309A motors, drives and controls practice questions.

Five free practice questions on motors, drives and controls: block D of the Red Seal 309A 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

1. Procedural · Installs motor control devices

   An emergency stop station is added to a machine's motor control circuit. How must it be implemented?

   1. A latching mushroom-head button with an NC contact hard-wired in the control circuit.
   2. A momentary NO push button wired to a PLC input that stops the motor in the program.
   3. A normally open contact in parallel with the stop button.
   4. A key switch that only the supervisor can operate.
   Show the answer

   A. Correct. An E-stop must remove power from the hazardous motion reliably: a red mushroom head on a yellow background, latching so it stays off until deliberately reset, and a positive-opening NC contact in series with the safety circuit or contactor coil. Routing it only through a PLC program is not acceptable because software or an I/O failure could ignore it. The RSOS lists emergency stop stations among control devices.

   Why not the others

   B. A momentary button releases as soon as the hand leaves it, and a PLC-only stop depends on the program running correctly; neither satisfies E-stop requirements.

   C. A NO contact in parallel does nothing when pressed in a stop circuit; stops are NC contacts in series.

   D. An E-stop must be operable instantly by anyone near the hazard.

2. Recall · Installs three-phase motors

   Which three-phase motor allows its starting torque and running speed to be adjusted by external resistance connected to the rotor circuit?

   1. Wound-rotor induction motor.
   2. Squirrel-cage induction motor.
   3. Shaded-pole motor.
   4. Synchronous motor.
   Show the answer

   A. Correct. The wound rotor's windings are brought out through slip rings to external resistors; high rotor resistance gives high starting torque at low starting current, and varying it changes the speed-torque curve, giving a measure of speed control. The RSOS lists wound rotor among three-phase motor types and slip rings among components.

   Why not the others

   B. A squirrel-cage rotor is a shorted cage with no external connections; its characteristics are fixed by design.

   C. Shaded-pole motors are small single-phase machines with no rotor connections.

   D. A synchronous motor runs at fixed speed; its rotor carries DC excitation, not adjustable resistance.

3. Critical · Performs servicing and maintenance of AC drives

   A drive on a large exhaust fan trips on 'DC bus overvoltage' each time the fan is commanded to stop, but runs normally otherwise. What is the cause and remedy?

   1. The motor is too small for the fan.
   2. The drive's carrier frequency is set too high.
   3. Regeneration raises the bus; lengthen the decel or add a braking resistor.
   4. The supply voltage is too high; ask the utility to lower it.
   Show the answer

   C. Correct. A high-inertia load driven below its running speed acts as a generator, pumping energy back through the inverter into the DC bus capacitors; the bus voltage rises until the drive protects itself. Extending the decel time reduces the rate of regeneration, or a dynamic braking resistor dissipates it. Coast-to-stop is another option if a quick stop is not needed. The RSOS lists DC circuits, inverters and rectifiers among AC drive components.

   Why not the others

   A. Motor size affects running current; the trip is specific to deceleration and is a regeneration problem.

   B. Carrier frequency affects heating and noise, not bus voltage during deceleration.

   D. A supply problem would trip the drive at any time, not only during deceleration.

4. Critical · Performs servicing maintenance of automated control systems

   A PLC output's status LED is on, commanding a solenoid valve, but the valve does not operate. The electrician measures 0 V at the output module's terminal for that point. What is the most likely cause?

   1. The solenoid coil is open.
   2. The program is not calling the output.
   3. The output module's fuse or switching device has failed.
   4. The input that triggers the output has failed.
   Show the answer

   C. Correct. The LED shows the logic has turned the point on; zero volts at the terminal shows the module is not passing power to it, which is a blown output fuse (many modules fuse each point or group) or a failed output device. Had the terminal shown voltage, the wiring and solenoid would be checked. The RSOS lists I/O systems among components and multimeters among diagnostic equipment.

   Why not the others

   A. An open coil would leave full voltage at the terminal; the module would still be switching.

   B. The LED confirms the program has energized the output.

   D. The output is already on in the logic; input devices are upstream of that decision.

5. Critical · Performs servicing and maintenance of motor starters

   An AC contactor in a starter pulls in but hums loudly and its coil runs hot, although the coil voltage measures correct. What is the most likely cause?

   1. The contactor is too large for the motor.
   2. The coil is rated for a higher voltage than applied.
   3. Dirt on the pole faces or a broken shading coil.
   4. The motor is overloaded.
   Show the answer

   C. Correct. An AC contactor's armature must close completely against the pole faces; the shading coil then keeps the magnetic pull steady through the zero crossings. Foreign material, rust, a bent armature or a cracked shading ring leaves an air gap, the magnet chatters or hums loudly and the coil draws inrush-level current continuously and overheats. Cleaning the pole faces or replacing the magnet assembly cures it.

   Why not the others

   A. An oversized contactor works normally; size does not cause humming.

   B. The voltage is correct; an under-voltage coil would fail to pull in rather than hold with a hum.

   D. Motor load does not affect the contactor's magnet.

02Answer key

03What block D covers

This block covers motors and the equipment that starts, stops and controls them. It includes motor starters and overload protection, control circuits with push buttons, limit switches and sensors, AC and DC drives, single-phase, three-phase and DC motors, alignment, vibration and insulation testing, and automated control with PLCs, ladder logic, analog signals and industrial networks.

The Red Seal Occupational Standard names this block “Installs, services and maintains motors 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.

04What the questions turn on

Motor circuit percentages

Branch-circuit conductors are sized at 125 % of the full-load current from the CEC tables. Overloads are set from the nameplate current: up to 125 % with a service factor of 1.15 or more, otherwise 115 %. Time-delay fuses go up to 175 %.

Two-wire versus three-wire control

Three-wire control seals in through a holding contact, so the motor stays off after a power failure until start is pressed. Two-wire control with a float switch restarts on its own. Added stop buttons go in series and added starts in parallel.

Finding the open device

In a series control circuit, full supply voltage appears across whichever device is open, while closed devices read near zero. Full control voltage across a coil that does not pull in means the coil itself is open.

Drive trips and long cables

A DC bus overvoltage trip when a high-inertia fan stops comes from regeneration, so lengthen the deceleration time or add a braking resistor. Long motor cables cause reflected-wave voltage peaks that call for inverter-duty motors or an output reactor.

05The other blocks

The 309A exam has five blocks. Each has its own page of free questions:

The 309A 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.