3 Ph 10Kv Oven Blues...

D

Thread Starter

denson1932

I am working on a 10 KVA 3 phase Brother (China) oven hooked up (wye) to 208 vac. The incoming wires are (3) #8 Ph1, Ph2, Ph3; plus chassis ground. So there is no neutral to insure balance in the loads. The scheme is (3) phases to the contactor with a typical latch circuit, then to (3) SCR or Triac (unsure of which) then to (3) 63A fuses (NT00) then to the top of the (3) primaries. The (3) lower primary wires are strapped together and mummy wrapped and go nowhere else. The primary windings of the triple core transformer appear to be about #8 wire.

The secondary windings are actually strap material about 7/8 X 1/4 aluminum. There is a current loop on each 'high strap' which feed (3) analog front panel AC current meters. Again the (3) low side secondary outputs are strapped together and mummy wrapped and go nowhere else.

The secondary high outputs feed the (3) heater banks which are M0Si2 (which are typically near zero resistance when cold). They are (4) in series each bank and again, the common of the (3) heater banks are tied together but go nowhere else.

Initially there were (2) of (3) front panel red lights lit pointing to (2) blown fuses. Let me digress and note the oven is 1 1/2 years old, it is a a research facility, it has never worked because it was shipped poorly packed and several heater elements and the special insulation bricks were damaged and broken 'on the boat ride'.

After factory replacement parts were FINALLY received and installed, the oven was fired up only to have:

1) HUGE current flow causing the wires in conduit to bang in protest.

2) Then the blown internal 63A fuses. (Circuit breaker in breaker box is 3-gang 50A with tie bar joining toggle levers).

I proceeded slowly using 40W then 100W then 250W light or heat bulbs; first in lieu of the transformer primaries, then in series to the (3) transformer primary wires. All I can report is that the control circuit can control and flash (dim-not full brightness) the (3) bulbs when substitute for the transformer primaries.

I question the wye without neutral scheme with MoSi2 elements...any unbalance is magnified...Also I don't know how to determine if the transformer or control circuitry is responsible for the huge current flow if all (3) phases are hooked up. I have verified over and over and over, no shorted path to chassis ground or across SCR's; no shorted or path to chassis ground in primaries.

The cold resistance of these MoSi2 heat elements is 60 milli-ohms each bank.

My clamp AC amp-meter captured 173Amps on one incoming phase...not a pretty picture. Really don't know where to go next.
 
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