Voltage drop in control circuit

[adamscb]

All,

This really isn't a question, but just something I learned today at work (I'm relatively new to the industry, so I'm learning something every day). I inherited a new MCC room from my predecessor who left the company, and the motors I just recently re-feed are a good distance away from the MCC room. It turns out that the start/stop wires are so long in length, that there's a considerable voltage drop (16 awg wire), so much so in fact that there's not enough voltage to pull in the contactor. What we did was add in a 120v relay (which can close with less than 120v), and then have the relay control the 120v going to the contactor. Nifty trick.

Lesson learned: the bigger the control wires the better (within reason of course)

 

[Ingenieur]

By replacing the contactor load with a relay I assume the current dropped alot
say a factor or 3 or 4
since V drop = current x R of wire
if current dropped to 1/3 of the contactors, so would V drop

so not only is the relay pick up lower, so is the V drop, ie, higher load V

 

[adamscb]

Here's a picture of what we did. The top is what we originally tried, but didn't work.

 

[topgone]

Here's a picture of what we did. The top is what we originally tried, but didn't work.

View attachment 16184

If you have a lot of motor starters, perhaps it will be much economical for you to change your control source with a 120V AC supplied by an inverter, which get its DC power from the normal 120VAC auxiliary power. Any voltage sag during starting will not disturb your inverter's AC output as it has battery backup.

 

[adamscb]

If you have a lot of motor starters, perhaps it will be much economical for you to change your control source with a 120V AC supplied by an inverter, which get its DC power from the normal 120VAC auxiliary power. Any voltage sag during starting will not disturb your inverter's AC output as it has battery backup.

I would try that, but it's a standard in our plant that all control power comes from the bucket. Before doing this I suggested that we use 120v power from a nearby 120v/240v panel, but in our plant that's unacceptable. Apparently people were throwing down bucket handles thinking everything inside was dead, but it wasn't.

 

[Ingenieur]

If you can get the va for each, contactor and relay

is the 88 v before and after or just before (w/contactor)?

quick search
contactor 5 va
relay 1.2 va
drop would be 1/4th as much

pu is about the same for both 0.80 to 0.85 x rated coil V

 

[adamscb]

Not sure about va ratings, but contactor is a Size 4. And the 88v was being dropped across the contactor (positive lead on one side, negative on the other), current was flowing

 

[Ingenieur]

Not sure about va ratings, but contactor is a Size 4. And the 88v was being dropped across the contactor (positive lead on one side, negative on the other), current was flowing

Measure the relay V for fun
how long is the run? Rough estimate

good to see your interest in this stuff

 

[adamscb]

Measure the relay V for fun
how long is the run? Rough estimate

good to see your interest in this stuff

Voltage across the relay was 2v, not sure if that reading was correct or not. And a rough estimate is about 1,400 feet.

 

[Fnewman]

Have you tried installing a larger CPT in the bucket?

 

[gar]

161130-1951 EST

adamscb:

Glad to see your interest in analysis of the circuit.

Some comments:

The overload contact should include your contactor coil. Under overload you want to be certain that the contactor drops out.

I believe your drawing logically describes what you did, but not physically, at least if the contactor coil wiring is still#16. #16 copper has a resistance of about 4 ohms per 1000 ft. This is 8 ohms per 2000 ft loop.

We don't know the resistance of your contactor coil, nor its impedance at 60 Hz when the contactor is de-energized.

Consider a #2 AB motor starter I have. It has a DC coil resistance of 40 ohms. Impedance at 60 Hz is higher, but when the contactor is in its de-energized state not as high as after the contactor closes. My impedance estimate at 60 Hz in the open state is 72 ohms.

On a resistance divider basis the initial coil voltage at the end of a 2000 ft loop of #16 copper wire is 120*40/48 = 100 V.

If I go thru the math with the above impedance, and no mistakes then the coil voltage is 112 V to use for the available voltage to pull in the contactor.

You may have a much larger contactor, and therefore a lower impedance.

We need more details on your circuit.

Need a precise drawing of where the contactor coil gets its power from in your modified circuit.

Your DC coil resistance can be measured with a Fluke 27 or 87. You can put a 1000 ohm 20 w resistor in series with the contactor coil apply 120 V to the series combination, and measure the voltage across the contactor coil and get a rough estimate of impedance. Current is approximately 120/(1000+DC coil resistance).

To get the voltage you see at the contactor means a very large contactor, or some other current on your neutral.

.

 

[adamscb]

Have you tried installing a larger CPT in the bucket?

We considered that, but we figured since we weren't blowing any fuses the transformer was rated correctly. Also getting a larger TX inside the bucket would be difficult, it's already packed as is.

 

[gar]

161139-2415 EST

adamscb:

My AB contactor pulls in about 75 to 80 V. But when closed is somewhat noisy.

At 123 V its measured closed state current is 0.3 A, power input 9 W, PF = 0.25, and VA = 33. The 33 VA times PF = 8.25 good correlation with measured power. The Kill-A-Watt instrument probably does this calculation so I would expect correlation.

Using 0.3 A and 40 ohms provides a coil power dissipation of 3.6 W. Thus, there is considerable shading coil and iron loss.

.

 

[gar]

161201-0825 EST

A typo in my previous post. "The 33 VA times PF = 8.25 good" should read "The 33 VA times PF = 8.25 W is a good" to be more clear.

.

 

[kwired]

Not sure about va ratings, but contactor is a Size 4. And the 88v was being dropped across the contactor (positive lead on one side, negative on the other), current was flowing

Size 4 contactor has much larger armature then a little ice cube style control relay, and takes a lot more power just to pull the armature let alone the higher "inrush" current it likely has when energizing it. I'd guess your voltage drop when the armature is pulled in isn't that bad, it just takes more current to pull it in which must happen first.

Some instances where the control voltage is derived either directly or indirectly from the motor circuit with voltge drop issues on the supply side of the circuit this problem can get even worse as the attempt of starting the motor itself also contributes to voltage dropped in the control circuit.

 

[steve66]

I had a similar situation where a thermostat controlled a motor starter/contactor. The thermostat went bad because there was too much current through it.

Relay coils normally draw a lot less current than a motor contactor does.

Definite purpose contactors also tend to draw less coil current than normal starter/contactors.

 

[don_resqcapt19]

Interposing relays are often required for long distance control circuits and/or large size starters.

You can also have the opposite issue with a very long 3 wire AC control circuit...that is where the start button will pull the contactor in, but the stop button will not cause it to drop out.

Under the right conditions, the capacitance of a long control circuit will be enough to hold the coil in even when you push the stop button and open the circuit. The increase in wire size that is often used to overcome the voltage drop on a long control circuit increases the likelihood that the stop button will not open the circuit.

 

[gar]

161201-1030 EST

Following are waveforms from my #2 starter with an external series resistance of 13 ohms.

.


Voltage across contactor coil, about 123 V AC source, 13 ohms resistance in series with contactor coil.

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Voltage across 13 ohm series resistance, about 123 V AC source, 13 ohms resistance in series contactor coil.



.

 

[dkidd]

There is a good Square D document discussing options: http://static.schneider-electric.us/docs/Motor Control/Lighting Contactors/M-379F.pdf

 

[Fnewman]

We considered that, but we figured since we weren't blowing any fuses the transformer was rated correctly. Also getting a larger TX inside the bucket would be difficult, it's already packed as is.

Might be worth an 'experiment'. I've seen it make a difference.