AC Generator Efficiency and Losses

When considering the efficiency and losses of an AC generator, crucial information can be found on the nameplate. This includes:

• Rated voltage
• Rated kVA
• Specific power factor and field current.
• Short-circuit ratio

Short-Circuit Ratio

Short Circuit Ratio (SCR) for the synchronous generator is the ratio of:

• Field current required for the rated voltage at OPEN circuit
• To the field current required for the rated armature current at the SHORT circuit. An indication of the quantity of copper and iron in the electrical and magnetic parts.

Machines with high short-circuit ratios will contain more materials.

The standard figure quoted for a turbo-alternator is between 0.8 and 1.0. The effect of a high short-circuit ratio gives an improved generator stability at times of load changes.

List of alternator losses:

• Friction
• Windage
• I²R losses in the stator and field windings
• Hysteresis and eddy current losses in the iron of the stator and rotor field circuits

Reasons Why Generators Fail

Foreign Object Damage

Problem:
Can come from external sources or failure of internal components, they pick up energy from the spinning rotor and do extensive damage.

Prevention:
Inspect on a regular basis all internal parts that are prone to failure or can be dislodged.
Inspection tests can be a combination of visual inspection along with ultrasonic or magnetic particle tests on rotating components.

Stator Winding Vibration

Problem:
Primarily a design-related problem.
Affects large (>300Mw) generators that have insufficient end winding bracing to limit the movement of end turns.

Prevention:
Proper bracing of the end winding is required to limit motion caused by electromagnetic forces.

Rotor Winding Distortion

Problem:
Caused by poor end turn blocking support design or by foreshortening of the rotor coils.
Foreshortening is caused by thermal forces that compress rotor coils.

Prevention:
Proper design of rotor coils and bracing to support the coils under axial load is essential.
Rotors should be tested for turn-to-turn shorts at operating speed.

Overheating Stator Or Rotor

Problem:
This can lead to insulation failure, shorting of turns, and ground faults.
Overheating can result from blocked ventilation passages caused by shifting insulation components or slot wedges.

Prevention:
Inspect on a regular basis to ensure all rotor wedges are “locked” in place preventing migration and thus blocking cooling passages.

Rotor Vibration

Problem:
Many causes: turn-to-turn shorts, rotor coil foreshortening, electrical grounds, mechanical imbalances, overheating, etc.

Prevention:
Comprehensive vibration measuring is effective when combined with a regular maintenance program.

Contamination

Problem:
Air-cooled machines, dirt, and dust cause tracking which can lead to electrical ground faults.

Prevention:
Inspect air filters on a regular basis, the filters must be checked and cleaned regularly.
Polarization index (PI) tests give a good indication of the overall cleanliness of the rotor winding.

Stator Core Damage

Problem:
Stator core looseness can occur over time as pre-tensioned bolts relax. A loose core results in insulation wear to coils and laminations resulting in hot spots and core-to-coil failures.

Prevention:
Inspect bolt tightness on a regular basis.