Introduction to Electrical Machines

Contents Basic construction and principles DC machines Synchronous machines Induction machines

Objectives When you have studied this chapter, you should: have an understanding of electrical machines construction. understand the principles of DC machines. understand the principles and application of synchronous machines. understand the principles and application of induction machines.

1. Basic Construction                                                  

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.

Conversion from mechanical to electrical: generator Electrical system Mechanical system Electric Machine e, i T, 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

DC machines (generators or motors) if the electrical system is DC. Electrical Machines DC machine AC machine Synchronous machine 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.

Coupling magnetic fields Electrical system Mechanical system e, i 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.

Electric Machines Basic Structure The structure of an electric machine has two major components, stator and rotor, separated by the air gap. • Stator: Does not move and normally is the outer frame of the machine. • Rotor: Is free to move and normally is the inner part of the machine. Both rotor and stator are made of ferromagnetic materials.

DC Machines Construction

Induction Machines Construction

Synchronous Machines Construction

2. DC Machines

3. Synchronous Machines

Classification of AC Rotating Machines Synchronous Machines: •Synchronous Generators: A primary source of electrical energy •Synchronous Motors: Used as motors as well as power factor compensators (synchronous condensers) Asynchronous (Induction) Machines: •Induction Motors: Most widely used electrical motors in both domestic and industrial applications. •Induction Generators: Due to lack of a separate field excitation, these machines are rarely used as generators.

Synchronous Machine Unlike induction machines, the rotating air gap field and the rotor rotate at the same speed, called the synchronous speed. Synchronous machines are used primarily as generators of electrical power, called synchronous generators or alternators. They are usually large machines generating electrical power at hydro, nuclear, or thermal power stations. Application as a motor: pumps in generating stations, electric clocks, timers, and so forth where constant speed is desired.

Synchronous Machines View of a two-pole round rotor generator and exciter

Synchronous Machine Round Rotor Machine •The stator is a ring shaped laminated iron-core with slots. •Three phase windings are placed in the slots. •Round solid iron rotor with slots. •A single winding is placed in the slots. Dc current is supplied through slip rings.

Round Rotor Machine

Synchronous Machine Salient Rotor Machine •The stator has a laminated iron-core with slots and three phase windings placed in the slots. •The rotor has salient poles excited by dc current. •DC current is supplied to the rotor through slip-rings and brushes.

Salient Rotor Machine

Synchronous Generator Principle of Operation 1) From an external source, the field winding is supplied with a DC current -> excitation. 2) Rotor (field) winding is mechanically turned (rotated) at synchronous speed. 3) The rotating magnetic field produced by the field current induces voltages in the outer stator (armature) winding. The frequency of these voltages is in synchronism with the rotor speed.

Parallel Operation of Synchronous Generator Generators are rarely used in isolated situations. More commonly, generators are used in parallel, often massively in parallel, such as in the power grid. The following steps must be adhered to: •when adding a generator to an existing power grid: 1) RMS line voltages of the two generators must be the same. 2) Phase sequence must be the same. 3) Phase angles of the corresponding phases must be the same. 4) Frequency must be the same.

4. Induction Machines

Induction Machine The induction machine is the most rugged and the most widely used machine in industry. Both stator and rotor winding carry alternating currents. The alternating current (ac) is supplied to the stator winding directly and to the rotor winding by induction – hence the name induction machine. Application (1f): washing machines, refrigerators, blenders, juice mixers, stereo turntables, etc. 2f induction motors are used primarily as servomotors in a control system. Application 3f: pumps, fans, compressors, paper mills, textile mills, etc.

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

Induction Machine General Construction of Induction Motor A typical motor consists of two parts: 1-An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, 2-An inside rotor attached to the output shaft that is given a torque by the rotating field. General •The induction machine is used as the most common motors in different applications. •It has a stator and a rotor like other type of motors. •2 different type of rotors: 1-squirrel-cage winding, 2-Wound-rotor •Both three-phase and single-phase motors are widely used. •Majority of the motors used by industry are squirrel-cage induction motors

Induction motor components. Basic principles: •An AC current is applied in the stator armature which generates a flux in the stator magnetic circuit. •This flux induces an emf in the conducting bars of rotor as they are “cut” by the flux while the magnet is being moved (E = BVL (Faraday’s Law)) •A current flows in the rotor circuit due to the induced emf, which in term produces a force, (F = BIL ) can be changed to the torque as the output. Induction motor components.

Induction Motor Stator construction Single-phase stator with windings. Stator construction –The stator of an induction motor is laminated iron core with slots similar to a stator of a synchronous machine –Coils are placed in the slots to form a three or single phase winding.

Induction Motors Magnetic Circuit

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 Squirrel-Cage rotor. One or 2 fans are attached to the shaft in the sides of rotor to cool the circuit.

Rotor of a large induction motor. (Courtesy Siemens). 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 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. Rotor of a large induction motor. (Courtesy Siemens).

Synchronous Speed 120f ns = p ns = synchronous speed [r/min] f = frequency of supply [hertz/Hz] P = total of magnetic pole

Slip and Slip Speed The slip s of an induction motor is the difference between the synchronous speed and the rotor speed, expressed as a Percent (per unit) of synchronous speed The per-unit slip is given by the equation S = slip ns = synchronous speed [r/min] nr = rotor speed [r/min] ns - nr S = ns

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 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 voltae induced in the rotor when at rest [V]

Active Power in a Induction Motor Poutput Efficiency () = Pinput

Motor Torque 9.55 Pm Tm = n 9.55 (1 – s) Pr = ns (1 – s) Tm = 9.55 Pr / ns

I2R losses in the rotor Mechanical Power Pjr = s Pr Pjr = rotor I2R losses [W] s = slip Pr = power transmitted to the rotor [W] Mechanical Power Pm = Pr - Pjr = Pr - s Pr = (1 – s) Pr

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

Knowing quantities: Source frequency = 50 Hz, number of poles = 20 Synchronous speed ns = 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.

Knowing quantities: 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 1200 r/min ns =

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

Example 3

Student Assignment 2 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