Ramsay Test Motors and Controls 5 — Questions and Answers
Question 1: When troubleshooting a three-phase motor, you measure voltages of 480V, 478V, and 461V across the three phases. What condition is present?
- Voltage unbalance (Correct answer)
- Overvoltage condition
- Normal operation
- Single-phasing
Correct answer: Voltage unbalance
Voltage unbalance occurs when the three-phase voltages differ significantly; even small unbalances can cause large current unbalances and motor heating.
Question 2: Which type of motor is most commonly used in variable frequency drive (VFD) applications because it is simple, rugged, and requires no brushes?
- Squirrel cage induction motor (Correct answer)
- Wound rotor motor
- DC series motor
- Universal motor
Correct answer: Squirrel cage induction motor
Squirrel cage induction motors are preferred for VFD applications because their simple, brushless construction withstands the variable frequency operation well.
Question 3: In a forward-reverse motor control circuit, what electrical interlock prevents both forward and reverse contactors from energizing simultaneously?
- Normally closed auxiliary contacts wired in series with the opposing coil (Correct answer)
- A time-delay relay
- An overload relay
- A phase monitor relay
Correct answer: Normally closed auxiliary contacts wired in series with the opposing coil
NC auxiliary contacts from each contactor are wired in series with the opposing contactor coil, ensuring one must de-energize before the other can pick up.
Question 4: What does a megohmmeter (megger) test on a motor measure?
- Insulation resistance between windings and ground (Correct answer)
- Winding resistance in ohms
- Phase-to-phase voltage balance
- Motor shaft speed
Correct answer: Insulation resistance between windings and ground
A megohmmeter applies high DC voltage to measure the insulation resistance, detecting moisture, contamination, or degraded winding insulation.
Question 5: A motor control circuit uses a 120V control transformer. The primary is connected to 480V and the secondary feeds the coil circuit. What is the transformer turns ratio?
- 4:1 (Correct answer)
- 1:4
- 2:1
- 1:2
Correct answer: 4:1
A 480V-to-120V step-down transformer has a turns ratio of 4:1 (480/120 = 4), meaning the primary has 4 times more turns than the secondary.
Question 6: Which braking method for an AC induction motor injects DC current into the stator windings after the AC supply is disconnected?
- DC injection braking (Correct answer)
- Dynamic braking
- Regenerative braking
- Plugging
Correct answer: DC injection braking
DC injection braking creates a stationary magnetic field in the stator that induces currents in the rotor, rapidly bringing it to a stop.
Question 7: A motor's nameplate lists 'Code G' for locked-rotor kVA. What information does this locked-rotor code provide?
- The inrush current per horsepower during startup (Correct answer)
- The insulation class rating
- The maximum ambient temperature
- The efficiency class of the motor
Correct answer: The inrush current per horsepower during startup
NEMA locked-rotor kVA codes indicate the range of starting inrush current in kVA per horsepower, used for sizing fuses and circuit breakers.
Ramsay Test Motors and Controls 5
When troubleshooting a three-phase motor, you measure voltages of 480V, 478V, and 461V across the three phases.
What condition is present?