Measuring motor starting current

Discussion in 'General Electronics Chat' started by David Meiland, Feb 6, 2011.

  1. David Meiland

    Thread Starter New Member

    Feb 6, 2011
    I am trying to learn about measuring motor starting current, using a couple of different Fluke digital multimeters and a current clamp. I am a general contractor who builds houses (Seattle, WA, USA area) and have very little electronics knowledge, so I'm hoping that someone will take pity on me and explain the basics.

    The motors in question are single phase AC motors, usually 120V but sometimes 240V, in the 1HP to 3HP range. They are the types used to run typical electric tools like air compressors, table saws, etc. This all started when I tried to figure out why a particular air compressor was causing nuisance trips of circuit breakers, when another similar compressor was not.

    The meters I have are models 179 and 87-V. The current clamp is a model i200. I use a line splitter at the wall plug to attach the clamp. Both of the meters have a "MIN MAX" mode, in which the minimum and maximum reading are captured and displayed. The difference, as I understand it, is that the 87-V has a faster response time (a shorter internal clock cycle?) and is going to be more accurate when trying to capture a short-term value. However, I have found that the 87-V consistently gives smaller motor start readings, which I find confusing. Example: 2HP 120V single phase air compressor, the running current is ~15A according to both meters, but the 179 says the start is 43A and the 87-V says the start is 35A. From what little reading I've done, it appears that start current is 4x running current, or even higher.

    Bottom line, I have little real understanding of how these meters work internally, but I believe I am operating them in the intended manner. I'm pretty sure an accurate reading of this type should be possible with this type of equipment, but I am aware that an oscilloscope or power analyzer could also be used (tools like these would be wasted on me).

    Any guidance would be greatly appreciated.
  2. mik3

    Senior Member

    Feb 4, 2008
    Your meters shall be specified for true RMS readings in order to measure the starting current accurately. If you want to estimate the circuit breaker rating, then assume a starting current of 5-10 times the rated current. Thus, use circuit breakers rated with the continuous current of the motor but they trip at 10 times the continuous current.
  3. someonesdad

    Senior Member

    Jul 7, 2009
    The measurement of the starting current is not a trivial measurement to make with digital multimeters because they are instruments that sample the signal -- and they may not sample at the right point or often enough to catch the peak current, which is what you seem to be interested in.

    The "proper" tools to perform this measurement are an oscilloscope and a current probe with a wide bandwidth. It can also be done with a shunt and an oscilloscope, but this wouldn't be safe for a beginner to try. A third way is with a current transformer and a scope. If you had access to a scope, your Fluke i200 could be used to make the current measurement (it also has a decent bandwidth).

    Probably the best you can do is to make multiple measurements and average the results. If you do so and compare the two loads with the same instrument, you may be able to distinguish why one blows the breaker and another doesn't. But then again, you might not, as your measurement tools really aren't adequate for this task.

    The peak current may not in fact be the thing of interest to explain why the breakers are blowing. Instead, the integral of the RMS current (area under the curve) may be what's relevant in whether the circuit breaker decides to trip. There are circuit breakers that can manage to deal with a significant overload (as mik3 mentioned) for a short period like a motor start-up, but then will trip if the continuous current level is too high.
  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    There are tools that can capture and display the cycle, but for your purposes, not important.

    On start, a motor is at standstill and the current leaps toward what is termed 'lock rotor', or in general terms, a stalled rotor. This can be a very high current, approaching the classic volts/resistance of the motor, but generally considered to be 6 times the FLA, or full load amps. This high current produces a high torque, which spins up the rotor. The higher the rotor speed, the greater it's generated voltage, which counters the source voltage, resulting in a much reduced current running current.

    Overload protection for the motor must take this high current startup into consideration, and does so based on a time regiment. Typically, in the overload, heat is generated due to the current flow. If the current flow is not reduced, the increasing heat causes the breaker element to activate the trip mechanism.

    One thing you want to be aware of, is that continued tripping/reseting of a breaker will injure it, as well, you may employ methods that can further injure a home's wiring system, such as increasing a breaker size to prevent tripping.

    You may want to consider looking at the options of fabricating a cord with overload for you specific purposes, that can plug into a higher current circuit, such as the range or dryer.
  5. timrobbins

    Active Member

    Aug 29, 2009
    David, maybe you also need to do some reading up on circuit breaker trip characteristics.

    Loads that have significant start-up/turn-on current level compared to their continuous levels usually need at least a D curve characteristic (where normal charcteristic is C curve - but different global regions have different terminology). Most people would just swap the cb (same current rating - different curve) to get around the 'problem'. If you get to higher current levels then the cb's get more exotic, and you can tune the thermal and magnetic characteristics of the cb, and there are specialist motor-start cbs.

    Maybe the easiest thing to do is google some example oscilloscope waveforms of motor start up currents, and just use them as the learning curve (rather than try and get an understanding from a meter than is not meant to measure what you are interested in).

    Ciao, Tim
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
    NEC 70 is a good reference, if you're in that business you should not only have a copy but also a qualified electrician to consult with.
  7. David Meiland

    Thread Starter New Member

    Feb 6, 2011
    Thanks to all for helping with this question.

    Part of what is confusing about this is that Fluke markets a line of clamping ammeters (the 330 series) as being able to measure "inrush current". Their tech rep explained to me on the phone (rightly or not...) that the internal workings of those meters were essentially the same as the 179 and 87-V and that the results would be the same. I would rather have a bench meter than a clamp meter simply because it's easier to read the display when you don't have to hang the meter from the wire, so that's why I have what I have.

    Further, the 87-V is said to have a clock time that's four times (?) as fast as the 179, so from a layperson's perspective it seems this would be more likely to return a higher (more accurate) value. With the FLA at ~15 I was expecting starting current in the range of 60A or more.

    Questions like this from me are strictly a matter of curiosity. I do not do electrical work on people's homes, I use a licensed electrician to do that, and I have little hope that he would be able to explain this stuff to me. Most residential service panels here are Square D QO, and while I believe there are two categories of breakers for this line I don't think there are any designed specifically for motor applications.

    The bottom line is that a lot of builders struggle with compressors tripping breakers. We take them to jobsites, and sometimes they work fine while other times they do not. Sometimes they trip one circuit while not tripping the adjacent circuit. I ended up with one compressor that tripped most circuits while a matching unit does not, so I started trying to figure out what might be going on (rather than mothballing a perfectly good piece of cast iron). Ultimately I rewired that one from 120V to 240V and if I take it out I have the electrician install a 20A/240V receptacle for it.

    What I'll take from this is that I really don't have the right tool for the job. I might ask my Fluke rep to loan me a scopemeter to play with. My wife would not tolerate seeing such a tool on my bench for long, because she knows I don't need it.