Correct contactors for a transformer

[conmgt]

Hello,
I need contactors for the primary side of a 30kVA transformer that is dedicated to an infrequently used motor (once a week-ish). I need to cut off the transformer to save on standby losses.
At 208V, the transformer draws about 84A. At first I figured a 100A contactor would be fine but while I was looking through a SqD catalog, I came across Table 16.49, Application Data for Selection which shows possibly that for switching a transformer, a NEMA size 5 contactor is recommended which is rated for 270A. Am I reading something wrong?

 

[gar]

130112-1357 EST

conmgt:

There is something strange in your data. There is not anything like 17,000 watts of losses in the transformer, it would be toast.

Your volts and amperes are more like the values when the transformer is loaded (motor doing some heavy work).

I won't give a judgement on what size your contactor should be. Partly it might depend upon whether it has to switch the motor load. However, switching transients (transformer inrush current unloaded) can be large and to prevent welding of contacts you might want a larger contactor than just an unswitched continuous load would imply.

If it were mine I would use the Sq-D recommendation.

.

 

[conmgt]

17,000W, How was that calculated?
I didn't say what the standby losses were...until now...600W.
The motor connected to the transformer will be switched on about 30 secs after the transformer is switched on so there won't be a simultaneous inrush of both the transformer and the motor.

 

[hillbilly1]

I would think you would be fine with a 100 amp contactor, if you were turning it off and on under load, that might be another issue since its a motor load. The arcing of the contacts upon start up is probably why they recommend the larger contactor.

 

[conmgt]

I would think you would be fine with a 100 amp contactor, if you were turning it off and on under load, that might be another issue since its a motor load. The arcing of the contacts upon start up is probably why they recommend the larger contactor.

Good point. I imagine a motor rated 100A contactor would see higher currents than an unloaded transformer.

 

[jim dungar]

Hello,
I need contactors for the primary side of a 30kVA transformer that is dedicated to an infrequently used motor (once a week-ish). I need to cut off the transformer to save on standby losses.
At 208V, the transformer draws about 84A. At first I figured a 100A contactor would be fine but while I was looking through a SqD catalog, I came across Table 16.49, Application Data for Selection which shows possibly that for switching a transformer, a NEMA size 5 contactor is recommended which is rated for 270A. Am I reading something wrong?

You are reading that table wrong, kind of.
It has two columns of data; one for 20-40X inrush and another for >40X. There is no way your standard run of the mill transformer has this much inrush (at worse you might be 14X, but it is more likely to only be 9X).

For most general duty power transformers (<150kVA), you could consider HP = 1.25 x KVA as a starting point for your NEMA HP rated contactor selection. Of course you should compare the actual inrush current with the device's ratings.

FWIW, output loading has very little impact on the inrush current of a transformer (in many cases a load helps to reduce inrush). During 'start-up' a transformer acts like a reduced voltage starter, limiting the voltage and current to the load.

 

[gar]

130112-2033 EST

conmgt:

You provided the figures ---
"At 208V, the transformer draws about 84A."
Which did not make much sense. You did not say the transformer and its load draws about 84 A. Apparently that is what you meant. But the import of your thread was transformer losses.

A totally unloaded transformer when switched off, even with a mechanical switch, will tend to leave the core near saturation. Because turn off of current in an AC circuit tends to occur near zero current, even with random turning off of a mechanical switch. In a highly inductive circuit (unloaded transformer) the voltage and current (basically only magnetizing current) are towards a 90 deg difference. Thus, at turn off the volt-time integral is near a maximum and thus flux is near maximum (meaning the core is towards saturation).

The same transformer when fully loaded with a resistor will tend to be nearer a zero flux state at zero current.

When a transformer with residual core flux near saturation has voltage applied that is at a phase angle that tends to send the core flux further into saturation, then you get a very large 1/2 cycle pulse of inrush current. This is typically much larger than the inrush current for many loads. I might judge the peak inrush peak at 4000 A for your transformer if the source is stiff enough, and it could be higher on a high efficiency transformer. These large peaks do not occur often because of the probability distributions of the various contributing factors.

.

 

[conmgt]

You are reading that table wrong, kind of.
It has two columns of data; one for 20-40X inrush and another for >40X. There is no way your standard run of the mill transformer has this much inrush (at worse you might be 14X, but it is more likely to only be 9X).

So...that table could be ignored to a degree?

For most general duty power transformers (<150kVA), you could consider HP = 1.25 x KVA as a starting point for your NEMA HP rated contactor selection. Of course you should compare the actual inrush current with the device's ratings.

HP = 1.25x30kVA = 45hp Is that what you mean, use a contactor rated for 45hp?

130112-2033 EST

A totally unloaded transformer when switched off, even with a mechanical switch, will tend to leave the core near saturation. Because turn off of current in an AC circuit tends to occur near zero current, even with random turning off of a mechanical switch. In a highly inductive circuit (unloaded transformer) the voltage and current (basically only magnetizing current) are towards a 90 deg difference. Thus, at turn off the volt-time integral is near a maximum and thus flux is near maximum (meaning the core is towards saturation).

The same transformer when fully loaded with a resistor will tend to be nearer a zero flux state at zero current.

When a transformer with residual core flux near saturation has voltage applied that is at a phase angle that tends to send the core flux further into saturation, then you get a very large 1/2 cycle pulse of inrush current. This is typically much larger than the inrush current for many loads. I might judge the peak inrush peak at 4000 A for your transformer if the source is stiff enough, and it could be higher on a high efficiency transformer. These large peaks do not occur often because of the probability distributions of the various contributing factors.

Thanks.

I think I'll contact the transformer manufacturer on Monday.

 

[jim dungar]

HP = 1.25x30kVA = 45hp Is that what you mean, use a contactor rated for 45hp?

Actually I said to use this as a starting point. 45HP is not standard, so you probably need a 50HP contactor.
Now compare the typical inrush currents: a motor normally has 6X times inrush, how much is that using at 50HP? A 30kVA delta-wye power transformers rarely exceed 14X inrush, unless it is being used 'backwards' as a step up device.

Remember this is for an a probable solution, leaning towards being oversized.

 

[jim dungar]

A few more comments, I mis-stated the column headings on the Digest table you referenced. I believe they should be "<20X" and "20-40X" for inrush.

Now back to my example:
30KVA at 208V = 83A, 14X inrush (my assumed extreme value) = 1162A
50HP at 208V = 150A, 6-8X inrush = 900-1200A.

The 900-1200A in comparable to the assumed inrush of the transformer, so you should be able to use a 50HP contactor. At 208V this would be a NEMA size 5 device.

As a sanity check, isn't this what you got from the Digest table in the first place?

 

[conmgt]

HP = 1.25x30kVA = 45hp Is that what you mean, use a contactor rated for 45hp?

Pardon me...1.25 x 30 = 37.5

The 900-1200A in comparable to the assumed inrush of the transformer, so you should be able to use a 50HP contactor. At 208V this would be a NEMA size 5 device.

As a sanity check, isn't this what you got from the Digest table in the first place?

It certainly was. Let's see what the manufacturer says this week.

 

[jim dungar]

Pardon me...1.25 x 30 = 37.5

Big oops.
So much for 'mental math'.

 

[conmgt]

I contacted the manufacturer with these questions in reference to the model that they made for me including it's serial number:
"I saw Table 16.49 of their (SqD) catalog which has a column for transformers. For a 30kVA transformer, a NEMA5 270amp contactor is specified at a considerable increase in cost. Is this due to inrush currents? Do you feel that such a large contactor is necessary? Do you have a recommendation? "

All I got was:
"In regard to your question, yes, the inrush current is the reason for the higher amperage requirements. The NEMA 5 should be fine."
I know it's fine!...and that only answers question 1, but is it overkill? What is the inrush current? Would a size 4 work or a size 3?

SqD said a 90A definite purpose contactor would work be that guy said that he wasn't a contactor guy and the person he forwarded my e-mail to also declared to not be a contactor guy.

!?!?!?

 

[jim dungar]

I contacted the manufacturer with these questions in reference to the model that they made for me including it's serial number:
"I saw Table 16.49 of their (SqD) catalog which has a column for transformers. For a 30kVA transformer, a NEMA5 270amp contactor is specified at a considerable increase in cost. Is this due to inrush currents? Do you feel that such a large contactor is necessary? Do you have a recommendation? "

All I got was:
"In regard to your question, yes, the inrush current is the reason for the higher amperage requirements. The NEMA 5 should be fine."
I know it's fine!...and that only answers question 1, but is it overkill? What is the inrush current? Would a size 4 work or a size 3?

SqD said a 90A definite purpose contactor would work be that guy said that he wasn't a contactor guy and the person he forwarded my e-mail to also declared to not be a contactor guy.

!?!?!?

What is the part number of the transformer you are using? Is it a new one or something that has 'been laying around'.
EE30T212H - 208V delta/480V Y has an inrush of 6.8X (since 2007)
EE30T3H - 480V delta/208V Y has an inrush of 7.5X (since 2007) if this is run backwards as a step up 14X may not be unreasonable.
30T3H - 480V delta/208V Y has an inrush of 12X (2006 and earlier), expect >20X if run backwards.

Yes, with your infrequent switching, there is no reason a definite purpose contactor cannot be used as long as it is HP rated with a LRA rating greater than the transformer inrush current.

 

[conmgt]

Hi Jim,

Attached is a photo of the label>

 

[brian john]

I would think you would be fine with a 100 amp contactor, if you were turning it off and on under load, that might be another issue since its a motor load. The arcing of the contacts upon start up is probably why they recommend the larger contactor.

I would think loaded or unloaded you would see the transformer inrush currents at the contacts instantaneously as the load is being energized. Discuss this with the contactor manufacture, they make different contactors for different loads. BUT I would believe the 100 amp designed for motor starting would be sufficient as it is designed to handle motor inrushes

 

[jim dungar]

Attached is a photo of the label>

Nameplates are rarely sufficient for determining the inrush of a transformer.

 

[conmgt]

EE30T212H - 208V delta/480V Y has an inrush of 6.8X (since 2007)
EE30T3H - 480V delta/208V Y has an inrush of 7.5X (since 2007) if this is run backwards as a step up 14X may not be unreasonable.
30T3H - 480V delta/208V Y has an inrush of 12X (2006 and earlier), expect >20X if run backwards.

I thought that these were industry standards of transformers and was looking for such info on my transformer. I see that they are SqD part numbers.

I'm going to post a new thread on transformer disconnects.

 

[jim dungar]

I thought that these were industry standards of transformers and was looking for such info on my transformer. I see that they are SqD part numbers.

I'm going to post a new thread on transformer disconnects.

You are correct, they are part numbers. My point was that the inrush current of transformers varies from one design to another.

 

[hillbilly1]

I would think loaded or unloaded you would see the transformer inrush currents at the contacts instantaneously as the load is being energized. Discuss this with the contactor manufacture, they make different contactors for different loads. BUT I would believe the 100 amp designed for motor starting would be sufficient as it is designed to handle motor inrushes

The Orange box uses lighting contactors extensively to turn large transformers off and on loaded, but its not a motor load, but then the OP is not starting the motor until after the transformer is turned on. They have a couple thousand stores like this, and don't appear to have a problem with it, some stores are over 20 years old and use this system. When I first saw this set up, I thought they would be issues, but I never had to change one due to contact failure, it's always a coil failure.