Shorting Transformer Taps?

[d21x]

I have an application where we are using a 480VAC Primary to 950VAC 7Amp Secondary to power a Mercury UV Lamp.

There are 3 taps on the secondary. These 3 taps are for the Low, Medium, and High settings (Changing voltages on the secondary). The way the machine came in, there was a relay that would switch between these 3 taps.

My question is what would happen if two taps were shorted for 1 second or .5 second? Is there potential for problems when switchting between taps?

Let me know what you guys think.

 

[rcwilson]

Shorting between taps lets a short circuit current flow between the tap positions. The amount depends on the voltage between the taps, the internal impedance of the transformer and the impedance of the external circuit. The transformer may have enough impedance to limit the current to a lower value. But most likely the current will be reliatively high compared to the rating of the tap winding and the external wiring. It may be OK for 0.5 seconds, if it only happened once. Most likely it will end up smoking the transformer.

The relay or switch that goes between taps will hae a problem breaking that highly inductive short circuit current. The contacts could weld with a permanent short that will definitely smoke the transformer.

Utility style tapchangers short out taps through an inductor or a resistor to limit the current during a tap change.

 

[Besoeker]

I have an application where we are using a 480VAC Primary to 950VAC 7Amp Secondary to power a Mercury UV Lamp.

There are 3 taps on the secondary. These 3 taps are for the Low, Medium, and High settings (Changing voltages on the secondary). The way the machine came in, there was a relay that would switch between these 3 taps.

My question is what would happen if two taps were shorted for 1 second or .5 second? Is there potential for problems when switchting between taps?

Let me know what you guys think.

What voltages are the taps?
Shorting out tappings would usually generate high currents. The transformer and "relay" might be designed for that if the circuit is intended to operate that way.
But a second seems like a rather long time from a design and operational perspective even if the if the system is designed for adjacent taps to be shorted to avoid open circuiting the load during the change.

 

[d21x]

Measured voltages on the taps are:
492V
592V
681V

The machine originally came with Telemecanique Relays, but we had some burn up so we switched to Mercury Relays. We are having problems with the UV Lamps staying on when switching from Low to High. We think the problem may be the relays are switching too slow causing an open circuit and are thinking of doing a make before break switching in the PLC program. This is why I questioned shorting two taps on the transformer for a short time.

 

[broadgage]

Shorting between taps might be acceptable if only very briefly, 0.5 of a second is quite a long time for such a short.
Apart from the risk of letting out the smoke from the transformer, remember that extra current will be drawn from the line during the short circuit and this may open the branch circuit breaker.

Options include going back to the original relays, or a very close equivalent, and regarding these as expendables, like lamps.

Or hardwire the equipment in the "full" or "high" position, and use a variable transformer on the input to alter the light output.

Or change the control system to one that conects the taps together via an impedance, before changing from one tap to another. This is usuall on grid transformers, but most unusual on small ones.

 

[dbuckley]

Theres 100V between them there taps, and a rated current of 7A, so I would think that you'll have significantly more than 7A flowing betwixt those terminals. As well as shorting part of the transformer, the overall secondary voltage will drop due to the transformer being overloaded.

I'd recommend you try using zero cross solid state relays (Crydoms) to avoid the interrupt you get from break before make, or the ugliness yoiu get with make before break.