hz,

Synchronous speed ns =

number of poles = 20 120 f p 120 x 50 = 20

ns = 300 r/min

Example 2 • A 0.5 hp, 6-pole induction motor is excited by a 3-phase, 60 Hz source. If the full-load is 1140 r/min, calculate the slip. Source frequency = 60 Hz,

number of poles = 6

Full load/rotor speed = 1140 r/min Synchronous speed ns =

120 f p 120 x 60 = 6

ns = 1200 r/min

Induction Motor Slip speed: ns – nR = 1200 – 1140 = 60 r/min Slip: s = (ns - nR) / ns = 60/1200 = 0.05 or 5%

Example 3 A single phase, 4 poles induction motor gives the following data: Output 373 W ; 230 V Frequency : 50 Hz., Input current 2.9 A Power factor: 0.71 ; Speed: 1410 r.p.m. a) Calculate the efficiency of the motor b) Determine the slip of the motor when delivering the rated output