Magnetron info needed

[elecdude]

How sensitive is a magnetron to ground problems and power quality. Customer has an industrial microwave system that trips out on anode over-current. When the unit is sent back to the factory it runs fine.

 

[busbar]

It may not be the magnetron itself, but the power supply and control/protection circuits. The design is probably proprietary the oven manufacturer, and up to them as far as power-quality requirements.

Your best bet may be to watch the AC line to the oven with a power-quality logger for a time, and see if there any obvious patterns associated with the oven malfunction. It may not be easy, for juggling alarm limits usually takes some patience.

 

[elecdude]

We had the electric utility check the PQ at the transformer, no unbalance or harmonics to speak of. The microwave drying unit has an on-board anode current relay and an on-board circuit breaker. One or the other of these will trip the unit as soon as it is started. (Except back at our plant where everything works fine). Today, at the customer site, we disconnected the magnetron tube from its power supply and the power supply did not trip. We then attached a new ground directly to the structural steel of the building, and the unit ran for a short period before tripping. Does this sound like it could be a system ground problem ? Nothing else in the customer's plant is malfunctioning. Maybe its poultergeists.

 

[busbar]

Are you able to {safely} measure anode current to verify in-spec operation? If an odd grounding arrangement changes the machine’s operation, then possibly a power-quality logger should be attached directly to the line terminals of the machine.

A methodical process of elimination is sometimes the only way to expose the problem.

 

[elecdude]

We will try this tomorrow. Thanks for your input. It is appreciated.

 

[elecdude]

We disconnected the ground that ran from the microwave cabinet to a transfer switch, which had been the original connection. We then grounded directly to a structural beam. The machine ran continuously but only at low power, but this was more than we'd seen in a while. I looked at the input AC with a Fluke harmonic analyzer and also a 3 phase Dranetz 4300. We saw about 18 to 20% THD on phases A and C but 28% THD on Phase B. The harmonic spectra looked the same , just more of it. Does it make sense that one phase would have more harmonics than the other 2 phases?

 

[busbar]

Offhand, 28-versus-20% current distortion is probably not that big a deal, considering the “pea soup” harmonic currents can be. So, there was no noticeable transient activity on the source circuit at the same time as anode-current tripping?

It may be wise to only very carefully disconnect equipment grounding that is routed with the normal circuit conductors. Adding alternate paths may be OK, as long as they only supplement {not replace} Code-mandated practices.

 

[elecdude]

We need to check the recorded data to see if any transient event can be discerned. In the meantime, our technician measured the voltage at a receptacle close to our machine and found a 10 volt differential between ground and neutral.