4th Class Power Engineering electricity, motors and transformers practice questions (4A).

Five free practice questions on electricity, motors and transformers: unit H of paper A (4A) of the 4th Class Power Engineer certificate exam, written to the SOPEEC syllabus. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Procedural · Apply Ohm's Law to parallel resistance circuits

   Three resistive tank-heater circuits of 48 Ω, 80 Ω and 120 Ω are connected in parallel across a 240 V supply. What total current does the supply deliver, to three significant figures?

   1. 0.968 A
   2. 2.90 A
   3. 5.00 A
   4. 10.0 A
   Show the answer

   D. Correct. In parallel each branch has the full 240 V: 240 ÷ 48 = 5.00 A, 240 ÷ 80 = 3.00 A and 240 ÷ 120 = 2.00 A, a total of 10.0 A. Check: 1/R = 1/48 + 1/80 + 1/120 gives R = 24.0 Ω, and 240 V ÷ 24.0 Ω = 10.0 A.

   Why not the others

   A. This adds the resistances as if they were in series: 48 + 80 + 120 = 248 Ω, and 240 V ÷ 248 Ω = 0.968 A. In parallel the equivalent resistance is lower than the smallest branch (24.0 Ω), so the current is 10.0 A.

   B. This averages the three resistances: (48 + 80 + 120) ÷ 3 = 82.7 Ω, and 240 V ÷ 82.7 Ω = 2.90 A. Parallel resistances combine by reciprocals: 1/R = 1/48 + 1/80 + 1/120, so R = 24.0 Ω and I = 10.0 A.

   C. This is only the current in the 48 Ω branch (240 V ÷ 48 Ω). The supply carries the sum of all three branch currents: 5.00 + 3.00 + 2.00 = 10.0 A.

2. Procedural · Describe magnetism and the relationship between magnetism and electricity

   The core of a control-relay coil must magnetize strongly when the coil is energized and let the armature drop out promptly when the coil is de-energized. The core is best made of

   1. soft iron
   2. hardened steel
   3. copper
   4. permanent-magnet alloy
   Show the answer

   A. Correct. Soft iron has high permeability, so a modest coil current produces a strong flux, and low retentivity, so it keeps almost no magnetism when the current stops and the armature springs back.

   Why not the others

   B. Hardened steel has high retentivity: it stays magnetized after the current stops. That suits permanent magnets but would hold the relay armature in after the coil is de-energized.

   C. Copper is an excellent electrical conductor but is non-magnetic, so as a core it would add nothing to the coil's flux. A magnetic core of soft iron concentrates the flux.

   D. A permanent-magnet alloy is magnetized all the time, so the armature would not drop out when the coil is de-energized. The core must lose its magnetism when the current stops: soft iron.

3. Procedural · Describe how voltage, current, and resistance are measured in an electric circuit

   To measure the current drawn by a DC control circuit, an ammeter is connected

   1. in parallel with the load, since its resistance is very low
   2. in series with the load, since its resistance is very low
   3. in series with the load, since its resistance is very high
   4. in parallel with the load, since its resistance is very high
   Show the answer

   B. Correct. All the circuit current must pass through the meter, so it goes in series, and its resistance is made very low so it adds almost no voltage drop and does not change the current it measures.

   Why not the others

   A. A low-resistance meter connected across the load or supply acts as a near short circuit: a large current flows, blowing the meter's fuse or damaging it. An ammeter goes in series.

   C. A high resistance in series would reduce the very current being measured. An ammeter in series must have a very low resistance.

   D. That describes a voltmeter: connected across the load, with a high resistance so it draws very little current. It measures voltage, not the circuit current.

4. Recall · Describe the construction and operation of AC generators (alternators) and motors

   Large plant alternators are built with a rotating DC field and a stationary armature mainly because

   1. the output frequency then stops depending on speed
   2. the high-voltage output then needs no sliding contacts
   3. a stationary field winding cannot use direct current
   4. the rotating field then needs no excitation once running
   Show the answer

   B. Correct. The armature carries the full output at high voltage and heavy current. Fixed armature windings connect straight to the output terminals and are easier to insulate and cool; only the small, low-voltage DC field current has to reach the rotor, through slip rings or a brushless exciter.

   Why not the others

   A. Frequency depends on speed and poles (f = pn/120) whichever part rotates; that is why generator speed is held so closely. The design reason is to take the high-voltage output from fixed windings.

   C. DC generators and motors have stationary field windings fed with direct current, so that is no obstacle. The field is put on the rotor so the high-voltage armature can stay still.

   D. The rotating field needs DC excitation the whole time it produces voltage; without it there is almost no flux and almost no output. The reason for the design is the stationary high-voltage armature.

5. Recall · Describe the construction and operation of three-phase transformers

   Compared with one three-phase transformer of the same total rating, a bank of three single-phase transformers has the advantage that

   1. one spare single-phase unit can replace any failed unit
   2. it weighs less and needs less floor space
   3. its total core losses are lower, so it is more efficient
   4. it costs less to buy and to install
   Show the answer

   A. Correct. If one unit of a single-phase bank fails, only that unit is changed, and one spare covers all three positions. With a single three-phase unit, a failure takes out the whole transformer and the spare must be a complete three-phase unit.

   Why not the others

   B. The single three-phase unit is the lighter and smaller of the two, because its three phases share one core and one tank. The bank's advantage is that one spare single-phase unit covers any failure.

   C. A three-phase unit is usually slightly more efficient, since its shared core uses less steel than three separate cores. The bank's advantage is spare-unit flexibility.

   D. Three separate tanks, cores and sets of bushings cost more than one three-phase unit of the same rating. The bank's advantage is that one spare unit can replace any failed unit.

02Answer key

03What unit H covers

Electrical theory for the plant: current, voltage, resistance and power in DC and AC circuits, series and parallel circuits, conductors and insulators, magnetism and induction, megohmmeters, wattmeters and kilowatt-hour meters, DC and AC motors and generators, slip and power factor, transformers and current transformers, and distribution systems, protection coordination, motor control centres and UPS supplies.

The SOPEEC 4th Class syllabus names this unit “Electricity” (ABSA numbers it A8) and lists six objectives under it. SOPEEC does not publish how many exam questions come from each unit, and neither do the ABSA, Technical Safety BC, TSASK or TSSA pages we checked. The right-hand column is how many questions our bank holds on each objective, Standard and Advanced together, not a count from the exam.

04What the questions turn on

Series adds, parallel shares voltage

In series the resistances add and the same current flows through each of them. In parallel every branch has the full supply voltage across it, so the branch currents found from I = V/R add up to the supply current.

Slip is measured from synchronous speed

Synchronous speed is 120 times the frequency divided by the number of poles, so 1 200 r/min for a 6-pole motor on 60 Hz. Slip is how far the rotor runs below that, as a percentage of synchronous speed.

Power factor is kW over kVA

Three-phase apparent power is √3 times line voltage times line current, and power factor is the true power in kW divided by kVA. A capacitor supplies kvar locally, cutting the kVA and current the supply delivers while the kW stays the same.

Never open a loaded CT secondary

A current transformer's secondary is meant to work into a near short circuit. Short it with a shorting link before a meter is removed; opened while the primary carries current, the secondary develops a dangerously high voltage.

05The other units

Paper 4A has twelve units, A to L. Each has its own page of free questions:

The 4th Class Power Engineering practice exam (4A) page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.