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

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ScottH

Sat May 13 2017, 07:20AM

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

Sulaiman wrote ...

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 ?

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?

Sulaiman

Sat May 13 2017, 11:43AM

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

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.

Uspring

Sat May 13 2017, 05:12PM

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

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.

Sulaiman

Sat May 13 2017, 07:14PM

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

ScottH

Sun May 14 2017, 09:14AM

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

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.

Uspring

Mon May 15 2017, 10:34AM

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

@ScottH:
Good idea.

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

klugesmith

Tue May 16 2017, 05:50AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Uspring wrote ...
...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.

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."

Uspring

Tue May 16 2017, 11:22AM

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

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.

Sulaiman

Tue May 16 2017, 01:05PM

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

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

hen918

Tue May 16 2017, 01:58PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

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. :)

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