Unit IV- Electrical Machines and Measuring Instrumentation Working Principle, Construction and application of DC machine & AC machines (Single Phase Transformer, Single Phase Induction Motor: Split Phase, Capacitor start and capacitor start and run motors). Basic Principles & Classification of Instruments – Moving coil and Moving Iron instruments.

Overview of Direct Current (DC) Machines Direct-current (DC) machines are divided into dc generators and dc motors. Most DC machines are similar to AC machines: i.e. they have AC voltages and current within them. DC machines have DC outputs just because they have a mechanism converting AC voltages to DC voltages at their terminals. This mechanism is called a commutator; therefore, DC machines are also called commutating machines. DC generators are not as common as they used to be, because direct current, when required, is mainly produced by electronic rectifiers. While dc motors are widely used, such automobile, aircraft, and portable electronics, in speed control applications…

DC Machine – Pictorial Representation

Construction of DC Machine.

Picture View of DC machine Cutaway view of a dc motor Stator with poles visible.

Yoke or Frame Poles: Field winding Made by silicon steel lamination. Silicon steel- reduce hysteresis loss Lamination- reduce eddy current loss. Pole shoe : Ensure uniform flux distribution and air gap between the poles and rotating armature. Field winding Winding is series connection to the other poles. Poles get magnetized to N and S poles alternatively.

Commutator: AC emf in to pulsating DC emf. Armature: Laminated silicon steel core supported on a solid shaft. Cylindrical drum consisting of thin circular stamping of silicon steel. Thickness of stamping – 0.35mm to 0.5mm. Winding: Lap winding : no of parallel path equal to no of poles; total current divides equal among them; no of brushes equal to no of poles. ( Large current machine) Wave winding: no of parallel path is 2; Commutator: AC emf in to pulsating DC emf. Brushes: made of carbon or graphite; gentle touch with commutator.

Working Principle – DC Generator

Use Commutator

Working Principle – DC Motor

Types of DC Generator Separately excited generator Self excited generator Series generator Shunt Generator Compound Generator Application Charging batteries Compensation in transmission line for line drop Arc welding.

Classification of DC motors DC motors are mainly classified into three types as listed below: Shunt motor Series motor Compound motor Differential compound Cumulative compound

Applications: Shunt Motor Blowers and fans Centrifugal and reciprocating pumps Lathe machines Machine tools Milling machines Drilling machines

Applications: Series Motor: Cranes Hoists , Elevators Trolleys Conveyors Electric locomotives

Applications: Cumulative compound Motor: Rolling mills Punches Shears Heavy planers Elevators

Transformer - Introduction A transformer is a static machines. The word ‘transformer’ comes form the word ‘transform’. Transformer is not an energy conversion device, but is a device that changes AC electrical power at one voltage level into AC electrical power at another voltage level through the action of magnetic field, without a change in frequency. It can be either to step-up or step down. Generation Station Distributions Transmission System 33/13.5kV 13.5/6.6kV 6.6kV/415V Consumer 17

Transformer- Types Core - type construction Shell - type construction

Transformer Lamination

Laminated Core: Winding: Transformer tank: Transformer oil: Made up of, L, E,I,U shaped stampings depends on the types. Silicon steel (reduced hysterics loss); Lamination (reduced eddy current loss) Winding: Primary and secondary winding made of copper are placed on the vertical limbs of the core. Transformer tank: Core and windings is placed in an aluminum tank. The tank acts as an enclosure and cooling surface. Transformer oil: Used mineral oil; cooling medium; insulator between tank and transformer assembly. Conservator: Its placed over a tank through the pipe connection; partially filled oil. Breather: Oil expanded the level – conservator raised and the air is released; It’s a small vessel containing drying agent silica gel. Buchholz Relay: Moisture entering the transformer – affect the insulating property of oil in transformer; This relay sense this condition.

Ideal Transformer An ideal transformer is a transformer which has no loses, i.e. it’s winding has no ohmic resistance, no magnetic leakage, and therefore no I2 R and core loses. However, it is impossible to realize such a transformer in practice. Yet, the approximate characteristic of ideal transformer will be used in characterized the practical transformer. V1 V2 N1 : N2 E1 E2 I1 I2 V1 – Primary Voltage V2 – Secondary Voltage E1 – Primary induced Voltage E2 – secondary induced Voltage N1:N2 – Transformer ratio

Transformer Equation Where, ‘a’ is the Voltage Transformation Ratio; which will determine whether the transformer is going to be step-up or step-down Step Down Transformer For a >1 E1 > E2 Step Up Transformer For a <1 E1 < E2

Applications

Step Up Transformer at Power Plant

Induction Motors

Introduction Three-phase induction motors are the most common and frequently encountered machines in industry simple design, rugged, low-price, easy maintenance wide range of power ratings: fractional horsepower to 10 MW run essentially as constant speed from no-load to full load Its speed depends on the frequency of the power source not easy to have variable speed control requires a variable-frequency power-electronic drive for optimal speed control

Construction - Stator

Stationary part of the motor. Outer solid circular metal part – Yoke or frame. Laminated cylinder drum – Stator drum. Required no of stator conductor are embedded in slots. Electrically connected in series; arranged to form balanced three phase winding.

Construction – Squirrel Cage Rotor

Each rotor slot accommodates a rod or bar made of a good conducting material. Rotor rod in short circuited in both ends Rotor circuit forms short circuited for a closed path in a current flow through it.

Construction – Slip Ring Wound Rotor

Large no of conductor embedded in a rotor slots. The conductor are electrically connected to form a balanced three phase winding. Another point connected the three slip ring for make a closed circuit.

Construction Squirrel cage rotor Wound rotor Notice the slip rings

Construction Slip rings Cutaway in a typical wound-rotor IM. Notice the brushes and the slip rings Brushes

AC Machines Single Phase Induction Motor: Split Phase Capacitor start Capacitor start and run motors

Split Phase Induction Motor Single-phase induction motor cannot develop a rotating magnetic field Needs an “auxiliary” method That method is another (auxiliary) winding

Single-Phase Squirrel-Cage Induction Motor There are two “Main Poles” Squirrel-Cage Rotor Single-Phase Mains Supply

Excite the Main Winding Stator flux is produced across the air gap – as shown, it is increasing in the downward direction. The squirrel-cage rotor responds with a mmf in the opposite (upward) direction. Magnetic axis of the rotor is in line with the magnetic axis of the stator – no rotation!

Current “out of” the page “Main” pole flux (Φ) increasing in the downward direction Rotor mmf develops in the upward direction Current “into” the page

Cause the rotor to turn clockwise Rotor conductors cut through the main pole flux. Current is induced in the rotor bars as shown, producing a magnetic flux perpendicular to the main pole flux. This is known as “Quadrature” flux.

The quadrature flux is sustained as the rotor conductors shift their positions – other conductors replace them.

Phase Relationship Between the Direct and Quadrature Flux The “speed” voltage is in phase with the flux that created it, and the flux due to current is in phase with the current that caused it. The instantaneous amplitudes of the direct and quadrature flux are shown above.

Animation

Split Phase Induction Motor Single-phase induction motor cannot develop a rotating magnetic field Needs an “auxiliary” method That method is another (auxiliary) winding

Split phase (or) resistance start induction motor

Construction: Operation: Main winding (M) and Auxiliary winding (A) (placed in stator). M – Running Purpose, Highly inductive; A – Starting purpose, Highly resistive. A – Centrifugal switch placed it; for the use of rest in motor. Operation: Current given to M & A; create phase angle ; the angle near to 90; starting torque proportion to sin  Motor attain 75% of speed switch is open and “A” is disconnected. the starting torque is 150 to 200% of full load torque.

CAPACITOR START INDUCTION MOTOR

Construction: Operation: Same type of the construction; replace the resistance with capacitor in Auxiliary winding. Operation: Phase angle () is equal to 90; Starting torque is high compare to previous type. Better starting power factor.

CAPACITOR START AND RUN INDUCTION MOTOR

Construction: Operation: Same construction in previous type Main winding added one capacitance permanently. Operation: Similar in previous one. The advantage are follows Low noise in running condition. Higher power factor. High Efficiency. Improved Over load capacity.

Applications Fans Pumps Refrigerator Air conditioners Blowers Washingmachines