Inrush Current

[1961MGA]

I have a fluke 87v but I am curious if this will measure inrush current. I noticed the 37x series will do it and has a 100ms test time listed, but so does the 87v. Anyone know if they are the same or is the inrush function on the 37x series different?

 

[scissorman]

I can't say for the Fluke 87V but I have a Fluke 375 amp clamp and it measures inrush current. I love my Fluke and need to buy another for home use.

 

[zkling]

Not by itself. You are comparing two different types of meters. Do you have a current clamp for your 87? A DMM isn't really the best way to measure inrush current. What range of current are you trying to measure?

 

[1961MGA]

Yes, I have a current clamp. I am testing below 50 amps. I am just not sure if I should get a separate clamp meter or if I am okay with the 87v. Both have a 100ms testing time.

On a side note, these current clamps are massive. I think a clamp meter is actually smaller!

 

[theoldwizard1]

Yes, I have a current clamp. I am testing below 50 amps. I am just not sure if I should get a separate clamp meter or if I am okay with the 87v. Both have a 100ms testing time.

I'm noy certain what your goal is, but IMHO, 100ms is nowhere near "fast" enough. You need a shunt and a 'scope that has better than 10 MHz capability.

 

[1961MGA]

Im just using it for startup amps on compressors

 

[May Pop]

The highest inrush current will be in the first cycle on a 60 HZ system often as much as 25 times the rated amperage. This will drop rapidly on the second and third cycle.
Inrush on a transformer drawing 50 amps may be as much as 12-1500 amps. This will not blow a 100 amp fuse if of the "slow blow" variety. This cannot be measured with a standard hand held ammeter.

 

[theoldwizard1]

The highest inrush current will be in the first cycle on a 60 HZ system often as much as 25 times the rated amperage. This will drop rapidly on the second and third cycle.

That first full sine wave (at 60Hz) will take 16.66 msec. 5 cycle would be 50 msec. Your 100 msec meter would never see this, Now you understand why I said you need a shunt and a 'scope. Actually a 'scope with differential input.

 

[1961MGA]

so what is the point of the fluke 37x series with inrush? The inrush is measured at 100ms according to the manuals.....

 

[Jdsmith]

As theoldwizard1 is alluding to, inrush can mean different things to different people. The current drawn in the first half cycle can be much higher than the locked rotor current depending on the design of the motor and a few other factors. Inrush is commonly taken to mean locked rotor current, or the current drawn while the load is accelerating. These values are slightly different but close enough that if you confuse them you may not get in trouble.

I have compared the inrush measured by my Fluke 337 with the inrush measured by an oscilloscope and similar higher speed instruments. The Fluke 337 accurately reports the current in the accelerating current/locked rotor current range - i.e. the inrush current most folks are interested in. The Fluke 337 does not report the first half cycle peak inrush value. I have not tried using an 87V with a current clamp for inrush measurements. Fluke's documentation is fairly consistent, so if you compare the verbiage in the 337 instruction manual with the 87V instruction manual and you can convince yourself that the inrush features work the same, it would be worth trying the current clamp on the 87V.

 

[Milton Shaw]

With a clamp amp meter the fluke should catch inrush with the max/min function. I have not tried it but that is one of the purposes of the max/min function.