A single-phase transformer has 800 primary turns and 200 secondary turns. If the primary is supplied at 230 V, what is the approximate secondary voltage?
Electrical Principles Science
Question · Easy
Same battery. Which lets more through?
Short
A four-pole three-phase induction motor is supplied at 50 Hz. What is its synchronous speed?
An induction motor has a synchronous speed of 1500 rev/min and runs at a full-load speed of 1440 rev/min. What is the percentage slip?
Question · Medium
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In a purely inductive AC circuit, what is the phase relationship between current and voltage?
A coil has a resistance of 6 ohms and an inductive reactance of 8 ohms. What is its impedance?
For the circuit with R = 6 ohms and Z = 10 ohms, what is the power factor?
Why is a low (poor) power factor undesirable for an industrial consumer?
Which component is normally connected in parallel with an inductive load to correct a lagging power factor?
A three-phase balanced load draws a line current of 20 A at 400 V line voltage with a power factor of 0.85. What is the approximate real power consumed?
Question · Hard
In a star (wye) connected three-phase system, how does the line voltage relate to the phase voltage?
Why is star-delta starting used for larger three-phase induction motors?
The EMF induced in a conductor is increased by which of the following?
A transformer is described as having a kVA rather than a kW rating because the rating is limited primarily by which factor?
An ideal step-down transformer has a primary current of 2 A and a turns ratio (primary to secondary) of 1:5. Neglecting losses, what is the secondary current?
What does the term 'reactance' describe in an AC circuit?
Inductive reactance XL of a coil will increase if which quantity increases?
In an R-L-C series circuit at resonance, what is the relationship between inductive reactance and capacitive reactance?
What is the function of the commutator in a DC machine?
A 230 V single-phase load has a resistance of 46 ohms in series with an inductive reactance of 23 ohms. What is approximately the circuit current?
What is meant by the 'efficiency' of an electrical machine?
Eddy current losses in a transformer core are reduced by which construction method?
Two 12-ohm resistors connected in parallel present what total resistance?
The power dissipated in a resistor can be calculated using which expression?
A capacitor of 100 microfarads is connected to a 50 Hz supply. What is its approximate capacitive reactance?
What is the main advantage of a three-phase supply over a single-phase supply for power distribution?
When designing a circuit to BS 7671, what does the symbol Ib represent?
Electrical Systems Design
In cable sizing to BS 7671, which correction factor accounts for an ambient temperature higher than 30 degrees C?
A circuit is protected by a 32 A device. The applicable correction factors are Ca = 0.87 and Cg = 0.7. What minimum tabulated current-carrying capacity (It) must the cable have?
BS 7671 limits voltage drop for a lighting circuit on a low-voltage installation supplied directly from a public network to what percentage of the nominal voltage?
A 2.5 mm2 cable has a voltage-drop value of 18 mV/A/m. It carries 20 A over a run of 25 m. What is the approximate voltage drop?
In the adiabatic equation S = square root of (I squared x t) / k, what does the factor k represent?
Using the adiabatic equation, a fault current of 800 A flows for 0.1 s through a 70 degrees C thermoplastic copper conductor (k = 115). What is the minimum required cross-sectional area?
What is the purpose of applying diversity when calculating the maximum demand of an installation?
For a TN-C-S (PME) supply, what is the typical maximum external earth fault loop impedance (Ze) declared by the distributor?
The earth fault loop impedance Zs for a final circuit is calculated using which relationship?
BS 7671 requires final circuits up to 32 A in a TN system to disconnect within what maximum time for additional protection by automatic disconnection?
When designing a circuit, what does the maximum permitted Zs value ensure?
Which BS 7671 reference method describes a cable clipped direct to a non-metallic surface?
A radial socket-outlet circuit in a domestic premises is to serve a floor area of up to 50 m2. Which arrangement complies with BS 7671 / the On-Site Guide?
What is the main reason for selecting an RCBO rather than a separate RCD and MCB on a final circuit?
BS 7671 requires additional protection by a 30 mA RCD for which of the following in domestic premises?
When selecting a protective device, the breaking capacity must be at least equal to which value?
A ring final circuit for socket outlets in a domestic dwelling is typically protected and wired as which arrangement?
Which document provides the standard final circuit arrangements and on-site reference data that supplement BS 7671 for typical installations?
A type B MCB to BS EN 60898 operates instantaneously at a fault current of how many times its rated current?
Why might a Type C or Type D MCB be specified instead of a Type B for a circuit supplying a motor?
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