Supply voltage undersized

[Jim Dungar]

Lancie1,

I did not say the transformer wouldn't work. What I said was, this is not what the taps are designed to do so it could lead to premature failure of the transformer. Among other things, there is the possibilty the transformer core will be put into an over-saturation condition if the supply voltage is above normal when the taps have been set for a below normal voltage.

 

[Lancie1]

Jim,
The transformer will have to supply a higher-than-normal KVA in either case, whether the input voltage is low and output is set for normal, or whether the input voltage is normal and output is set for higher voltage. But then, it was designed to do this, or it would not have the taps!

 

[Jim Dungar]

Lancie1,
Any time a transformer supplies higher than normal kVA it is working harder and heating up more than it was designed to. Transformer taps are "designed" to provide the correct amount of magnetic flux (which is dependent on a specific volts/hertz ratio) in order to provide the rated output even with a low input voltage. Effectively a transformer is a motor that doesn't turn, so if the motor doesn't like the voltage differential than a transformer won't either.

Let's get back to PLCs and control systems and leave the detailed power engineering to other forums.

 

[TConnolly]

You could do as I usually do in this situation: Go to the transformer supplying the 208 volts and kill the power, remove the cover, loosen the wires and move them up to the next (or second) higher voltage tap. Adjust so that you get 218 volts instead of 208. Then quit worrying about the motor.

Absolutely not.

That 208 transformer is a Y transformer and it is probably also supplying the facility's 120 VAC. If you retap, you will raise the voltage on your 120VAC outlets as well and start frying the microwave, coffee pot, lights, computers, furnace motor, and other 120V equipment, or worse, burn down the building.

 

[Lancie1]

Big deal, so the 120 volts goes to 126 volts. It will still be within the 10% tolerance. Is blowing a few bulbs worse than burning up some expensive motors? Buy some 130 volt bulbs!

 

[elevmike]

I vote for installing the right motor...

This is beyond me. The thrid item on our job survey sheets is Measured Power Supply Voltage.

 

[darrenj]

LOL..

If you look at any large photocopier in use in a large office tower it is 240v..guess what..large toers only have 208..look at any Large computer server..240v..but we only give it 208..

a 240volt motor dosnt care if its running at 208..(Most of the time unless, as stated, it is fully loaded) Howver a 208volt motor will not like running at 240v..

Now to the transformer..

If you have say a 600-120/208v transformer and you are only getting 200 volts out..you tap it up right?.. Now if you want extra voltage even if you have 600 the tapping it up isnt going to hurt the transformer..It will affect the KVA.. Instead of doing the 208*sqr 3 you will be doing 215 or 216..

I mean come on we use transformers backwards to get 600 in place we only have 208 volts right??

I will agree that the right way would be to get a transformer wound for the correct voltage..OR to replace the 240 motor with a 208..but me..I would sleep sound at night running the motor at 208..

d

 

[Jim Dungar]

The rated kVA does not change it is the output amps that change. As the volts go up the load current must go down, if not the transformer will be overloaded.

Electrically a transformer looks like motor that does not turn (0% slip). If the combination of voltage and loading is bad for a specific voltage motor then it is bad for the same specific voltage transformer.

To get the secondary voltage to rise by 5% you would chose a primary tap that is 5% below normal, effectivelly making the transformer a 454->208V (or a 570->208V) device. Now you are going to feed it normal voltage, so it will be in a 5% overvoltage condition (would you normally run your motors at this high of a voltage?). Now if the utility exercises their right to supply +5% voltage your transformer will see a 10% overvoltage. How about during a no/low load situation when a typical industrial park sees a 10-15% overvoltage condition (this condition is one of the reasons electronic boards fail during shutdowns)?

I am not saying "do not use the taps", in fact I have even done it myself. What I am saying is "do not use the taps blindly, understand what you are doing and the consequences of doing it".

 

[darrenj]

The rated kVA does not change it is the output amps that change. As the volts go up the load current must go down, if not the transformer will be overloaded.

".

Sorry Jim..you are right..Kva=Kva=Kva..it dosnt change..just the current changes...

As for running 5% above?...Most motors here in canada are rated at 575v..so yes we do run them 5% higher..600v...

 

[Jim Dungar]

For some archaic (?) reason in all of North America the nominal supply/generation voltages and the motor utilization voltages differ by about 4.2%. Nominal supplies are 120, 208, 240, 480, and 600V while the corresponding motor nameplate voltages are: 115, 200, 230, 460, and 575V.