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Small transformer question

thread248-491969

Small transformer question

Small transformer question

(OP)
I ran into some industrial saws that I think are not properly wired. It has a transformer wired for 480 volt primary and a 240 secondary and that is all well and good. However some of the controls on the inside use 120 volt coils and there is no connection to x2-x3 on the secondary so they have to be flowing current through ground. Center tap is not grounded and one one machine they got 75v to ground on one leg and 220v on the other and was tripping a breaker after 7 years of running ok. A similar machine next to it had 120v to ground. It doesn't make since to me at all how either are working since the secondary isn't grounded. Is the power returning through the core?



https://res.cloudinary.com/engineering-com/image/upload/v1644846983/tips/transformer_xczmgx.bmp

RE: Small transformer question

Having trouble with some controls being 220V and others being 120V? Never seen that mess before. Are you sure some are 220V and not just indicator lights?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Small transformer question

The 240 V secondary is serving 120 V loads that are connected in series, through the ground connections. The voltage across each "leg" depends on the impedance of each leg to ground. Say you have two 120 ohm loads, one connected from X1 to ground and one from X4 to ground. The voltages will be 120 V across each since the loads are equal. Current flowing from X1 to X4 will be 120/240 = 0.5 A. If you had 120 ohm from X1 to ground and 360 ohm from X4 to ground, total 480 ohm drawing 120/480 = 0.25 A. Voltage from X1 to ground would be 0.25 x 120 = 30 V. Voltage from X4 to ground would be 0.25 x 360 = 90 V. Total 120 V.

There will be some capacitance to ground, making things a little different and causing some current flow and voltage to ground even without loads.

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