JQR 251.07 Amp Meter, Clamp-On

Use of the Clamp-On Ammeter

Learning Objectives
You will be able to:

Follow the safety precautions when using the clamp-on ammeter/multimeter.

Read voltage measurements with a clamp-on ammeter/multimter.

Read resistance measurements with a clamp-on ammeter/multimter.

Conduct a current balance test of a motor winding.

Record current readings in the appropriate section of the motor data sheet (electrical).

Determine if motor condition is satisfactory.

Background

It is essential that electricians be able to use clamp-on ammeters, or multimeters, for in-place troubleshooting of motors and controllers as well as other circuits. Many clamp-on ammeters can be used to measure voltage and resistance as well as current. They come in various sizes with selections of scales that make them extremely versatile tools.

Typical clamp-on ammeter components and functions

METER SCALE. Will indicate amperage as well as voltage and resistance depending on model of meter.

RANGE SELECTOR. Selects amperage scale as well as voltage and resistance scales depending on model of meter.

TRANSFORMER JAWS. Used to measure current; jaws are opened and then closed around one energized conductor.

TRIGGER. Used to open transformer jaws.

OHMMETER RECEPTACLE (optional). Used to plug in test leads on meters equipped with a resistance function.

POINTER LOCK. Freezes meter needle after readings are taken.

ZERO ADJUST. Used to zero meter prior to use to ensure accuracy of reading.

OHMMETER ZERO ADJUST. Used to zero meter with test leads shorted.

VOLTAGE RECEPTACLE (optional). Used to plug in test leads on meters equipped with a voltage function.

Use of the clamp-on ammeter

Preparation

Ensure that the test leads are removed from the meter before testing for current. Release the ammeter lock by sliding it to the right. Set the RANGE SELECTOR switch to the highest range to prevent damage to the meter.

Ensure that all test leads, if used, are removed from the meter.

Release the meter pointer by sliding the POINTER LOCK to the right. Set the RANGE SELECTOR to the highest scale to avoid possible damage to the meter.

Follow all local safety instructions for working on energized electrical circuits. Do NOT expose the TRANSFORMER JAWS to bare connectors. Always wear rubber gloves to protect hands from contact with bare connectors. Use only one hand in live equipment measurements.

Taking amperage readings

Properly connect an operational three-phase motor to a test panel or other electrical source.

Ensure that test lead connections are properly insulated and that test leads and stator leads are spread so that the TRANSFORMER JAWS can clamp around them individually.

Open the TRANSFORMER JAWS by pressing the trigger on the side of the meter.

If the jaws are closed around more than one conductor, an inaccurate reading will result.

Close the jaws around one conductor, making sure that they are completely closed. Energize the power supply and start the motor.

Observe the amperage reading. If the reading appears in the lower part of the scale, shift the range select to the next lower scale until the reading appears in the middle part of the scale where the meter is most accurate.

Open the TRANSFORMER JAWS and remove the meter. Take readings on the other two conductors as previously described.

Click on image to enlarge.

Typical Clamp-On Ammeter/Multimeter, Model SPR-300

The current in any phase, at rated load, shall not differ from the arithmetic average of the maximum and minimum phase current by more than 10 percent for 1-1/2 horsepower (hp) motors or less; by more than 7.5 percent for 2 and 3 hp motors; and by more than 5 percent for motors over 3 hp. For submarine motors at no load, the maximum deviation of any phase shall not exceed 3 percent of the average no-load current. The above values apply to installed motors only.

Record readings in the appropriate section of the motor data sheet (electrical).

Use of the voltage feature (on meters so equipped)

Preparation

Plug test leads into the jacks provided. Set the RANGE SELECTOR switch to the highest current range scale. De-energize the circuit to be tested and tag it out.

Avoid holding the meter in your hand to eliminate possible electrical shock.

Connect test leads to the circuit.

Taking voltage readings

Tag in and energize the circuit. Observe the reading. De-energize the circuit and tag it out.

If the reading was in the lower part of the scale, shift the range selector scale to the next lower scale and repeat the procedure.

Energize the circuit and retake the reading as necessary until the reading appears in the upper part of the scale. When a satisfactory reading has been obtained, De-energize the circuit and disconnect the test leads. Record the reading in the appropriate section of the motor data sheet (electrical).

Use of resistance feature (on meters so equipped)

Preparation

Ensure that the equipment or circuit to be tested is de-energize and tagged out. Set the RANGE SELECTOR to any resistance ( S ) scale. Plug test leads into the jacks provided.

Use the ZERO ADJUST screw to align the pointer with the "0" at the bottom of the scale. Short the leads and use the ohmmeter ZERO ADJUST knob to align the pointer with the "O" at top of the scale. This must be done with the leads shorted to compensate for battery resistance.

Taking the resistance reading

Place one lead to one end of circuit or component to be tested. Observe and record the reading on motor data sheet (electrical).

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.