Module I

Electric drives

The System which control the motion of machine is called the drives. It is called the

drives. It is the combination of prime moves, transmission equipment and mechanical loud. In

electrical drive , electric motor is need as primemover.

An electric drive may define as the combination of electric motor with its controlling

devices, power transmission equipments and mechanical loud.

Parts of Electric drive.

(Picture)

A controller is a device introduced in the system to modify the eraser signal and to

produce a control signal. The manner in which the controller produces the control signal is called

the control action. The controller modifies the output of the system. The controller may be

electrical, electronic, hydraulic as pneumatic, depending on the nature of signal and system. The

output of the controller is given to the input of electric motor, it converts electric energy into

mechanical energy and it is mechanically coupled to the mechanical loud. A feedback is connected

between loud and the controller.

Advantages of Electric drives :

1. It is quiet clear due to the absence of fuel and fumes.

2. Electric drives available in wide range due to the availability of wide range motor.

3. it is more flexible.

4. Due to the source is electrical energy, there is no need of free storage, easily transmitted and

economical.

5. Less and easy maintenance.

6. Controlling starting are very easy.

7. It occupies less floor area.

8. Life of drives are more.

9. Regenerative breaking only possible in electric drives.

10. It can remote controlled and its operating characteristics can easily modified.

11. It is reliable source of drive.

Factors influencing the choice of electric drives :

The following are the factors to be devoted to any or all of them before final selection of

the motor is made.

1. Nature of Supply.

a. A.C Supply

b. Pure D.C Supply

c. Rectified A C Supply

2. Electrical Characteristic

a. Starting characteristics

b. Running Characteristics

c. Speed Control

d. Braking characteristics.

3. Mechanical Characteristics :

a. Type of enclosure

b. Methods of power transmission

c. Type of bearing

d. Type of cooling systems

e. Noise level

4. Nature of Drive

a. Individual drive

b. Group drive.

5. Size and rating of motors

a. Whether the motor is to drive continuous, intermittent, or variable load cycle.

b. Whether its overload capacity and pull out torque are adequate.

6. Cost :

a. Capital (or initial) cost.

b. Running cost.

Load equalization

The load fluctuations taken place in many of the industrial drives. These fluctuations vary

widely within a span of few seconds. The sudden and peak load require large current from the

supply. This involve lot of voltage drop in the system, or alternately would require very large size

cables. It is desirable to smooth out these load fluctuations in the interest of economy. The process

of smoothing out these load fluctuations is commonly referred to as load ‘equalization’. The load

euqalisation involves the storage of energy during the light-load period and gives out during peak

load period. Thus the demand from the supply is approximately remain same. Fig shows the load

curve of a non continuous rolling mill

Load (kw)

Time(S)

Fig. : Load Curve of Rolling Mill.

The load varies from a very heavy, lasting for a few seconds while the billet is between the rolls,

to a light –load during which the motor has to supply only friction loss in the rolls.

Module II

Servomotors

They are also called control motors and have high torque capabilities. They are not used

for continuous energy conversion but only for precise speed and precise position control at high

torques. Basic principle of operation is the same as that of electromagnetic motors. However, their

construction, design and made of operation are different. Their power ratings vary a fraction of a

watt upto a few 100W. Due to their low inertia they have high speed of response. So they are in

smaller in diameter but longer in length. They generally operate at very low speeds or sometime

zero speed.

These motors look like the usual electric motors. Their main difference from industrial

motors is that more electric wires come out of there for power as well as for control. The

servomotor wires go to a controller and not to the electrical line through contactors. A tachometer

is mechanically connected to the motor shaft. Sometimes, blower or fans may also be attached for

motor cooling at low speeds.

Types of Servomotors

1. DC Servomotors

2. AC Servomotors

a. Two phase AC Servomotors

b. Three phase AC Servomotors

1.DC Servomotors :

These motors are either separately excited dc motors or permanent magnet dc motors.

The schematic diagram as shown below.

(Picture)

The Speed of dc servomotors is normally controlled by varying the armature voltage. Their

armature us designed to have large resistance so that torque speed characteristics are linear and

have large negative slop as shown in fig (c). The negative slope serves the purpose of providing the

viscous damping for the servo drive system fig (b) , the armature mms and excitation field are in

quadrature. This fact provides a fast torque response because torque and flux become decoupled.

Accordingly, a step change in the armature voltage or current produces a quick change in the

position or speed of the rotor.

2. AC Servomotors

Presently, most of the ac servomotors are of the two phase squirrel cage induction type

and are used for low power applications. However, recently 3 phase induction motors have been

modified for high power servo systems which had so far been using high power dc servomotors.

a. Two phase AC Servomotor

Such motors normally run on a frequency of 60 Hz or 400Hz. The stator has two

distributed windings which are displaced from each other by 90 ̊. The control phase voltage is

controlled by an electronic controller. The speed and torque of the rotor are controlled by the

phase difference between the main and control windings. Reversing the phase difference from

leading to lagging reverses the motor direction.

Since the rotor bar have high resistance, the torque speed characteristics for various

armature voltages are almost linear over a wide speed range particularly near the zero speed. The

motor operation can be controlled by varying the voltage of the main phase while keeping that of

the reference phase constant.

(Picture)

b. Three phase AC Servomotors.

A three phase squirrel cage induction motor use in high power servo systems. Normally

such a motor is a highly non linear couple circuit device. Recently, this machine has been operated

successfully as a linear decoupled machine by using a control method called Vector control or field

oriented control. In this method, the currents feels to the machine are controlled in such a way that

its torque and flex become decoupled as in a dc machine. This results in a high speed and a high

torque response.

Application:

1. Radar

2. Tracking and guidance systems

3. Process controllers

4. Computers and machine tools.

DC Motor :

Operation Principle

An electric motor is a machine converts electric energy into mechanical energy. Its action is

based on the principle that when a current carrying conductor is placed in a magnetic field, it

experiences a mechanical force whose direction is given by Flemming’s Left hand Rule and whose

magnitude is given by F = BIl Newton.

where B → flux density in Tesla or Weblm².

I → Current through the conductor in Ampere

l → Length of the conductor in meter

Flemming’s left hand rule states that stretch middle finger forefinger and thumb mutually

perpendicular, middle finger represents the direction of current forefinger direction of magnetic

flux and thumb represents the direction of motion (force).

Torque equation of dc motor:

Consider a conductor, is rotating with a speed of N rpm using a force ‘F’ Newton. Let ‘r’ be

the radius of the armature.

F

Torque is called moment of twist (T)

T = Force x r distance. T= F x r Nm Work done for one rotation = force x distance. work done = F x 2 𝜋r jule

time for one revolution = 60

𝑁 seconds

Power = Work done/ time = Fx2 𝜋r

60

𝑁

= 2 𝜋N. f x r 60 P = 2 𝜋N T watt 60 T = 60P NM

2 𝜋N

Armature torque Ta = 60P Pa 2 𝜋N

where 3 Pa → armature power

Let ‘Eb be the back emf and Ia be the armature current them Pa = Eb Ia

Ta = 60P 2 𝜋N

Ta = 60P ZN P

2 𝜋N 60 A Ia Eb = ZN . P 60 A

= Z P Ia 2𝜋 A

Ta = 1 PZ Ia 2 𝜋N A

Ta = 0.159 PZ Ia. Newton meter A

r

Ta & Ia

Shaft torque Tsh:

Tsh = 60 (Pa –Mechanical loss) 2 𝜋N

Tsh = 60 Psh Psh = Pa –Mechanical loss 2 𝜋N

Type of DC Motor

DC Motor

Shunt Series Compounds

Long Shunt Short shunt

Level Cumulative Differential

Shunt Motor:

If the field winding is connected in parallel with armature winding.

(Picture)

Current equation :

Let V be the supply voltage, it is the line current.

Il =Ia + Ish

Ia =Il – Ish

Voltage equation

Eb = V-IaRa –Vbrush.

Series motor :

If the field winding is connected in series with armature winding it is called series motor.

(Picture)

Current Equation :

Ia = Il =Ise

Voltage equation:

V-IaRa –Ise Rse- V brush

Eb = V-Ia (Ra+Rse) –Vbrush

DC Compound Motor :

A motor with both series field and shunt field winding.

Long shunt compound motor :

The shunt field winding is connected in parallel with the series combination of armature and series

field winding.

(Picture)

Eb =V-Ia (Ra+Rse) –Vbruah

Ia =Il –Ish

Ish = v/Rsh

Short shunt dc compound motor :

Parallel combination of shunt field and armature is connected in series with a series field winding.

(Picture) (Picture)

Eb = V –IaRa –IlRse-Vbrush

Ia =Il =ish

il =Ise

Ish = V-IlRsh Rsh

Cumulative Compound Motor I the series field aids shunt field such type is called cumulative compound motor.

Differential Compound Motor If the series field oppose the shunt field such type is called differential compound motor.

Level compound Motor If the full load speed is equal to no load speed. Such type of motor is called level

compound motor.

Speed Control

Speed Control of dc Shunt Motor

We know that N & Eb/ or N & 𝑉 −𝐼𝑎𝑅𝑎

∅, by varying voltage or armature current or flux, the

speed of motor can be varied. 1. Flux control method or field rheostatic method

(Picture)

When Eb remains constant, th speed of motor is inversely proportional to flux is any decrease

in this flux caused to increasing speed and vice versa. The flux is directly proportional to Ish ie

decrease in ish results in increase in speed. Field current Ish can be decrease by inserting a field

rheostat in the field circuit are shown above the rated speed. Here Ish is very low there for copper

loss is very less.

2. Armature control method :

When flux remain constant N & Eb and N &V-IaRa, increase in armature drop will cause the

reduction in the speed. Let Re, & Rez are two resistance volume connected in series with armature

them (Re, Rez).

R1 = Ra + Re1

R2 = Ra + Rez

(Picture)

Then speed –Ia Chara. of a same motor with different external resistance Re,and Rez is

shown above. This method is used to control the speed below its rated speed. The main

disadvantage of this method is high Cu. loss efficiency reduced.

3. By controlling change in supply voltage :

(Ward Leonard Method)

(Picture)

Let M, be the main motor whose speed to be controlled. M2 –G be the motor generator set.

The field winding of motor M1- & M2 are connected directly to the supply. Field winding of

generator is connected to the supply to the field regulator and a reversible switch. The input of

the motor M2 is connected to the supply. The output of the generator is fed to motor M1 as input.

Motor M2 and generator G are mechanically coupled.

Working :

1. Motor M2 act as a prime moves for the generator when it is connected to the supply.

2. The terminal voltage of generator is applied to the motor M1

3. These terminal voltage can be varied by varying the field of generator.

4. The field of generator can be varied using field regulation which is connected in series with

field winding

5. By varying the terminal voltage of the G, can be changed by reversing the field of

generator by reversing the switch Rs.

Advantages

1. Very accurate and precise control.

2. No resistance is inserted. efficiency is high.

3. Direction of rotation can be change

Disadvantages

1.unavailability of identical motor generators.

2. Expensive.

Speed Control of Series motors :-

1. Field diverted method :

(Picture)

By connecting a rheostat parallel to series field winding, the current through the field can be

reduced as shown above. But flux directly proportional to field current and inversely proportional to

speed.

N x 1

∅ N ∝ 𝑉 − (𝑅𝑎 +

𝑅𝑠𝑒

∅)

∞ Ise These reduction in the field current causes the reduction in flux and increasing the speed. This method is used to control the above rated speed.

2.Armature diverted method :

(Picture)

Armature current can be diverted by connecting a rheostat parallel to the armature winding. By

decreasing the armature current speed increases. This method is used to control the speed below rated

speed.

3. Taped field method :-

In this method field winding is taped into several numbers when it is tapped at last sted the motor

runs at its minimum speed. The speed can be increase step by step cutting out sted one by one. These

method is used in electric train to control the speed. The main advantage of this method is power loss is

negligible compared to field diverted and armature diverted method.

4. Series resistance method ( Voltage control)

By connecting a rheostat in series with field and armature winding the applied voltage to the

motor can be reduced these by speed also reduced. Main disadvantage is high copper loss.

(Picture)

Applications

Shunt Motor 1. For driving constant speed line shafting 2. Lathes 3. Centrifugal pumps

4. Machine tool 5. Blowers and fans 6. Reciprocating pumps.

Series Motor 1. For traction work 2. Electric locomotives 3. Rapid transit systems 4. Trolly cars etc. 5. Cranes and hoists 6. Conveyors

Cumulative Compound motor 1. Far intermittent high torque loads. 2. Far shears and punches. 3. Elevators 4. Conveyors 5. Heavy planers 6. Rolling mill; ice machines 7. printing presses 8. Air compressors

Electric Breaking :

A motor and its load may be brought to rest quickly by using either (i) Friction Braking or (ii) Electric Braking. The commonly used mechanical brake has one drawback ; it is difficult to achieve a smooth stop because it depends on the condition of the braking surface as well as on the skill of the operator.

Electric Braking of Shunt Motors : a. Rheostatic or Dynamic braking

In this method, the armature of the shunt motor is disconnected from the supply and is connected across a variable resistance R as shown below. The field winding is however, left connected across the supply undisturbed. The breaking effect is controlled by varying the series resistance R. Obviously, this method makes use of generator action in a motor to bring it to rest.

(Picture)

b. Plugging or Reverse current Braking :

This method is commonly used in controlling elevators, rolling mills, printing presses and machine tools etc. In this method, connections to the armature terminals are reversed so that motor tends to run in the opposite direction. Due to the reversal of armature connections, applied voltage V and Eb start acting in the same direction around the circuit. In order to limit the armature current to a reasonable value, it is necessary to insert a resister in the circuit while reversing armature connections. Plugging gives greater braking torque then rheostatic braking. Obviously, during plugging, power is drawn from the supply and is dissipated by R in the form of heat. It may be noted that even when motor is reaching zero speed, there is some braking torque.

(Picture)

Regenerative Braking This method is used when the load on the motor has overhauling characteristic as in the lowering of the cage of hoist or the downgrade motion of an electric train. Regeneration takes place when Eb becomes greater than V. This happens when the overhauling load acts as a prime mover and so drives the machine as a generator. Consequently, direction of Ia and hence of armature torque is reversed and speed falls until Eb becomes lower them V. It is obvious that during the slowing down of the motor, power is returned to the line which may be used for supplying another train on an upgrade thereby relieving the powerhouse of part of its load. For protective purposes, it is necessary to have some type of mechanical brake in order to hold the load in the event of a power failure.

(Picture)

Electric Braking of series motor :-

a. Rheastic (or Dynamic ) Braking : The motor is disconnected from the supply, the field connections are reversed and the motor is connected in series with a variable resistance R as shown in fig. Obviously, now, the machine is running as a generator. The field connections are reversed to make sure that current through field winding flows in the same direction as before in order to assist residual magnetism. In practice the variable resistance employed for starting purpose is itself used for braking purposes.

b. Plugging or Reverse current Braking. As in the case of shunt motors, in this case also the connections of the armature are reversed and a variable resistance R is put in series with the armature as shown in fig.

(Picture)

c. Regenerative Braking : This type of braking of a series motor is not possible without modification because reversal of Ia would also mean reversal of the field and hence of Eb. However, this method is sometimes used with traction motors, special arrangements being necessary for the purpose.

Module III Three Phase Induction Motor : Conversion of electrical power into mechanical power takes place in the rotating part of the an electric motor. In dc motors, the electric power is conducted directly to the armature through brushes and commutator. In ac motor, the rotor does not receive electric power by conduction but by induction in exactly the same way as the secondary of a 2-winding transformer receives it’s power from the primary. That is why such motors known as induction motors. AW induction motor can be treated as a rotating transformer is one in which primary winding is stationary but the secondary is free to rotate. Induction motor consist of mainly two parts stator and the rotor. Stator is made up of a number of stampings, which are slotted to receive the windings. Rotor are of two types, squirrel-cage rotor and slipping rotor. 90% of induction motors are squirrel –cage type.

Working Principle of induction motor When a 3 supply is given to the stator winding, a magnetic flux of constant magnitude but rotating at synchronous speed is setup. The flux passes through the air gap, sweeps past the rotor surface and so cuts the rotor conductors which as yet are stationary. Due to the relative speed between the rotating flux and the stationary conductors an emf is induced in the latter, according to Faraday’s laws of electromagnetic induction. The frequency of the induced emf is the same as the supply frequency. Its magnitude is proportional to the relative velocity between the flux and the conductors and its direction is given by flemming’s Right hand rule. Since the rotor bars or conductors form a closed circuit, rotor current is produced whose direction as given by Len’s law, is such as oppose the very cause producing it. In this case, the cause which produces, the rotor current is the relative velocity between the rotating flux of the stator and the stationary rotor current. Hence to reduce the relative speed, the rotor starts running in the same direction as that of the flux and tries to catch up with the rotating flux.

Synchronous speed (Ns) : It is the speed of the stator field, a rotating magnetic flux with constant speed & magnitude is set up.

Ns = 120𝑓

𝑃

Slip speed It is the difference between synchronous speed and actual speed.

Slip Speed = Ns-N

Actual Speed It is the speed of the rotor or speed of the motor. Ship

It is the ratio of the difference between synchronous speed and the actual speed to the synchronous speed.

S = 𝑁𝑠 −𝑁

𝑁3

S= 1, at starting ie when N = 0 S= 0 , When N=Ns

Module IV Synchronous motor : A Synchronous motor is electrically identical with an alternator or ac generator. Most synchronous motors are rated between 150 KW and 15 MW and run at speeds ranging from 150 to 1800 r.p.m. Some characteristic features of a synchronous motor are given below. 1. It runs either at synchronous speed or not at all. ie, while running it maintains a constant

speed. The only way to change speed is to vary the supply frequency (because Ns =120𝑓

𝑝 )

2. It is not inherently self-starting. It has to be run upto synchronous (or near synchronous) speed by some means, before it can be synchronized to the supply.

3. It is capable of being operated under a wide range of power factors, both lagging and loading. Hence , it can be used for power correction purposes, in addition to supplying torque to drive loads.

Salient pole and Non Salient Pole Two types of rotors are used in alternators and synchronous motor. ie, smooth cylindrical type and salient pole type.

Salient Pole It is rotor type, projecting from rotor care. It is used for low speed hydroelectric generator. The pole phase is so shaped that the radical air gap length increases from the pole centre to pole tip. This makes the flux distribution over the armature uniform to generate sinusoidal wave form of emf.

Features 1. They have large diameter and short axial length. 2. Poles are laminated to reduce eddy current losses. 3. The speed is 100 to 375 rpm.

4. Pole shoe covers about 2𝑟𝑑

3 of pole pitch.

Cylindrical rotor (Non salient pole) The rotor is in cylindrical form dc winding embended in its rotor slots. It is used for high speed turbo generators.

Features

1. They are of small diameter and very long axial length. 2. Robust in construction 3. High operating speed 3000 rpm. 4. Noiseless operation. 5. Dynamic balancing is better 6. Less vintage loss. 7. No need to provide damper winding 8. The vector emf waveform.

Torque Equation

(Picture)

Except for small machines, the armature resistance of a synchronous motor is negligible as compared to its synchronous reactance. Hence the equivalent circuit for the motor becomes as shown above fig. (a). From the phasor diasysam fig.(b), it is seen that

𝐴𝐵 = 𝐸𝑏 sin = 𝐼𝑎 𝑋𝑠 𝑐𝑜𝑠

or 𝑉𝐼𝑎 𝑐𝑜𝑠∅ = 𝐸𝑏 𝑉𝑠𝑖𝑛

Now, 𝑉𝐼𝑎 𝑐𝑜𝑠∅ = 𝑚𝑜𝑡𝑜𝑟 𝑝𝑜𝑤𝑒𝑟𝑖𝑛𝑝𝑢𝑡

𝑝ℎ𝑎𝑠𝑒

Pin = 𝐸𝑏𝑉

𝑋𝑠 Sin Per phase

= 3 𝐸𝑏𝑉

𝑋𝑠 Sin for three phase.

Stator Cu losses have been neglected, Pin also represents the gross mechanical power Pm developed by the motor.

Pin = 3𝐸𝑏𝑉

𝑋𝑠𝑆𝑖𝑛

The gross torque developed by the motor is Tg

Tg= 9.55 𝑃𝑚

𝑁𝑠 𝑁 − 𝑀 Ns is in rpm.

Different types of Torque : 1. Stating Torque :

It is the torque developed by the motor when full vintage is applied to its stator winding. It is also sometimes called breakaway torque. Its value may be as low as 10 % as in the case of centrifugal pumps and as high as 200 to 250 % of full load torque as in the case of loaded reciprocating two cylinder compressors.

2. Running Torque As its name indicates, it is the torque developed by the motor under running conditions. It is determined by the horse-power and speed of the driven machine. The peak horse power determines the maximum torque that would be required by the driven machine.

3. Pull in Torque A synchronous motor is started as induction motor till it runs 2 to 5% below the synchronous speed. Afterwards, excitation is switched on and rotor pulls into step with the synchronous rotating stator field. The amount of torque at which the motor will pull into step is called the pull in torque.

4. Pull out Torque The maximum torque which the motor can develop without pulling out of step or synchronism is called pull-out torque.

Methods of Starting

Almost all synchronous motors are equipped with dampers or squirrel cage windings consisting o f Cu bars embedded in the pole shoes and short circuited at both ends. Such a motor starts readily, acting as an induction motor during the starting period. (Picture)

The line voltage is applied to the armature towards and the field circuit is left unexcited. Motor starts as an induction motor and while at reaches nearly 95 % of its synchronous speed, the

dc field is excited. At that moment the stator and poles get engaged or interlocked with each other and hence pull the motor into synchronize two points should be noted ie, 1. At the beginning, when voltage is applied, the rotor is stationary. The rotor field of the stator winding induces a very large emf in the rotor during the starting period, through the value of this emf goes on decreasing as the rotor gathers speed. Normally, the field windings are meant for 110v but during starting period there are many thousands of volts induced in them. Hence the rotor windings have to be highly insulated for withstanding such voltages. 2. When full line voltage is switched on to the armature at rest, a very large current, usually 5 to 7 times the full load armature current is drawn by the motor. In some cases, this may not be objectionable but where it is the applied voltage at starting, is reduced by using auto transformers. However, the voltage should not be reduced to a very low value because the starting torque of an induction motor varies approximately as the square of the applied voltage. Usually a value 50 % to 80 % of the full line voltage. For reducing the supply voltage, the switches S1 opened to cut out the transformers. Hunting It is otherwise called surging or phase swinging. When a synchronous motor is used for driving a varying load, then a condition known as hunting is produced. Hunting may also be caused if supply frequency is pulsating. The damper windings are used to prevent hunting.