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When wiring 4 MOTs in parallel, do the specs have to be identical?

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ScottH

Tue May 09 2017, 12:25AM

Registered Member #61373 Joined: Sat Dec 17 2016, 01:45PM
Location: San Antonio, TX
Posts: 87

I am planning to add 2 Mots to my dual Mot stack, Hv connected in parallel. The 2 new ones are from 2 different microwaves, and are slightly different in size. The 2 new Mots are almost as large as the Mots I have now.

With resonant caps, do the caps have to have identical specs too (uF)? What will happen if one cap is .85uF and the other is .75uF?

Dr. Slack

Tue May 09 2017, 05:03AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Sounds like you're gathering together all the elements for a mourning family.

Post a schematic of what you have now, specs for what you think it does now, what you want it to do, and a schematic of how you're intending to modify it.

Sulaiman

Tue May 09 2017, 08:43AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Whenever you put similar but not identical transformers in parallel there are two main situations to consider;
. on load,
each primary voltage will be greater than the load voltage so will contribute a current of (Vsec - Vload)/(secondary impedance) each.
so the lower voltage and/or higher impedance transformers of the group will supply less current each to the load than the higher voltage and/or lower impedance transformers
. off load,
due to slightly dissimilar secondary voltages, one or more secondaries will be above the common voltage, and one or more below it
so one or more transformers will be 'sourcing' current and one or more will be 'sinking' current.
MOTs have effectively high output impedances so the circulating currents will be smaller than full load currents,
so no problem.
Normal transformers with no deliberate flux leakage paths (or MOTs with the shunts knocked out) have a low output impedance,
so the circulating currents in mis-matched transformers could easily exceed rated currents.

I consider ONE MOT to be potentially LETHAL
four in parallel just makes your corpse smoke quicker

P.S. How are you going to power the four MOTs in parallel ?
can your a.c. supply provide the required power ?

ScottH

Thu May 11 2017, 11:50PM

Registered Member #61373 Joined: Sat Dec 17 2016, 01:45PM
Location: San Antonio, TX
Posts: 87

Dr. Slack wrote ...

Post a schematic of what you have now, specs for what you think it does now, what you want it to do, and a schematic of how you're intending to modify it.

What I have now, 2.8kVAC 2.7A:

How I'm going to modify it, 2.8kVAC 5.4A:

I'm using induction values as an example. I don't know the exact inductance. My mots are identical now, but the 2 newbies are slightly different in size, and a different brand. They should both be in the range of 2,100 - 2,200v

Sulaiman wrote ...

P.S. How are you going to power the four MOTs in parallel ?
can your a.c. supply provide the required power ?

I have 2 outlets, both adjacent and connected to separate fuses. One outlet can supply 2 MOTs (resonant config. only) without blowing the 30A fuse.

teravolt

Fri May 12 2017, 02:45AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

you could phase 1/2 the transformers and get 4kv. so you would have a 4kv with a grounded center tap like neon sign transformer

Kolas

Fri May 12 2017, 02:49AM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

What you have appears to be something that will generally work. However, I cannot fathom what you could possibly want to do with all that current.
I agree with Sulaiman here: you are creating something far past what is considered to be lethal.

DerAlbi

Fri May 12 2017, 04:38AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Stop whining about safety, 4 mots arent more dangerous than 1 alone if mishandled.
More important is, that his simulation lacks the coupling factor statement. It feels like this config will have strange load dependent behavior. Try to actually simulate it (like to share the .asc?)

Uspring

Fri May 12 2017, 08:48AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

ScottH, what you are trying here can spell out to be disastrous.

Sulaiman wrote:

MOTs have effectively high output impedances so the circulating currents will be smaller than full load currents,
so no problem.

This is generally correct, but by adding caps in series, the leakage inductance might be cancelled, which can lead to large currents between the transformers and consequently very high voltages due to resonance conditions. For an unlucky choice of caps and transformers, the whole thing can blow up spectacularly.

Omit the caps and you are much safer. Relatively.

Dr. Slack

Fri May 12 2017, 01:59PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Uspring wrote:

Omit the caps and you are much safer. Relatively.

So if Scott wanted to avoid the possibility of cancelling out the shunt-enhanced leakage inductance that would otherwise protect his parallel MOTs, he should parallel the MOTs directly. Then if he wanted to play with the resonant caps, add a parallel connection of them between the load and the commoned MOT HV terminal? Just a thought. Not as good a thought as putting the MOTs back in the box and having a cup of tea instead, but a thought.

Sulaiman

Fri May 12 2017, 02:37PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

ScottH wrote ...

Sulaiman wrote ...

P.S. How are you going to power the four MOTs in parallel ?
can your a.c. supply provide the required power ?

I have 2 outlets, both adjacent and connected to separate fuses. One outlet can supply 2 MOTs (resonant config. only) without blowing the 30A fuse.

I asked the questions that way because I'd anticipated your answer and wanted to see if you are learning to think safety yet .......

You do realise the potential hazard when you unplug one of the two circuits ?

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