Solid State Drives

Solid State Drives

7th Semester - Final Year EEE

UNIT I FUNDAMENTALS OF ELECTRIC DRIVES 9

Advantage of Electric Drives – Parts and choice of Electrical Drives – Status of DC and AC drives – Torque-speed characteristics of motor and load – Selection of Motor power rating – Thermal model of motor for heating and cooling – Classes of duty cycle – Determination of motor rating – Control of Electric drives – Modes of operation – Speed control and drive classifications – Closed loop control of drives.UNIT II CONVERTER / CHOPPER FED DC MOTOR DRIVE 9

Steady state and transient analysis of the single and three phase fully controlled converter fed separately excited D.C motor drive – Continuous and discontinuous conduction mode – Multiquadrant operation – Converter control – Chopper-fed D.C drive – Steady-state analysis – Block diagram of closed loop dc drive.UNIT III INDUCTION MOTOR DRIVES 9

Analysis and performance of three-phase induction motor – Operation with unbalanced source voltage, single-phasing and unbalanced rotor impedance – Starting – Braking – Transient analysis – Stator voltage control –Adjustable frequency control of VSI and CSI fed induction motor – Static rotor resistance control – Slip-power recovery drives – Open loop V/f control – Principle of vector control – Vector control of induction motor – Block diagram of closed loop drive.UNIT IV SYNCHRONOUS MOTOR DRIVES 9

Open loop V/f control and self-control of CSI and VSI fed synchronous motor -Cycloconverter fed synchronous motor – Microprocessor based synchronous motor control –Marginal angle control and power factor control – Permanent magnet (PM) synchronous motor – vector control of PM Synchronous Motor (PMSM).UNIT V BLDC, STEPPER AND SWITCHED RELUCTANCE MOTOR DRIVES 9

Brushless DC motor drives and its applications – Variable reluctance and permanent magnet stepper motor Drives – Operation and control of switched reluctance motor – Applications, modern trends in industrial drive.

Total: 45TEXT BOOKS1. Bimal K. Bose, “Modern Power Electronics and AC Drives”, Pearson Education, 2002.2. Dubey, G.K., “Fundamentals of Electrical Drives”, 2nd Edition, Narosa Publishing

House, 2001.REFERENCES1. Pillai, S.K., “A First Course on Electrical Drives”, Wiley Eastern Limited, 1993.2. Krishnan, R., “Electric Motor and Drives Modelling, Analysis and Control”, Prentice

Hall of India, 2001.

UNIT I FUNDAMENTALS OF ELECTRIC DRIVES

Advantage of Electric Drives – Parts and choice of Electrical Drives – Status of DC and AC drives – Torque-speed characteristics of motor and load – Selection of Motor power rating – Thermal model of motor for heating and cooling – Classes of duty cycle – Determination of motor rating – Control of Electric drives – Modes of operation – Speed control and drive classifications – Closed loop control of drives.

Drive:Drive is a system which supplies mechanical energy for motion control.

Electric Drive:An electric drive is a system that converts electrical energy to mechanical energy.

Applications of Electric Drives:1. Transportation Systems2. Rolling Mills3. Paper Mills4. Textile Mills5. Machine Tools6. Fans and Pumps7. Robots8. Washing Machines etc.

Block Diagram of Electric Drive:

Sources: Mostly Single and Three phase 50Hz supply is used base on the power

requirements. When 50Hz Supply is used, maximum of 3000 RPM can only be obtained. For

increased speed, higher frequency supply is required. For low and medium power ratings: 400V used. For Higher power ratings: 3.3kV, 6.6kV, 11kV and higher voltages are used. Some drives are powered from battery and even solar powered drives are also

used. But these drives are not economical and having restrictions. Type of motor used is independent of the supply available.

Power Modulator: The switches within the converter are controlled by the modulator which

determines which switches should be on, and for what time interval, normally on a micro-second timescale. An example is the Pulse Width Modulator that realizes a required pulse width at a given carrier frequency of a few kHz.

Source Power Modulator Load

Control Unit

Motor

Sensing Unit

Input Command

The power modulators are classified as follows:o Converters

AC to DC Converters – Variable DC Voltage AC Voltage Regulators – Variable AC Voltage Choppers – Variable DC Voltage Inverters

i. Stepped wave inverter – Variable Frequency Fixed Voltageii. PWM Inverter – Variable Frequency Variable Voltage

Cycloconverters – Variable Frequency and Variable Voltageo Variable impedances: Variable resistors are commonly used for the

control and dynamic breaking of low cost AC and DC drives. In high power applications, liquid rheostats are employed to get stepless variation of resistance. Two step (full and zero) inductors are employed for limiting starting current of AC motors.

o Switching Circuits: Switching operations are performed by high power electromagnetic

relays. Now a day, thyrister switches are used. Switching operations are required for the following:

i. For changing motor connections to change its quadrant of operation

ii. For changing motor circuit parameters in discrete steps for automatic starting and braking control

iii. For operating motors and drives according to predetermined sequence

iv. To provide interlocking to prevent maloperation andv. To disconnect motor when abnormal operating conditions

occur.Control Unit:

Controls for a power modulator are provided in the control unit. The control unit, typically a digital signal processor (DSP), or micro-controller

contains a number of software based control loops which control, for example, the currents in the converter and machine.

In addition torque, speed and shaft angle control loops may be present within this module. Shown in the diagram are the various sensor signals which form the key inputs to the controller together with a number of user set-points (not shown in the diagram). The output of the controller is a set of control parameters which are used by the modulator.

Motors: DC Mots:

o Shunt, series, compound and permanent magnet motors Induction motors:

o Squirrel cage, slip ring and linear induction motors Synchronous Motors:

o Wound field and permanent magnet motors For variable speed operations, DC motors are preferred. The development of solid

state devices helps to use AC motors in variable speed applications. Because of numerous advantages of AC motors, AC drives have succeeded DC

drives in a number of variable speed applications.

Advantages of Electric Drives:1. They have flexible control characteristics.2. They are available in wide range of torque, speed and power.3. Electric motors have high efficiency, low no load losses and considerable short

time overloading capacity.4. They are adoptable to almost any operating conditions such as explosive

environment, submerged, vertical mountings and so on.5. They do not pollute the environment.6. Can operate in all the four quadrants of speed torque plane.7. They can be started instantly and can immediately be fully loaded.

Choice of Electrical drives:Selection of electrical drive depends on number of factors such as,

1. Steady state operation requirements: Nature of speed torque characteristics Speed regulation Speed range Efficiency Duty cycle Quadrants of operation Ratings

2. Transient operation requirements: Acceleration and deceleration Starting Braking and reversing performance

3. Requirements related to the source: Types and capacity of the source Magnitude of voltage Voltage fluctuations Power factor Harmonics and effect on other loads

4. Capital, running and maintenance needs, life.5. Space and weight restrictions if any.6. Environment and location.7. Reliability.

Status of AC and DC drives: Previously induction and synchronous motors were only preferred for constant

speed and DC motors were dominated in variable speed applications. The development of thyristors in 1960s brought the induction motor widely in to

variable speed applications with increased efficiency and equivalent performance compared to DC drives. It was expected that the efficient induction motor may occupy the place of DC drives in variable speed applications.

But the expectation prohibited by the following reasons:i. The cost of converters and controllers made the induction motor drive

expensive than those of DC drive.ii. The control technology of DC drives was well established than the new

technology of AC drives.iii. AC drives were not reliable than DC drives.iv. Implementation of the developed digital and VLSI design were helpful in

improving the performance of AC drive and the same led to improvements in DC drives.

Use of recent developments in power electronics have resulted into reduction in cost, simplified controller, increased efficiency and reliable performance of AC drives.

Even now-a-days, in between widely used variable speed DC drives, induction motors are used in low to high power ranges and synchronous motor drives are used in medium and very high power applications.

The permanent magnet synchronous motors (PMSM) and brushless DC (BLDC) motors are replacing DC servomotors for fractional HP range.

Fundamental Torque Equations:Motors are coupled to the load with transmission system. During rotation of motor shaft, the load may rotator or may undergo translational motion. The speed of the load may differ from that of the motor and the speed and motion of the different parts of the load may also differ. But the motor load system is conveniently discussed with the following representation:

Let,J – Moment of inertia of the motor-load system referred to the motor shaft (kg-m2)ωm - Instantaneous angular velocity of motor shaft (rad/sec)T- Instantaneous value of developed motor torque (Nm)Tl- Instantaneous value of load torque (Opposing torque including friction and

windage) referred to motor shaft (Nm)By the fundamental torque equation,

(1)

The above equation is applicable to variable inertia drives used in mine winders, industrial robots etc., but for constant inertia,

Then,

From equation (2), it is clear that the torque developed by the motor (T) is counter balanced by the load torque (Tl) and a dynamic torque J(dωm/dt). The term J(dωm/dt) is called dynamic torque because it is present only during the transient operations.

Acceleration and deceleration of the drive depends on the values of T and T l. For acceleration, T> Tl and for deceleration, T< Tl.During acceleration, the motor torque not only overcome the load torque but it supplies to dynamic torque J(dωm/dt) to overcome drive inertia.

In drives with larger inertia, like electric trains, motor torque must exceed the load torque by a large amount in order to get sufficient acceleration. When fast transient response required, motor torque should be maintained at larger rate and the system should be designed with lowest inertia.

The energy associated with dynamic torque is stored as kinetic energy assists the motor torque T and maintains drive motion.

Components of Load Torque: Friction torque, (Tf), results from relative motion between surfaces, and it is

found in bearings, lead screws, gearboxes, slideways, etc. Windage torque,(Tw), is caused by the rotating components setting up air (or

other fluid) movement, and is proportional to the square of the speed. Load torque,(Tl) required by the application.

Loads with rotational motion:

ωm

The motor is driving two loads. One load is directly driven and other is a through gear with ‘n’ and ‘n1’ teeth.

Let,o J0 – Moment of inertia of motor and directly coupled loado ωm – Speed of the motoro Tl0 – Torque of the directly coupled loado J1 – Moment of inertia of load coupled through gearo ωm1 – Speed of the load coupled through gearo Tl1 – Torque of the load coupled through gear

Now,

Neglecting transmission losses, the kinetic energy due to equivalent inertia must be same as the kinetic energy of various moving parts. Thus,

Power at the loads and motor must be same. If transmission efficiency of the gears be 1, then

where Tl is the total equivalent torque referred to motor shaft.Then,

Motor Load Tl0

Load Tl1

ωm ωm

ωm1

n

n1

(a) Loads with Rotational Motion

If in additional load directly coupled to the motor with inertia J0 there are m other loads with moment of inertias J1,J2,.....,Jm and gear ratios of a1,a2,.....,am then

and

If the load is driven by belt drive instead of gears, then, neglecting slippage, the equivalent inertia and torque can be obtained from the above equations by considering a1,a2,.....,am each to be the ratios of diameters of wheels driven by motor to the diameters of wheels mounted on the load shaft.

Loads with Translational Motion:

The motor is driving two loads. One load is directly driven and other is a through a

transmission system which is converting rotational motion in to linear motion. Let,

o J0 – Moment of inertia of motor and directly coupled loado ωm – Speed of the motoro Tl0 – Torque of the directly coupled loado M1- Mass of the load with translational motion(kg)o v1 – Velocity of the load with translational motion (m/sec)o F1 – Force of the load with translational motion (N)

Neglecting transmission losses, the kinetic energy due to equivalent inertia must be same as the kinetic energy of various moving parts. Thus,

Similarly, power at the motor and load should be same, thus if efficiency of transmission be 1, then

where Tl is the total equivalent torque referred to motor shaft. If in addition to one load directly coupled to the motor shaft, there are ‘m’ other

loads with translational motion with velocities v1,v2,…..,vm and M1,M2,…..,Mm, respectively, then

Motor Load Tl0

ωm ωm

v1

(b) Loads with Translational Motion

Rotational to Linear motion Transmission

Mass M1 Force F1

and

Torque Speed Characteristics of Electrical Motors:

The main advantage of electrical drives is that their wide range of speed variation. They are explained below:

Synchronous or Reluctance motor:

o They exhibit constant speed characteristics as shown in curve I.

o At steady state conditions, these motors operate at constant speed irrespective of the value of load torque.

DC Shunt/Separately excited motor:

o The characteristic curve II shows that the speed is slightly reduced when the load torque increases.

DC Series Motor:

o The speed is high at light load and low at heavy loading condition as shown in the curve III.

Induction Motors:

o They exhibit complex characteristics as shown in curve IV.

o During steady state, they operate at the linear proportion of speed torque characteristic, which resembles the characteristic of a DC shunt motor.

o The maximum developed torque of induction motors is limited to Tmax.

In electric drive applications, the selection of the motor should match with the required performance of the loads. For example, the synchronous motor is probably the best option for the constant speed applications. Other motors, such as induction or DC shunt motors can also be used in constant speed applications,

provided that feedback circuits are used to compensate for the change in speed when load torque changes.

Selection of Motor Power Rating:

Selection of power rating is important to achieve economy with reliability. Improper selection of motor power rating results extra initial cost and extra loss of

energy du tot operation below rated power makes the choice uneconomical. Furthermore, induction and synchronous motors operate at a low power factor

when operating below the rated power. During operation of the machine, heat is produced due to losses and temperature

rises. An amount of developed heat is dissipated into the atmosphere. When the dissipation of heat is equal to the developed heat, then it is said to be equilibrium condition. Motor temperature then reaches a steady state value.

Steady state temperature depends on power loss, which in turn depends on the output power of the machine. Since temperature rise has a direct relation with the output power, it is termed thermal loading on the machine.

Steady state temperature varies in different parts of the machine. It is usually high is the windings because loss density in conductors is high and dissipation is slow; sind the conductors which are wrapped in insulating material are partly embedded in slots and thus are not directly exposed to the cooling air.

Insulating materials have lowest temperature limit. They are classified based on temperature limit as follows:

Insulation Class Temperature ( in 0C) 90

A 105

E 120

B 130

F 155

H 180

C >180

Motor rating should be selected in such a way that the insulation temperature never exceeds the prescribed limit; otherwise it loses its thermal stability.

It is simple to calculate the motor power rating of the motor which operate at a constant power and speed. But most loads operate at variable power and speed at different applications.

Thermal model of Motor for Heating and Cooling: It is difficult to predict the heat flow and temperature rise inside an electrical

motor because of its complex geometry and heterogeneous materials. Assuming nearly equal thermal conductivity in all the materials (i.e, assuming

homogeneous), simple thermal model can be obtained. It helps us to select the motor for a particular application.

Let the homogeneous body machine has following parameters at time t:p1 – Heat developed (W)p2 – Heat dissipated to the cooling medium (W)W – Weight of the active parts of the machine (kg)h – Specific heat (Joules/kg/0C)A – Cooling Surface (m2)d – Coefficient of heat transfer or specific heat dissipation (Joule/sec/ m2/0C) - Mean temperature rise (0C)

During a change in time dt, the temperature rises to d. Since,

Heat absorbed in the machine = Heat developed – Heat dissipated to atmosphereor Wh d = p1 dt - p2 dt (1)Since p2= dA (2)Using (1) and (2) we get,

where, Thermal capacity of the machine, C=Wh (Watts/0C) (4) and heat dissipation constant, D=dA (Watts/0C) (5). The first differential equation of the equation (3) is,

where, ss=(p1/D) (7) and = (C/D) (8)

The value of K is obtained by substituting t=0 in (6). When the initial temperature rise is 1,=ss(1-e-t/)+1e-t/ (9)where, - Heating (or Thermal) time constant of the machine.When t= in equation (9), =ss. Thus ss is the steady state temperature when the machine is continuously heated by the power p1. Up to this temperature all the heat produced by the machine is dissipated to the surrounding medium.

Let the load on the machine be removed after reaching temperature 2,. Heat loss will reduce to a small value p1’ and cooling operation of the motor will begin.Let the new value of heat dissipation constant be D’. If time is measured from the instant the load is removed, then

Solving (10) with the initial condition =2 at t=0, we have

where, steady state temperature for new conditions of operation, ’ss=(p’1/D’)

(12) and Cooling (or Thermal) time constant of the machine, ’ = (C/D’) (13).

If the motor were disconnected from the supply during cooling then p’1=0 gives ’ss=0, suggesting that the final temperature attained by the motor will be ambient temperature. Therefore equation (11) becomes,

Equations (9) and (14) suggest both heating time constant and cooling time

constant ’ based on respective heat dissipation constants D and D’, which in turn depend on the velocity of the cooling air.

In self cooled motors, fans are mounted at the shaft and hence the velocity of the air varies with the speed, thus varying cooling time constant ’. Cooling time constant at standstill is much larger than when running. Therefore, in high performance, medium and high power variable speed drives, motor is always

provided with separate forced cooling, so that motor cooling be independent of speed.

The above figure shows the variation of motor temperature rise with time during heating and cooling. Thermal time constants of a motor are far larger than electrical and mechanical time constants. While electrical and mechanical time constants have typical ranges of 1 to 100ms and 10ms to 10s, the thermal time constants may vary from 10 min to couple of hours.

Classes of Motor Duty:1. Continuous Duty:

It denotes the motor operation at a constant load torque for duration long enough for the motor temperature to reach steady state value. This duty cycle is characterized by constant motor losses. Ex: Paper mill drives, Compressors, Conveyers, centrifugal pumps and fans.

2. Short Time Duty:The operation of the drive is less than the heating time constant in this case and the machine is allowed to cool to ambient temperature before to operate again. In this case, the machine can be overloaded to permissible limit. Ex: Crane drives, drives for house hold appliances, turning bridges, valve drives etc.

3. Intermittent Periodic Duty:It consists of periodic duty cycles, each consists of a period of running at a constant load and a rest period. Neither the period of operation is sufficient to raise the temperature to a steady state value, nor the rest period long enough for the machine to cool off to ambient temperature. Ex: Pressing, Cutting and drilling machines etc.

4. Intermittent periodic duty with starting:In this case, the heating during starting cannot be ignored. It consists of a period of starting, a period of operation at constant load and a rest period being too short for the respective steady state temperature to be attained. Ex: Metal Cutting and drilling tool drives, drives for fork lift trucks, mine hoist etc.

5. Intermittent periodic duty with starting and braking:In this case, the heating during starting and braking cannot be ignored. It consists of a period of starting, a period of operation at constant load, a braking period with electrical braking and a rest period; with operating and a rest periods being too short for the respective steady state temperature to be attained. Ex: Billet mill drive, manipulator drive, ingot buggy drive, schrewdown mechanism of blooming mill etc.

t0

Heating

Cooling

Heating and Cooling Curves

6. Continuous duty with intermittent periodic loading:It consists of a period of running at constant load and a period of running at no load, with normal voltage. The no load period is too short for the respective steady state temperature to be attained. Ex: Pressing, Cutting, shearing and drilling machine drives etc.

7. Continuous duty with starting and braking:It consists of a period of starting, a period of running at a constant load and a period of electric braking without rest period. Ex: Blooming Mill.

8. Continuous duty with periodic speed changes:It consists of a period of running at one load and speed, and another period of running at different speed and load; again both operating periods are too short for respective steady state temperatures to be attained. There is no period of rest.

Determination of Motor Rating:For calculation of motor rating, the duty cycles can be mainly classified in to,

1. Continuous duty2. Fluctuating loads3. Short time and intermittent duty

1. Continuous Duty:A motor with next higher power rating compared with the maximum power demand is selected with consideration of the following factors:

o Motor and load speedso Starting torque of the motoro Ability of the motor to withstand normal disturbances of the power supplyo Transient and steady state reserve torque capacity of the motor

2. Fluctuating and Intermittent loads:This method can be employed for Intermittent Periodic Duty and Continuous duty with periodic speed changes. It is based on approximation that the actual variable motor current can be replaced by an equivalent Ieq which produces same losses in the motor as actual current.Motor losses = Constant losses+ Copper loss.For fluctuating loads with n value of motor currents I1,I2,…,In for durations t1,t2,…,tn

respectively, the equivalent current Ieq is given by

or

or

If the current varies smoothly over a period T can be written as,

DC Motor:This motor can be allowed to carry larger (2 to 3.5 times) than the rated current for a short duration. This is known as short time overload capacity of the motor. Ratio of maximum allowable current (short time overload capacity) to rated current be denoted by . Then

If the above condition is not satisfied, then the motor current rating is calculated from

Induction and Synchronous Motor: For stable operation of induction and synchronous motors, the maximum load

torque should be within the breakdown torque. In normal design of induction motors, the ratio of breakdown to rated torque

varies from 1.65 to 3 and for synchronous motors 2 to 2.25. Then the rating can be selected by

When the load has high pulses, selection of motor rating based on this will be too large. Load equalization is made by mounting flywheel on the shaft.

When torque is directly proportional to current, then

When the motor operates at constant speed, its power will be directly proportional to torque. Therefore,

Control of Electrical drives:

Four Quadrants of operation:

The following conventions govern the power flow analysis of the electric drive systems:i. When the torque and speed of the machine are in the same direction, then the

machine is operating as a motor (consume electric energy from the source and delivers mechanical power to the load).

ii. If the speed and torque of the machine are in the opposite directions, the machine is acting as a generator (consume mechanical energy from load and delivers electric power to the load).

1st Quadrant (Forward Motoring): The torque and speed of the motor are in the same direction. Of course, the load

torque is opposite to the machine torque. The electrical machine in this case is operating as a motor. The flow of power is

from the machine to the load.2nd Quadrant (Forward Braking):

The speed direction is unchanged while the direction of the torque is reversed. Since the load torque direction is in the same direction of speed, the mechanical

load is delivering power to the machine. The machine then receives mechanical energy, converting it in to electrical

energy and returning it back to the electric source. The electric machine is thus acting as a generator.

3rd Quadrant (Reverse Motoring): Compared to the first quadrant, the system speed and torque are reversed in the

third quadrant. Since the torque and speed of the machine are in the same direction, the power

flow is from the machine to the load. The machine is therefore acting as a motor rotating in the reverse direction to the speed of the first quadrant.

Bidirectional grinding machine is the good example of the 1st and 3rd quadrant operation. The direction of the load torque of the grinding load is reversed when the speed is reversed (3rd quadrant). A horizontal conveyor belt is another example of this type of operation.

4th Quadrant (Reverse Braking): The torques remains unchanged as compared to the first quadrant. The speed,

however, changes the direction. From the load perspective, the load torque and the speed are in the same

direction. Hence the power flow is from the load to the machine. The machine is in this case acting as generator delivering the electric power to

the source. The first and fourth quadrant of operation can be explained with the elevator.

When the elevator is going upward or downward, the direction of the load torque remains unchanged but the direction of the speed only reversed.

Any electric drive system operates in more than one quadrant. In fact, most versatile systems operate in all the four quadrants. The converters of these systems must be designed to allow the electric power to flow in both directions.

Modes of Operation:

Operation in all four quadrants of the speed-torque plane can be achieved: motor and generator (braking) operation in both rotational directions.

The direction of the armature current is changed for reversing the torque direction

An electric drive operates in three modes:i. Steady stateii. Acceleration including startingiii. Deceleration including stopping

The steady state operation is realized by adjusting the speed torque characteristic such that the motor and load torques are equal at that speed. When the torque opposes motion, the motor works as motor in quadrant I and III.

The active load can reverse its sign and assist the motion in some cases like lowering of loaded hoist. For such a case, the steady state can be obtained by adding mechanical brake which will produce a torque in a direction oppose the motion.

Drive operates in quadrant II and IV depends on the direction of rotation. Acceleration and deceleration modes are transient operations. Drive operates in

acceleration whenever an increase in its speed is required. For this speed-torque curve is changed so that the motor torque exceeds the load torque. The time taken for a given change in speed depends on inertia of motor load system and the amount by which motor torque exceeds the load torque.

Motor operation in deceleration mode is required when a decrease in its speed is required. Deceleration occurs when load torque exceeds the motor torque. Whenever the reducing the motor torque to zero does not provide enough deceleration, mechanical brakes may be provided. Alternatively, electrical braking may be employed.

Speed Control and Drive Classifications: Drives where the motor runs at nearly fixed speed are known as constant speed

or single speed drives. Multi-speed drives are those which operate at discrete speed settings.

Drives needing step less change in speed and multispeed drives are called variable speed drives.

When a number of motors are fed from a common converter, or when a load is driven by more than one motor, the drive is called as multi-motor drive.

Speed range of a variable speed drive depends on the application. In some applications, it can be from rated speed to 10% of rated speed. In some applications, speed control above rated speed is also desired, and the ratio of maximum to minimum speed can be as high as 200. There are also applications where the speed range is as low as from rated speed to 80% of rated speed.

A variable speed drive is called constant torque drive if the drive’s maximum torque capability does not change with a change in speed setting. The corresponding mode of operation is called constant torque mode.

ωm

ωm2

ωm1

T

Tl

2

1

Principle of Speed Control

It must be noted that the term ‘constant torque’ refers to maximum torque capability of the drive and not to the actual output torque, which may vary from no load to full load torque. Constant power drive and constant power mode are defined in the same way.

In ideal case, the motor speed should be remains constant as the load torque is changed from no load to full load. But in practical case, the speed drops with the increase in load torque. Quality of a speed control system is measured in terms of speed regulation.

If the open loop control fails to provide the desired speed regulation, drive is operated as closed loop speed control system.

Closed-Loop Control of Drives:

Feedback loops in an electrical drive may be provided to satisfy one or more of the following requirements:

i. Protectionii. Immediate Speed responseiii. Improvement in steady state accuracy

1. Current – limit Control:

This method limits converter and motor current below a safe limit during transient operations. It has a current feedback loop with a threshold logic circuit.

As long as the current is within a set maximum value, feedback loop does not affect the operation of the drive.

During transient period, feedback forces the increase of current beyond the set value to the set value and become inactive. Further the operation repeats in every transient condition. Thus the current fluctuate around the set value.

When the operation close to the steady state point, current will not have a tendency to cross the maximum value, consequently, feedback loop will have no effect on the drive operation.

2. Closed loop torque control:

This technique mainly used in battery operated vehicles, rail cars and electric trains.

Driver presses the accelerator to set torque reference T*. Because of feedback, the actual torque T follows the reference T*.

-

Threshold logic circuit

Contoller Converter

Current Sensor

Motor Load

Imax0

I

If

V*

+

-

Torque Contoller

Converter

Torque Sensor

Motor Load

T

T*

+

Speed feedback loop is present through the driver. By giving appropriate pressure on the accelerator, driver adjusts the speed depending on traffic, road condition, car condition etc.

3. Closed loop speed control:

Above figure shows the widely used closed loop speed control method in electrical drives. It consists of inner current loop and outer speed loop.

Current loop controls the converter and motor currents to safe limit and hence the torque. Inner current loop also reduces the effect on performance of any non-linearity present in converter-motor system.

An increase in speed reference m* produce a positive error Δm. Speed error is processed through a speed controller and applied to current limiter which saturates even for a small speed corresponding to the maximum allowable current.

Drive accelerates at the maximum allowable current. When close to the desired speed, limiter desaturates. Steady state is reached at the desired speed and at current for which motor torque is equal to the load torque.

A decrease in speed reference m* produce a negative speed error. Current limiter saturates and sets current reference for inner current loop at a value corresponding to maximum allowable current. Consequently, drive decelerates in braking mode at maximum allowable current. When close to the required speed, current limiter desaturates. The operation is transferred from braking to motoring. Drive then settles at a desired speed and at a current for which motor torque equal to the load torque.

Current and speed controllers may consists of PI, PD or PID controller depending on steady state accuracy and transient response requirements.

Δm

-+

-

SpeedContoller

Current Sensor

Motor Load

m

+

m*

ConverterCurrent Controller

Speed Sensor

I*

ICurrent Limiter

Current Limiter

UNIT II CONVERTER / CHOPPER FED DC MOTOR DRIVE

Steady state and transient analysis of the single and three phase fully controlled converter fed separately excited D.C motor drive – Continuous and discontinuous conduction mode – Multiquadrant operation – Converter control – Chopper-fed D.C drive – Steady-state analysis – Block diagram of closed loop dc drive.

Single Phase fully controlled rectifier control of DC separately excited motor:

Fig.1(a) shows the separately excited DC motor fed from fully controlled rectifier. The ac input voltage is given by vs=Vmsint.

(a)

(c)

(b)

Fig.1.(a).Single phase fully controlled rectifier fed DC separately excited motor

Fig.1.(b). Discontinuous conduction waveformFig.1.(c). Continuous conduction waveform

In a cycle of voltage, thyristors Q1 and Q2 are given gate signals from to , and thyristors Q3 and Q4 are given gate signals from (+) to 2.

When the armature current doesn’t flow continuously, the motor is said to operate in discontinuous conduction. When the current flows continuously, the conduction is said to be continuous.

Discontinuous Conduction: In this mode, the current starts flowing with the turn-on of thyristors Q1 and Q2 at

t=. Motor connected to the source and its terminal voltage equals vs. The current, which flows against both Vc and source voltage after t=, falls to zero at .

Due to the absence of current Q1 and Q2 turn-off. Motor terminal voltage is now equal to its induced voltage Vc. When thyristors Q3 and Q4 are fired at (+), next cycle of the motor terminal voltage vd starts.

In a cycle of motor terminal voltage vd, the drive operates in two intervals:o Duty interval ( t ) when motor is connected to the source and

vd=vs.o Zero current interval ( t +) when id=0 and va=Vc.

Drive operation can be described by the equations:

Solution of Eq.(2) has two components:1. Due to the AC source (Vm/Z)sin(t-), and2. Due to back EMF (-Vc/R)Each of these components has in turn a transient component. Let these be represented by a single exponent K1e-t/a, then

where,

Constant K1 can be evaluated subjecting eq.(3) to the initial condition id()=0. Substituting value of K1 so obtained in Eq.(3) gives

Since id()=0,

‘’ can be evaluated by iterative solution of eq.(7).Since voltage drop across the armature inductance due to DC component of armature current is zero.

Vd=Vc+IdR.

Plug reversal and plug braking:

Because the rotor always tries to catch up with the rotating Weld, it can be reversed rapidly simply by interchanging any two of the supply leads. The changeover is usually obtained by having two separate 3-pole contactors, one for forward and one for reverse. This procedure is known as plug reversal or plugging, and is illustrated in Figure.

The motor is initially assumed to be running light (and therefore with a very small positive slip) as indicated by point A on the dotted torque– speed curve in Figure (a). Two of the supply leads are then reversed, thereby reversing the direction of the field, and bringing the mirror-image torque–speed curve shown by the solid line into play. The slip of the motor immediately after reversal is approximately 2, as shown by point B on the solid curve. The torque is thus negative, and the motor decelerates, the speed passing through zero at point C and then rising in the reverse direction before settling at point D, just below the synchronous speed.

The speed–time curve is shown in Figure (b). We can see that the deceleration (i.e. the gradient of the speed–time graph) reaches a maximum as the motor passes through the peak torque (pullout) point, but thereafter the final speed is approached gradually, as the torque tapers down to point D.

Very rapid reversal is possible using plugging; for example a 1 kW motor will typically reverse from full speed in under 1 s. But large cage motors can only be plugged if the supply can withstand the very high currents involved, which are even larger than when starting from rest.

Frequent plugging will also cause serious overheating, because each reversal involves the ‘dumping’ of four times the stored kinetic energy as heat in the windings. Plugging can also be used to stop the rotor quickly, but obviously it is then necessary to disconnect the supply when the rotor comes to rest, otherwise it will run-up to speed in

reverse. A shaft-mounted reverse rotation detector is therefore used to trip out the reverse contactor when the speed reaches zero.

It should be noted that, whereas, in the regenerative mode (discussed in the previous section) the slip was negative, allowing mechanical energy from the load to be converted to electrical energy and fed back to the mains, plugging is a wholly dissipative process in which all the kinetic energy ends up as heat in the motor.

Injection braking

This is the most widely used method of electrical braking. When the ‘stop’ button is pressed, the 3-phase supply is interrupted, and a d.c. current is fed into the stator via two of its terminals. The d.c. supply is usually obtained from a rectifier fed via a low-voltage high-current transformer.

(c)

It is known that the speed of rotation of the air-gap field is directly proportional to the supply frequency, so it should be clear that since d.c.is effectively zero frequency, the air-gap field will be stationary. Also known that the rotor always tries to run at the same speed as the field. So, if the field is stationary, and the rotor is not, a braking torque will be exerted. A typical torque–speed curve for braking a cage motor is shown in Figure (c), from which we see that the braking (negative) torque falls to zero as the rotor comes to rest.

This is in line with what we would expect, since there will be induced currents in the rotor (and hence torque) only when the rotor is ‘cutting’ the flux. As with plugging, injection (or dynamic) braking is a dissipative process, all the kinetic energy being turned into heat inside the motor.

VSI Controlled Induction Motor Drives:

B

C

E

B

C

E

B

C

E

B

C

E

B

C

E

B

C

E

M

Vd

Tr1 Tr3 Tr5

Tr4 Tr6 Tr2

A BC

Induction Motor

VSI can be operated as a stepped wave inverter or PWM inverter. In stepped wave inverter, transistors are switched on in the sequence of their

numbers with a time difference of T/6 and each transistor is kept ON for duration of T/2, where T is the time period of one cycle.

Frequency is varied by varying T and output voltage is varied by varying DC input voltage.

When the input is DC, a chopper circuit can be used between the supply and inverter.

When the input supply is AC, controlled rectifier converts AC to DC and feeds to the inverter. A large electrolytic capacitor is connected in DC link to make inverter operation independent of rectifier and to filter out harmonics in DC link voltage.

The RMS value of the fundamental phase voltage,

The torque for a given speed can be calculated by considering only fundamental component.

Drawbacks of stepped wave inverter drives:o Large harmonics in lower frequency operationso Low frequency harmonics increases the motor losses at all speeds and

causing derating of the motor.o Motor develops pulsating torques due to 5th, 7th, 11th, and 13th harmonics

which cause jerky motion of the rotor at low speeds.o Harmonic content in motor current increases at low speeds. The machine

saturates at light loads at low speeds due to high V/f ratio. These two effects overheat the machine at low speeds, thus limiting lowest speed to around 40% of base speed.

The PWM inverter overcomes all the above drawbacks. And also no additional arrangement is required variation of DC voltage, hence inverter can be directly connected when the supply is DC and through a diode rectifier when the supply is AC.

The fundamental component in the output phase voltage of PWM is

where m-Modulation index

Braking of VSI induction motor drive: The power input to the motor is

where,V - Fundamental component of the motor phase voltageIs - Fundamental component of the motor phase current - Phase angle between V and Is.

In motoring operation, >90o, therefore Pin is positive i.e., power flows from the inverter to the machine.

A reduction in frequency makes the synchronous speed less than the rotor speed and the relative speed between the rotor conductors and air gap rotating field reverses.

The reverse of rotating field reverses the rotor induced emf, rotor current and component of stator current which balances the rotor ampere turns.

Consequently, angle becomes greater than 90o and power flow reverses. The machine works as generator feeding power into the inverter, which in turn feeds power into dc link by reversing the dc link current Id.

Regenerative braking is obtained when the power flowing from the inverter to the DC link is usefully utilized and dynamic braking is obtained when it is wasted in a resistance.

CSI Induction Motor Drives: CSI fed induction motors have an advantage over the VSI fed induction motor that

it is having very good control of electromagnetic torque and drive dynamics because the torque is directly proportional to the current.

Current Source variable frequency supplies are realized by Auto Sequentially Commutated Inverter (ASCI) or current regulated inverter drives.

Diodes D1-D6 and Capacitors C1-C6 provide commutation of thyristors T1-T6, which are fired with a phase difference of 60o in sequence of their numbers.

Inverter behaves as a current source due to the presence of large inductance DC link.

The fundamental component of motor phase current is

The torque is controlled by varying dc link current Idc by changing the value of Vr

for a given speed. The maximum value of dc output voltage of fully-controlled rectifier is chosen so

that the motor terminal voltage saturates at rated value. The major advantage of CSI is its reliability. In case of VSI, a commutation failure

will cause two devices in the same leg to conduct. This connects conducting devices directly across the supply. Consequently, current through the devices suddenly rises to dangerous values. Expensive high speed semiconductor fuses are required to protect the devices.

In CSI, conduction of two devices in the same leg does not lead to sudden rise of current through them due to the presence of a large inductance Ld. This allows time for commutation to take place and normal operation to get restored in subsequent cycles.

As seen in figure, the motor current rise and fall are very fast. These rapid rise and fall through the leakage inductance causes large voltage spikes. Therefore, a motor with low leakage inductance is used.

The commutation capacitances C1-C6 reduce the voltage spikes by reducing the rate of rise and fall of the current. Large value of capacitors is required to sufficiently reduce the voltage spikes. Large range of capacitors reduces the frequency and hence the speed of the drive.

2

Idc

0

iA

t

iA

iA

Ld

Due to large value of capacitors and inductors, the CSI drive is expensive and has more weight and volume.

Regenerative Braking and Multiquadrant operation: When inverter frequency is reduced to make synchronous speed less than motor

speed, machine works as a generator. Power flows from machine to DC link and DC link voltage Vr reverses. The power

supplied to DC link will be transferred to ac supply and regenerative braking will take place. Thus, no additional equipment is required for regenerative braking of CSI drive.

Change of phase sequence of CSI will provide motoring and braking operations in the reverse direction.

Static Rotor Resistance Control of Induction Motor:

The AC voltage induced in the rotor circuit is rectified and given to the parallel combination of the resistance R and the switching transistor Tr.

The effective value of the resistance R can be changed by the duty ratio of the transistor, which in turn varies the rotor circuit resistance.

The inductance Ld is added to reduce the harmonics and discontinuity in the dc link current Id.

The rotor current is shown in fig without ripple. The rms rotor current will be

Resistance between A and B will be zero when transistor is ON and it will be R when it is OFF. Therefore, the average value of resistance between the terminals is given byRAB=(1-)Rwhere is the duty ratio of the transistor.

Power consumed by RAB is given by,

Power consumed per phase is

The above equation suggests that the rotor circuit resistance per phase is increased by 0.5 (1- ) R. Thus, total rotor circuit resistance per phase will now be RrT=Rr+0.5 (1- ) R.

Slip Power Recovery Schemes:

Scherbius Drive:

Id

2 t0

Ir

The slip power is dissipated in the external resistance and leads to poor efficiency of the drive.

The slip frequency power is converted to DC voltage – converted to line frequency. That frequency is pumped back to the ac source.

The inductor smoothens the ripples in the rectified DC voltage.

The phase controlled bridge1 with a firing angle less than 900 is allowed to function as a controlled rectifier and bridge 2 with a firing angle more than 900 is allowed to function as a line commutated inverter which offers subsynchronous motor control.

The power flow is from Rotor Bridge 1 to bridge 2 – transformer – AC supply.

If bridge 1 is made to work as a line commutated inverter with a firing angle of more than 900 and bridge 2 as a controlled rectifier with a firing angle less than 900. It offers super synchronous motor control power flow is from ac supply-transformer-bridge 2 – bridge 1 – rotor circuit – motor becomes doubly fed motor in this mode.

Near synchronous speed, slip frequency emfs are insufficient for natural commutation of thyristors.

The difficulty can be overcome using forced commutation.

Static Kramer Drive:

Rotor power is converted in to DC by a diode bridge as shown in figure. The DC power now fed to dc motor mechanically coupled with the induction motor.

Torque supplied to the motor is the sum of torque developed by the induction motor and dc motor.

Speed control can be obtained by controlling field current of DC motor. Speed control is possible from synchronous speed to around half of synchronous

speed. When larger speed range is required, Diode Bridge is replaced by a thyristor bridge.

The relationship between Vd1 and speed can be altered by controlling firing angle of thyristor rectifier. Speed can now be controlled up to standstill.

Open loop V/f Control:

The above figure shows the implementation of the constant V/f control strategy in open loop.

This type of variable speed drive is used in low performance applications where precise speed control is not necessary.

The frequency command fsn* is enforced in the inverter and corresponding DC link voltage is controlled through the front end converter. The offset voltage, Von, is added to the voltage proportional to the frequency, and they are multiplied by 2.22 to obtain the dc link voltage.

Some problems associated with the open loop drive are listed below:o The speed of the motor cannot be controlled precisely, because the rotor

speed will be less than the synchronous speed. Note that the stator frequency, and hence the synchronous speed, is the only variable controlled in this drive.

o The slip speed, being the difference between the synchronous and electrical rotor speed cannot be maintained the rotor speed is not measured in this drive scheme. This can lead to operation in the unstable region of the torque speed characteristics.

o The effect discussed in the above point can make the stator currents exceed rated current by many times, thus endangering the inverter-converter combination.

Vector Control of Induction motor drive:Principle of Vector Control:Let the rotor flux linkage r is at f (field angle) from a stationary reference. The three stator currents can be transformed in to q and d axes currents in the synchronous reference frames by using the transformation

from which the stator current phasor, is, is derived as

and the stator phasor angle is

where, and are the q and d axes currents in the synchronous reference frames that

are obtained by projecting the stator current phasor on the q and d axes respectively.

The current is produces the rotor flux r and the torque Te. if is the field producing component and iT is the torque producing component.i.e, r if (4) and Te r iT (5)Since the relative speed with respect to that of the rotor field is zero in steady state. Therefore, if and iT have only dc components and are ideal for use as control variables; the bandwidth of the computational control circuits will have no effect on the processing of these dc signals.Crucial to the implementation of vector control, then, is the acquiring of the instantaneous rotor flux phasor position, . This field angle can be written as

where is the rotor position and is the slip angle.

The field angle can be written in terms of speeds as,

Vector control schemes are classified as,i. Direct Vector Control – Field angle is calculated using terminal voltages and

currents.ii. Indirect Vector Control – Field angle is obtained using rotor position

measurement and partial estimation with only machine parametersImplementation of Vector Control in induction motors:

Vector control is the most popular control technique of AC induction motors. In special reference frames, the expression for the electromagnetic torque of the smooth-air-gap machine is similar to the expression for the torque of the separately excited DC machine. In the case of induction machines, the control is usually performed in the reference frame (d-q) attached to the rotor flux space vector. That’s why the implementation of vector control requires information on the modulus and the space angle (position) of the rotor flux space vector. The stator currents of the induction machine are separated into flux- and torque-producing components by utilizing transformation to the d-q coordinate system, whose direct axis (d) is aligned with the rotor flux space vector. That means that the q-axis component of the rotor flux space vector is always zero:

To perform vector control, follow these steps:i. Measure the motor quantities (phase voltages and currents)ii. Transform them to the 2-phase system (α ,β) using a Clarke transformationiii. Calculate the rotor flux space vector magnitude and position angleiv. Transform stator currents to the d-q coordinate system using a Park

transformationv. The stator current torque- (isq) and flux- (isd) producing components are

separately controlledvi. The output stator voltage space vector is calculated using the decoupling blockvii. An inverse Park transformation transforms the stator voltage space vector back

from the d-q coordinate system to the 2-phase system fixed with the statorviii. Using the space vector modulation, the output 3-phase voltage is generated

The components isα and isβ, calculated with a Clarke transformation, are attached to the stator reference frame α and β. In vector control, all quantities must be expressed in the same reference frame. The stator reference frame is not suitable for the control process. The space vector is is rotating at a rate equal to the angular frequency of the phase currents. The components isα and is depend on time and speed. These components can be transformed from the stator reference frame to the d-q reference frame rotating at the same speed as the angular frequency of the phase currents. The isd and isq

components do not then depend on time and speed. The component isd is called the direct axis component (the flux-producing component) and isq is called the quadrature axis component (the torque-producing component). They are time invariant; flux and torque control with them is easy.Knowledge of the rotor flux space vector magnitude and position is key information for AC induction motor vector control. With the rotor magnetic flux space vector, the rotational coordinate system (d-q) can be established. There are several methods for obtaining the rotor magnetic flux space vector. The flux model implemented here utilizes monitored rotor speed and stator voltages and currents. It is calculated in the stationary reference frame (α, β) attached to the stator. The error in the calculated value of the rotor flux, influenced by the changes in temperature, is negligible for this rotor flux model.For purposes of the rotor flux-oriented vector control, the direct-axis stator current isd

(the rotor flux-producing component) and the quadrature axis stator current isq (the torque producing component) must be controlled independently. However, the equations of the stator voltage components are coupled. The direct axis component vsd also depends on isd and the quadrature axis component vsq also depends on isq. The stator voltage components vsd and vsq cannot be considered as decoupled control variables for the rotor flux and electromagnetic torque. The stator currents isd and isq can only be independently controlled (decoupled control) if the stator voltageequations are decoupled and controlling the terminal voltages of the induction motor indirectly controls the stator current components isd and isq.

Speed Sensor

isqis

ia

ic

ib

vsq

vsd

vs

vs

Motor Flux Command

vsd,ln

vsq,ln

+-

SpeedContoller

Space Vector Modulation

3 Phase Controller

3 AC

Speed Command

Dec

oupl

ing

Torque Controller

Flux Controller

Inve

rse

Park

Tr

ansf

orm

ation

3 Im

Forw

ard

Clar

ke

Tran

sfor

mati

on

Rotor Flux Calculation

is

Forw

ard

Park

Tr

ansf

orm

ation

isd