JQR 251.38 Measuring System, Precision Resistance

310.2

Use of the Valhalla 4100 ATC
Low-Range Ohmmeter

Learning Objectives
You will be able to:

Identify the major controls of the Valhalla Model 4100 ATC and describe their functions.

Observe the safety precautions involved in using the Valhalla Model 4100 ATC.

Use the Valhalla Model 4100 ATC to conduct a resistance balance on a motor winding.

Record the resistance balance on the motor data sheet (electrical).

Determine whether resistance balance is satisfactory or unsatisfactory.

Background

NAVSEA 0900-LP-060-2010 Series recommends that the internal resistance balance of a three-phase motor be measured with a Kelvin bridge. The Valhalla Model 4100 ATC is a Kelvin bridge that measures the voltage drop across an unknown resistance. It displays the voltage digitally as a resistance measurement.

The Valhalla Model 4100 ATC digital ohmmeter is capable of precise measurement from 10 micro-ohms to 19.999 kilo-ohms. The lower range of the instrument will allow measurement of the resistance of windings in large motors. It will also detect small variations of resistance. Unsatisfactory differences in phase resistance in large motors will be in milliohms. The precision of an instrument like the 4100 ATC is required to detect such faults. This measurement range permits it to be used instead of the Wheatstone bridge or the General Electric portable double bridge.

Valhalla 4100 ATC components and their functions

1. The DIGITAL READOUT provides direct reading of the resistance of the component under test. The lowest reading the instrument is capable of is 10 micro-ohms. This display will show 0.00001. The highest reading the instrument will read is 19.999 kilo-ohms.

2. The OHMS selector switch is a six-station pushbutton selector for range selection. The ranges are: 0.1, 1, 10, 100, 1,000, and 10,000.

3. The STD/TC selector switch is a two-station pushbutton used to select the internal zero temperature coefficient or an external temperature sensor.

4. The ZERO adjustment control is used to adjust the zero offset of the instrument.

5. The red POWER pushbutton switch is used to energize and deenergize the instrument.

6. The TEMP SENSOR (temperature sensor) is the connection for the temperature compensating probe.

The special leads provided with the instrument must be used

7. The VOLTAGE terminal provides for connecting the leads being used for the voltage measurement across the tested component to the instrument.

8. The CURRENT terminal provides for connecting the leads used to supply a constant current across the tested component to the instrument.

Making resistance measurements

Place the instrument near a 115-volt, 60-hertz, single-phase conveninece outlet. Plug the test leads into the voltage and current plugs on the front of the instrument.

If individual leads are used, the HI CURRENT lead and the HI VOLTAGE lead must connect to the same end of the unknown. The LO CURRENT lead and the LO VOLTAGE lead connect to the other end of the unknown.

Plug the instrument into the convenience outlet. Depress the POWER pushbutton to turn on the instrument.

If the display is flashing, select a higher resistance range by pressing the OHMS pushbuttons until the the flashing stops, then read the display. If the flashing does not stop and any range is selected, it indicates an open circuit. Check your connections and/or megger the windings.

Read the display.

Measuring and determining the resistance balance between motor leads

Mark motor leads 1, 2, and 3. Place the 4100 ATC next to the motor. Plug the 4100 ATC into the convenience outlet. Connect the instrument leads to the VOLTAGE and CURRENT terminal post on the instruments.

Connect HI VOLTAGE and HI CURRENT instrument leads to same motor lead when separate leads are used.

Connect the instrument leads to leads 1 and 2 on the motor to be tested. Depress the STD pushbutton.

Depress the POWER pushbutton to turn on the 4100 ATC. Depress the OHMS 0.1 pushbutton.

When the display is flashing, press higher range OHMS selector pushbuttons sequentially until the flashing stops, then read the display. If the display continues to flash at every range it indicates an open circuit.

Click on image to enlarge.

Valhalla 4100 ATC Digital Ohmmeter

Read the display.
Record the reading in the appropriate resistance balance with double bridge section of the motor data sheet (electrical).

Disconnect the instrument leads from motor lead 1 and connect them to motor lead 3. Read the display (if steady) and record the reading in the appropriate section of the motor data sheet (electrical).

Disconnect the instrument leads from motor lead 2 and connect them to motor lead 1. Read the display (if steady) and record the reading in the appropriate section of the motor data sheet (electrical).

Depress the POWER pushbutton to turn off the machine. Unplug the power cord. Disconnect the instrument leads and stow the 4100 ATC.

Balancing calculations

EXAMPLE: Assume the phase resistance balance readings are those below:

TERMINAL	OHMS
1 - 2	0.325
2 - 3	0.326
3 - 1	0.333

The maximum imbalance between phases in a three-phase motor is 10 percent for motors up to 1.5 hp, 7.5 percent for 2- and 3-hp motors, and 5 percent for motors over 3 hp.

• Multiply the highest reading by 0.05. The calculated figure is maximum allowed inbalance between phases.

0.333 (highest reading)
0.05
0.01665

• Subtract the lowest reading from the highest.

0.333 (highest reading)
0.325 (lowest reading)
0.008 (difference)

• When the difference is greater than the calculated allowed imbalance, the winding is bad and will require rewinding. In this example, 0.008 is less than 0.01665; therefore, the winding is good.

Click on image to enlarge.

Typical Motor Data Sheet (Electrical)

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.