Measuring In-rush Current Using a Graphical™ MultiMeter

Introduction

In-rush current is the initial surge of current drawn by a circuit when voltage is first applied. Motors, fluorescent lighting and High Intensity Discharge (HID) lamps all have in-rush current characteristics that can be used to help identify circuit problems.

Although the examples in this application note are based on a motor, the procedure would be the same for any circuit. The measurements shown in the illustrations are actual measurements taken on a 3 horsepower, 208 volt, three phase motor.

In a motor, the initial current after voltage is applied or locked-rotor current, charges the motor windings which in turn, create the magnetic fields that start the rotor and shaft in motion. As the motor increases speed, the current drawn from the source decreases, until normal operating speed is reached. At this point, the current is primarily a function of the load.

Three-phase motors obtain their starting motion from the phase difference between the three voltages. The three phase voltage creates a rotating field that causes the stator to turn. A single-phase motor, however, requires an auxiliary winding and a capacitor to create a rotating field for initial turning torque. This starting circuit is switched out after the motor reaches a specified speed.

Each motor has unique starting characteristics which must be taken into consideration when analyzing in-rush current. Not only the internal motor design but external factors such as voltage amplitude, ambient temperature, and load affect in-rush current. All these factors should be recorded and considered when trending or analyzing a motor over time. In the case of three-phase motors, it is important to measure the in-rush current on the phase that is applied first.

Looking at the peak current drawn at startup, the time it takes to go from lock-rotor to normal running current and the amount of normal running current can help identify excessive loads and defective starter circuits.

The 860 Series Graphical MultiMeters (GMM) not only measure the peak, maximum, minimum, average, or rms current, but they capture and display waveforms as well. Viewing the captured waveform will allow you to determine how fast the current drops from startup current to running current levels.

Setting Up the GMM

A motor's physical and electrical characteristics can be determined from the information printed on its nameplate. The motor for this application has the following nameplate information:

Horsepower:	3
Voltage:	200V 3 phase
Amps:	9.4
Class:	B
RPM:	3475
Type:	K
Code:	J

This information indicates the motor draws a running current of 9.4 amps rms, at full load. However, the maximum current at startup will be much greater. The locked-rotor current for a 3-phase motor is calculated using the following formula:

amps = ((kVA/hp) x hp x 1000) / (volts x ÷3)

The kVA/hp rating is a locked-rotor kilovolt-Amperes value that is obtained from a table using the motor code. For code J, this motor has a 7.1 - 8.0 kVA/hp rating. Therefore, the minimum locked-rotor current will be:

(7.1 x 3 x 1000) / (208 x 1.73)= 59.2A

The maximum locked-rotor current will be:

(8.1 x 3 x 1000) / (208 x 1.73)= 66.7A

To measure 66.7 amps with the GMM without overloading its input, a current transformer that steps down the current is required at the meter's input. A Fluke 80i-400 Current Probe that produces 1 mA for every 1 amp of current sensed through the probe's jaws, was used for this application. This means all displayed readings must be multiplied by 1000. e.g. 55.6 mA is interpreted as 55.6 Amps.

Input Connections
To set up the GMM for an rms amps measurement:

1. Connect the current probe to the mA and COM jacks.
2. Clamp the current probe around one of the motor's three phase input leads.
3. Turn the GMM selector to mA and allow the meter to display in the COMBO mode.
4. Press the AC/DC softkey (5) to select an AC measurement.

Setting the Range
The GMM's autorange circuitry sets the meter's range based on the amplitude of the input signal. However, when measuring in-rush current, the autorange circuit cannot react quickly enough to the jump from zero to 66.7 amps. As a result, an overload condition will be displayed by the meter. Manually selecting the range that will measure the maximum expected current is the only way of obtaining an accurate reading of the in-rush current. To set the range manually:

1. Press the Range key to activate the range selection softkeys. The selected range is displayed just above the softkey labels in the lower right-hand corner of the display.
2. Press the up (2) or down (3) arrow softkeys to change the range. This application requires a range setting of 300 mA.
3. Press Exit (5) to return to the mA function softkey labels.

The GMM is now set up to take an in-rush current measurement. Because the in-rush current is present and gone so quickly, use Min Max, Peak Hold, or Single Shot to "capture" or hold the measurement for inspection. These are explained next.

Using the GMM

Measuring the Maximum Current with MIN MAX

1. Press the MIN MAX key. The window and secondary display area is replaced with the minimum, average, and maximum readings.
2. Apply power to the motor.

The GMM display updates as new minimum, average, and maximum readings are stored. After the motor has reached its operating speed, the maximum reading is the maximum current drawn by the motor during the startup period.

Figure 1. A set of maximum, average, and minimum reading for a motor at startup

A maximum current of 63.46 amps rms is well within the locked-rotor current range calculated earlier for this motor. A measurement higher than maximum locked-rotor current could be an indication of a defective motor.

Another useful motor measurement that can be made with the Min Max feature is line voltage drop caused by the motor's in-rush current.

1. Setup the GMM for an AC volts measurement.
2. Place the meter leads across the motor's source voltage at the input to the power switch.
3. Activate the Min Max mode.
4. Apply power to the motor.

The minimum reading will be the lowest voltage sensed on the line while the motor was starting.

Capturing the Peak Current with Peak Hold

1. With the function softkey labels displayed, press the Peak Hold softkey. The window and secondary display area is replaced with the peak maximum and peak minimum readings.
2. Apply power to the motor.

Figure 2. Peak current readings of an induction motor.

Displaying Waveforms

The primary, secondary and graphical window displays are replaced with a two dimensional grid. Time is plotted horizontally and signal amplitude is plotted vertically. Setting the range for the vertical divisions and the timebase for the horizontal divisions directly influences waveform appearance.

In the View mode, the GMM allows you to capture and examine a single waveform that has met your trigger criteria. This is called Single Shot operation.

Single Shot operation uses a trigger circuit to initiate the storage and display of a waveform. When an input signal meets the trigger criteria, the GMM stores 25 time divisions of the input signal: 6 time divisions before the trigger and 19 after. The amount of time within each division is set through the time base setting described later in this application note.

Setting the Trigger

To set the trigger:

1. Rotate the function switch to SETUP.
2. Move the highlight bar by pressing the Next Item softkey until the Trigger selection is highlighted.
3. Press the left (2) or right (3) arrow softkey to change the trigger setting to Single.
4. Rotate the function switch to the desired measurement mode, in this case mA. Set the softkey selections and range for the desired measurement and select the View mode.
5. Press the trigger (2) softkey.
6. Press the up (2) or down (3) arrow softkey to adjust the trigger level (Figure 3).
   
   
   Figure 3. Trigger indicators in View mode.
   

   The trigger level is displayed in the upper right corner of the display and the trigger icon moves along the left vertical axis of the display grid. In this case, a setting just above zero is adequate for in-rush current. The two trigger levels indicated with a "Hz" label are the dual trigger settings for the counter circuit. These two levels are automatically set while the waveform trigger mode is set to single trigger. These settings can be ignored for this application.

7. Return the View mode soft-keys by pressing the Exit (5) softkey.

Setting the Timebase

1. Press the Timebase (1) softkey.
2. Press the Slower (2) or Faster (3) softkey to set the desired Time Base. The Time Base setting is displayed along the right edge of the display grid.
3. Return to the View mode softkeys by pressing the Exit (5) softkey.

Capturing the Waveform with Single Shot Mode

1. Press the Single Shot (3) softkey. The Arm softkey label (1) will be highlighted.
2. Apply power to the circuit.

In the Single Shot mode, the GMM acts like a video recorder. In the GMM, there is enough memory to store 25 time divisions worth of input data. When the memory is full, the next piece of input data is stored and the first piece is pushed out of mem-ory, always retaining the last 25 time divisions of data.

When armed, the GMM starts the "recorder", storing the input data in memory. When the input signal exceeds the trigger level, the GMM records nineteen more time divisions worth of input and then displays the waveform.

Figure 4. Captured current waveform from an induction motor.

Up to 6 time divisions before the trigger and 19 time divisions after the trigger can be viewed using the left (2) and right (3) softkeys. This is analogous to the forward and reverse buttons on a video recorder.

Another Single Shot waveform can be captured simply by pressing the Arm (1) softkey and reapplying voltage to the motor.

Storing Configurations and Waveforms

1. Press the Print/Save key.
2. Press the Save Screen (1) softkey.
3. Press the up (2) or down (3) softkey to move the highlight bar to one of the three waveform storage locations.
4. Press the Save (3) softkey. The label used for the stored screen is a combination of the display mode (Meter, Combo, View, etc.) and the measured value.

Storing the waveform only captures an image of what is inside the confines of the display. This is analogous to placing a camera on the display and snapping a picture. The waveform information that is off screen is NOT saved in memory.

To speed up the setting of range, trigger levels, and time base, you can save the GMM configuration for future recall. To save the GMM's configuration:

1. Press the Save Print key.
2. Press the Save Config (4) softkey.
3. Move the highlight bar to one of the seven configuration memories by pressing the down (1) or up (2) arrows.
4. Press the Save (3) softkey.
5. Press Exit (5) to return to the selected display mode.

To recall the configuration:

1. Ensure the rotary function switch is set to the function the configuration is based on.
2. Press the Save Print key.
3. Press the Recall (2) softkey.
4. Move the highlight bar to the desired configuration by pressing the down (1) or up (2) softkey.
5. Press the Load Config (4) softkey. The Load Config softkey will only appear when the function selected by the rotary switch is compatible with the highlighted configuration.

Summary

There are two models in the 860 Series: the full featured 867B and the 863. Both of the models will make the measurements mentioned in this application note. Contact your nearest Fluke representative for further information.