# Daily Archives: January 30, 2021

## What does a star–delta starter used with an induction motor do

1. Atwo-pole 60 Hz induction motor has a slip of2%on full load. The full-load

speed is

(a) 58.8 r/s (b) 3528 r/min

(c) 60 r/s (d) 2880 r/min

2.The slip of an induction motor can never be zero in a stable state because

(a) the rotor would be at standstill

(b) induced EMF and rotor current would be excessive

(c) it would then be a synchronous motor

(d) there would be no rotor EMF, rotor current or torque

3. The torque provided by the rotor of an induction motor is at a maximum when

(a) speed is maximum

(b) rotor resistance equals rotor reactance

(c) the connected mechanical load is at a minimum

(d) starting

4. If an electronic speed controller feeds an induction motor, the machine….

## A rotor-resistance starter for a three-phase wound-rotor induction motor

1. A rotor-resistance starter for a three-phase wound-rotor induction motor

(a) adds external resistance to the wound rotor via sliprings

(b) is connected in series with the machine stator

(c) can only be used in conjunction with a double-cage rotor

(d) applies only to DC motors

2. A large synchronous motor is often started by

(a) connecting it directly to the three-phase supply

(b) pushing the rotor in the required direction before connecting the supply

(c) running it up to speed using a ‘pony’ motor before energising the rotor

(d) first switching on the rotor supply then applying the stator voltage

3.A single-phase induction motor

(a) can only be used for loads rated at less than 0.5 kW

(b) will run in whichever direction the rotor is….

## Calculate the resistance of a shunt required to convert it for use as a 0–10 A ammeter.

1.The instrument of Exercise 7 is to have a multiplier connected in series with it

to convert it for use as a voltmeter to read up to 50 V. What will be the multiplier resistance? 2. A permanent-magnet moving-coil instrument has a resistance of 3 _, but has a 27 _ swamping resistor connected in series with it. The movement requires a current of 15 mA for full-scale deflection. Calculate the resistance of a shunt required to convert it for use as a 0–10 A ammeter. What will be the resistance of a multiplier to convert the unit for use as a 0–300 V voltmeter if the swamping resistor remains in circuit?

## Draw a circuit diagram showing a single-phase wattmeter measuring the power in a single-phase 6.6 kV 500Acircuit by means of current and voltage transformer

1.The resistance material used for meter shunts is chosen for its very low value of

temperature coefficient of resistance. Why is this so? 2.State briefly with the aid of circuit diagrams

(a) how to extend the range of a 0–110 V AC voltmeter to read (i) 220 V and

(ii) 22 kV;

(b) how to extend the range of a 0–5 A AC ammeter to read 500 V;

(c) how to extend the range of a 0.5 A DC ammeter to read 500 A.

3.Draw a circuit diagram showing a single-phase wattmeter measuring the power in a single-phase 6.6 kV 500Acircuit by means of current and voltage transformers.

4.Calculate the sensitivity in ohms/volt of voltmeters giving full-scale deflection with currents of (a) 10 mA, (b) 0.5 mA….

## find the resistance of a resistor, various currents were passed through the resistor and the corresponding voltage drops across it were measured.

1.To find the resistance of a resistor, various currents were passed through the resistor and the corresponding voltage drops across it were measured. The results obtained were

(a) Draw to a suitable scale the graph connecting these quantities, with current values horizontal, and from the graph estimate the resistance value. (b) One serious reading error has been made; estimate what the voltage reading should be.

2.A resistor R is measured by the ammeter-and-voltmeter method, as in Figure 11.51. The ammeter and voltmeter readings are shown on the figure and the resistance of the voltmeter is 400_. Find the apparent and the true values ofR.

## Describe briefly how the bridge would be used to determine the value of the resistor, X .

1. Figure 11.52 shows a Wheatstone-bridge circuit. The ratio arms P and Q can each be set at 10, 100, 1000 or 10 000 _ and the variable resistor R can be varied from 1 _ to 10 000 _ in steps of 1 _.

(a) Describe briefly how the bridge would be used to determine the value of the resistor, X .

(b) Suggest values of P, Q and R if the bridge is at balance and the resistor X is 2.864 _.

2. A slide-wire bridge has a wire 1mlong and is arranged as shown in Figure 11.26. What is the value of the unknown resistor when the value of the known resistor and the distance of the slider from the left-hand end of the wire for balance, are, respectively

(a) 10 _, 27 cm (b) 2 _, 68 cm (c) 60 _, 42 cm

## Explain the principle on which the Wheatstone bridge works. Show how it could be used to measure the resistance of a busbar joint by comparison with a standard 0.1 _ resistor.

1.Explain the principle on which the Wheatstone bridge works. Show how it could

be used to measure the resistance of a busbar joint by comparison with a standard 0.1 _ resistor.

2. Draw a schematic circuit diagram of a Wheatstone bridge arranged for resistance measurement. Prove the relationship between the resistances at ‘balance’. Set out resistances, labelling each with its value, representing a bridge at balance when the measured resistance is 1.752 _. The variable-resistor dials give units, tens, hundreds and thousands, 0–10 of each; the ratio arms can each be set at 10, 100, 1000 or 10 000 _.

## Describe how you would find the resistance of a coil of insulated wire

Describe how you would find the resistance of a coil of insulated wire using

either (a) a slide-wire bridge or (b) an ammeter and voltmeter. 2.Atwin-core PVC-insulated and armoured cable is known to have a low-resistance fault to earth on one core only. The second core is in sound condition. Describe, with diagrams, the Murray-loop method of finding the position of the fault, and explain how the test results would be used for this purpose. 30 Figure 11.53 shows the various pieces of apparatus to be used in determining the position of an earth fault on a metal-sheathed two-core cable AB, by means of a Murray-loop test. Complete the diagram to show the apparatus connected to the cable ready for the test.

## Calculate the RMS values of a sinusoidal alternating voltage applied to the Y plates of a CRO if it gives the following vertical lengths of trace, the Y amplifier control setting for each display being shown after the trace length

1. Calculate the RMS values of a sinusoidal alternating voltage applied to the Y plates of a CRO if it gives the following vertical lengths of trace, the Y amplifier control setting for each display being shown after the trace length

(a) 5 cm, 10 V/cm

(b) 9 cm, 50 V/cm

(c) 4 cm, 100 V/cm.

2. The current in a load is displayed on a CRO by passing it through a 1.5 _ resistor and applying the PD across the resistor to the Y amplifier of the CRO. The peak-to-peak amplitude of the display is 7.5 cm when the amplifier setting is 200 mV/cm. Assuming the current wave-shape to be sinusoidal, calculate the RMS value of the current.

## what is the biggest disadvantage of the permanent-magnet moving-coil instrument

1. The biggest disadvantage of the permanent-magnet moving-coil instrument is that it

(a) has very low power consumption

(b) will not read alternating values

(c) has a linear scale

(d) has built-in damping

2. The biggest disadvantage of the rectifier permanent-magnet moving-coil instrument is that it

(b) can be adapted as a multi-range instrument

(c) is extremely large and heavy

(d) is in error if the applied wave-shape is not sinusoidal

3.The biggest advantage of the moving-iron instrument is that is

(a) reads true RMS values (b) is inaccurate

(c) needs special damping (d) has a non-linear scale

4 The most usual application of the dynamometer instrument is as

(a) an ammeter (b) a voltmeter

(c) a wattmeter (d) a frequency meter

….