Induced voltage on Motor Power Wires? 2

[nodoubt]

Application:

We have six 5hp 3ph 460V motors.
The wires are all in the same conduit trunkline back to the MCC. (longest run 225ft.) We are using
Siemens Sirius soft starters 3rw3024. We had a situation where one of the softstarters shutdown. The amperages readings for the motors were all in the proper range. To further debug, the motor leads were disconnected from the motor and from the soft starter. We were getting about 60-100VAC (T-lead to ground) on the wires. Ungrounded delta system.

Basically with all the soft starters off you may have 40mV on one set of T-leads. As you turn one soft starter on (just powering up 120VAC control) at a time you can see the voltage rising on the disconnected T-leads.

A ground wire is run to each motor. And the conduit system is continuous throughout.

I was thinking about using some type of coil or a resistor.

All suggestions are welcome.

 

[itsmoked]

If you are using a modern DMM this could be capacitive coupling. Hook up (carefully) a drop light across where you are measuring, make a measurement to confirm the same reading then turn ON the drop light and take the reading. If the reading is near zero and the drop light doesn't actually light up it is capacitive leakage.

 

[electricpete]

I agree with itsmoked. You are probably measuring a floating conductor. As suchj it is very easy to induce voltage on it. You need to put a little load to help evaluate it. If digital meter, a light as he mentioned or even a large resistor in parallel with the meter (large enough so it won't fry if the conductor is somehow energized). Or the easier approach is to find an analog meter like a fluke. Most likely you will see that voltage go to 0. For safety, treat it like it is actually energized until you confirm otherwise.

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[electricpete]

I meant analog meter like an old Simpson.

Also note the ground wire you mentioned is likely going to the motor frame and doesn't serve to provide any ground of the motor windings.

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[nodoubt]

Thank you both for the prompt responses.
I'm not sure if we have any analog meters
but I will check. If not I will add a resistive load
and verify accordingly. My main concern was that an electrician would not get shocked while troubleshooting.

Thanks again.

 

[jraef]

What they are refering to is the leakage through the SCRs, and possibly the voltage across the dv/dt snubbers in the soft starter. They will leak a voltage that you can read, but any resistnce in the circuit drops it to nearly nothing. That is why an old analog meter burden is even enough. Digtal meters have no appreciable burden and so are capable of reading these tiny leakages.

Your problem is likely very different. You may not be aware of it, but that particular soft starter does not have 3 sets of SCRs that turn off when you remove the Run command. It has 2 sets of SCRs and one piece of busbar that goes directly through the unit, Line to Load, so one leg of your soft starter IS ALWAYS HOT!! Even when you think it is off. So if, by saying you "apply 120V control power", you mean that you turned on the line power to energize the control ower transformer, you are also energizing that uncontrolled leg through the soft starter!

This design is a common trend in the industry, accomadating the "smaller-cheaper-slicker" tendancy we have all come to expect from technology. This solution to size reduction however is considered to be unsafe for the above reason, and many US users insist upon Line Isolation Contactors as a result, but that of course increases the size and cost, defeating the original purpose of the design. The concept has been used in other countries for years, but when Siemens and others introduced it to the US market, they explained that it was "OK, because there was no code saying you couldn't do it" (paraphrased from a now defunct Siemens introduction website for the Sirius).

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[electricpete]

Wow, that would be a scary design, but I can believe it.

But in case one input is sent directly to the output, we would see full line-to-ground voltage of 277vac, right?

(That's what I measure to ground on our ungrounded 480vac systems... must be due to symmetry of capacitance and other ground-connected impedances).

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[jraef]

If grounded, yes (although he said it was an ungrounded delta, so it may not be exactly 277). But even if he is not grounded, with one leg being hot all the time it may be skewing the voltage readings because instead of reading a potential difference from leak to leak across the SCR or dv/dt snubber, it is reading from leak to line on 2 of the 3 phases. And to boot, that leakage may in fact induce a measurable voltage even on the 2 lines with SCRs on them, I don't know. I have never had one of these in my hands long enough to power it up and measure everything. They scare me too much.

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[nodoubt]

Jraef,

You are so true. A couple of weeks ago, we changed out
an ACL starter with a softstarter and we had to modify the push button controls etc. That's when we discovered that
the softstarter does allow the roughly 277-280VAC (Line to ground) to bleed through. My electricians didn't like that too much Anyways, if this is something we have to live with, what are some good safety measures I can implement to
protect my guys. One guy said he was shocked once already.

Do they make filters, resistors, or some sort of device we can install?

Thanks again!

 

[itsmoked]

How is this different from ALL the refrigeration control panels that use 2 pole contactors to control all the 3 phase loads?

 

[briand2]

Don't you just install an isolator on the supply side of the soft starter? That's what we would do in the UK.

 

[jraef]

Itsmoked,
It is no different at all. As I said, it is not illegal, just a bad idea in my opinion. The fact that it does not violate any codes is because it is assumed that you must have disconnected power prior to opening the cabinet, and once you re-energize for testing, it is always assumed that a qualified electrician understands that everything in that panel has the potential for being hot. Good electricians are taught that early on. What makes it a bad idea is that not everyone who opens a control panel is necessarily a good electrician, and this type of system is one that has dangerous potential (pun intended). The victim will take the blame, but he will be a victim of something that could have been avoided. You know the saying;

There are old electricians, and there are bad electricians, but there are no old bad electricians.

nodoubt,
The only truly safe method is a line isolation device as briand2 mentioned. The voltage you are reading is already through a high resistance, and the current potential is very very low, in the neighborhood of 4-10ma. As your electricians have already experienced that is something that you can feel, but typically below the threshold of what even a class B GFCI would trip at (i.e. swimming pool light fixture trip level). Many industries require a line isolation contactor because it is a voltage that can be read by a meter, but most people consider it acceptable as is, other than the 2 pole device issue mentioned above.

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[Barry1961]

If you don't have a Simpson analog meter you can get a "Wiggy" for about $35. Sort of a crude device but works well in cases like yours.

Barry1961

 

[itsmoked]

Thanks J.

 

[nodoubt]

My siemens rep found this for me: But it basically says what everyone elese has been saying.


3RW30: Electrically isolation and switching off in the event of an emergency off

QUESTION:How can the power system and the motor be electrically isolated from each other? How is the motor switched off in the event of an emergency off?ANSWER:Soft starters cannot electrically isolate the power system and the motor terminal from each other. No electronic switching device does this. There can be a measurable and detectable voltage across the motor terminals even if the switching device has been switched off. This also applies to frequency converters. There is, however, no standard which prescribes such electrical isolation. It is also not necessary because, if work has to be done on the motor terminals, the switching device with disconnector function must be switched off in any case. Simple opening of a contactor would not be enough here. Electrical isolation is therefore irrelevant to a normal switching device like a soft starter.In order to switch off the complete motor feeder circuit in the event of an emergency off, only an undervoltage trip has to be fitted to the circuit-breaker. This is operated by the emergency off button and it then disconnects the complete motor feeder circuit from the voltage supply.

http://support.automation.siemens.c...cslib.csinfo&lang=en&objid=21657127&caller=nl

 

[jraef]

LOL, that is a more "politically corrected" version of the way they described it when they rolled out this product a few years ago, but it is essentially saying the same thing. "We can do it because nobody says we cannot, and if you are too ignorant to understand these issues, you deserve your fate".

I love that. They are so matter-of-fact about these things.

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[itsmoked]

Yes, the "Evolution-In-Action", method.