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MOT magnetising / idle current - Zebra TAN-100TT

Marco Polo

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Nov 2, 2012
Messages
618
Points
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I decided to apply full voltage to one of my two MOT's today and noticed that above 80 volts, I could feel the variac dial vibrating as I adjusted the setting. Clearly at that point, the current draw begins to ramp up substantially. I decided to measure and record the idle current at various input voltages. I figured it would be interesting to know, if nothing else.

The MOT in question is a Zebra TAN-100TT. It is rated for 120V and 60Hz input.

The data on the label is:
MOT label data said:
(ZEBRA)
Tan-100TT
RTRN-A512WRE0
120V 60Hz
3V8ZTT4
The variac is a brand-name Variac rated for 115 volts input, maximum current 5A. The actual mains input is 124 volts.

The "Variac" column is the variac dial setting. The V2 column is the actual output voltage. The IMOT is the MOT idle current for that voltage. I decided to end the test at the 120V setting because going farther would probably damage the variac. Readings were taken every 5 volts according to the variac dial. The current numbers for 125 to 135 were extrapolated from the data, but the voltage numbers were measured on the variac.

It looks like the transformer begins to saturate above 80 volts actual, and is well into saturation by 110 volts. I think if I were to wire the primaries of two of these MOTs in series, saturation would be more or less avoided since each unit would only be seeing 60 volts or so.

I'll do another test like this later, using the Basler Electric MOT that I harvested from a very old (30+ years) microwave that was being thrown out. It should be interesting to see the difference between then and now, because that old MOT is a lot bigger and heavier!

ETA: Btw, formatting that table was a bit of a pain because the forum ignores repeated normal spaces. I had to use U+2004 spaces to make it work but it still looks a bit choppy. I can see why DTR just posts photos when doing his diode current tests. Is there a better way to format a table like this?


Data for Zebra TAN-100TT

Variac           V2, V              IMOT, A
-------         ------            ------
  0                  0.60                0.00
  5                  6.18                0.08
10                  11.45              0.12
15                  17.03              0.17
20                  22.0                0.20
25                  27.2                0.23
30                  32.7                0.26
35                  37.7                0.30
40                  42.8                0.33
45                  48.5                0.37
50                  53.5                0.41
55                  59.3                0.46
60                  64.4                0.51
65                  69.5                0.57
70                  75.2                0.64
75                  80.1                0.72
80                  85.6                0.83
85                  91.0                1.01
90                  96.3                1.26
95                  101.4              1.66
100                106.7               2.34
105                111.8               3.23
110                117.2               4.54
115                122.2               6.14
120                126.7               7.69
125                132.3               9.3
130                137.4               10.9
135                143.0               12.5

 
Last edited:

djQUAN

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I believe MOTs are designed to run full load all the time which prevents the saturation. Have you removed the shunts?
 

Marco Polo

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I haven't modified them at all.

I'm not sure what removing the shunts would do to the idle current. AFAIK their main function is allow some of the flux to bypass the secondary, which limits the output current. Although, maybe the flux that bypasses the secondary puts some back emf on the primary and limits the input current, in which case removing them would increase it.

Mainly, I was curious as to what the effect running two of them in series would be, as far as preventing them from burning up. Every video I've seen of them running a single MOT flat-out results in the unit overheating and/or burning up. It doesn't seem to matter if the output is kilovolts or kiloamps. Since I don't see much point in destroying equipment if I don't have to, it's something I'm interested in knowing how to prevent. It looks like running two or more in series would accomplish that. Also would avoid having to mess with winding extra turns onto the primary and the potential issues that would pose.

Will be interesting to see the numbers for the older unit and compare then to now.
 
Last edited:

Cyparagon

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Sep 12, 2007
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Every video I've seen of them running a single MOT flat-out results in the unit overheating and/or burning up...

I don't see much point in destroying equipment if I don't have to, it's something I'm interested in knowing how to prevent.
Easy. Don't run them flat-out. Of course shorting the secondary will pop it eventually. NO transformer will last long when you abuse them. Be aware MOTs are incredibly small for a "1kVA" device, and they only get away with that rating by being fan-cooled and running for a few minutes at a time. Here's a MOT compared with a properly(conservatively) rated industrial transformer. The MOT is "1kVA" and the GE is 0.75kVA.

Big picture link
 




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