Motor and Generator Testing Flashcards

Vibration analysis on electric motors identifies which type of problem through characteristic frequency signatures?

Answer: Mechanical faults including bearing defects, unbalance, misalignment, and loose components

Vibration analysis uses frequency spectra to identify specific mechanical fault signatures: bearing defect frequencies, rotational frequency for unbalance, 2 times rotational for misalignment, and natural frequencies for resonance.

When testing a motor with a locked rotor test, the technician should be aware that:

Answer: Locked rotor current is typically 5 to 7 times rated current, requiring a test duration short enough to prevent winding overheating

Locked rotor current (LRC) is 5 to 7 times full-load current. The motor windings overheat rapidly at this current; therefore, locked rotor tests should be brief (typically under 3 to 5 seconds) to prevent thermal damage.

The DC step voltage test for motor insulation involves:

Answer: Applying voltage in equal increments while monitoring insulation resistance to identify moisture or contamination

The step voltage test applies DC voltage in equal increments (steps), measuring insulation resistance at each step. Healthy dry insulation shows increasing resistance; contaminated or degraded insulation shows decreasing resistance with increasing voltage.

A high winding temperature detected by RTDs in a motor may indicate:

Answer: Overcurrent due to mechanical overload, ventilation blockage, cooling system failure, or winding fault

Elevated winding temperatures above design values indicate problems: excessive current from overload, reduced cooling from blocked ventilation or cooling system failure, high ambient temperature, or winding faults increasing resistance and losses.

The efficiency of an induction motor at partial load is generally:

Answer: Lower than at full load, with maximum efficiency typically occurring at 75 to 80% of rated load

Induction motor efficiency peaks at approximately 75 to 80% of full load. At lower loads, fixed losses (core, friction, windage) become proportionally larger relative to output power, reducing efficiency.

Bearing insulation testing on large electric motors is important because:

Answer: Stray shaft currents can flow through uninsulated bearings and cause bearing fluting (electrical erosion)

Variable frequency drives and other power electronics can generate common-mode voltages that drive high-frequency currents through motor shafts. These shaft currents discharge through bearings, causing electrical erosion (fluting) of races and balls.