JQR 251.31 Tester, Surge, Portable Page 2

310.2

Use of the Baker ST112E Surge Comparison Tester/
DC High-Potential Tester (Continued)

DC hi-pot (high-potential) test

Computing the amount of voltage applied during dc high-potential testing

High-potential test voltage = 2E + 1000, where E is the operating voltage of the motor.

Rewound (new) stators will be tested at a voltage equivalent to 1.6 x (2E+1000) high-potential test voltage.

Reconditioned stators will be tested at a voltage equivalent to 1.1 x (2E+1000) high-potential test voltage.

The following are examples of how test voltages will be computed.

440 x 2 = 880 + 1000 = 1880

Rewound three-phase stator with operating voltage of 440 V

1.6 x 1880 = 3008 V

Reconditioned three-phase stator with operating voltage of 440 V

1.1 x 1880 = 2068

A dc high-potential test is only to be conducted if the polarization index is 2 or above or 1-minute reading on the polarization index is 2(1 + E/1000) or above.

Operation of the Baker ST112E dc high-potential tester

Check that the ON/OFF switch is in the OFF position.

On multilead equipment, connect all leads together with lead 1. Connect the black lead of the tester to a clean ground on the frame or the core under test.

Connect the test leads to the winding to be tested. Connect the line cord to a 110/120 Vac outlet.

Preset the following controls as indicated.

FOCUS
INTENSITY
VERTICAL
HORIZONTAL
SWEEP
OUTPUT

Midscale
Midscale
Midscale
Midscale
Midscale
Minimum (full counterclockwise)

Rotate the TEST LEAD SELECT switch to the HIPOT position.

Turn the FUNCTION switch to the 500 microamps per division setting.

The VOLTS/DIV switch is always 500.

Turn the ON/OFF switch to the ON position, allow a brief period for the CRT to warm up, and adjust it to midscale.

Adjust the FOCUS for a sharp, well-defined trace.

Do NOT come into contact with the leads during testing. Do NOT attempt to preset the output.

Make sure that all the switches and controls are in the proper position for the testing mode desired.

The voltage output of a high-potential test is read on the screen as with the surge test. The high-potential leakage current is also displayed on the screen. The display will be three flat lines (items 5 and 6 on the figure at right); one full-width stationary line is the zero reference, the half-width line on the left indicates the output voltage, and the half-width line on the right indicates the leakage current.

Rotate the VERTICAL POSITION control so that the line is positioned near the bottom of the screen behind one of the grid lines etched on the face of the oscilloscope.

Do NOT allow the overcurrent relay to trip. Tripping the relay produces an inductive voltage surge which could damage the insulation. Increase the test voltage slowly enough to see an abrupt rise in the slope of the leakage current curve and to stop the test.

The unit is equipped with a HIPOT overcurrent safety trip. If the HIPOT leakage current trace goes very high (off the screen), or approximately 10 times the high potential leakage current microamps/div setting of the FUNCTION switch, this overcurrent trip will operate. If the HIPOT trips, the high voltage is turned off and the HIPOT TRIP lamp comes on. To reset the high-potential trip, release the TEST switch.

Depress the TEST switch and slowly raise voltage with the OUTPUT CONTROL. Some deflection of the leakage current trace will be noted as the voltage is increased. This is due to the charging current necessary to saturate the winding.

Click on image to enlarge.

Waveform Patterns

Using a graph like that in the figure at right, plot a curve of the measured leakage current values, taking at least eight equally spaced voltage points up to full voltage. Record the data for each point after the current stabilizes. Approximately 25 percent of the calculated test voltage or 500 volts, whichever is less, should be applied as the initial test voltage and leakage recorded.

Click on image to enlarge.

High-Potential Test Curve Worksheet

The test should be stopped when a sharp rise in current is obtained.

When the dc high-potential test is used as either a preventive or troubleshooting maintenance action, conduct an insulation resistance test at 500 volts (or as recommended by the equipment technical manual) before each high-potential test and after the last high-potential test is completed. Record the test reading on the curve sheet. An insulation resistance test need not be conducted by repair activities following the dc high-potential tests conducted during the refurbishment process because the test will be conducted as the initial test in each series of electrical tests that follow. After performing the insulation resistance and dc high-potential tests, ground the copper for a sufficient time to complete discharge the winding. Following a dc high-potential test, hold the ground for at least four times the accumulated test period or 1 hour, whichever is longer. Failure to completely discharge the machine being tested will produce inaccurate megger readings taken after dc high-potential test.

DC high-potential curves should be obtained before starting overhaul and after each major step in the overhaul procedure. In this manner, improvements can be noted as the work progresses.

Click on image to enlarge.

DC High-Potential Test Curves

Review Questions
If you can correctly answer the questions below, you have learned the key points in this lesson. Choose the correct answer for each question, then look at your score below the questions. Review the lesson if you miss any questions.