Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
Dr. Slack, Tue May 09 2017, 05:03AM

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.

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
Sulaiman, Tue May 09 2017, 08:43AM

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 ?

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
ScottH, Thu May 11 2017, 11:50PM


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


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.

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
teravolt, Fri May 12 2017, 02:45AM

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

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
Kolas, Fri May 12 2017, 02:49AM

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.

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
DerAlbi, Fri May 12 2017, 04:38AM

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?)

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
Uspring, Fri May 12 2017, 08:48AM

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.

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
Dr. Slack, Fri May 12 2017, 01:59PM

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.

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
Sulaiman, Fri May 12 2017, 02:37PM


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 ?

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
ScottH, Sat May 13 2017, 07:20AM


I was going to use 1 large lever switch to switch the mots on and off simultaneously, connected to both power cords. I plan to upgrade my entire mot build, wires, and chicken stick when doing this.

I am curious, if I unplugged one of the 2 circuits, wouldn't the active mots back feed the unplugged mots secondaries, making the primary voltage 120+ on the cord that is unplugged?

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
Sulaiman, Sat May 13 2017, 11:43AM

Precisely ;)

I was just concerned for your safety as this is exactly what another member was considering doing some time ago.
(except it was for 2-0-2 secondaries in series for 4-0-4 kV rms, for a TC if I remember correctly)(4 MOT stack)

A suitably rated 2-pole switch or relay is sufficient, knife switches are too exposed for my liking,
consider something like a lamp/bulb/neon/led/indicator across each a.c. input to indicate danger ?

P.S. the increase in output voltage due to resonance with a capacitor is limited by the volts/turn of the transformer winding before saturation,
which is not much more than the normal operating voltage
as manufacturers do not add 'spare' capacity to anything due to cost.

i.e. if you want to use transformer windings for resonance experiments,
your drive voltages should not exceed (Vnominal)/Qmax

e.g. if you expect Q = up to 100, for a 110 V winding your drive voltage should be 1.1 Vrms or less, etc.

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
Uspring, Sat May 13 2017, 05:12PM

Dr. Slack wrote:

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?

Looks, like it could work.

Sulaiman wrote:

P.S. the increase in output voltage due to resonance with a capacitor is limited by the volts/turn of the transformer winding before saturation,
which is not much more than the normal operating voltage
as manufacturers do not add 'spare' capacity to anything due to cost.

That's certainly a limiting factor. I've read reports about "much more power", though. Admittedly, I don't have any specifics.

When wiring up two MOTs with a cap as Dr. Slack describes there is a problem if the MOTs are powered from 2 outlets with two separate circuit breakers. If one of them triggers, you will see a back feed coming from the other MOT. This can be much higher than 120 V due to resonant rise. Other devices connected to that line might not be happy.

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
Sulaiman, Sat May 13 2017, 07:14PM

?

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
ScottH, Sun May 14 2017, 09:14AM

Those 2 outlets are dedicated to the washer and the dryer, separately. I'll have them unplugged.

As for the indicator lights, the hum of the transformers is audible enough to know that the setup is very Hot. I may decide to incorporate a small bulb or led though.

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
Uspring, Mon May 15 2017, 10:34AM

@ScottH:
Good idea.

@Sulaiman:
You got me puzzling. Please don't use invisible ink when replying. Which statement are you wondering about?

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
klugesmith, Tue May 16 2017, 05:50AM

Huh?
Even if so-called resonant rise existed*, we are talking about a pair of more-or-less identical circuits. Whatever goes up, from primary to secondary on the powered circuit, would come back down from secondary to primary on the disconnected circuit.

That might be over-simplifying things a bit, and I haven't really thought this through. Suppose the circuit impedance were resonant as viewed from the 120V port. The powered instance is connected to an extremely low impedance (the mains). The backfeeding instance is connected to an extremely high impedance (the open circuit breaker), or to an intermediate impedance (the "other devices" on same branch circuit). I guess it's plausible that the voltages could be pretty different.

Uh, that would be wrong. The only source of voltage in the primary winding of the backfeeding MOT(s) is magnetic flux change in the core. That can never induce a voltage much higher than nominal, at nominal frequency. There isn't enough room in the core for much extra flux.

*That page by loneoceans, in Uspring's link, appears to be an early work. With utmost respect for the author, it does have mistakes. E.g. "MOTs are designed to have what is known as a large leakage inductance. When a heavy load is applied on the secondary of the MOT, instead of dropping to 0 inductance, the MOT core is designed in such a way for the inductance to drop to this leakage inductance."

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
Uspring, Tue May 16 2017, 11:22AM

I confess, that I don't know, how much leakage inductance a typical MOT has and to give justification to Sulaimans point about saturation I am also unclear to when it sets in.

Consider a MOT with some leakage inductance connected to mains. Now load the secondary with a cap. For small capacitances this will cause a rise in secondary voltage. It can be seen as an LC series circuit made out of the leakage inductance and the cap. This circuit is connected to a low impedance AC source. Increasing the capacitance will cause higher voltages up to a max from where the voltages drop again. The amplitude of the max depends on winding resistances and saturation effects.

Now add a second MOT with its secondary parallel to the first one, but not connected to mains. It will create a primary voltage, which is dependent on the max of the first MOT. If that is considerably larger than the unloaded first MOT voltage, than you might get a higher than mains voltage on the primary of the second MOT. If the second MOT is connected to a mains with an open circuit breaker, that might cause harm to other devices on this part of the mains.

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
Sulaiman, Tue May 16 2017, 01:05PM

Google "ferroresonant transformer" for a blast from my past
(yes, we really used them ... state of the art)
the principles are similar.

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
hen918, Tue May 16 2017, 01:58PM

Yeah, I kind of agree with both Uspring and klugesmith; MOTs don't have a particularly large leakage inductance, however current limiting is provided by the magnetic shunts, and this creates an alternative path for current to flow through. The electrical model would be a series inductance, so loneoceans is kind of right in saying they have a large leakage inductance, as long as the shunts remain in place.

Whether there is a high enough Q-factor to cause significant voltage rise remains to be seen. AFAIK there is nothing about a MOT that is particularly good quality. :)

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
Uspring, Thu May 18 2017, 09:48AM

@Sulaiman
I see what you mean, particulary the stuff about stabilising transformers. Assuming MOTs to be near saturation under normal operating conditions, i.e. without capacitive loads, would adding a cap lead to increased power output?

Re: When wiring 4 MOTs in parallel, do the specs have to be identical?
Sulaiman, Thu May 18 2017, 10:55AM

I can't give a definite answer because I've not tried,

but I can not imagine how the transformer output VOLTAGE can be significantly increased, due to core saturation,

POWER output will depend upon load matching, so the load needs to be specified,
but it is easy to imagine how capacitors could increase real POWER output for some loads,
and apparent power for others.

So I think that in this case we should consider the capacitors mainly as a load impedance matching device.

... but practice beats theory ... and I'm only theorising here.

and frankly, MOTs still scare me :)