Lesson Learned: Check Your Remotes!

[John McClanahan]

We have an old freight elevator at work that I rewired 20+ years ago. It has a 208 volt, 3 HP, 3 PH motor that draws something like 5 amps full load. The power wires travel in horizontal conduit for about 25 feet. When the elevator starts, the wires jump in the conduit. It sounds like someone slapped the conduit with a piece of chain.


John

[Mike Henderson]

Every AC motor draws higher current when it's first energized, a fact referred to by the terms "starting current," "startup surge" or "inrush current." Different motor designs behave differently in this respect, and High-Efficiency motors like the 3hp Leeson on my new cyclone are especially fond of high inrush current. I read somewhere that the inrush current on these high-efficiency motors is roughly equal to the locked-rotor current, which in the case of my Leeson is around 100 Amps (vs 10-12 amps in normal operation). Because the inrush lasts only a few seconds, the #12 wire that supplies this motor handles it fine--with one little exception.D.

If you had 100 amps for several seconds through a breaker that was rated at say 20 amps, it would trip the breaker. The inrush current on a motor decays with the rotation speed. On a normal unloaded motor of less than 5 HP you're probably talking milliseconds of really high current (a few Hz), with a very rapid decay. By one second the current should be pretty close to unloaded running current. I know the motors in my shop "jump" to speed in no more than a second.

Like Dan, I cannot imagine a single wire, in the kind of voltage and amperage we're talking about for woodworking tools, generating sufficient magnetic field to move the wire.

Mike

[Thomas Canfield]

I had a problem with my Oneida Dust collector starting on its own after I got my Powermatic 3520B. It happened about 800 rpm on the lathe and the frequency off the inverter. It turned out that the remote for the 220V dust collector used the wiring in the walls for transmission, and the problem was solved by changing to a magnetic starter for the 220V that was controlled by the 120V side of the remote switch I got from Oneida. Somewhere here is a post of the problem back about 7 years. Needless to say, it was a nuisance to get resolved, but has not been a problem since.

[Brad Swanson]

John, your worries are founded. This has happened to me with my CV1800, although I caught it the same day. Its not fun coming home from work to find the DC on (twice!).

[Steve Peterson]

Like Dan, I cannot imagine a single wire, in the kind of voltage and amperage we're talking about for woodworking tools, generating sufficient magnetic field to move the wire.

I am skeptical also. I do remember an experiment in college physics class where 2 wires were running parallel to each other. Current flowed one direction in one wire and the opposite direction in the other wire, causing them to be attracted to each other. It took a very sensitive scale to detect the force caused by current flowing through a single wire in each direction.

Steve

[Carl Weber]

Hello all, I can tell you from experience it is indeed possible to create "wire slap" in conduit due to high current starting of a motor. I have done it with 460 volt 800 amp 3 phase. 500hp across the line. (I am a Project Manager at a international Automation company) Here is a quote from a white sheet from Yaskawa "Typically, when a NEMA Design B induction motor is started across-the-line, an inrush current of 600 to 800% FLA occurs"

I will also include the link If anyone else is curious. https://www.google.com/url?sa=t&rct=...91071109,d.cWc

[Dan Hintz]

I have done it with 460 volt 800 amp 3 phase. 500hp across the line.

That's a far cry from a 120V, single-digit-amperage, single-phase line, don't you think? 500hp vs. 3hp

[Carl Weber]

I agree it is a far cry. But the principals the same. The 5hp lesson is FLA of 20.8 Amps worst case 20.8 x 800% = 166.4 amps (for a few milliseconds). This is basically a near short circuit condition for a few (10-100 milliseconds) This causes the wires to move. (caused by the opposing magnetic fields in the wires). During normal operation the magnetic fields in the conductors will be in the same direction. I have included the links where I found the information.

Example http://ocw.mit.edu/ans7870/8/8.02T/f...Series_640.mpg

http://dev.physicslab.org/Document.a...ryingWires.xml
If two current carrying wires are parallel to each other, their respective magnetic fields either attract or repel each other.

As you can see in the diagram above, if two parallel wires have currents traveling in opposite directions, the magnetic fields generated by those currents between the wires will both point in the same direction, in this case, into the plane of the page. These wires would repel each other. An animation showing this result can be view from MIT's OpenCourseWare.

[Dan Case LR]

If you had 100 amps for several seconds through a breaker that was rated at say 20 amps, it would trip the breaker.

For several seconds, yes. For one second, only if you're using a fast-trip breaker. If you are using the right type of breaker, it takes it just fine. The Square D QO breakers I use in my shop are all HACR rated, designed to handle inrush current.

D.

[Mike Henderson]

For several seconds, yes. For one second, only if you're using a fast-trip breaker. If you are using the right type of breaker, it takes it just fine. The Square D QO breakers I use in my shop are all HACR rated, designed to handle inrush current.

D.

Unless you're running a very big motor you're not going to have 100 amps going to that motor for several seconds. Motors that we use in woodworking will not draw that kind of current for more than a fraction of a second - the fall off is probably exponential. Within about a second, the current will fall to essentially operating current. Within about 2 seconds you're at steady state.

Mike