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EEE Dept./Mahalakshmi Engineering College,Trichy-621213

MAHALAKSHMI ENGINEERING COLLEGE

TIRUCHIRAPALLI – 621213

QUESTION BANK WITH ANSWER

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Sub. Code : EE2352 Semester : VI

Subject : SOLID STATE DRIVES Unit : II

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CONVERTER / CHOPPER FED DC MOTOR DRIVE

1.What are the advantage and disadvantages of D.C. drives?

The advantages of D.C. drives are,

a. Adjustable speed

b. Good speed regulation

c. Frequent starting, braking and reversing.

The disadvantage of D.C. drives is the presence of a mechanical commutator which limits the maximum power rating and the speed.

2. Give some applications of D.C. drives.

The applications of D.C. drives are,

Rolling mills

Paper mills

Mine winders

Hoists

Machine tools

Traction

Printing presses

Excavators

Textile mils

Cranes.

3. Why the variable speed applications are dominated by D.C. drives?

The variable speed applications are dominated by D.C. drives because of lower

cost, reliability and simple control.

4. What is the use of flywheel? Where it is used?

It is used for load equalization. It is mounted on the motor shaft in dc compound motor.


5. What are the advantages of series motor?

The advantages of series

motors are,

a. High starting torque

b. Heavy torque overloads.

6. How the D.C. motor is affected at the time of starting?

A D.C. motor is started with full supply voltage across its terminals; a very high

current will flow, which may damage the motor due to heavy sparking at commuter and

heating of the winding. Therefore, it is necessary to limit the current to a safe value

during starting.

7. Define and mention different types of braking in a dc motor?

In braking the motor works as a generator developing a negative torque which

opposes the motion. Types are regenerative braking, dynamic or rheostat braking and

plugging or reverse voltage braking.

8. List the drawbacks of armature resistance control?

In armature resistance control speed is varied by wasting power in external resistors that are connected in series with the armature. Since it is an inefficient method of speed control it was used in intermittent load applications where the duration of low speed operations forms only a small proportion of total running time.

9. What is static Ward-Leonard drive?

Controlled rectifiers are used to get variable d.c. voltage from an a.c. source of

fixed voltage controlled rectifier fed dc drives are also known as static Ward-Leonard drive.

10. What is a line commutated inverter?

Full converter with firing angle delay greater than 90 deg. is called line commutated

inverter; such an operation is used in regenerative braking mode of a dc motor in which

case a back emf is greater than applied voltage.

11. Mention the methods of armature voltage controlled dc motor?

When the supplied voltage is ac,

Ward-Leonard schemes

Transformer with taps and uncontrolled rectifier bridge Static Ward-Leonard scheme or controlled rectifiers

When the supply is dc:

Chopper control

12. How is the stator winding changed during constant torque and constant horsepower operations?

For constant torque operation, the change of stator winding is made form series –


star to parallel – star, while for constant horsepower operation the change is made from

series-delta to parallel-star. Regenerative braking takes place during changeover from

higher to lower speeds.

13. Define positive and negative motor torque.

Positive motor torque is defined as the torque which produces acceleration or the

positive rate of change of speed in forward direction. Positive load torque is negative if it

produces deceleration.

14. Write the expression for average o/p voltage of full converter fed dc drives?

Vo=(2Vm/Π)cosα.................continuous conduction

Vo=[Vm(cosα-cos β)+(Π+α+β)]/Π]........discontinuous conduction

15. What are the disadvantages of conventional Ward-Leonard schemes?

Higher initial cost due to use of two additional m\cs.

Heavy weight and size

Needs more floor space and proper foundation

Required frequent maintenance

Higher noise and higher loss

16. Mention the drawbacks of rectifier fed dc drives?

Distortion of supply.

Low power factor.

Ripple in motor current

17. What are the advantages in operating choppers at high frequency?

The operation at a high frequency improves motor performance by reducing

current ripple and eliminating discontinuous conduction.

18. Why self commutated devices are preferred over thyristors for chopper circuits?

Self commutated devices such as power MOSFETs, power

transistors, IGBTs, GTOs are preferred over thyristors for building choppers

because they can be commutated by a low power control signal and don’t need

commutation circuit.

19. State the advantages of dc chopper drives?

Dc chopper device has the advantages of high efficiency, flexibility in control, light

weight, small size, quick response and regeneration down to very low speed.

20. What are the types of control strategies in dc chopper?

Time ratio control.

Current limit control.

21. What is meant by mechanical characteristics?


The curve is drawn between speed and torque. This characteristic is called

mechanical characteristics.

22. What is called inversion?

Rectifier takes power from D.C. terminals and transfers it to A.C. mains is called

inversion.

23. What are the advantages of armature voltage control?

The advantages of armature voltage control are,

a. High efficiency

b. Good transient response

c. Good speed regulation.

25. What are the limitations of series motor? (Or) Why series motor is not used in

traction applications now a day?

1. The field of series cannot be easily controlled. If field control is not employed, the

series motor must be designed with its base speed equal to the highest desired speed of

the drive.

2. Further, there are a number of problems with regenerative braking of a series

motor.

Because of the limitations of series motors, separately excited motors are now

preferred even for traction applications.

PART –B

1)Explain the classification of DC Drives.

Classification of DC drives:

1. Single phase drives

a) 1 - Ө half wave drives

b) 1 - Ө full wave drives i) 1 - Ө fully controlled converter fed drives ii) 1 - Ө half controlled converter fed drives

c) 1 - Ө dual converter fed drives

2. Three phase drives

a) 3 - Ө half wave drives b) 3 - Ө full wave drives

i) 3 - Ө fully controlled converter fed drives ii) 3 - Ө half controlled converter fed drives

c) 3 - Ө dual converter fed drives


3. DC chopper drives

Classification of chopper fed DC drives:

1) First quadrant chopper or type A chopper

2) Second quadrant chopper or type B chopper

3) Two quadrant type A chopper or type C chopper.

4) Four quadrant chopper or type E chopper.

Advantage of DC chopper Drive:

1) High efficiency

2) Flexibility in controls

3) Light weight

4) Small size

5) Quick response

2)Describe the steady state analysis of single phase fully controlled converter fed

separately excited dc motor drive in continuous and discontinuous modes.

STEADY STATE ANALYSIS OF SINGLE PHASE DC DRIVES

In single phase drives, single phase supply used to drive the motor. The drive has poor speed regulation and it can be overcome by closed loop operation.

Single Phase Fully Controlled Bridge Converter Control Of DC Separately

Excited Motor

The input voltage is defined by Vs = vm sinωt


Driver Circuit

Discontinuous conduction waveforms continuous conduction waveforms

Discontinuous conduction I) Duty interval (α ≤ ωt ≤ β) II) Zero current interval (β ≤ ωt ≤ π+ α)

Drive operation is described by the following equation:

Va= Ra ia+ La dia/dt + E = Vm sinωt, for α ≤ ωt ≤ β Va = E and ia = 0 for β ≤ ωt ≤ π+ α

Let these component represented by a single exponent K1e-t/ζa then

Ia = (ωt) = vm/Z sin (ωt - Ө) – E/Ra + K1e-t /ζa for α ≤ ωt ≤ sin β


Where Ө = tan-1 (ωLa/Ra)

Ia = (ωt) = vm/Z [sin (ωt - Ө) – sin (α - Ө) e-(ωt – α) cot Ф] - E/Ra [1- e-(ωt – α) cot Ф],

for α ≤ ωt ≤ β

Since ia(β) = 0 then

vm/Z sin (β - Ө) - E/Ra + [E/Ra - vm/Z sin (α – Ф)] e-(β-α)cotФ = 0

Since voltage drop across the armature inductance due to dc component of armature current is zero Va = E + IaRa

Va = 1/ π[⌠αβ

Vm sinωt d(ωt) + ⌠αα+ π E d(ωt)]

= Vm ( cos α- cos β)/k(β – α) + (π + α – β)E/ π

ωm = Vm ( cos α- cos β)/k(β – α) - π Ra/K2(β – α) T

ωmc which separates continuous conduction from discontinuous conduction for a given α as ωmc = Ra Vm /ZK sin (α – Ф)[1+ e-πcotФ / e-πcotФ-1]

Continuous conduction

Va = 1/π⌠απ+α

Vm sinωt d(ωt) = 2 Vm/π cosα

ωm = 2 Vm/πk cosα- Ra/K2T

No load speed is given by ωmo = Vm/k, for 0 ≤ α ≤ π/2

= Vm sin α/K, for π/2≤ α ≤ π

Boundary between continuous and discontinuous conduction shown in the fig


Single Phase Half Controlled Bridge Converter Control Of DC Separately Excited Motor

Driver circuit


Discontinuous conduction

A cycle of motor terminal consist of three intervals

i) Duty interval ( α ≤ ωt ≤ π)

ii) Freewheeling interval ( α ≤ ωt ≤ β) : operation governed by the following equation:

Ia Ra+ La dia/dt+E=0

The initial current yields

Ia(ωt) Vm/Z[ sinФ . e – (ωt-π)cotФ –sin (α- Ф). e – (ωt-π)cotФ]-E/Ra[1- e – (ωt-α)cotФ] iii) Zero current interval(β ≤ ωt ≤ π+α)

eβcotФ=Ra Vm/ZE[sinФ e πcotФ- sin (α-Ф) e αcotФ]+ e αcotФ


Va = 1 Vm Sin wt d(wt) +

+

Ed(wt)

wm = Vm(1 + cos ) K( – ) – Ra Ka(( – ) T

Boundary between continuous and discontinuous conduction is reached when = +

substituting = + above equation gives the critical speed wmc

wmc = Ra K

Vm

k

sin . e– cot

– sin( – )e– cot

1 – e– cot

Continuous conduction

Va = 1 Vm sinwt d(wt) = Vm (1 + cos )

wm = Vm

K (1 + cos ) –

Ra

K2 T

Speed torque curves of single phase half controlled rectifier fed separately excited motor

3)Three phase fully controlled rectifier control of DC separately excited

motor

Thyristor are fired in the sequence of their numbers with phase difference of 60º by gate pulse of 120 º duration. Each thyristor conduct for 120 º and two thyristor conduct at


time one from lower group and another from upper group applying voltage to the motor

respectively.

Driver circuit

Motoring operation = 30º

If the line voltage taken as the reference voltage, then

VAB = Vm sinwt

= wt – 3

For the motor terminal voltage cycle from +

3

to + 2

3


Va = 3

+3

+ 23

Vm sinwt d(wt)

= 3 Vm cos

wm = 3vm

K cos –

Ra

K2 T

Speed torque curve of drive after neglecting discontinuous conduction

Three phase half controlled rectifier control of DC separately excited

motor

For the rectifier above shown in the, under continuous conduction

Va = 3Vm

2 (1 + cos )

wm = 3vm

2 K (1 + cos ) –

Ra

K2 T


4)MULTIQUADRANT OPERATION

Here, multiquadrant is considered with regenerative braking. In these drives, current control is always is provided to limit the current within safe

limit.

Three schemes are used 1. Single fully controlled rectifier with a reversing switch.

2. Dual converter 3. Single fully controlled rectifier in the armature with fields current reversal.

Single fully controlled rectifier with a reversing switch.

Four quadrant employing single converter and reversing switch

A fully controlled rectifier feed the motor through reversing switch.

A fully controlled rectifier can provide the operation of quadrant I and IV

And reversal armature connection of can provide the operation of quadrant II and III.

Thyristor switch is realized in the circuit over the relay contactor to avoid the

maintenance.


Dual converter

Dual converter

Dual converter consists of two fully controlled converters in anti parallel across the armature.

Rectifier A, provides the positive motor current and voltage in either direction and allows motor control in I and IV . Rectifier B, negative motor current and voltage in

either direction and allows motor control in II and III. There are two methods of control

a) In simultaneous control both the rectifiers are controlled together. Thus

VA + VB = 0

cos + cos = 0

A + B = 180°

b) n non circulating current control method only on e thyristor controlled at a time.

CHOPPER FED DC DRIVES Choppers, also commonly known as dc- to dc convertrers, are used to get variable

voltage from the fixed voltage.

Chopper control separately excited DC motor

Motoring control Transistor Tr is operated with periodically time T and with on for a period of ton


Chopper control of separately excited motor

During the on period of the transistor, 0 ≤ t ≤ton, the motor terminal voltage is V. The operation is described by

Ra i a + La d i a

d t+ E = 0

At t=ton Tr turned off and motor current freewheels through the diode and motor terminal voltage is zero during the interval ton ≤ t ≤T, Motor operation during the interval is known as

freewheeling interval, is described by

Ra i a + La d i a

d t+ E = 0

Ratio of duty interval ton to chopper period T is called duty ratio or duty cycle (δ). Thus

δ = Duty interval/T = ton/T


Va = 1T

0

t°n

Vdt = V

Ia = V – ERa

wm = VK

– Ra

K2T

Regenerative Braking

Transistor Tr operated periodically with a period T and on period of ton.

The mechanical energy converted into electrical energy by the motor, now working as generator, partly increase the stored magnetic energy in armature circuit inductance and reminder is dissipated in armature resistance and transistor.

If the δ is again defined as the ratio of duty interval to period T, then

δ = Duty interval/T = T- ton/T


Va = 1T

t°n

T

Vdt = T – t°n

T

Ia = E – VRa

S ince Ia has reversed

T = K Ia

= VK

– Ra

K2 T

Speed torque curves of chopper controlled separately excited motor


5)Explain the chopper control of DC series motor. Chopper Control of Series Motor

Motoring:

During the on period of the transistor, 0 ≤ t ≤ton, the motor terminal voltage is V. The

operation is described by

Ra i a + La d i a

d t+ E = 0

At t=ton Tr turned off and motor current freewheels through the diode and motor terminal

voltage is zero during the interval ton ≤ t ≤T, Motor operation during the interval is known as freewheeling interval, is described by

Ra i a + La d i a

d t+ E = 0

Ratio of duty interval ton to chopper period T is called duty ratio or duty cycle (δ). Thus δ = Duty interval/T = ton/T


Va = 1T

0

t°n

Vdt = V

Ia = V – ERa

wm = VK

– Ra

K2T

However, e is not constant but varies with ia, The nature of speed torque curves is shown in

the fig:

Motoring and regenerative braking characteristics of chopper controlled series motor

6)Explain how regenerative braking is obtained in series motor with chopper control.

Regenerative braking Regenerative braking of series motor also obtained. During the regenerative braking, series motor function as a self excited series generator. For self excitation, current flowing through the field winding should assist as residual

magnetism.


= V + IaR(a)Ka

T = – KaIa BLOCK DIAGRAM OF CLOSED LOOP DC DRIVE

Block diagram of closed loop control

The converters are made up of semiconductor materials, which have low thermal capacity.

DC motor can carry 2 to 3 times of rated current.

Converter and motor is chosen with equal current to allow the maximum permissible rated current.

The above diagram shows the closed loop control with inner current loop control and outer speed control loop.

Basic approach for the closed loop control is above and below the base speed.

The field current is constant and armature voltage is variable for below the base speed and for above base speed field current is variable and armature current is

constant. The emf is chosen as 0.85 or 0.95 of the rated armature voltage. The speed will

be very and it will be compared with reference speed and then speed will be

increased according to reference speed.

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