Motor Controls & PLCs — Construction Electrician (309A) Practice Questions

Free Motor Controls & PLCs practice questions

1. 1.A 3-phase motor nameplate reads FLA 28A. What size overload heater is required?

   • A.28A✓
   • B.25A
   • C.30A
   • D.115% of FLA = 32.2A

   Why: Overloads are sized at or near FLA (Full Load Amps). Select the heater element from the manufacturer's table that corresponds to 28A. CEC allows up to 125% for standard motors.

2. 2.A star-delta (wye-delta) starter reduces starting current by:

   • A.25%
   • B.33% (to 1/3 of DOL current)✓
   • C.50%
   • D.75%

   Why: Wye-delta starting reduces inrush to approximately 1/3 of direct-on-line (DOL) current. Motor starts in wye (lower voltage per winding) then switches to delta for full speed.

3. 3.What does a PLC input module do?

   • A.Powers the motor
   • B.Converts field device signals to logic levels the CPU can process✓
   • C.Runs the program
   • D.Stores data long-term

   Why: Input modules interface field devices (switches, sensors, transmitters) to the PLC CPU by converting real-world signals to digital logic levels.

4. 4.A motor drawing 45A on 2 phases and 0A on the third has:

   • A.Normal operation
   • B.A single-phasing condition (lost phase)✓
   • C.Excess load
   • D.A grounded conductor

   Why: Single-phasing (loss of one phase) causes remaining phases to carry excess current. The motor overheats rapidly. Phase loss relays protect against this.

5. 5.What does a motor's service factor of 1.15 mean?

   • A.15% below rated power
   • B.The motor can safely run at 115% of rated HP continuously✓
   • C.The motor is 15% efficient
   • D.15% voltage tolerance

   Why: Service factor 1.15 means the motor can operate at 115% of nameplate HP without damage under rated conditions. It provides a safety margin but reduces motor life.

6. 6.What is the advantage of a soft starter over a DOL starter?

   • A.Lower cost
   • B.Reduced starting current and mechanical stress on the driven equipment✓
   • C.Higher speed
   • D.Better for small motors only

   Why: Soft starters ramp voltage gradually, reducing inrush current and mechanical shock to belts, couplings, and driven equipment. Ideal for pumps, fans, and conveyors.

7. 7.In a PLC timer instruction TON, the output is TRUE when:

   • A.Timer starts
   • B.The accumulated value equals or exceeds the preset value while input is TRUE✓
   • C.The input goes FALSE
   • D.The PLC powers up

   Why: TON (Timer On-Delay): output energizes after the accumulated time reaches the preset, AND the input remains TRUE. If input goes FALSE, the timer resets.

8. 8.A reversing motor starter uses:

   • A.One contactor and a switch
   • B.Two mechanically and electrically interlocked contactors✓
   • C.Three contactors
   • D.A VFD only

   Why: Reversing starters have two contactors (forward/reverse) that swap two phases. Mechanical AND electrical interlocks prevent both from energizing simultaneously (dead short).

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