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Registered Member #1316
Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365
Today I was viewing the waveform of the output of a MOT on my oscilloscope to see how the saturation of the core changes the waveform and I discovered that the when turned off, the MOT generates quite a large voltage spike. From the two MOT's I tested it its on the order of 8 Kv. I believe that under loading, this spike would mostly disappear. However, those who do not already know of the large spike should take it into account if they use a MOT to power high impedance loads that are sensitive to over voltage.
From these spikes, one of two things can be determined. One is that microwave transformers are designed with a 8kv core to winding insulation to withstand these spikes, and are actually rated to reliably withstand the high voltages associated with resonant operation. The other possibility is that microwave transformers are only designed to be operated with a load, and should never be turned off at a full primary voltage without a load to prevent a potentially damaging voltage spike.
Here are two pictures from my DSO showing the normal waveform of a MOT under saturation and the turn off spike.
Looks like the forum doesn't let me upload .bmp images . I will try to covert them and post the pictures tomorrow.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
All transformers show this behavior. When you interrupt the primary current the energy stored in the primary inductance must go somewhere, and a pulse is generated. A suitable capacitor or a surge supressor in parallel with the primary winding will reduce the spike.
Registered Member #1316
Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365
Antonio, I understand that all transformers display this voltage spike. However, I was astonished by the magnitude of the spike, which I believe is made worse due to the extra leakage inductance of the shunts in a MOT compared to a standard transformer. And like I said, the large magnitude of the spike can tell us that we must never turn off a MOT while it is unloaded, such as when drawing arcs, or that a MOT is built with extra insulation and is a bit better made then what most people complain.
As promised here are the scope waveforms of the spikes, and the icky output waveform caused by saturation of the core.
Registered Member #1617
Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139
The other possibility is that microwave transformers are only designed to be operated with a load
That seems most likely!
I once decided to push my luck and hook 3 mots in series in open air, with all the cores floating. When the secondaries were open circuit the thing hissed and crackled with corona very loudly! I stopped then, but I reckon theyd quickly die like this.
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Turnoff Voltage Spikes from Microwave Oven Transformers
. I will try to covert them and post the pictures tomorrow. 
