Background

The Baker Model ST112E is a combination surge comparison tester and dc high-potential tester. The surge comparison tester uses the principle of impedance balance to simultaneously test turn to turn, coil to coil, phase to phase, and coil to ground insulation. In addition, qualitative evaluations are made of a winding's likelihood of satisfactorily passing resistance, impedance, turn balance, and high-potential (hi-pot) tests.

DC high-potential tests are made by applying dc voltage in steps and recording leaking current (micro amperes) through the insulation. The voltage and current are plotted on cross-sectional paper and the shape of the resultant curve is used for checking the cleanliness and moisture content of the windings being tested.

1. CATHODE RAY TUBE (CRT)/OSCILLOSCOPE. Displays the test patterns.

2. POWER LINE CORD ATTACHMENT. Primary power input line for the tester.

3. POWER ON/OFF. Activates the logic and display circuitry. A built-in white lamp indicates the tester is turned on.

4. TEST SWITCH. Pressed to activate the tester circuitry.

5. FOCUS. Used to adjust the focus, or clarity, of the trace.

6. INTENSITY. Used to adjust the display intensity (brightness).

7. HORIZONTAL POSITION. Used to adjust the horizontal position (starting point) of the trace.

8. VERTICAL POSITION. Used to adjust the vertical position of the trace.

9. SWEEP. Used to adjust the oscilloscope sweep time duration (trace length).

10. FUNCTION. Used to select surge or microamp scale for a high-potential test.

Do NOT operate the VOLTS/DIV control while a test is in progress.

11. VOLTS/DIVISION (VOLTS/DIV). Used to control the vertical amplitude of the oscilloscope display during testing.

12. OPEN GROUND. Indicates an open ground in the 115 Vac supply line.

13. HIPOT TRIP LAMP. Lights when overcurrent causes the instrument to trip during a high-potential test.

14. OUTPUT CONTROL. Used to adjust the tester output voltage to the winding under test.

15. TEST LEAD SELECT. Controls which of the output leads are activated and is also used to energize lead 1 during a high-potential test.

16. and 17.  TEST LEADS. Alligator clip type with three leads and a ground used for connecting the instrument to the stator. Only leads 1 and ground (17) are used in the high-potential position.

Computing the amount of voltage applied during a surge comparison test

Rewound stators will be tested at a voltage equivalent to (2E + 1000) x 1.4, where E is the operating voltage of the motor.

Reconditioned stators will be tested at 2/3 [(2E + 1000) x 1.4].

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

Rewound three-phase stator with operating voltage of 440 V

(2 x 440) + 1000 = 1880 V
1880 V x 1.4 = 2632 V

Reconditioned three-phase stator with operating voltage of 440 V

(2 x 440 + 1000) = 1880 V
1880 V x 1.4 = 2632 V
2/3 x 2632 = 1754 V

Operation of the Baker ST112E surge comparison/dc high-potential tester

Check that the ON/OFF switch is in the OFF position. Connect the test leads to the windings to be tested. Connect the ground lead to a "good" ground on the motor being tested. Connect the surge comparison/dc high-potential tester line cord into a 110/120 Vac outlet.

Preset the following controls as indicated.

Turn the ON/OFF switch to ON, allow a brief period for the CRT to warm up,

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 positions for the desired testing mode.

Adjust the SWEEP control to obtain the proper sweep duration for easy observation.

Do NOT operate the tester with a vertical display that exceeds the size of the CRT grid.

Adjust the OUTPUT CONTROL switch for the desired test voltage.

Adjust the VOLTS/DIV to keep the waveform on the CRT grid when the OUTPUT CONTROL is increased.

Enter the results on the motor data sheet (electrical) when the surge test has been completed.

The following data must be considered in conducting any test comparing observed waveforms:

• Most testing is comparison testing in which the inductance of two identical windings (or two windings of a three-phase winding) are tested simultaneously. The output waveforms are compared with each other.

• The tester can be used to test for voltage breakdown of a single winding. However, results are difficult to judge due to lack of a comparison.