Motors and Controls Flashcards

A 3-phase induction motor is running at rated load when one phase of the supply suddenly opens. Which of the following best describes what happens next?

Answer: The motor continues to run but draws excessive current in the remaining two phases and overheats

When single-phasing occurs under load, the motor does not stop — it continues to run on the momentum of the rotor and the pulsating (not rotating) field from the two remaining phases. The motor draws very high current in those two phases trying to maintain torque, generating excessive heat that will damage the windings if not quickly interrupted by overload protection.

A wye-delta (Y-Δ) reduced-voltage starter transitions a motor from wye to delta connection during starting. At the moment of transition, what is the primary electrical hazard if the transition is not timed correctly?

Answer: A high transient inrush current spike can occur if the motor back-EMF and line voltage are out of phase at the moment of switching

At the transition point, the motor has a back-EMF that may be out of phase with the supply. If the timer is too short, the motor has not decelerated its back-EMF enough, and when the delta contactor closes, the phase difference creates a large transient inrush — sometimes exceeding direct-on-line starting current. Proper transition timing or open-transition management is critical to avoid this transient.

A variable frequency drive (VFD) maintains constant volts-per-hertz (V/Hz) ratio below base speed. What is the primary reason for maintaining this ratio?

Answer: To maintain approximately constant air-gap flux density and prevent magnetic saturation or underexcitation

The V/Hz ratio controls the air-gap magnetic flux in an induction motor. If voltage drops faster than frequency, the motor becomes underexcited and loses torque-producing capability. If voltage is too high for a given frequency, the core saturates, causing excessive magnetizing current and heat. Keeping V/Hz constant maintains roughly constant flux and thus constant torque capability across the speed range.

An overload relay on a motor control center (MCC) has a service factor (SF) setting of 1.15. At what percentage of the relay's full-load amp (FLA) rating will it typically be set to trip?

Answer: 115% of FLA — the relay is set to match the motor's service factor rating

Per NEC 430.32(A)(1), when a motor has a nameplate service factor of 1.15 or greater, the overload device shall be set to trip at no more than 125% of the motor's FLA. However, in practice the relay's thermal trip is calibrated to the motor's FLA, and the motor is allowed to run at 115% of rated current (its service factor load) without nuisance tripping, because the 125% trip threshold accommodates SF 1.15 motors running at their service factor load plus a thermal margin. The relay is set at 115% of FLA to match and protect the motor's rated service factor continuous capability.

A synchronous motor is operating at rated load with a leading power factor. If the field excitation is suddenly reduced, what is the most accurate description of the immediate effect?

Answer: The power factor shifts toward lagging and the armature current increases as the motor absorbs reactive power from the line

Synchronous motors can operate stably with under-excitation (lagging power factor) as long as the load angle does not exceed the pull-out torque limit. Reducing field excitation causes the motor to absorb reactive power (inductive behavior) from the supply, shifting the power factor from leading toward lagging. To supply the same real power, the armature current magnitude increases. The motor does not immediately lose synchronism — it simply transitions from a capacitive to an inductive reactive power mode.

A motor's thermal overload relay trips repeatedly under normal operating load. The motor and relay are correctly matched to nameplate FLA. Which of the following is the LEAST likely cause of nuisance tripping?

Answer: The motor's insulation resistance has degraded to 2 MΩ, slightly above the minimum acceptable threshold

Degraded insulation resistance (even down toward 1 MΩ minimum thresholds) primarily indicates a risk of insulation failure but does not directly cause elevated line current that would trip an overload relay under normal load conditions — leakage current from marginal insulation is typically in the microamp-to-milliamp range, far too small to affect thermal relay trip behavior. High ambient temperature, supply harmonics causing additional bimetallic heating, and running current that is marginally below trip threshold are all plausible direct causes of nuisance thermal tripping.