Journeyman Electricianâs Exam Journeyman Electrician's Motors and Motor Controls 5 â Questions and Answers
Question 1: A three-phase induction motor running at 1,750 RPM on a 60 Hz supply has a synchronous speed of 1,800 RPM. What is the motor's slip?
- 1.75%
- 2.78% (Correct answer)
- 3.5%
- 5%
Correct answer: 2.78%
Slip = (Synchronous speed â Rotor speed) Ă· Synchronous speed = (1800 â 1750) Ă· 1800 = 50 Ă· 1800 â 2.78%.
Question 2: What is the purpose of a 'control transformer' in a motor control circuit?
- To step up voltage for high-power contactors
- To reduce line voltage to a safer control voltage (typically 120V) for the control circuit (Correct answer)
- To convert three-phase power to single-phase for the control circuit
- To provide isolation between the motor and the power supply
Correct answer: To reduce line voltage to a safer control voltage (typically 120V) for the control circuit
A control transformer steps down line voltage (e.g., 480V) to a safer level (typically 120V) for operating pilot devices, relays, and coils.
Question 3: Under NEC 430.102(B), a motor disconnecting means must be located within sight of the motor and the driven machinery, OR:
- The disconnect may be omitted entirely if lockout/tagout is used
- The disconnect may be out of sight if it is lockable in the open position (Correct answer)
- The disconnect must be located at the motor control panel only
- A second disconnect must be installed at both the panel and motor
Correct answer: The disconnect may be out of sight if it is lockable in the open position
NEC 430.102(B) Exception allows the disconnect to be out of sight from the motor if it is capable of being locked in the open (off) position.
Question 4: Which motor protection scheme uses a set of normally-closed auxiliary contacts from one contactor wired in series with the coil of the other contactor to prevent both from energizing simultaneously?
- Sequence control
- Electrical interlocking (Correct answer)
- Time-delay control
- Reduced-voltage starting
Correct answer: Electrical interlocking
Electrical interlocking uses NC auxiliary contacts from each contactor in the opposing contactor's coil circuit to prevent simultaneous energization (common in reversing starters).
Question 5: A motor with an NEMA Design D characteristic would be most appropriate for:
- Centrifugal pumps requiring low starting torque
- General-purpose fan and blower applications
- High-inertia loads like punch presses requiring very high starting torque (Correct answer)
- Applications requiring constant speed across all load conditions
Correct answer: High-inertia loads like punch presses requiring very high starting torque
NEMA Design D motors provide very high starting torque (275%+ of FLC) with high slip, making them ideal for high-inertia, impact, or peak-load applications.
Question 6: According to NEC 430.6(A), which current value must be used for determining conductor ampacity and device ratings for AC motors â the nameplate current or the NEC table current?
- Always use the nameplate current
- Use the NEC table value, not the nameplate current (Correct answer)
- Use whichever value is higher
- Use the nameplate current only if it is within 10% of the table value
Correct answer: Use the NEC table value, not the nameplate current
NEC 430.6(A) requires using the current values from NEC Tables 430.247â430.250 for conductor and device sizing, not the motor nameplate FLA.
Question 7: A motor running in an ambient temperature of 50°C when its insulation class is rated for 40°C ambient must have its ampacity-corrected conductors selected based on:
- The 40°C column of NEC Table 310.15 with no correction factor
- The standard 75°C conductor ampacity table with a temperature correction factor applied (Correct answer)
- Doubling the FLC to compensate for elevated temperature
- Replacing the motor with a higher temperature-rated model before wiring
Correct answer: The standard 75°C conductor ampacity table with a temperature correction factor applied
Conductors must be derated using the temperature correction factors in NEC Table 310.15(B)(1) when ambient temperature exceeds the conductor's rated ambient.
Journeyman Electricianâs Exam Journeyman Electrician's Motors and Motor Controls 5
A three-phase induction motor running at 1,750 RPM on a 60 Hz supply has a synchronous speed of 1,800 RPM.
What is the motor's slip?