Five free practice questions on motor starters, drives and motors: block D of the Red Seal 442A Industrial Electrician exam, about 21 of its 100 questions. 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
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On a two-speed motor starter, what does a compelling relay do?
- It stops the low and high contactors from closing at the same time.
- It delays a change from high to low speed until the motor slows.
- It makes the motor start in low speed before high can be selected.
- It switches the motor from low to high automatically after a set time.
Show the answer
C. Correct. Compelling control forces the operator to start in low speed; high speed can be selected only once the motor is running in low, so the motor and load never take a direct high-speed start from rest. It suits high-inertia loads that would draw too much current or stress the drive train.
Why not the others
A. Keeping both speed contactors from closing at once is done by electrical and mechanical interlocks between them, because closing both would connect the windings wrongly. That is separate from the start sequence.
B. A delay before dropping from high to low is the job of a decelerating relay. It lets the motor coast down so the low-speed winding does not brake it hard.
D. Stepping from low to high automatically after a time delay is an accelerating relay: the operator presses High and the starter passes through low by itself. A compelling circuit instead refuses High until low has been started.
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While a brush-type synchronous motor on a pulp refiner accelerates on its damper (amortisseur) winding, its DC field winding is connected across a field discharge resistor until near synchronous speed. Why?
- To slow acceleration so the rotor pulls into step smoothly.
- To hold field current at its rated value while starting.
- To set the power factor the motor runs at after pull-in.
- To limit the voltage induced in the field winding.
Show the answer
D. Correct. At standstill the stator's rotating field cuts the many-turn field winding at slip frequency and, if the winding were left open, could induce a voltage high enough to break down its insulation. The resistor keeps that voltage down and adds some starting torque; DC is applied near synchronous speed to pull the rotor into step.
Why not the others
A. The resistor does not brake the rotor; by giving the field winding a closed circuit, it adds a little induction-motor torque during acceleration.
B. No DC is applied during acceleration; the resistor only loads the winding. Excitation is switched on when the rotor nears synchronous speed.
C. Running power factor is set by the DC field current after pull-in; the resistor is switched out by then and plays no part.
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An electrician wants to compare a drive's output voltage to its motor, running at 45 Hz, with the value on the drive's display. Which meter setting gives a comparable reading?
- True-RMS AC volts with the low-pass filter on.
- Average-responding AC volts with no filter.
- DC volts measured across two output phases.
- True-RMS AC volts with the low-pass filter off.
Show the answer
A. Correct. The drive's output is a train of pulses at the carrier frequency, several kilohertz, whose fundamental is 45 Hz. The meter's low-pass (drive-output) filter strips the carrier, so the true-RMS reading is the fundamental voltage the motor works on, which is what the drive displays.
Why not the others
B. An average-responding meter assumes a sine wave and scales its reading for one; on a pulse-width-modulated output that assumption fails and the reading can be far off. A true-RMS meter with a low-pass filter is used.
C. The voltage between two output phases alternates at 45 Hz, so a DC setting averages it to about zero. DC volts is used on the DC bus, not on the output.
D. Without the filter the meter includes the carrier-frequency pulses, so its reading is usually higher than the drive's display and can jump around. The filter removes the carrier and leaves the 45 Hz fundamental.
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Since a repair to its controller, a crane's lifting magnet holds full loads normally, but small scrap keeps clinging to the magnet face for several seconds after the operator selects drop. What is the most likely fault?
- The magnet coil has developed shorted turns.
- The magnet supply voltage is now set too high.
- The reverse release circuit has failed.
- The coil's discharge resistor has gone open.
Show the answer
C. Correct. When the lift supply opens, the core and the load keep some residual magnetism that holds light scrap. The controller normally applies a short, reduced reverse current at drop to cancel it; if that circuit no longer works after the repair, light pieces cling until the residual field fades.
Why not the others
A. Shorted turns weaken the magnet while it is energized, yet full loads are lifted normally; the problem appears only after the supply is removed.
B. A higher supply would make lifts stronger and heat the coil, but the clinging happens after the supply is off; the residual field, not the lift voltage, is holding the scrap.
D. An open discharge resistor lets a high voltage appear across the coil and contacts when the lift opens, a risk to insulation, but it does not make scrap cling; without that path the coil current collapses faster, not slower.
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Why are the main contacts of a DC contactor fitted with a magnetic blowout coil and an arc chute?
- The coil absorbs the voltage spike when the operating coil opens.
- A DC arc has no current zero to extinguish it.
- The blowout field holds the contacts closed on a fault.
- The coil lowers the holding current once the contacts close.
Show the answer
B. Correct. An AC arc goes out by itself at each current zero, 120 times a second at 60 Hz. DC never passes through zero, so the arc would hold. The blowout coil, in series with the contacts, sets up a magnetic field that drives the arc up into the arc chute, where it is stretched, cooled and broken.
Why not the others
A. The operating coil's switching spike is absorbed by a suppression diode, RC network or varistor across that coil. The blowout coil carries load current in the main circuit.
C. Holding contacts closed under fault current is a blow-on arrangement found in some breakers and contactors. The blowout coil does the opposite job: it pushes the arc away from the contacts and into the chute.
D. Reducing coil current after pickup is the job of an economizer (a resistor or electronic circuit) in the operating-coil circuit, not of the blowout coil in the main contact circuit.
02Answer key
| Question | Answer | Sub-task |
|---|---|---|
| Q1 | C | Installs motor control devices |
| Q2 | D | Installs three-phase motors |
| Q3 | A | Maintains AC drives |
| Q4 | C | Maintains other fixed equipment and associated controls |
| Q5 | B | Maintains motor starters |
03What block D covers
Block D covers what makes plant machinery run: motor starters, contactors, overload relays and control circuits, AC drives and DC drives, and single-phase, three-phase, synchronous and DC motors. It also covers fixed non-rotating equipment and its controls, such as welders, lifting magnets, electrostatic precipitators and x-ray inspection units.
The Red Seal Occupational Standard names this block “Installs and maintains rotating and non-rotating equipment and control systems”. It carries 21% of the exam, about 21 of the 100 questions, split across four tasks. The share column is each task's weight within the block, as the standard publishes it.
| Task and its sub-tasks | Share of block |
|---|---|
| Installs and maintains motor starters and control devices Installs motor starters; Maintains motor starters; Installs motor control devices; Maintains motor control devices | 34% |
| Installs and maintains drives Installs AC drives; Maintains AC drives; Installs DC drives; Maintains DC drives | 23% |
| Installs and maintains other fixed equipment and associated controls Installs other fixed equipment and associated controls; Maintains other fixed equipment and associated controls | 14% |
| Installs and maintains motors Installs single-phase motors; Maintains single-phase motors; Installs three-phase motors; Maintains three-phase motors; Installs DC motors; Maintains DC motors | 29% |
04What the questions turn on
AC coils burn out on DC
On AC most of a contactor coil's impedance is inductive reactance, which drops to zero on DC. Only the winding resistance is left to limit current, so the coil draws many times its rating and burns out.
Manual reset on two-wire control
With two-wire control the run command stays present, so an overload relay set to automatic reset would restart the machine as soon as it cooled. Set manual reset where an unexpected restart could injure someone clearing the jam.
One drive, several motors, several overloads
A drive feeding several motors in parallel sees only their total current, so it cannot detect one jammed or single-phased motor. Each motor needs its own overload relay on the drive output, set from its nameplate.
Armature voltage sets speed below base
At full field a DC motor's counter-EMF is proportional to speed, so below base speed the drive controls speed with armature voltage. Half of base speed needs roughly half of rated armature voltage, plus a little for the armature's IR drop.
05The other blocks
The 442A exam has six blocks. Each has its own page of free questions:
The 442A Industrial Electrician practice exam 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.