A seemingly unlikely tranformer failure?

A single phase transformer has a 480V primary, 50 V secondary. The transformer secondary is connected to a resistive heating load. The transformer primary voltage is controlled by a triac. Under normal operation the primary voltage might be around 315 volts and primary current at around 9 amps, with the secondary current at around 80 amps at about 31 Volts. So far everything is about like I would expect it to be. However, if the primary voltage is raised just slightly with the triac to about 320 - 330 volts then suddenly the primary current jumps significantly to more than 50 amps. There also must be a thermal component associated with it, the jump takes longer to happen if the transformer was off for a period of time.

But now here is the mystery, the secondary output current and voltage does not change when the primary current jumps. Wouldn't the secondary output be expected to change someway if for example a soft-short was developing in the primary winding. A short would have the effect of changing the winding ratio, so shouldn't it affect the output? Is there a way that a transformer can fail that increases the current draw at the primary without affecting the secondary? How unlikely is that?

If i get you...

you have 480v that is basicaly converted into 30volts DC..
If thats the case then perhaps the secondary has regulators on it..in which case it dosnt matter if the incoming goes hight the out going will stay the same..

secondary current will stay the same if the voltage holds..(resitive load)..as for what happening on the primary side..i would ahve to see it..(I assume this is heating and they are pulsing the heaters?)

There are no regulators on the secondary, output is AC, they connect directly to the heating elements. Its a vacuum furnace, elements are tungsten strips. The triac is phase angle fired, not pulsed.

DC component to the current

Put a scope across the primary winding and see if the positive and negative waveforms are similiar. It could be that the firing angle is different for the positive and negative halves of the cycle. If they are significantly different, the primary current will have a DC component to it which will cause the primary current to increase. If a scope is not available, put a digital meter set to DC Volts across the primary. You should not read more than a few volts if the positive and negative halves are equal.

My guess is you're saturating the transformer core causing loss of inductive reactance.

I agree with Gerry. Once you reach saturation, the impedance goes out of the window and the dc resistance of the primary dictates the current drawn.

Regards,
Nigel Betteridge

Try disconnecting the load, then bypass the triac. Make sure you have suitable fusing in the event of failure.

If you have a variac (variable voltage transformer) rated for 480 volts then connect it to the transformer. Power it up set at a low voltage and increase gradually to see if the no load current remains low.

My guess is the triac is firing unequally as others have said.

Sid

Alaric said:

A single phase transformer has a 480V primary, 50 V secondary. The transformer secondary is connected to a resistive heating load. The transformer primary voltage is controlled by a triac. Under normal operation the primary voltage might be around 315 volts and primary current at around 9 amps, with the secondary current at around 80 amps at about 31 Volts. So far everything is about like I would expect it to be. However, if the primary voltage is raised just slightly with the triac to about 320 - 330 volts then suddenly the primary current jumps significantly to more than 50 amps. There also must be a thermal component associated with it, the jump takes longer to happen if the transformer was off for a period of time.

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If heat is a factor, sounds like the windings are breaking down...try megging it.

[/QUOTE]But now here is the mystery, the secondary output current and voltage does not change when the primary current jumps. Wouldn't the secondary output be expected to change someway if for example a soft-short was developing in the primary winding. A short would have the effect of changing the winding ratio, so shouldn't it affect the output? Is there a way that a transformer can fail that increases the current draw at the primary without affecting the secondary? How unlikely is that?[/QUOTE]

no, the primary spike will not affect the secondary load. How many times is it that the secondary fuse blows when the primary shorts? The secondary is wrapped around the primary windings, not 'bonded' to it.

You did not mention the KVA rating of the transformer. Is it rated for the current you are drawing on the secondary?

I calculated a 9.6:1 turns ratio. Under the "NORMAL" condition, your running currents and voltages are acceptable levels. Current being the inverse of voltage.

I am inclined to believe the primary windings are shorting. If you have a 480 volt primary and are only putting 350 volts, it should have normal turns ratio reaction on the secondary.

I do not recall a triac failing like a diode causing DC. Usually shorted or open. However, I would not be supprised.

Is the transformer a single winding or dual winding primary? If it is dual, measure each winding and see what the ohmic values are. One will be some what higher due to the longer length of wire used as the core becomes larger.

Leadfoot said:

You did not mention the KVA rating of the transformer. Is it rated for the current you are drawing on the secondary?

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Good question.

Leadfoot said:

I am inclined to believe the primary windings are shorting. If you have a 480 volt primary and are only putting 350 volts, it should have normal turns ratio reaction on the secondary.

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The supply to the primary is phase controlled, not attenuated. A voltmeter (d'Arsonval type) measures average voltage. When the triac supplies less than a full cycle, the meter reads a lower average than for the full cycle. The peak voltage supplied to the transformer will always be equal to the peak mains supply to the triac, unless the firing angle is greater than 90 degrees. So, whether supplying an average 315 or 330 volts, the windings see the same peak voltage and insulation break-down doesn't fit the scenario.

More about meters: although measuring the average, the AC scale is typically graduated in RMS. That is only accurate for a full sine wave. For a partial sine wave, it is likely higher than actual. There are digital meters available that will measure "true" RMS.

So, whether supplying an average 315 or 330 volts, the windings see the same peak voltage and insulation break-down doesn't fit the scenario.

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Exactly. Thats partly why this had me stumped. It seemed so unlikely, but one thing I didn't do was look for DC components from the SCR power controller. So I sent a maintenance spark (we have two new guys) down there with an o-scope this afternoon to look at it. He was having trouble trying to figure out the scope but I was a good boy and stayed out of the way while he tried to work it out. We limited the command signal (PID max out) to the power controller so right now the furnace is running and just barely able to make temperature, but sometimes its tough to stay out of the way and let these new guys work their way thorugh the problems (but those are the orders). If he does find a DC component on the output I'll let ya all know.

sounds like the iron is being saturated, use a surge tester look for failing of winding turns, as the coils saturate you get into more of a ir2 loss