While consuming less electricity than their older, less efficient counterparts, high-efficiency motors are much more likely to trip their respective circuit protectors when they start. Initial start-up current — or inrush current — is the culprit. This current can be several times greater than the operating or steady-state current of the motor. In a 3-phase motor, for example, inrush current generally lasts between 75 msec and 150 msec, with a current spike between 500% and 1,200%.
Although short-lived, inrush current can create serious problems. For example, if the protector isn't designed to handle the amount of inrush current present, the device can trip upon energizing the circuit or during circuit operation. Excessive inrush current can also shorten the life of switches and circuit protectors. Switches are most susceptible because the current spike occurs as the contacts are closed, causing the contacts to become pitted. In severe cases, the excess current can weld switch contacts.
The precise measurement of inrush current is more than just a convenience — it's a critical element of motor installation and maintenance. However, making this measurement with good repeatability can be a point of frustration because existing methods of measuring inrush current just don't have the repeatability attribute.
Several established methods exist for measuring inrush current. By using any one of them, you could possibly extrapolate a close approximation of the effect of inrush current. But this may not be adequate. Ideally, you want to see what actually happens when the motor goes through its startup and how the current draw profile affects the motor's circuit breaker and/or overload devices. It's also important to understand the terminology and methodologies for measuring inrush current so you can decide which method is most appropriate for your needs. Let's start with a review of the tools available to make inrush current measurements.
Three tools are available for measuring inrush current: the clamp meter, the digital multimeter, and the power quality analyzer. The types of measurements you need to take, the number of tools you wish to carry to do your job, and the price you're willing to pay will dictate your choice.
Clamp meters measure current, voltage, resistance, and continuity. They're used primarily to troubleshoot electrical and HVAC systems. Since the clamp is an integral part of its design, the clamp meter needs no other accessory. The good news is that a class of clamp meters in the market can accurately measure inrush current. They use high-speed, digital signal processing to filter out electrical noise and capture inrush current as the circuit protector sees it.
Digital multimeters (DMMs) measure two or more electrical values, such as voltage, amperage, or resistance. They require a clamp accessory to measure motor inrush current.
Power quality analyzers are sophisticated tools capable of a wide range of power measurements. Like DMMs, they require a clamp accessory to measure inrush current.
Asynchronous vs. synchronous measurements
The most commonly used methods of measuring inrush are peak, peak hold, and min/max hold. However, in most cases these readings won't accurately depict what the circuit protector is experiencing because they aren't necessarily synchronized with the motor startup.
You can solve this problem by starting the inrush measurement function precisely when the inrush current starts. Clamp meters capable of this operation are equipped with an inrush function that triggers the meter and takes the measurement when the motor starts. Once triggered, the meter will take a large number of samples during a 100 msec period and then digitally filter and process the samples to calculate the actual starting current. The result is a more accurate, synchronous indication of the start current.
Some power quality analyzers have inrush functions. While they share the same name, they're different from clamp meters with inrush modes. When you select the inrush mode on a power quality analyzer, it digitally captures the current waveform. You can then use a cursor to pick out the instantaneous ampere value at any point on the sampled current waveform. While you can estimate the effect of inrush current using this method, it's not as precise or as easy to use as a clamp meter with an inrush mode.
The basics of measurement functions
Adding to the confusion, manufacturers of clamp meters use different terms to describe the same measurement function. In fact, the actual operation of the feature may be significantly different from what the name would imply. Although these terms can vary from one manufacturer to the next, the following sample offers an idea of what's out there.
This records the highest peak value on the negative or positive half-cycle that exceeds 1 msec or 250 microseconds (µsec), depending on the meter. This is useful for capturing voltage transients and measuring distorted voltage and current waveforms. While the peak function is capable of capturing an inrush reading, it typically uses too narrow a measurement window in the start-up process to capture true inrush.
This DMM recording mode can capture voltage sags and other intermittent events. A meter with this feature will display the highest, lowest, and average of all readings during the period it's recording. A meter usually won't record an event that lasts less than 100 msec, or 6 cycles at 60 Hz, so it will typically miss the startup current.
Newer generations of meters have a function more correctly labeled Digital Min/max. This measurement looks at a portion of the analog-to-digital conversion to obtain its value. The meter takes measurements based on the meter's system clock, which, in many cases, may not coincide with the startup event. So, you could miss some or all of the event.
Min/max measurements are most useful for longer-term events like those that occur on heavily loaded or long wire run circuits to record more regular voltage drops or load increases. They're not, however, ideal for inrush measurements.
Although used as an analog track and hold in AC systems, this function on some meters looks at the output of the AC analog rms converter. This slows down the response rate significantly so these meters may be useful only for events that last several hundred seconds or longer. This is too slow for short-term events, like inrush current.
On some older Fluke instruments, this measurement captures the maximum readings of waveform peak values. The earliest DMMs had an analog peak hold circuit, labeled Peak Hold that captured the highest peak value that lasted for 10 msec or longer, regardless of when it happened. From a strict definition point of view, this was a correctly labeled feature because it did capture the actual peak value. But as with Peak, it was asynchronous to the inrush event, so it didn't necessarily measure the inrush current.
Beware the meter with misleading function terms, especially when it comes to measuring inrush current. Few measurement functions are capable of accurately tracking and measuring inrush, so it's important to be able to decipher the varying terminology and choose the correct option for identifying this damaging power quality event.
Greenberg is a product planner for Fluke Corp., Everett, Wash.
Sidebar: A Case for Efficiency
An estimated 65% of the electrical energy consumed in the United States comes from electric motors and more than 78% of the electricity consumed by industry is used to power electric motors. If all the motors sold in this country met the energy efficiency goals set by the Consortium for Energy Efficiency, companies could save 23.3 billion kWh annually, worth about $1 billion. Add to that the energy shortages around the country and it's no wonder the urgency has increased to install high-efficiency motors in plants.