3 phase Heater on soft start .

[lefty51]

A project was done in house, where they had to install a 480 V - 3 phase tank heater ( I am not sure of the KW rating ), in a spare tub, which is a soft start.
I believe it normally draws 92 amps....or very close to that.
The main MCC breaker tripped, and someone wondered if it was because the soft start caused excessive amperage draw, when it started. I said not likey. On an electric baseboard heater for example, if you cut the voltage in half, the current also drops by 1/2, and you have 1/4 of the wattage. This should also be applicable on a 3 phase heater as well, I would assume ? Its a resistive load, so 92 amps, should not peak, or vary....its either on or off....correct ?

 

[fangas]

Cold heaters have a very low initial resistance, that increases with temp.

I don't know what you're refering to as a soft starter, but I'll assume it a controller with an adjustable "On" ramp. It that ramp time is short enough, you might see inrush in excess of your CB rating. Extend the ramp time some & see what happens

Ed

 

[DRWeig]

Lefty,

Not sure how your "soft start" works -- is it a wye-delta transition starter, or autotransformer starter, a VFD-style soft starter, or some other reduced-voltage device?

You're right about how the heater should act at reduced voltage.

Is your 480 system grounded, is it 3-wire or 4?

From the amps you quote, it's about 75 KW.

Feed us some more data -- this could be anything from a ground fault to a wiring error...

Good on ya,

Goober Dave

 

[lefty51]

A soft - start motor controller, for slowly ramping up a motor up to speed. This is what is controlling the heater. Yes, I know that it should be used for motor loads, but apparently, it was the only spare cell that they had, and decided to use the soft start instead of a contactor. Theoretically, it should work fine....if it will work for an inductive load, it should be fine for a resistive load..........I would think.

 

[lefty51]

Its an Allen Bradley SMC solid state soft - start, if that helps at all.

 

[DRWeig]

Hi again Lefty,

I'm familiar with the AB devices. Here are some thoughts.

If the heater is purely a resistive one (not using induction), it should work.

Fangas is right in that there will be an inrush due to lower resistance at lower temperature when starting, but it shouldn't be as severe as a light-bulb filament (temp difference from cold to hot isn't as great).

What's the trip setting on the CB that is tripping? Does it have ground fault protection? Since your main circuit breaker is tripping, it seems that a ground fault is possible.

Let us know what you find out!

Good on ya,

Goober Dave

 

[fangas]

Two questions, Lefty,

Are there any transformers between the controller and heaters?

How are the heaters configured? Specifically, are they in a Wye configuration with the center neutral?

Ed

 

[lefty51]

Im not sure of the configuration as we speak. I may have to get some info. off the engineers if we have to trouble shoot this. There are no transformers between the soft - start and the unit. It feeds into a local disconnect, and then to the heater. I was thinking that the unit may be shorted somewhere, and going to ground, and as DRweig said, this may be the cause of the main going out.
Engineer said he took and ohm reading between phases and all seemed balanced, but that does not verify our insulation factor either.

 

[jraef]

The AB soft starter can operate in Current Limit start or Voltage Ramp start. If it is set in Current Limit and the load is resistive, you will not likely see any ramping effect, it will immediately allow current up to the limit setting, which on a resistive heater with a PTC or resistance may end up being too high for the instantaneous trips on your circuit breaker. You need to make sure it is set for pure voltage ramping if used on a resistive load.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[lefty51]

Am I wrong in thinking, that the load cannot go over 92 amps ?? I can see this on a motor starting and building up to speed. Heres another clue to the mystery.....I found out that they blew ( 2 ) 175 amp time delay fuses at the soft start, as well as trip the mcc breaker.
If I remember correctly, time delay fuses will allow 500 % of the rating for 10 seconds. So, a ground , ( if ground fault protection is provided on the breaker,) is definitely an option.....you think ?
Or possibly a short between windings ?

 

[itsmoked]

I'd guess short between heater elements. Or miss-wiring if these heaters are made up of a whole bunch of multiple elements that need to be paralleled and series'd. It's easy to mess that up.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[fangas]

Lefty,

The resistance of heating elements can be very dynamic. They'll have both a hot and cold resistance.

If (for simplicity) an element has a cold resistance of 1 Ohm and you apply 100V to it, it will draw 100A initially. If it has a hot resistance of 2 Ohms, it will draw 50A when it warms up.

A 100W lamp at 100V will draw 1 A, showing a hot resistance of 100 Ohms. If you take that same lamp and measure its resistance cold, it will appear almost like a short.

Your elements may not be as dynamic, but they will change. So yes, for some period of time, under full voltage, your elements can draw more then 92A.

That's where Jraef was going. If your control is set up for current limit, it will only throttle back the voltage when that limit is met. And might be set too high for your load, or not fast enough to respond. That's why he recommended making sure you're setup for a voltage ramp.

Itsmoked, was refering to the fact, if you have miswired anything, such you have some parallel elements where there isn't supposed to be some, your resistance will be reduced even further.

Ed

 

[itsmoked]

Hey isn't "Fangas" the name of a heater or heater control company?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

"not fast enough to respond"

Definitely another possibility. Some soft starters, especially digital ones such as the AB SMC Series, might take a half second or so to initialize before they can begin limiting current. Normally not a big deal in a motor circuit. In addition, AB soft starters have a feature called "Kick Start" which is supposed to be used to "break free" a motor load that might be stuck due to excessive friction or icing etc. If that feature is enabled, it does NO control of voltage (or current) for the first few seconds (adjustable from .2 - 2 seconds), then it drops down to the initial ramp settings In a heater, that may be long enough to make the breaker trip or fuses blow.

Another issue: you didn't say which of the AB SMC products it is. In that size range, it could be the SMC Flex, which has built-in bypass contacts. In a purely resistive load, ie.e no inductive properties, the soft starter may interpret the current profile as if it is a motor that is getting to full speed very quickly and it will override any ramp settings, going to full voltage almost immediately and closing the bypass contactor.

Remember, you have taken a device intended to be used on a motor application and are using it on something that it was not designed to work on. It CAN work, but you must be vigilant on making sure it is used correctly.

The problem is in the soft starter setup IMHO and it is not likely a bad component either. If it were, the soft starter would see that and fault.


By the way, if the soft starter is working properly, a shorted load would NOT likely cause the current spike you are seeing until AFTER the soft starter was done ramping because the current is going to be artificially limited by the soft starter. That is something seen in motor applications all the time; people discover they have a problem in the motor when they add a soft starter because the current limitation allows the effects to last long enough to be seen, whereas when they started across-the-line, it was happening too fast.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[fangas]

I'm not sure about any heater Cos. Keith. Fangas is a "term of endearment" my maintenance crew gave me.

Like all good main't, they were a bit lazy and "F'in Mangas" As in "That F'in Mangas (My last name), I'm gonna kill him" quickly got shortened to Fangas.

Ed

 

[waross]

I have never seen a problem with a cold heater drawing excess current when used for heating water. The temperature change is just not enough and the heater should start across the line successfully in any event. If the heaters were intended for wye connection and connected in delta you would have over 173% of the current that you expect.
I would check the rated voltage of the heaters, the connections (wye or delta) and see if any drive experts can suggest why a soft start won't drive a resistive load. The slight extra current when cold is insignificant compared to the starting current of a motor.
Another strong possibility is that the soft start is bad. That may be why it is 1) still available, and 2) not cannibalized or removed from the MCC tub.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

OK Ed.

I must have been thinking of Furnas.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[lefty51]

The mystery has been solved. A termination wore through in the end junction box.....should have had more tape.
Thanks for the help ! This opened up quite a discussion on controlling heaters through a soft - start.

 

[itsmoked]

Thanks for the "comeback".

Keith Cress
kcress -

http://www.flaminsystems.com