Question

The figure shows an electrical circuit used to measure the temperature of a gas. When the resistance R Is heated by the gas to a certain temperature, a current of 100 mA is observed to flow through the galvanometer G.

Determine the temperature of R If the circuit was originally balanced at 30° C

The temperature coefficient of resistance ft Is 0.0035 per° C.

 

Question

Determine the magnitude and direction of the current in the 60Ω resistor shown connected between Band D.

 

Question

In the circuit the batteries P and Q are made up of lead acid cells, each having an e.m.f. of 2V and an Internal resistance of 0,001Ω . Battery P has 60 cells and battery Q has 50 cells.

Calculate the current flowing In each battery and In resistor R.

 

Question

A two-wire distributor system is fed 440V at end X and at 425V at end Y, the distance XY being 1650m. Two loads of 110A and 7OA are supplied from points, 300m and 900m respectively from end X. The resistance of the cable is 0.33Ω per 1000m.

Calculate the current flowing in each section of the distributor network and determine the potential difference across each load.

 

Question

A vessel has a d.c. ring main system 3600m In length to which the current is supplied at 220V. Loads of 60A, 96A and 120A are placed: at 1080m, 1800m and 2880m respectively from one end of the main,

Sketch a diagram of the ma in and indicate there on the amount and direction of the current in each section if the resistance of the cable is 0.01Ω/1000m.

 

Question

A battery has an e.m.f. of 6V and an internal resistance of O.5Ω. Determine, graphically or otherwise, the greatest power which this battery can deliver to a load resistor and estimate the value of external circuit resistance at which this delivery of maximum power will occur.

 

Question

The voltmeter of internal resistance 8Ω, shown in the figure, reads 6V with the switch open aid 12V with the switch closed. Neglecting the internal resistance of the batteries, determine the following:

a)    the current that would flow in a short circuit between x and Y,

b)    the value of resistance R3,

c)    the value of resistance R2,

 

Question

Two generators supply loads X and V as shown in the figure.

Determine:

a)   the current distribution In the system, and

b)    the potential difference across each of the two loads.

 

Question

A d,c, generator of emf. 113V and internal resistance 0.1Ω    is connected to a parallel circuit consisting of an 11 Ω   resistor in one branch and a battery in the other branch. The battery has 50 cells, each of 2V e.m.f, and 0.01 Ω internal resistances. Determine the current flowing In each branch of the parallel circuit when:

a)    the generator and the battery voltages are in opposition,

b)    the generator and the battery voltages are not in opposition.

 

Question

The circuit shown In the figure is used to charge two batteries B1 and B2 with different values of charging current, B1 consists of 50 cells, each with e.m.f. 2.2V and internal resistance 0.12 Ω   and B2 consists of 40 cells each of e.m.f. 1.8V and internal resistance of 0.10 Ω . The generator has an open circuit e.m.f. of 220V and an armature resistance of 0.2 Ω, The resistors R1 and R2 have values of 3.8 Ω   and 80 Ω   respectively.

Determine the magnitude of the current charging each battery

 

Question

In the circuit shown, determine:

a)    the p.d, between points X and Y stating clearly which is at the lower potential

b)    power dissipated In each of the 1 Ω   resistor

c)    the energy In watts-hr and In joules supplied to the circuit in a period of 20 minutes.

 

 

Question

A magnetic circuit is built up or rectangular laminated iron plates of width 60 mm.

The combined depth of the plates Is 75 mm, the Insulation material between the laminations accounting for 10% of the depth. The magnetic circuit has a mean length of 1.5 m and contains an air gap of 30 mm In length, The effective cross-sectional area of the air gap is 5000 mm2 .

Determine the magnetomotive force required to produce a flux of 0.005 Wb across the air gap, given that the leakage factor is 1,1, the relative permeability of iron is 2500, and the permeability of free space Is 4π   x 10 -7   H / m.

 

Question

A magnetic circuit Is to be constructed to satisfy the following:

Air gap length                                 0-001 m

Flux set up in air gap                      0,01 Wb

Flux density in air gap                      1Tesla

Relative permeability of steel core at working density       1200

No. of exciting coil turns                300

Supply current                                   5 A

Assuming no leakage or fringing occurs, determine the length of the steel portion of the circuit to satisfy the above requirements, given that permeability of air is 4π x10-7   H/m

 

Question

A coil of 1500 turns Is uniformly wound on an iron ring having a mean diameter of 0.4 m and a cross-sectional area of 0.0009 m2 . Given that the permeability of air is 4π x10-7   H/m, determine:

a)    The relative permeability of the iron If a current of 0.6 A produces a flux of 0,0015 Wb, and

b)    The current required to maintain the same flux density if an air gap of length 0.007 m is made in the iron ring, assuming that no flinging occurs.

 

 

Question

An iron ring has internal and external diameters of 0.1 m and 0.12 m respectively and axial thickness of the ring is 0.015 m. The ring is uniformly wound with 300 coil turns. Given that the permeability of free space Is 4π x10-7   H / m, and the following data;

Flux density (Tesla) :            1.2                1.3                1.4               1,5

Relative permeability :        2000             1560                1150            750

Determine:

a)    The current required to set up a flux of 200 μ Wb,

b)    The current required to maintain the same flux of 200μ Wb, and the

corresponding reluctance of the magnetic circuit; if a radial gap 3 mm wide is made in the ring,

 

 

Question

The magnetic circuit shown in the figure Is the arrangement for the overload trip of a starter, The gaps are set for 25 per cent overload current. If the armature lifts when the flux density in the air gap is 0,8 Tesla and the relative permeability of the steel of the magnetic circuit is 2140 at this flux density, determine the full-load current to the motor for which the starter is designed. Neglect leakage and fringing. The cross-section of the magnetic circuit can be taken as being uniform throughout and the permeability of air taken as 4π x10-7H /m.

 

Question

A wooden ring has a mean diameter of 0.3 m and a cross-sectional area of 0,00065 m2   .A coil "A"' having 300 turns is wound uniformly on this ring, A coil "B"' having 700 turns is wound uniformly over the coil A. A steady direct current of 2 amperes in coil A is reversed In 0.002 seconds. Given that the permeability In the wooden core is that of free space (4π x10-7H / m ), and assuming that all the magnetic flux produced by the current in coil A links all of the turns of coil Br determine,

a)    the average value of the e.m.f. Induced in coil B as a result of the current reversal, and

b)    the value of the mutual Inductance between the two coils.

 

Question

The ferromagnetic part of a magnetic circuit shown has a mean length of 0.4 m, a cross-sectional area of 0,001 m2 and the relative permeability of 600.

The exciting coil is wound with wire of resistivity 0,018 μΩm and cross-sectional area 0.5 x 10-6 m2    and the mean length of turn Is 0.21 m.

Neglecting fringing, determine the direct voltage which must be applied to the coil in order that a magnetic flux of 1.2 mWb is obtained in the air gap. Given μo      =   4π x10-7   H/m.

 

 

Question

A current of 1.5 A passing through a coll of 120 turns produces a magnetic flux of 12μWb. Calculate the self inductance of the coll.

If an identical coil is so arranged that 60 per cent of the flux from the first coil links the second coil, determine their mutual inductance.

If both coils are arranged in series, with a current of 1.5 A flowing through each coil and having a 60 per cent mutual flux linkage, determine the total inductance of the series circuit when:

a)    the mutual fields are additive, and

b)    the mutual fields are In opposition.

 

Question

(a)    Explain what is meant by the mutual induction of a pair of coils and define the unit in which It Is expressed,

(b)    A ring of non-magnetic material having a mean diameter of 0.12 m and a cross-sectional area of 2 x 10-4m2'    is uniformly wound with a coil of insulating wire comprising 750 turns. A second coil having 400 turns is then wound on top of the first coil,

Assuming that all the magnetic flux produced by the current In the first coil links with the second coil, determine the mutual inductance.

The permeability of free space, μo      =   4π x10-7   H/m

 

Question

A 3300 / 400 V, single phase transformer has 300 turns on the primary winding. Determine the magnitude of the maximum magnetic flux in the coil under normal operating conditions.

If the frequency Is Increased from 50 Hz to 60 Hz, determine the magnitude of

the primary applied voltage to maintain the maximum flux density In the core at its previous value,

 

Question

A single phase transformer with a rated output of 175 KVA at 2 KV and

operating at 60 Hz has a primary winding of 1000 turns and a secondary winding of 200 turns. Neglecting the magnetizing current and ignoring the winding volt drops resulting from the winding resistances and leakage reactance’s, determine:

a)    the magnitude of the primary voltage

b)    the magnitude of the primary current corresponding to the rated output, and

c)    the peak value of the magnetic flux in the core.

 

Question

A single phase transformer has 200 primary and 480 secondary turns. The effective cross-sectional area of the core is 40 x 10-4 m 2. If the primary winding is connected to a 100 V, 50 Hz supply, calculate:

a)    the secondary voltage, and

b)    the maximum value of the magnetic flux density in the core.

Determine the efficiency of the transformer given that the primary and secondary currents are 5 A and 2 A respectively, both at unity power factor,

 

 

Question

A transformer has an efficiency of 97.5 per cent at unity power factor when its load is 3 KVA, the copper and iron losses being equal at this load, Estimate its efficiency for a performance over 24 hours if it has no load for 7 hours, quarter load for 5 hours, half load for B hours and full load for 4 hours.

The Iron loss and power factor may be assumed to remain constant throughout the period of 24 hours.

 

Question

A 440 V / 110 V transformer with an effective primary resistance of 0.5Ω and a secondary resistance of 0.03Ω has an Iron loss of 200 W on normal Input voltage. Given that maximum efficiency occurs when the copper loss Is equal to the Iron loss, determine:

a)    the secondary current when maximum efficiency occurs, and

b)    the value of the maximum efficiency on a unity power factor load.

 

Question

The primary and secondary windings of a 500 KVA transformer have resistance of0.42Ω and0.011Ω; respectively. The primary and secondary voltages are 6600 V and 400 V respectively and the iron loss is 2,9 KW, Determine the efficiency on:

a)   full load, and

b)    half load, assuming the power factor of the load to be 0.8.

 

Question

The voltage output from an unstabilized power supply can vary between 20V and 30V. This is to be used with a resistor and a zener diode, as shown in the figure, to provide a stabilized voltage output from which currents of up to 50mA will be drawn. The zener diode has a "breakdown" voltage of iov and should be used with a current flowing through it of at feast 1mA; the resistance of the diode to voltages In excess of 10V Is 8Ω. Determine:

a)    a suitable value of the resistance R when E Is at Its lowest value and the current to the load Is at its greatest value, and

b)    the stabilized voltage output under these conditions.

 

Question

The figure shows a voltage stabilizing circuit. The breakdown voltage of the voltage referents of the zener diode in this circuit is 8V, and above this voltage its slope resistance is 15Ω IL isthe current drawn from the circuit.

Determine:

I)   the value of V when E = 10V and IL  = 0 mA

II)   the value of V when E = 50V and IL  =40mA

iii) the minimum value of E which will give a stabilized voltage output, v, when

IL = 40 mA,

and

iv) the power dissipation In the diode and in the resistor when E = 50V, and IL = 20 mA.

 

Question

The figure shows a circuit diagram with a bridge rectifier from which the diodes have been omitted.

a)    Reproduce the diagram with the diodes correctly connected to give the polarity of the output voltage as shown.

b}   Determine the magnitude of V, given that the voltage drop across each diode when conducting is 0.75V,

c)   Estimate the peak value of the ripple voltage If a constant current of 0.5A Is taken from the rectifier output.

 

 

Question

A 230V, SOHz supply is to be transformed to 40V, then converted to a D.C. supply by a bridge rectifier and then smoothed by means of a "reservoir capacitor" of 2500μ F,

Sketch an appropriate circuit diagram and estimate;

a)    the minimum value required for the "peak inverse voltage" of the rectifier diodes used In the circuit and

b)    the peak to peak value of the ripple voltage for the circuit when delivering a small current of 250 mA.

 

Question

A transistor Is alternately connected In common-base circuit and common-emitter circuit and the current amplification factors are found to be α and β   respectively,

Show that:

β = α / 1- α

A transistor Is connected In common-base configuration and when the collector voltage is constant, a change of 3.26 mA in the emitter current produces a change of 3 mA in the collector current.

Determine the current amplification factor of the transistor when It is connected in common-emitter configuration.

 

Question

The following data refers to an n-p-n transistor: Base current flowing   IB = 20 μ A

Collector/Emitter voltage VCE (V)

1

5

9

Collector Current            Ic (mA)

2.6

2.8

3.0

a)    Determine the d,c. common emitter current gain at SV and also at 9V.

b)    Determine the "slope" output resistance of the transistor.

c)    Sketch a circuit diagram to show the position of the instruments, and the means of varying VCE   and Ic  , to obtain the above data.

 

Question

For the circuit shown In the figure, determine:

a)    the collector current flowing,

b)    the voltage at the emitter, at the base and at the collector of the transistor All voltages being measured with respect to earth, and

t)    the power dissipated in the transistor.

The voltage between the base and the emitter of the transistor, when conducting, may be taken as 0.6V. The current gain of the transistor is very high.

 

 

Question

The forward characteristics of a junction diode are as follows;

I   (mA)

0.1

0.3                        1.0

3.0

V  (V)

1

2                                        3

4

Estimate::

a)    the forward d.c. resistance when the current is 2.5 mA, and

b)    the voltage and current when the forward d.c. resistance Is 2200Ω .

 

Question

The negative terminal of an 16V battery Is connected to the emitter of an n-p-n transistor and the positive terminal Is connected through a 5KΩ resistor to the transistor collector,

I)     Draw a circuit diagram and determine the value of the base current needed to make the potential difference between the collector and emitter equal to 3V, giver that the transistor current gain is 120,

ii)    Determine the magnitude of the base current at which the voltage between the collector and emitter would dminlsh almost to zero,

 

Question

The output characteristics for a common-emitter connected transistor are set out in the following table:

Voltage between collector and emitter

3

7

10

Collector current (mA) when base current is 30μA

1.0

1.20

1.5

Collector current (mA) when base current Is 60μA

2,1

2.55

2.9

Collector current (mA) when base current Is 90μA

 

3,88

4,4

a)    Draw the characteristic curves for this transistor and construct load lines to show the operation from a 6,5V battery with load resistors of 1000Ω and 1500Ω  respectively.

b)    If a suitable value of base bias current Is 60μA for an Input signal of ± 30μ A, determine the current amplification for each load.

 

Question

The equivalent T-circuit of a transistor used In the figure, where re  =  20Ω, rb =  500Ω and rc   =  1.5 MΩ.

The transistor Is used with a load resistance   R =  12 KΩ,

Given that the amplification factor α  is 0.98, determine:

a)    the current gain,

b)    the voltage gain,

c}   the input resistance Rs

 

Question

The output voltage from an amplifier is sinusoidal and its magnitude Is limited to ± 24V. Estimate the maximum output power that can be obtained from this amplifier given that the load resistance Is 8Ω ,

If the Input voltage and resistance of the amplifier are 0.1V r.m.s. and 1 KΩ respectively, determine the maximum possible:

a)    voltage amplification factor, and

b)    power gain ratio,

 

 

Question

A fluorescent lamp, rated at 100W is equivalent to a pure resistance of 220Ω The lamp Is to be operated at the rated value from a 250V, 50Hz supply, by having an Inductive coil (of negligible resistance) connected in series with the lamp, Determine the Inductance of the coll.

Sketch the phasor diagram of the circuit and evaluate the phase angle between the current and the supply voltage

 

Question

Two coils, A and B, are connected in turn across a 150V, 50Hz supply. Coil A takes 12 amperes from the supply and dissipates 270W. Coil B takes 15 amperes from the supply and dissipates 1125W. When the two coils are connected in series across the supply, determine:

a)   the current flowing from the supply,

b)    the power taken from the supply,

c)    the overall power factor, and

d)    the value of a pure capacitor, connected In series with the two coils, to give an overall power factor of 0,6 lead,

 

Question

A coil of inductance 0.52 H and resistance 120Ω Is connected In parallel with a 30μ F capacitor across a 240V, 50Hz supply.

Calculate the total current and the power factor.

 

Question

A copper inductor which has an iron core, takes a current of 2A at a lagging power factor of 0.4, from a 240V, 50Hz supply. After the iron core is completely removed, the inductor takes a current of 4A at a lagging power factor of 0.8, from a 100V, 50Hz supply.

Estimate the Iron core loss sustained by the Inductor at 50Hz,

 

Question

An electric circuit consists of a 15Ω resistance in parallel with a series combination of resistance R and capacitance C, The circuit is supplied from a 50Hz sinusoidal a.c, source, The current taken from the supply Is 8A, the current in the 15Ω  resistance branch is 7 A and the current in the R-C combination branch is 3A.

Determine the phase angle between the branch currents and evaluate the magnitudes of R and C.

 

Question

A capacitor and a resistor In series are connected across a 150V, a.c, supply. When the frequency is 40Hz the current is 5A and when the frequency Is 50Hz the current Is 6A. Determine the values of the capacitor and resistor.

If the resistor is connected In parallel with the capacitor across a 150V, 50Hz a,c. supply, determine the total current and the power factor

 

Question

A two-branch parallel circuit is connected across the terminals of a sinusoidal

a.c. supply. One branch contains a resistance of 4Ω  and an inductive reactance of 8Ω  , the power dissipated in the branch being 800W. The other branch contains two pure circuit elements and the total power dissipated in the whole circuit is 3.15KW at a leading power factor of 0.9.

a)    Identify the elements in the second branch.

b)    Determine the ohm ic magnitude of each circuit element in the second branch.

c)    Sketch a phasor diagram for the circuit.

 

Question

A circuit comprising two branches In parallel takes a current I from a 240V, 50Hz supply. One branch is a pure capacitor of reactance 60Ω and the other Is an Inductor of resistance Rand Inductance L The Inductor can be varied, but Its ratio R/L remains constant at 7 5/1.

If the Inductor is adjusted to make,

a)    I a minimum and

b)    I in phase with the supply voltage

Determine each of these values of I and the corresponding currents In the Inductor.

 

Question

In the figure, the potential difference across the 8Ω resistor Is 96V, Determine:

a)    the magnitude of the current drawn from the

b)    the mean value of the power drawn from the supply,, and

c)    the magnitude of the potential difference across the parallel combination of three resistors.

 

Question

A coil of resistance 15Ω and inductance 0.05H is connected in series with a 100 μF capacitor across a 240V, 60Hz supply.

Calculate;

a)    the value of the voltage across the coil,

b)    the value of the voltage across the capacitor,

c)    the total power taken from the supply,

d)    the resonant frequency for the circuit and

e)    the voltages across the capacitor and the coil during resonance

 

Question

A load of 1.8 KVA at a leading power factor of 0.6 If In parallel with a load of 1.2 KVA at a lagging power factor of 0.8. Given that the applied voltage Is 240V, calculate the line current and the power factor

Sketch a phasor diagram for the circuit.

 

Question

Two motors, A and 6, are supplied with current from a 120V, 50Hi mains source, Motor A takes 1755 Watts and 0.8 power factor lagging and motor B takes 933 Watts at 0,707 power factor lagging.

Calculate the following;

a)    the current taken by each motor

b)    the total current and overall power factor, and

c)   the capacitance required to raise the overall power factor to unity when connected in parallel with the load,

 

Question

Equal currents flow when two coils X and Y are tested separately across an alternating supply. Coil X is found to have a power factor of 0,8 and coil Y a power factor of 0.75.

If coils X and Y are now coupled in series across the same supply, calculate the overall power factor of the circuit.

 

Question

An a,c. electrical installation consists of;

a)    one synchronous motor taking 60 KW at a leading p.f. of 0.6

b)    one induction motor taking 80 KW at a lagging power factor of 0.7 5

c)    a bad of 60 KW at a lagging p.f. of 0.8

Calculate the resultant p.f. of the installation and the total KVA. Sketch a phasor diagram showing all the power and KVA components.

 

Question

A 190 KW load at a lagging power factor of 0,5 Is taken from an11KV, 3-phase supply.

When an additional 166 KW load Is taken from the same supply, the overall p,f becomes unity,

Determine:

a)    the line current of the 190 KW load when It Is connected to the supply alone,

b)    the power factor of the 16S KW load,

c)    the line current of the 166 KW load, and

d)    the line current supplying the combined loads.

 

Question

An unbalanced 3-phase, 4 wire star connected load is supplied by a line voltage

of 415V and non-inductive loads of 1 KW, 0.6 KW and 0.8 KW are connected between the central line, and the red, yellow and blue lines respectively. Determine:

a)    the magnitude of the current in each line, and

b)    the magnitude of the current In the central line and Its phase displacement from the current In the red line,

 

Question

A three-phase, 3300V Industrial load consists of:

(i)      a 59 KW motor of B4 per cent efficiency and 0.9 power factor lagging,

(II)     a load of 120 KVA at unity power factor, and

(III)    a load of 160 KW at 0,8 power factor lagging.

Calculate:

a)    the total power

b)    the supply current, and

c)    the overall power factor.

Sketch a phasor diagram showing all of the power and KVA components,

 

Question

A 400V, 3-phase balanced load takes 4DA at a power factor of 0.8 lagging, An over excited synchronous motor is connected parallel to the load to raise the power factor of the combination to unity. The mechanical output and mechanical efficiency of the synchronous motor is 12 KW and 91 per cent respectively.

Determine:

a)    the power factor of the synchronous motor, and

b)    the total power taken from the supply mains.

 

Question

A three phase system supplies the following loads:

(I)    30 KW of heating and lighting load at unity power factor, (it)    40 KW of induction motor loading at 0.8 p.f, lagging,

A 37.3 KW synchronous motor, efficiency 80%, Is to be used to raise the overall power factor to 0.95 lagging.

Determine the power factor at which the synchronous motor must operate:

a)    when used In conjunction with the loads shown in (I) and (II), and

b)    when used in conjunction with the induction motor loading only.

 

Question

A 240V, single phase, 50Hz, 500W-output motor has a full load efficiency of

70 per cent and a power factor of 0.6 lagging. If the power factor Is to be increased to 0.9 by means of a capacitor connected in parallel with the motor, determine the minimum required value of the capacitor,

Estimate also the value of a capacitor which connected In similar manner would improve the power factor to unity.

 

Question

A 220V, 4-pole wave wound, shunt generator has an armature resistance of 0.1Ω and a field resistance of 50Ω . Calculate the flux per pole, if the machine has 700 armature conductors, runs at 800 rpm and is supplying a 38 KW load.

 

Question

A shunt generator Is to be converted Into a level compound generator by the

addition of a aeries field winding. From a test on the machine with shunt excitation only, it is found that the shunt current is 4.1A to give 440V on no load and 5.8A to give the same voltage or full load of 200A.

The shunt winding has 1200 turns per pole.

Find the number of series turns required per pole.

 

Question

A 240V, 70KW d.c. generator has 1400 turns per pole on its shunt field winding. When on no load, a field current of 4A is required to produce terminal voltage of 240V. On full load, the field current must be Increased to 6A In order to maintain

the terminal voltage.

Calculate the number of series field turns to produce:

a)    level compounding and;

b)    6 per cent over compounding.

State and explain any assumptions which have to be made.

 

Question

The open-circuit characteristic of a d,c, shunt generator operating at 15 rev/s is as follows:

Generated e.m.f, (V)     100     210     300     360     400     420 Field Current (A)             0.5     1.0       1,5      2,0      2.5      3.0

Plot the open-circuit characteristic of the generator for a speed of 20 rev/s and determine, for this operating speed:

a)    the open-circuit generated e.m.f. If the shunt field resistance Is 200Ω and

b)    the critical resistance value of the shunt field.

The effects of armature resistance and brush volt drop may be neglected.

 

Question

A short shunt compound d,c. generator has a terminal potential difference of 125V on no-load, when driven at rated speed. If the terminal potential difference is to be 130V when the load-circuit current Is BOA, determine the number of  series connected turns per pole given that:

Armature winding resistance                                 =    0.3Ώ

Shunt field circuit resistance                                 =    125Ώ

Number of turns per pole for shunt winding         =    1100

Total brush - commutator contact voltage loss      =    1V

Series field winding resistance                              =    0.1 Ώ

Estimated field-weakening effect of

armature-reaction at 80A loading                         -    50 ampere-turns per pole

 

Question

Two similar d.c. machines are mechanically coupled so that one machine acting as a motor drives the other as a generator. The field coils of both machines a re connected in series with each other and in series with the motor armature circuit, and the motor is supplied at a supply at 230V, the effect being that the motor has a series field and the generator has a separately excited shunt field.

Given the following data, determine the friction and windage loss of each machine and hence estimate the efficiency of each machine.

Motor circuit current                                 16.2 A

Generator current                                     14.3 A

Generator terminal voltage                      155 V

Resistance of each machine armatu re     0,7 Ώ

Resistance of each machine field             0.4Ώ

 

Question

A 50KW, 250V, 750 rev/mln long shunt d,c. compound generator has shunt field, series field and armature resistance of 100Ώ, 0.05Ώ and 0.25Ώ respectively. If the prime mover falls and the machine continues to run as a motor drawing 50A from the 250V bus bars, estimate the speed of the machine while running in the motor mode.

 

Question

Two shunt generators are connected in parallel. Their external characteristics are rectilinear over their normal working range. The terminal voltage of one generator falls from 270V on no-load to 240V when delivering 300A to the busbars, whilst the terminal voltage of the other generator falls from 260V on no-load to 245V when delivering 200A to the busbars,

If the machines share a common load of 500A, determine:

a)    the current which each machine delivers

b)    the busbar voltage

c)   the power delivered by each machine,

 

Question

A 240V d.c. shunt motor has an armature resistance of 0.5Ώ and an armature current of 60A on full-load. Determine how the magnetic flux must be reduced in order to increase the speed by75percentlf the developed torque is constant.

The effect of armature reaction may be neglected.

 

Question

Ad,c, machine has an armature resistance of 0,2 Ώ and a field winding resistance of 0.3Ώ.The machine is driven as a series connected motor on a 500V d.c. supply when It takes a current of 79.2A and develops a torque of

500 Nm.

The machine Is then stopped and a 0.2 Ώ resistor is connected In parallel with the series field winding from outside the machine and the machine Is again driven as a motor from the same supply until the torque developed Is 550 Nm,

Determine for the final condition, the power Input to the machine given that ϕ   =   0.08 Ip   , for the operating range where ϕ is the magnetic flux and If is the field current, producing magnetic flux.

 

Question

A shunt motor supplied from a 220V d.c. supply takes a armature current of 55A when operating at 10 rev/s, The armature resistance is 0,2 0 ,

Determine the speed at which the motor would operate if the magnetic flux is increased by 10 per cent and a 0.1Ώ  resistance is connected in series with the armature given that the torque developed by the motor remains unchanged.

 

Question

A d.c. shunt generator has an overall efficiency of 70% when supplying a current of 80A at 120V. Given that the friction and windage losses amount to 350W and that the value of the shunt resistance Is 40 Ώ , determine the armature resistance.

Estimate the torque of the prime mover given that the rotational speed of the generator armature Is 6 rev/s.

 

Question

A d.c, shunt wound machine gives a n output of 100KW at 440V when run as a generator at 20 rev/sec. The resistances of the armature and the shunt field are 0.05 Ώ  and 200 Ώ   respectively.

When the machine Is run as a motor on a 440V supply, the Input power Is 100KW and the iron and mechanical losses total 3.24KW. When the machine Is run as a motor, calculate:

a)   the motor speed,

b)    the output power, and

c)    the motor efficiency.

 

Question

While running without load, the speed of a 100V d.c. shunt motor Is 1000 rev/min and the armature current Is 1,2A, The armature circuit resistance Is 1Q aid the field resistance Is 200 Q ,

Calculate the speed and the efficiency of the motor if the armature current increased to 30A. The windage and friction losses are proportional to the armature speed squared.

 

Question

A shunt motor running at normal speed takes a current of 2A while en no load from a mains supply of 24QV. The motor has a field circuit resistance of 240 0 and an armature resistance of 0.4 0 .

a)    Estimate the windage and friction loss of the machine and assuming this loss and the speed of the machine to be constant, determine the motor power output and efficiency when the current taken from the mains by the machine is 41A.

b)    If the motor was driven by a prime mover as a shunt generator at the same speed as before and the terminal voltage and output current delivered by the generator were 240V and 40A respectively, determine the efficiency of the machine when running as a generator.

 

Question

A d.c. series connected motor operates on 350V dc supply. The machine Is directly coupled to a load, the torque of which Is proportional to the value of the speed at which it is driven. At a certain speed the current taken by the motor is 40A.

a)    Calculate the value of resistance to be Inserted into the motor circuit In order to reduce the speed by 20 percent. The magnetic flux per pole may be assumed directly proportional to the current in the field winding. It is assumed in this calculation that the resistance of the field and armature windings are zero.

b)    Determine the value of the power In KW, supplied to the motor at the reduced speed.

Rotational losses In the d.c. motor may be ignored In all calculations.

 

Question

A4KW, 220V d,c> shunt motor has a full-load efficiency of $5 percent and a full-load speed of 12,5 rev per second. The shunt field current Is 1.6A.

Calculate the full load armature current and the value of starting resistance required if the armature current at start is to be limited to 1.3 times full-load

value,

If the first step of the starter is to be cut out when armature current has fallen to full-load value, determine the speed of the motor when this step Is cut out. The armature resistance Is 0,55 ohm,

 

Question

Two alternators, A and 6, connected In parallel, supply 100 KW at 0.8 p.f. lagging. Machine A supplies 60 KW at 0,7 p.f. lagging. Calculate for machine B:

a)   the KW output,

b)   the KVA output,

c)    the KVAr output, and

d)   the operating power factor.

 

Question

A load of 8000 KW at 0.8 power factor lagging is taken from two alternators running in parallel. The load on one alternator is 2000 KW at 0.543 power factor lagging. Calculate the power factor of the second alternator and its KVA output Sketch a phasor diagram of the system.

 

Question

Two 3-phase star-connected alternators share a bad of 210 KW, at power factor 0.75 lagging, In the ratio 2:3, The line voltage Is 2.2 KV. If the current loading on the first machine (I.e. the number supplying the least power) Is

28 A, determine:

a)    the current loading on the second machine, and

b)    the power factor at which each of the machine is operating.

 

Question

Two 1100 KVA alternators operating tn parallel with similar power factors share the following loads equally:

500 KW at unity power factor

Induction motor taking 1000 KW at 0,7 lagging.

Synchronous motor taking 500 KW.

Determine:

a)    the power factor at which the synchronous motor should operate so that the alternators are not overloaded, and

b)    the KVA loading at which the synchronous motor should operate to raise the power factor of the combined load to unity.

The maximum loading on each alternator is 1000 KW.

 

Question

Two Identical 3-phase, star-con nee ted alternators, X and Y, are connected In parallel and share a load of 3Q0 kva at p.f. 0.85 lagging, The line p.d. Is 2,2 KV. When the power output of machine X Is 90 KW, with a lagging power factor and a line current of 35 A, determine;

a)    the p.f. at which each machine operates, and

b)    the current supplied by machine Y.

Voltages and currents may be assumed to vary slnusoidally with time.

 

Question

Two 6600 V, 3-phase, star-connected alternators supply the following loads:

500 KW at unity power factor

1200 KWatO.7B power factor lagging

800 KW at 0.7 power factor lagging

One alternator supplies current of 150 A at a power factor of 0.8 lagging, For the other alternator, determine:

a)    the power supplied

b)    the power factor

c)    the line current.

 

Question

A 3-phase, 6-pole, induction motor is supplied from a 3-pfiase, 50 Hz source and operates at unloaded condition with 4 per cent slip.

Determine:

a)    the synchronous speed

b)    the rotor speed, and

c)    the frequency of the rotor current.

If the motor is now loaded and operates at a steady speed of 10 rev/s, determine the altered frequency of the rotor current.

 

Question

A 6-pole, three-phase, 415 V, 60 Hz, Induction motor has a slip of 5 per cent and takes a current of 31 A when providing a shaft torque of 100 Nm. Determine the motor speed and Its brake power.

Given that the torque lost due to windage and friction Is 10 Nm and that the Iron and copper losses are 900 watts, estimate the efficiency and power factor of the machine.

 

Question

a)    State why "slip" is always present in an induction motor, even when the

motor is running unloaded,

b)   A 3-phase,. 4-pole Induction motor Is supplied from a 50 Hz supply and runs unloaded with 4% slip.

Determine:

1)     the synchronous speed, 2)    the rotor speed, 3)   the frequency of the rotor current, 4)   the speed of rotation, in space, of the rotor m.m.f., and 5)    the rotor e.m.f./phase when the motor Is running unloaded with 4% slip, if the rotor e.m,f./phase at standstill Is 150 V.

 

 

Question

A 4-pole induction motor has its stator windings connected in delta, the motor obtaining Its energy from a 415 v, 60 Hz, three-phase supply, The shaft-power Is 10 KW and the power factor, overall efficiency and slip are 0.9, 0.83 and 0,5 respectively.

Determine:

a)    the rotor speed.

b)    the rotor current frequency,

c)    the current flowing In each phase of the stator windings,

d)    the equivalent per phase resistance and inductance of the motor, expressed in terms of stator windings.