TESTING OF AC GENERATORS BASIC AND TUTORIALS


Tests are performed on generators to establish conformance with projected performance and dynamic performance parameters. Details of such tests are contained in IEEE Standard 115, IEC 60034-2 and IEC 60034-4 standards.

Resistance
Field and armature resistances are typically small, so measurements should be made using a 4-wire technique. It is important that resistance be measured at a known temperature so that correction can be made to actual operating temperature.

Open-Circuit Saturation Curve
The generator is driven by a motor to rated speed and excitation varied to produce terminal voltage over a range, typically from perhaps 30% to 120% that of rated. Some caution is required here, particularly for large machines in which excessive flux can damage the core. Open-circuit losses may be established by this test if the drive motor is well characterized and input power is measured.

Short-Circuit Saturation Curve
This test is similar to the open-circuit test, except the armature terminals are short-circuited and excitation varied to produce armature current over some convenient range. Windage and friction losses may be inferred from power input at zero excitation.

Stray load loss may be estimated as the difference between input power at rated armature current and the sum of friction and windage and armature I2R.

Zero Power Factor Saturation Curve
For a relatively small generator, the zero power factor saturation curve can be determined by running the machine with its shaft unloaded, driven by a second generator. By adjusting the excitation on the ac generator under test and excitation on the second generator, it is possible to measure the zero power factor saturation curve.

Rather extensive discussion of this method is described in IEEE 115. For large generators for which this “back-to-back” method is not practical, the zero power factor curve is usually determined by numerical methods. Often those methods employ finite elements.

Deceleration
Deceleration may be used for determining losses if the inertia of the machine is known. Since, if the shaft of a machine is unloaded, power dissipated is


where wm is mechanical speed, deceleration through synchronous speed can give a good measure of dissipation. The test may be run with the machine operating either at open-circuit or short-circuit conditions, or at zero excitation. It is usually run from a slight overspeed.

This test can be used to determine an unknown inertia from known losses and observed deceleration.

Heat Runs
These are tests performed by operating the generator at some condition until the temperature stabilizes. Heat runs at open-circuit, short-circuit, and zero power factor may be combined to estimate temperature rise in actual operation.

In large machines, good estimates of dissipation may be made by measuring the temperature rise of coolant (e.g., water). This is an alternative or supplement to measuring input power to the drive motor or machine deceleration. wm

0 comments:

Post a Comment

PREVIOUS ARTICLES