Background

NAVSEA 0900-LP-060-2010 series recommends the use of the General Radio Type 1644A, 1863, or an equivalent instrument for ground testing electric motors during troubleshooting, or prior to and after reconditioning or rewinding. Using these units for ground testing permits the application of voltage over an extended time period to determine the condition, and total value, of the motor's insulation resistance to ground.

To use the Type 1644A, you must know the parts of the unit, be able to hook the unit to the motor to be tested, know how to convert the insulation resistance to ground readings to megohms, and know where to record the readings.

Instrument setup

Place the instrument on a table with the handle away from the operator.

Do NOT try to force the instrument open.

Pull the gray handle locks toward the operator. Reach over the unit with the left hand and push the handle all the way down.

With the right hand, grasp the side closest to the operator and lift the unit up to a convenient reading position. Release the handle.

1. DeltaR%. Inserts R% adjustment in the measurement circuit; balances bridge ±5% range.

2. FUNCTION. Turns the instrument on, selects DISCHARGE, CHARGE-ZERO, or MEASURE function.

3. COARSE ZERO. Provides for coarse zero adjustment of detector.

4. VOLTAGE ON UNKNOWN. Selects magnitude of internal voltage applied to the uninown or connects an external voltage source.

5. EXT GEN. Allows for connection of an external power supply.

6. EXT ADJ. Allows connection of a resistor to adjust the voltage applied to the unknown to values between those supplied.

7. GALVANOMETER. Indicates balance of bridge.

8. FINE ZERO. For sensitive zero adjustment of the detector.

9. GUARD. An insulated binding post for connection to points to be guarded, such as shields of leads.

10. GROUND. Uninsulated binding post for ground connection to instrument chassis.

11. - UNKNOWN +. Pair of insulated binding posts for connection of component to be measured.

12. RESISTANCE MULTIPLIER. Selects the measurement of range.

13. SENSITIVITY. Adjusts the sensitivity of the detector circuit.

14. R. This continuous rotary control with dial balances the bridge.

Operation of the megohm bridge

Ensure that the bar guard is in the proper position. Attach it to the guard terminal if the unknown to be measured is a small, separate component. Unknown refers to the component being tested. Attach it to the negative terminal for testing large motors. Attach leads to the terminal posts on the right side of the instrument.

Ensure that the FUNCTION switch is in the OFF position.

Plug the instrument into a convenient 115-volt, 60-cycle outlet.

Always set the function knob to DISCHARGE before connecting or disconnecting the unknown component.

Turn the FUNCTION knob from OFF to DISCHARGE, and allow 15 minutes for the instrument to warm up.

Never move the VOLTAGE ON UNKNOWN knob when the function switch is in the MEASURE position.

Ensure that the VOLTAGE ON UNKNOWN knob is set at 500 volts. Attach one lead to the motor frame and one lead to the motor winding. Move the FUNCTION knob to CHARGE ZERO.

Use the COARSE ZERO knob to approximately null the meter. The R dial should be in the zero position. The RESISTANCE MULTIPLIER control should be in the 1K position.

Use the FINE ZERO knob to completely null the meter. Move the FUNCTION knob to the MEASURE position. If the needle swings to the left, there is a ground in the motor and the resistance cannot be measured.

If the needle swings to the right, proceed to the next step.

Adjust the RESISTANCE MULTIPLIER control to move the needle to the null position.

When the needle swings past the null position, move the RESISTANCE MULTIPLIER control back one step.

The null point will change as a charge builds up on the winding insulation.

Adjust the COARSE ZERO control for a meter zero (NULL).

Then use the vernier resistance to completely null the meter.

Since a charge will continue to build up, take the reading quickly. Record the reading.

The value of the unknown resistance is the reading on the resistance dial when the needle is at the null position multiplied by the quantity indicated on the RESISTANCE MULTIPLIER dial. For example: 5 x 10G = 50G. Convert the reading to megohms.

Polarization index

  A polarization index is conducted to appraise the cleanliness and dryness of the winding.

Connect the megohmmeter in the same manner as previously described.

Observe the 1-minute meter reading and record this reading on the motor data sheet (electrical). The 1-minute reading is also used as an insulation resistance-to-ground test. Observe the 10-minute meter reading and record the reading on the motor data sheet (electrical).

Use the insulation resistance values obtained at 1 minute and 10 minutes to calculate the polarization index.

To calculate the polarization index, divide the 10-minute value by the 1-minute value.

If the polarization index is less than 2, or the 1-minute insulation resistance measurement (corrected to 25°C) is less than 2 (1 + E/1000) megohms where E is the machine's rated voltage, the winding is not suitable for application of the dc high-potential test and should be reconditioned.

Securing the megohm bridge

Return the R dial to zero. Return the RESISTANCE MULTIPLIER dial to 1K. Turn the function switch to DISCHARGE. Turn the function switch to OFF. Disconnect the leads. Reattach the guard bar between the guard terminal and the metal terminal. Disconnect the power. Place the test leads in the top cover and close the instrument.

Keep the right hand on the face of the instrument to prevent sudden tilt forward.