industrial drives & applications (06ee74) · web viewthe drive circuit is shown in fig...

INDUSTRIAL DRIVES & APPLICATIONS (06EE74)

SOLUTION TO QUESTION BANK

Unit 3 & 41) Explain the working of Single-phase half controlled rectifier for continuous mode of

operation. (June-2015),(Jun-2013)

Fig: Single-phase fully-controlled rectifier-fed dc separately excited motor

The drive circuit is shown in Fig Motor is shown by its equivalent circuit. Field supply is not

shown. when field control is required. Feld is fed from a controlled rectifier, otherwise from an

uncontrolled rectifier. The ac input voltage is defined by Vs = Vrn sin wt

(a) Drive circuit (h) Discontinuous conduction (c) Continuous conduction waveforms .Waveforms in

a cycle of source voltage. Thyristors T1 and T3 are given gate signals from a to tr. and thyristors T2

and Ts are given gate signals from + a) to 2ir. When armature current does not flow continuously, the

motor is said to operate in discontinuous conduction. When current flows continuously, the conduction

is said to be continuous. The drive under consideration, predominantly operates in discontinuous

conduction.

Discontinuous conduction has several modes of operation. The approximate, but a simple,

method of analysis is obtained when only the dominant mode of discontinuous conduction is taken into

account. The motor terminal voltage and current waveforms for the dominant discontinuous

conduction and continuous conduction modes are shown in Figs. 5.26(b) and (c). In discontinuous

conduction mode, current starts flowing with the turn-on of thyristors T1 and T3 at cot = a. Motor gets

connected to the source and its terminal voltage equals v1. The current, which flows against both, E

and the source voltage after an = rr, falls to zero at Due to the absence of current Ti and T3 turn-off.


Motor terminal voltage is now equal to its induced voltage E. When thyristors T2 and T4 are fired at

Or + a). next cycle of the motor terminal voltage v. starts. In continuous conduction mode, a positive

current flows through the motor, and T2 and Ts are in conduction just before a. Application of gate

pulses turns on forward biased thyristors T1.

2) Explain the dynamic braking operation of separately excited dc motor. Draw its speed torque

characteristics. (Dec-2013)

Fig: Circuit & speed-Torque characteristics of dc shunt motor depicting four-quadrant operation


The supply to the field winding is maintained, but the armature is disconnected from the supply

voltage and reconnected to an external resistor Fig.The machine now acts as a generator, convening

kinetic energy stored in its moving parts to electrical energy, which is dissipated as heat in the resistor.

This method of braking is called the dynamic or rheostatic braking. Since, V= 0 and the polarity of the

back emf remains unchanged, the armature current during dynamic braking is determined from the

equation

3) With dynamic equivalent circuit, explain the transient analysis of separately excited dc motor.

(Dec-

2013) DC motor operates on the principle that when a current carrying is placed in a magnetic field, it

experiences a mechanical force given by F = BIL newton. Where the current and ‘L’ is the length of

the conductor. The direction of force can be found by left hand rule. Constructional, there is no

difference between a DC generator and DC motor Conductors. The collective force produces a driving

torque which sets the armature into rotation. The function of a commutator in DC motor is to provide a

continent DC generator the work done in overcoming the magnetic drag is converted into electrical

energy.

Conversion of energy from electrical form to mechanical form by a DC motor takes place by

the work done in overcoming the opposition which is called the BACK EMF: is the dynamically

induced emf in the armature conductors of a dc motor when the armature is rotated. The direction of

the induced emf as found by Flemings right hand rule is in Opposition to the applied voltage. Its value

is same . This emf is called as back opposition is converted into mechanical energy.

4) Explain the reverse voltage braking with diagrams of D.C of separately excited dc motor.

(June-2014)

Field, it experiences a mechanical force given by F = BIL newton. Where ‘B’ = flux density in

wb/m2 is the length of the conductor. The direction of force can be found by Left hand rule.


Constructionally, there is no difference between a DC generator and DC motor. Armature conductors

are carrying current downwards under North Pole and upwards under South Pole. When the field coils

are excited, with current carrying armature conductors, a force is experienced by each armature

conductor whose direction can be found by Fleming’s left hand rule. This is shown by arrows. The

collective force produces a driving torque which sets the armature into rotation. The function of a

commutator in DC motor is to provide a continuous and unidirectional torque.

In DC generator the work done in overcoming the magnetic drag is converted into electrical

energy. Conversion of energy from electrical form to mechanical form by a DC motor takes place by

the work done in overcoming the opposition which is called the ‘back emf’. is the dynamically induced

emf in the armature conductors.

5) Explain the plugging of D.C of separately excited dc motor and draw its speed torque

characteristics. (Dec-2014)

If the polarity of the supply voltage to the armature is reversed, while the motor is running the

type of breaking in the motor is termed counter current breaking or plugging due to reversal of

armature connection, applied voltage ‘V’&’Eb’ starts 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

which reversing armature connection

With the reversal of supply voltage, the normal direction of rotation is also reversed, shifting the point

corresponding to no-load speed from Wo to –Wo.

The direction of motor torque is also reversed .The speed torque characteristics may be written as


The motor is operating at speed corresponding to point A in 1stQuadrant. The armature Vg is reversed

with a suitable resistance in the armature circuit. The operating point is transferred from A point to B

in 2nd Quadrant on the characteristics curve drawn according to equation (2).As breaking torque is

developed motor speed drags down along the characteristics curve BC in 2nd Quadrant.

If the purpose of breaking is to stop motor, supply voltage has to be switched off as soon as

speed is zero at point c. If the supply is not switched off, the motor accelerates in opposite direction in

the 3rd Quadrant, since the direction of the rotation has changed, motor EMF also reversed thus

apposing supply voltage. Plugging gives greater breaking torque than Rheostatic Breaking

6) Explain the motoring control and regenerative braking of chopper control of separately

excited dc motor. (Jun-2013)


If the voltage drop across the stator is negligible relative to the voltage drop across the

magnetizing reactance, it is reasonable to re-draw the equivalent circuit with the magnetizing branch at

the terminals of the circuit.

The primary advantage of using the approximate circuit shown above is that the calculation of rotor

current is greatly simplified.

Substituting into the torque equation

Fixed Frequency Operation

If the stator supply frequency is held at the rated value fe b, the stator voltage cannot be increased above

the rated valueVs b, it can only be reduced. Considering the torque equation above, it can be seen that

the magnitude of torque is proportional to voltage squared. The shape of the torque-speed curve will


be independent of voltage. Torque speed curves for an induction motor with a variable voltage supply

are sketched in the figure.

7) Explain the multi-quadrant operation of D.C of separately excited dc motor using Singlephase

fully controlled rectifier with a reversing switch. (Dec-2013)

The motor speed can be varied by

– controlling the armature voltage Va, known as voltage control;

– controlling the field current If, known as field control; or

– torque demand, which corresponds to an armature current Ia, for a fixed field current If.

The speed, which corresponds to the rated armature voltage, rated field current and rated armature

current, is known as the rated (or base) speed.

In practice, for a speed less than the base speed, the armature current and field currents are maintained

constant to meet the torque demand, and the armature voltage Va is varied to control the speed. For

speed higher than the base speed, the armature voltage is maintained at the rated value and the field

current is varied to control the speed. However, the power developed by the motor (= torque X speed)

remains constant.

Figure above shows the characteristics of torque, power, armature current, and field current

against the speed.

8) Explain the chopper control of separately excited dc motor for regenerative braking.


(Dec-2013)

In variable-speed applications, a dc motor may be operating in one or more modes:

motoring, regenerative

braking, dynamic

braking, plugging

Motoring: The arrangements for motoring are shown. Back emfEg is less than supply voltage

Vy. Both armature and field currents are positive. The motor develops torque to meet the load demand.

Regenerative braking:

• The motor acts as a generator and develops an induced voltage E g. Eg must be greater than supply

voltage Va.

• The armature current is negative, but the field current is positive.

• The kinetic energy of the motor is returned to the supply.

• A series motor is usually connected as a self-excited generator.

• For self-excitation, it is necessary that the field current aids the residual flux. This is normally

accomplished by reversing the armature terminals or the field terminals.

9) With a neat circuit diagram and waveform, explain the chopper control of series motor.

(June-2014)

• The arrangements shown in Figure are similar to those of regenerative braking, except the supply

voltage Va is replaced by a braking resistance Rb,.

• The kinetic energy of the motor is dissipated in Rb.

Plugging:

• Plugging is a type of braking. The connections for plugging are simple


• The armature terminals are reversed while running. The supply voltage Va and the induced voltage

Eg act in the same direction.

• The armature current is reversed, thereby producing a braking torque. The field current is positive.

• For a series motor, either the armature terminals or field terminals should be reversed, but not both.

10) Explain the rectifier control of d.c series motor and draw its speed-torque curve (Dec-2014)

DC to DC converters operating under certain conditions. The use of such converters are

extensive in automotive applications, but also in cases where a DC voltage produced by rectification is

used to supply secondary loads. The conversion is often associated with stabilizing, i.e. the input

voltage is variable but the desired output voltage stays the same. The converse is also required, to

produce a variable DC from a fixed or variable source. The issues of selecting component parameters

and calculating the performance of the system will be addressed here. Since these converters are

switched mode systems, they are often referred to as choppers.

The basic circuit of this converter is shown in figure connected first to a purely resistive load.

If we remove the low pass filter shown and the diode the output voltage vo(t) is equal to the input

voltage Vd when the switch is closed and to zero when the switch is open, giving an average output

voltage Vo: Ts = D, the duty ratio. The low pass filter attenuates the high frequencies (multiples of the

switching frequency) and leaves almost only the DC component. The energy stored in the filter

inductor (or the load inductor) has to be absorbed somewhere other than the switch, hence the diode,

which conducts when the switch is open. We’ll study this converter in the continuous mode of

operation i.e. the current through the inductor never becomes zero. As the switch opens and closes the

circuit assumes one of the topologies of figures.


11) Explain the dynamic braking of separately excited by chopper circuit. (Dec-2014)

A single-phase half-wave converter feeds a dc motor, as shown

• The armature current is normally discontinuous unless a very large inductor is connected in the

armature circuit.

• A freewheeling diode is always required for a dc motor load and it is a one-quadrant drive.

• The applications of this drive are limited to the 0.5 kW power level.

• Figure shows the waveforms for a highly inductive load.

• A half-wave converter in the field circuit would increase the magnetic losses of the motor due to high

ripple content on the field excitation current.

12) With circuit diagram and waveforms explain three phase fully controlled rectifier control of

separately excited dc motor. (Jun-2015)

During the interval 0<t<ton ia increases from ia1 to ia2. A part of generated energy is stored in

inductance &rest is dissipated in Ra & Tr. During interval ton<t<T, ia decreases from ia2 to ia. The

energy generated & stored in inductance are dissipated in braking resistance RB, Ra & diode D.

Transistor tr controls the magnitude of energy dissipated in RB& therefore controls its effective value.

Average power consumed by Rb P=

, )

, or ,


If ia is assumed to be ripple less dc, then energy consumed En by RB during a cycle of chopper

operation is

Effective value of RB=

The above equation shows that the effective value of the breaking resistance can be changed

steplessely from 0 to RB as δ is controlled from 1 to 0. As the speed falls, δ can be increased

steplessely to brake the motor at a constant max torque as shown.

Any useful motor relationships between time, current, voltage, speed, power factor and torque

can be obtained from equivalent circuit analysis. The equivalent circuit is a mathematical model used

to describe how an induction motor's electrical input is transformed into useful mechanical energy

output. A single-phase equivalent circuit representation of a multiphase induction motor is sufficient in

steady-state balanced-load conditions.

Neglecting mechanical inefficiencies, the basic components of the induction motor equivalent circuit

are:

• Stator resistance and leakage reactance (

• Rotor resistance and leakage reactance ( )

• Rotor slip ( )


• Magnetizing reactance ( )

• Inertia of the motor and mechanical load.


Paraphrasing from Alger in Knowlton, an induction motor is simply an electrical transformer the

magnetic circuit of which is separated by an air gap between the stator winding and the moving rotor

winding. It is accordingly customary to either separate equivalent circuit components of respective

windings by an ideal transformer or refer the rotor components to the stator side as shown in the

following simplified equivalent circuit and associated table of equations and symbols:

industrial drives & applications (06ee74) · web viewthe drive circuit is shown in fig...

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