Motor and Generator Testing Flashcards

During generator commissioning, the synchronizing check relay (Device 25) function verifies that before closing to the grid:

Answer: The generator voltage magnitude, frequency, and phase angle match the bus within acceptable tolerances before closing the circuit breaker

Synchronizing check relays prevent closing a generator breaker unless voltage magnitude, frequency, and phase angle are within acceptable windows to prevent the sudden power surge from an out-of-synchronism close.

The purpose of a generator stator core iron test (EL-CID test) is to:

Answer: Detect localized shorted laminations in the stator core that cause eddy current hotspots

The EL-CID (Electromagnetic Core Imperfection Detector) test excites the stator core to approximately 4% rated flux and uses a search coil to detect localized high-current spots from shorted laminations, which cause hotspots during normal operation.

A motor's service factor (SF) listed on the nameplate indicates:

Answer: The permissible overload factor: a 1.15 SF motor can operate continuously at 15% above nameplate kW

Service factor is a multiplier applied to nameplate horsepower/kW indicating allowable continuous overload. SF = 1.15 means the motor can run at 115% of rated load continuously, with higher temperature rise.

What is the significance of critical speed in large motor and generator applications?

Answer: A rotor speed at which the rotational frequency coincides with a natural mechanical resonant frequency, causing excessive vibration

Critical speed is the rotor speed that matches a natural lateral mechanical resonance frequency of the rotor-shaft-bearing system, causing resonant vibration amplification that can cause catastrophic damage if sustained.

Excitation current testing of a synchronous generator with no load measures the field current required to produce rated terminal voltage. An increase in required excitation current compared to factory data may indicate:

Answer: Increased air gap due to bearing wear or rotor movement, or demagnetization of the rotor poles

Higher excitation current needed to produce rated terminal voltage indicates a reduced magnetic coupling between rotor and stator — from increased air gap (bearing wear allowing rotor drop) or demagnetization of rotor field poles reducing flux per ampere.

The slip of an induction motor is defined as and indicates:

Answer: The difference between synchronous speed and rotor speed as a fraction of synchronous speed, and increases with increasing load

Slip s = (Ns - Nr) / Ns, where Ns is synchronous speed and Nr is rotor speed. Slip is near zero at no load and increases to a few percent at full load for standard motors, as the rotor must lag synchronous speed to develop torque.