Motors and Controls Flashcards
7 cards from real Ramsay Test practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Motors and Controls flashcards as text
What is the primary function of a contactor in a motor control circuit?
Answer: To switch high-current motor power circuits using a low-power control signal
A contactor uses electromagnetic coil energization to open and close high-current power contacts, allowing a low-voltage control circuit to safely switch motor-level currents.
In a forward-reverse motor control circuit, what prevents both the forward and reverse contactors from energizing at the same time?
Answer: Electrical and mechanical interlocks installed between the two contactors
Interlocking uses normally-closed auxiliary contacts from each contactor wired in series with the opposite contactor's coil, and mechanical interlocks physically prevent both from closing simultaneously.
Why does an AC induction motor draw very high current at the instant of startup?
Answer: The stationary rotor produces no back-EMF, so only winding resistance limits current
At startup, rotor speed is zero so there is no counter-electromotive force (back-EMF); the motor acts like a short-circuited transformer, drawing 6–8 times full-load current.
What is the purpose of a wye-delta (star-delta) reduced-voltage starter?
Answer: To reduce starting current and torque by initially connecting windings in wye, then switching to delta at speed
Wye starting reduces the voltage across each winding to 58% of line voltage, cutting starting current and torque to one-third; once near full speed, the circuit switches to delta for full voltage operation.
A motor nameplate shows 'Insulation Class F.' What does this indicate?
Answer: The motor windings can withstand a maximum temperature of 155°C (311°F)
Insulation class designations (A, B, F, H) indicate the maximum total temperature the insulation can withstand — Class F is rated for 155°C total temperature (40°C ambient + 105°C rise).
What does the power factor of an electric motor represent?
Answer: The ratio of real power (kW) to apparent power (kVA) drawn from the supply
Power factor is the ratio of real (working) power to apparent power; induction motors have lagging power factor because their magnetic fields require reactive (VAR) current that does no useful work.
What type of motor or drive system is most appropriate when precise speed control over a wide range is required in an industrial application?
Answer: AC induction motor paired with a Variable Frequency Drive (VFD)
A VFD varies the frequency and voltage supplied to an AC induction motor, enabling smooth, precise speed control from near-zero to above base speed with good torque characteristics.