Ramsay Test Motors and Controls 6

Question 1

A 3-phase induction motor is running but develops only about 1/3 of its rated torque and draws unbalanced line currents. The most likely cause is:

Accepted Answer

Single-phasing due to an open fuse or contactor contact on one phase

Answer

Single-phasing — the loss of one phase — causes a 3-phase induction motor to continue running (unlike starting, where it cannot self-start) but at severely reduced torque (roughly 1/3 rated) and with highly unbalanced currents on the remaining two phases. This is the classic symptom set described. Low voltage reduces torque but keeps currents balanced; excessive load increases slip but doesn't unbalance currents; a shorted rotor bar causes vibration and reduced torque but not the dramatic current imbalance pattern.

Question 2

A DC shunt motor is running at no load when its field circuit suddenly opens. What happens?

Accepted Answer

The motor accelerates to a dangerously high (runaway) speed

Answer

In a DC shunt motor, speed is approximately proportional to back-EMF divided by flux (N ∝ Eb/Φ). When the field circuit opens, flux (Φ) drops to nearly zero (only residual magnetism remains). With essentially zero flux, the motor accelerates to a dangerously high 'runaway' speed trying to maintain the back-EMF relationship, which can mechanically destroy the motor. This is why loss-of-field protection is critical for DC shunt motors, especially at light or no load.

Question 3

A two-speed motor wired in a consequent-pole (one-winding) configuration is being re-examined. At low speed, the winding is connected in parallel-delta. To achieve the high speed, the winding must be reconnected to:

Accepted Answer

Parallel-wye, which halves the number of poles

Answer

In a consequent-pole two-speed motor (Dahlander connection), switching from low speed (parallel-delta) to high speed requires reconnecting to parallel-wye. The parallel-wye connection effectively halves the number of active poles by creating 'consequent' poles magnetically, which doubles the synchronous speed (since Ns = 120f/P). This is the standard Dahlander speed-switching arrangement. The other options do not achieve the correct pole-halving effect for this winding type.

Question 4

A motor control circuit uses a latching (memory) rung with a normally-open auxiliary contact of the main contactor wired in parallel with the START pushbutton. If this auxiliary contact fails OPEN while the motor is running, what is the immediate effect?

Accepted Answer

The motor continues running; the fault only appears when STOP is pressed and a restart is attempted

Answer

The auxiliary (seal-in) contact only carries the control circuit current to maintain the coil energized after the START button is released. If it fails open while the motor is already running and the START button is also released, the coil de-energizes and the motor stops. However — and this is the subtle point — if the contact fails open while a maintained START signal is somehow held (or the failure goes unnoticed), the symptom appears the next time the operator releases START or cycles the stop/start sequence. In practice on a correctly operating circuit, a failed-open seal-in contact means pressing and releasing START will not latch the motor on; it only runs while START is held. The failure is not apparent until the next start attempt reveals only jog capability, not during uninterrupted running where coil excitation is maintained through START being released.

Question 5

A variable frequency drive (VFD) is programmed with a V/Hz ratio set for 460V at 60Hz. The drive is then commanded to run the motor at 30Hz. Which statement BEST describes the motor's condition at 30Hz under full load?

Accepted Answer

The motor develops full rated torque because the V/Hz ratio is maintained at 230V/30Hz

Answer

The fundamental principle of V/Hz (scalar) VFD control is that maintaining a constant volts-per-hertz ratio keeps air-gap flux constant, which in turn preserves the motor's torque-producing capability. At 30Hz, the drive outputs approximately 230V (half of 460V), maintaining the same 7.67 V/Hz ratio. With constant flux, the motor can develop approximately rated torque across the speed range (below base speed). Overheating is a concern at sustained very low speeds only if the motor lacks independent (forced) cooling; at 30Hz (half speed) many motors operate acceptably. Impedance does drop at lower frequency but flux — not impedance alone — governs torque.

Ramsay Test Practice Test

Ramsay Test Practice Test

Ramsay Test Motors and Controls 6