Transformer stay on/turn off?

[Steve in SoCal]

I have a pedestal grinder that requires two small transformers to boost voltage from 480 to 550. I just finished an overhaul of the grinder and while wiring thought of adding a contactor to switch the transformers off when the machine will be idle. The idea is to switch the transformers with a contactor using the light switch. Durring a work session the machine would be up and using the stop/start switch would cotrol the grinder. When the machine was idle the work lights would be off and the transformers shut down? Would the cycling be more harm than good? The expected use is a few to several work sessions a week.

Steve

 

[jim rozen]

Do the transformers stay warm when the grinder is off?

 

[Rob F.]

I would turn them on on each day you use the machine and probably leave them on all day long if you might use the grinder again later that day. That way they are off and not adding a small amount to your electric bill all the time. I would not turn them on and off multiple times per day, and if you were to need the grinder every day then they might stay on 24-7.
jim rosens question is a good one.

 

[Steve in SoCal]

To further this, yes, the transformers would stay on while the machine was in use daily.

The transformers would be "warm" all the time without this switching scheme. The grinder is pluged in to an always on circuit. There is a bus duct disconnect 10 feet above it.

The frequency of use is inconsistant, there may be weeks where it is not used and others where it plays a major role.

 

[JST]

If you have 3 phase service, they will add a tiny amount to your low power factor charge. If they are the only thing connected overnight, they may lower the power factor quite a bit for all that time. Then it depends on whether you are charged all the time based on the lowest "average" power factor, or charged for power drawn at the low power factor.

If your lowest power factor is a "trap door" that increases your whole bill, then it is worthwhile to shut them off.

If you are charged based on the power factor at the time the power is drawn, then it won't save you very much.

 

[EmGo]

Had a small transformer for something, I forget what. Also California .. maybe it was just backup power for the control so it didn't need to be reloaded ? Anyway, at California electric prices it cost $2 a day. That's $60 a month - for nothing.

I'd kick them on when you plan to use it.

 

[rklopp]

I have single-phase residential service. I have three encapsulated transformers for my machines. They get up to maybe 120 degF idling. I figure why pay to heat the room when it’s hot enough already. I turn them off at the end of the day or sooner, depending on machine use.

 

[SomeoneSomewhere]

I'd consider just putting the TXs on the load side of the machine contactor. Might need to up the contactor a size, but you'll avoid needing a separate switch to turn stuff on/off.

If you do want to switch it separately, a load-break disconnect is probably a better choice than a contactor. Otherwise you'll end up with the losses of a contactor too.

 

[Steve in SoCal]

Switching the transformers WITH the load maybe OK, IS the inrush each time the grinder is started an issue?

My electrical tariff is peak KW in 15 minutes windows any time, I would not leave them on all the time. By the Manny, the low voltage trnsformers for my shop 240/120 eat about 125.00 a month to kee warm!

This is my shop in Kansas with Evergy as the POCO

 

[JST]

It makes sense to turn them off when they are not needed, of course.

Control transformers might be about 250VA. That's roughly an amp at 240V, a bit less at 208, or a half amp at 480. (less if smaller)

They may draw as much as 5% of that when unloaded, so 0.05A at 240V. Of that, only maybe 25% is "power" current due to the low power factor.

If you were paying 40 cents per kW hour, then on a kW basis you would pay for about 2.2 kW hours per month, or about 90 cents per transformer. It doesn't do anything for you, but it's not a ton of money. Might take a lot of months to pay for a new contactor. A disconnect may be cheaper in months to payback.

I'm estimating high, so if a transformer is costing you more than that per 250VA of capability @$0.4 per KWh, you are being charged on a different basis (which would be normal for 3 phase service).

Any transformer will have a rotten power factor when not supplying a load, possibly as low as 0.1 or lower. If nothing else is drawing power, then that is your facility power factor when the place is closed, and the powerco may hit you with a surcharge.

A 480 to 208/120 transformer might cost many times the amount calculated above, based on lousy power factor, because it will draw a lot more idle current. Any old-style fluorescent fixtures connected to it won't help at all either.

You can fix that, if the transformer cannot be shut off, by connecting enough capacitors to raise the power factor to whatever the powerco says is their minimum. That has a cost also, due to parts and install charges.

If you have a new type meter, it may give you the information as to what kW and kVA you are drawing. They have a series of display readings they cycle through. Sometimes they say what the number is, or your powerco may have a "how to read your meter" information page on the website, etc.

 

[Phil in Montana]

Put the transformers on the load side of the main contactor, then the the issue is moot...Phil