Transformer Output Voltage High -- Will it hurt my machine?

Just moved from a building with 208V 3-phase with equal voltage legs, to one with a 240V 3-phase with 120/120/240 high-leg system. Still trying to wrap my head around how a high-leg system works.

We have been using a transformer to step up voltage to 480V to run a grinder with a 20HP motor. This grinder has a heavy wheel (120 lbs), and draws a lot of current on startup. This 30KVA transformer is for 480V primary, 208V secondary.

When we moved to the new 240V system, we connected the transformer up with the same wiring, but used the lowest voltage jumper setting. The previous 208V input was set on Jumper 3 to yield 480V output. We set the Jumper to the lowest setting - for 208V, should yield 432 volts, but for 240V, is now yielding about 505V between each leg going to the machine.

Questions:

1. Will this voltage damage our grinder (running about 5% above required voltage), which requires?
2. Should we seek out a 240V-480V transformer, or will this one continue to work well, even though designed for 208V-480V.
3. The transformer is located about 90 feet from the panel. The input voltages to the transformer are 122V to ground on 2 legs, and 211V to ground on the high leg. On the output voltage, they read 508, 506, 506 leg-to-leg, but 236V, 270V, and 313V when measuring each output leg to ground? Stumped by this, and electrician was too? Any ideas?
4. For any gluttons for punishment or electrical sleuthing hobbyists, when we started up this grinder, immediately blew three 30A fuses on the disconnect box on the transformer. Electrician had to see this for himself, so we replaced the fuses, started up the grinder, and immediately blew 2 of 3 fuses again (different pair this time.) He was stumped, but tried switching the wiring to the disconnect box to heavier gauge wire (even though this wiring worked well on our 208V system), and we were able to start the grinder up no problem. He didn't know why this worked. Any ideas? We have only run this grinder for about 5 mins, and 2 starts, so don't know if this permanently solved the blowing fuse issues.

Any ideas much appreciated, and thanks in advance.

Palak said:

...is now yielding about 505V between each leg going to the machine...

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Was that measured with the grinder running?

All transformers have internal losses.
Output voltages are defined with the transformer fully loaded, i.e. supplying the full rated current.
If the current drawn by the load (the grinder) is less than the full-load current spec for the transformer, its output voltage will always be higher than the nameplate value.

As a general rule, I would prefer not to run a 208v motor on 240v.
That can be done (I've done it here in my shop for years), but the motor will be happier at nominal line voltage.

- Leigh

What you probably need is time delay fuses.

Let me get this straight. You strapped the 208 side to the lowest tap which which, if it were operated normally with 480 on the primary, would have given the lowest secondary voltage. Is this correct? Or is it strapped to produce 230 or whatever would be the highest voltage with 480 on the primary?

To Leigh's point, the normal design procedure is have the output high unloaded so that when loaded to design, the voltages are at design. 480 in at 30KVA will yield 208 secondary.

Tom

TDegenhart said:

To Leigh's point, the normal design procedure is have the output high unloaded so that when loaded to design, the voltages are at design. 480 in at 30KVA will yield 208 secondary.

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That's what I said.

- Leigh

The grinder won't care if the voltage is 5% high at no load, it will probably draw less amps. (Edit: at more than say, 16 hp load.)

Your transformer will have significantly higher no load losses because you are running it at 15% higher voltage.

Whether or not you need to look for a 240 to 480 volt transformer depends on how long you need to run it, and whether or not the transformer is saturating and overheating. If you keep the transformer powered off all the time, you will probably not reach 190C temperatures in say, 4 hours. who knows, stick a thermocouple in the windings and find out.

I'm assuming you have a dry indoor transformer. probably 40C temperature rise at no load, and 150C temperature rise at full load.

Thanks for the replies.

To answer your questions:

1. Voltages were measured both at the transformer, and at the grinder when it was on, but NOT running. Both sets of voltages were about the same.

2. Jumpers were set at tap 7, which if we had 208V input, would have resulted in 432V output (according to transformer panel). However, when 240V input was jumpered to this same tap 7, approx. 505V was output.

3. We are running this transformer to get a higher voltage of 480V, not inputting 480V to get 240 or 208. Grinder runs at 480V.

4. Grinder will be running up to 10 hours a day, but usually for 4-5 hours a day, stopped every 1-2 hours for 15-20 mins.

5. It is a dry indoor transformer.

Thanks all.

Palak said:

Voltages were measured both at the transformer, and at the grinder when it was on, but NOT running. Both sets of voltages were about the same.

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If the grinding is not running, it's not ON.

The electronics and support motors draw a trivial amount of power.

The motor that turns the wheel is the load that the transformer sees.
Plus pwerhaps the motor that moves the table, depending on size.

The only valid voltage measurement is with the grinder wheel turning and grinding something.

- Leigh

If you replace the motor with a 240v motor, eventually the 4% energy savings from getting rid of the transformer (plus additional energy savings due to newer motor, say 2%) will pay for themselves.

As for the transformer, as long as the temperature rise is acceptable then its not a problem.

Relocating 20HP 480V Grinder to a new voltage

Palak,
Good to see you found the message, seemed it was pretty much on the mark, but now your back, with a new building. You asked "For any gluttons for punishment or electrical sleuthing hobbyists" I'll offer some more babbling that comes to mind.

Taken the information given and filling in the blanks with some assumptions and prior knowledge:

Grinder problem: 20HP 480V 27FLA Hard starting grinder
30KVA 208/480V Transformer 83.27/36.09 FLA Taps 2.5%(2 above normal and 4 below normal)(+5% to -10%) Standard 480V Delta Primary / 120/208V Wye Secondary, Used in the reverse direction.

New Building service voltage 120/240 center tapped delta 3Φ (hi leg) Measured voltage at site 244V (122v X 2)

Existing 480V Fuse size 30A Max.

Prior knowlege of AB 509 starter overload relay block burned up from excessive heat, since been replaced and working at old site (208V).
---------------------
Ramblings begin:

Transformer ratio 480/208V =2.307 ratio, 2.307-10% (maximum tap adjustment on 480 winding) 2.307 x .9 = 2.076 adjusted ratio.

244V Input x 2.076 Adjusted ratio = 506.6V This is what you measured, so far good.

Transformer Input Over-voltage. 244/208V = 1.17 or +17% Higher than design.Tolerance range before saturation is +10%, so 7% above the allowable tolerance limit. That number could go much higher if the utility voltage goes higher, as the local grid becomes unloaded. Utility is allowed to go up to +10% or 264V. At the existing 244V, running the input coil 17% above design (208) then the utility allowed to go another 10% higher on their end, could be problematic for the input winding. It could be driven well past its saturation point under the right conditions.

A word about voltage balance on center tapped delta systems. These system voltages are unbalanced by design. With one transformer winding of the delta, center tapped, to provide for 1Φ loads. The L to L voltages are almost never exactly equal, more voltage drop on the 1Φ winding than the other 2 phases. The voltages to ground would be 120, 120. 208. Your reading 122, 122, 211 pretty close to the design voltage. 240V L to L, your reading 244V

Next up the 480V output winding. Measuring 506 to 508 L to L, and 236,270,313, to ground.
This tells me that the 480V output winding is ungrounded. The voltage your measuring is a ghost voltage produced by the capacitive coupling inside the transformer windings. If my assumption above was correct that the transformer 480V coil is a delta winding. The only way it could be grounded is by corner grounding. If it was corner grounded, as it should be, to be safe and meet code, the readings to ground would be 506, 506, 0. Your not getting that, so that's why I think your running ungrounded 506V. Best look into a situation like that to be sure.

Next up the 20HP motor and overvoltage. 506 L to L overvoltage. 506.6/480 = 1.055 or 5.5% overvoltage to the motor for a 480V rated motor. Usually they have the 10% margin and 5.5% is well within that. But if the motor is an old cruiser rated at 440V things look differently. 506.6/440 = 1.15 or 15% over design, 5% past the 10% limit. Don't forget that the utility has the right to crank it up another 10% if they see the need. Again a case where your spindle could easily be driven into saturation.

Next up transformer size. 30KVA ratings 208/240/480V 83.27/72.17/36.09A
480V output coil rating 36.09/27FLA (20HP)= 1.33 or 33% bigger than the load.

Minimum suggested size for normal starting loads. Load x 1.5 (50%), x 2 for really hard starting loads. 27 x 1.5 = 40.5A, 27 x 2 = 54A
This is larger than your existing 36.9A winding. Should really step up to the next standard size as a minimum, which would be 37.5KVA rated at 45.11A output. That size might be hard to come across used as its usually uncommon.

The next size up would be 45KVA, rated at 54.13A. This may be cheaper to obtain used, as it's more common. A little extra reserve from the bare minimum, spinning up that 120LB chunk of rock couldn't hurt things in any way. What your running is very much on the small side for what is reliably needed for your heavy start condition.

During starts your voltage drops, as the voltage drops, more time is required to accelerate that rock. More time at large starting currents, equals a lot more heat. This may be a partial cause to your AB 509 overload going up in crumbles. Too much heat developed during prolonged starts take it's toll after a while.

Moving on to fuse sizing. Dual element time delay fuses, Protecting 480V motor circuit. NEC code sizing. Min code size for normal starting loads, FLA x 1.25 (25%),27FLA x 1.25 = 33.75A (35A standard size min), Recommended size for hard starting FLA x 1.5 (50%), 27 x 1.5 = 40.5A (45A Std), Max Size for hard starts FLA x 1.75, 27 x 1.75 = 47.25 (45A Std, rounding down from max)

Your existing fuses are 30A, that means your disconnect is 30A max. You need a 60A disconnect to provide the correct size fuses for the application.

A tip for when your fuses blow from long time over-currents. If 2 of 3 burn as you mentioned, be sure to dispose of the last conducting one as well. It's just as heat melted as the other 2, but they opened a split second before the last one had a chance to open. If you try and be cheap and reuse it, on the next trial start, it will open, and cause the load to single phase on the remaining legs, until the new ones melt as well. Its cheaper to replace them as a set when they blow from over-currents.

Not asked but I'll go there. Supply circuit rating feeding the transformer. Same rules apply to the input fuses as the output. 30KVA @ 240V. 72.17A x 1.25 = 90.21 (90 or 100A Std), 72.17 x 1.5 = 108.25A (100 or 110A Std)

Conclusion. Seems to me that you have been running that rock on the bare margins at your old 208V service. And at the new 240V service, the transformer primary 208V will be running very hot and saturated at the measured voltage, let alone if the utility voltage goes up during periods of low demand. All of these weak links add up to much longer starting times and more heat damage trying to do so. I think that getting the proper circuits would be cheaper and easier to do than removing that rock spinner and sending it out for a rewind, or burning up the transformer in the middle of a hot project.

Make sure you get that 480V secondary winding grounded for your own safety, at a bare minimum.

Is this transformer similar to what your running?


SAF Ω

SAF,

Wow. Just wow.

Thanks for the reply and analysis. I have printed it out and will go over it carefully today. I will have to make this a research project, as I am a novice reading what appears to be a PhD dissertation.

The grinding motor is rated for 460V -- here is a picture of the data plate.

Also, yes, the transformer is similar to the plate you posted. I have attached a picture of mine here.

Will post more when I have had a chance to study your post.

Thanks to you, and to all.

Palak said:

SAF, Wow. Just wow. Thanks for the reply and analysis. I have printed it out and will go over it carefully today. I will have to make this a research project, as I am a novice reading what appears to be a PhD dissertation. ...

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I think you just got several hundred dollars of consulting for free. PM is great, and SAF is very generous.

so... next question is why not wire the motor for 230 volts and use it without a transformer?