How to use a clamp meter to measure inrush current

The effect is rather short-lasting. For example, for a 3-phase motor it lasts 75 to 150ms, but the current increases by 500% to 1,200%, which may cause serious problems and malfunctions. Besides, the measurement of the inrush current is required in the maintenance of electric motors.

Inrush current in switched- mode power supplies

The effect of inrush current occurs also in switched-mode converter-based power supplies. Such devices have the elements of crucial importance placed at the input, namely rectifiers and capacitor-based filters. When the power supply is switched on, discharged capacitors at the input have small impedance that causes short overload – the rush of flowing current. Charging time of capacitors is short, but the current that flows at the time may reach even several dozen amperes at peak.

An additional NTC thermistor is often included into the power supply line. It can diminish the inrush current due to its initial resistance. While the power supply in the thermistor heats up and its resistance decreases, limiting the loss of power.

Unfortunately, many manufacturers deem NTC thermistors unnecessary and do not include them in their devices in order to cut the cost. When you need to choose a power supply, it is advisable to find out whether your selected model has a built-in thermistor.

How a clamp meter works

• UNI-T UT216C clamp meter
• Axiomet AX-215TIC clamp meter
• FLUKE 376 FC clamp meter
• EXTECH EX655 clamp meter

A clamp meter is a device with a metal ring, which can be clamped around two coils. The first one (the primary circuit) is the wire in which the current flows. The second one (the secondary circuit) is inside the meter. The secondary circuit consists of a much higher number of coil turns, so the flowing current is lowered and can be easily and safely measured. Modern clamp meters can also be used for DC measurements. It is possible due to the Hall effect, which allows the measurement of the magnetic field intensity.

Many clamp meters can be used not only for current measurements, but also as traditional multimeters. Thanks to additional test lead sockets, we can e.g. test the circuit continuity or perform a diode test.

The measurement of current with a clamp meter is very simple. The meter probe must be clamped around the wire of the tested circuit and the knob must be set appropriately, depending on the current type: AC or DC. In the case of distorted waveform current, it is advisable to find out whether the meter has a function of the true effective value (True RMS) measurement. This functionality gives much more reliable results when the waveforms are distorted.

Advantages

The main advantage of using the clamp ammeter is the non-invasiveness of the procedure. The measurement may even be performed on the operating machine, and no interference in the supply circuit is required. It is a very convenient feature, especially in the case of production lines, which should not be interrupted without a good reason.

Moreover, safety is another important aspect. Due to the non-invasiveness of the measurement, the operator is not directly exposed to the impact of the electric current.

Offers

There are a number of offers on clamp meters available at TME.

UNI-T UT216C clamp meter

UT216C is one of the devices from UNI-T. It is a classic clamp meter which enables the measurement of AC/DC voltage, AC/DC current, resistance, capacitance, frequency, temperature and inrush current.

Moreover, UT216C has several additional functions that make it even more user-friendly. These are:

• Automatic change of the measuring range (autoranging)
• Function of diode test and continuity test
• Mode of relative measurement (relative mode)
• Backlit LCD display
• True effective value (true RMS) measurement function
• Data Hold function – keeping the last measurement result
• Automatic power-off after 15 minutes
• Low Battery Display (≤3,6V)
• Non-Contact Voltage (NCV) – the function of contactless voltage detection
• Built-in torch

The device is powered with three 1.5V LR03 (AAA) batteries and all measurements are compliant with EN61010 1000V CAT II, EN61010 600V CAT III.

Axiomet AX-215TIC clamp meter

The device that represents the AXIOMET brand is the AX-215TIC clamp meter. This low-cost meter is a perfect tool for DIY hobbyists. It has basic functions which enable measurement of AC/DC voltage, AC/DC current, resistance, capacitance, frequency, temperature and inrush current.
Moreover, the meter has several additional functions. These are:

• Automatic change of the measuring range (autoranging)
• Function of diode test and continuity test
• Automatic power-off
• HOLD function (hold of displayed results)
• True effective value (true RMS) measurement function
• Low battery indicator
• Overflow indicator
• Built-in torch

AX-215TIC, just like UNI-T, is powered with three batteries 1.5V LR03 (AAA).

FLUKE 376 FC clamp meter

The FLUKE 376 FC can be a perfect choice for car service workshops or other places with difficult environmental conditions. The device is extraordinarily durable, which is the trademark of all FLUKE products. The FLUKE 376 FC enables the measurement of AC/DC voltage, AC/DC current, resistance, capacitance, frequency, temperature and inrush current.

The device also has several more functions:

• Automatic change of the measuring range (autoranging)
• Function of diode test and continuity test
• Precise true effective value (true RMS) measurement function
• MIN/MAX/MEDIUM function
• Compatibility with the Fluke Connect application (wireless transfer of measurement data via Bluetooth)
• HOLD function (hold of displayed results)
• Low battery indicator
• Low-pass filter for measuring drive parameters.

The meter is powered with three 1.5V LR6 (AA) batteries. All measurements are compliant with EN61010 1000V CAT II and EN61010 600V CAT III. It should be noted that the meter is equipped with a flexible current probe, which make the measurements easier to perform.

EXTECH EX655 clamp meter

The EX655 clamp meter by EXTECH can be an alternative to the FLUKE device. It has similar features, but is more affordable. The meter enables the measurement of AC/DC voltage, AC/DC current, resistance, capacitance, frequency, temperature and inrush current.
Moreover, EX655 has several additional functions:

• Automatic change of the measuring range (autoranging)
• The function of diode test and continuity test
• Automatic power-off
• True effective value (true RMS) measurement function
• Backlit LCD display with bargraph
• Non-Contact Voltage (NCV) - the function of contactless voltage detection
• HOLD function (hold of displayed results)
• Low battery indicator

The meter is powered with three 1.5V LR03 (AAA) batteries and the measurements conform to the standards EN61010 1000V CAT II and EN61010 600V CAT III.