Voltages not matching through transformer

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wirenut1980

Senior Member
Location
Plainfield, IN
I have a customer with a milling machine that takes 480 V 3 phase supply from a 480Y/277 V solidly grounded service. The machine has an internal transformer that steps 480 V down to 200 V 3 phase for the motors and 100 V single phase for the controls (the machine is Japanese or something). They have fried 2 control boards in the past several months in this machine. The board operates at 24 V DC, and is fed by a power supply which is fed from the 100 V single phase. I measured incoming phase to phase voltages to the machine at 498 V, 495 V, 495 V. At the most, that is less than 4% above nominal. At the transformer secondary, I get about 219 V on the 3 phase voltages and 111 V on the single phase. So I have 4% high on the 480 V primary, and 11% high on the 100 V secondary. We opened up the compartment to look at the transformer and it does have taps on the primary (see attached pictures). The transformer appears to be tapped correctly for 480 V incoming voltage. It did not come out well in the picture I took, but for 480 V input tap, the connections are 1-4, 5-8, and 10-11.

I checked continuity between the incoming equipment grounding conductor and the machine ground bar and it checked good. They were not connected directly together by a wire, but rather through the machine enclosure. The resistance measured 0.2 ohms.

Any ideas on what I could do other than try to work with the machine manufacturer? The manufacturer is telling the customer that the incoming 480 V is too high. I did set a monitor at the incoming 480 V, just to make sure voltage was not getting much higher.
I think I have convinced the customer that there is something wrong with the transformer, but they do not know what to do now. The manufacturer states that they control devices can't handle more than 10% over the nominal 100 V rating.

Thanks in advance.
 

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GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
I It did not come out well in the picture I took, but for 480 V input tap, the connections are 1-4, 5-8, and 10-11.

I think I have convinced the customer that there is something wrong with the transformer, but they do not know what to do now. The manufacturer states that they control devices can't handle more than 10% over the nominal 100 V rating.

Thanks in advance.

The fact that the tap list shows different connections for 200, 220 and 230 input (but same for 230 and 240?) suggests that the machine really is very sensitive to input voltage (and that since 230 and 240 share a single tap config, applying 240 is already on the high side of the tolerance band). I assume that the 400/440/460/480 lines in the table have a similar relationship?

The difference between 480 and 495 is only 3%, and even added to a hypothetical 2-5% for the shared setting for 460 and 480 still leaves you within 10% of nominal.

Are you able to measure the 24 volts DC? If the supply is regulating reasonably, I do not see the difference in AC input causing the 24V DC supply to go out of regulation and fry the board. After two bad boards, I would consider replacing the 24 volt supply just on general principles.

I would be much more suspicious of some sort of stray voltages on the sensor leads inputs and control outputs of the boards, but do not have any good advice to you on what to do to troubleshoot from there except to verify grounding and bonding. And I definitely would use a wire from EGC to ground bus rather than relying on the metal of the enclosure. The reading of .2 ohms is suspiciously high if accurate.
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
Are you able to measure the 24 volts DC? If the supply is regulating reasonably, I do not see the difference in AC input causing the 24V DC supply to go out of regulation and fry the board. After two bad boards, I would consider replacing the 24 volt supply just on general principles.

I agree with this. If the board is fed from a DC power supply, then I'd check voltage there first. If you're frying the board, and not the rest of the equipment, that power supply is what I'd be focused on. If it's a decent PS you should be able to adjust your DC voltage up/down with the turn of a screw. That should take any worries of high incoming voltage on the line side right out of the equation.
 

mike_kilroy

Senior Member
Location
United States
my take is 495/460=nominal 8% hi

Since they have same for 460 as 480 tap, and pix shows wired right to max voltage level, it is really 460v input xfmr u r putting 495 into.
I suggest at some time in the day the input is higher than 495 thus blowing the sensitive 24v boards since mfgr admits it cannot handle more than 110v - which u already measured at 111v during a visit?

solution? Onbe would be to buy a small 3ph 460-460v AUTOTRANSFORMER with +/-5,10,15% taps to allow you to reduce ur customer's 496v input to 450v input to help the japanese machine survive this slight higher voltage. remember, a lot of stuff overheats with high input voltage and lives a lot longer and cooler with voltages closer to the low tolerance level. if you cannot find a source for simple low cost auto xfmr pm me and I will give you some sources in US and Canada.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Onbe would be to buy a small 3ph 460-460v AUTOTRANSFORMER with +/-5,10,15% taps to allow you to reduce ur customer's 496v input to 450v input to help the japanese machine survive this slight higher voltage.

Or possibly, if you are comfortable wiring inside the machine, put a buck transformer just on the 100 volt winding feeding the controls?
(May not be permitted by code and listing of the machine or by warranty.)
 
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