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Modding MOTs, Need Advice.

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Patrick

Sat Mar 26 2011, 05:21AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I need help making some decisions regarding my most recent MOT aquisition.

Background:
I have determined the "as manufactured primary" is 103 turns, the label stated 120Vac, at 60Hz. I presume from its size I can get 1500W out of it. The primary consists of 14 awg mag wire. Magnetic shunts have been pressed out. The intent for this MOT is to use a common setup with a variable turn secondary.

Math:
Given the label said 120 Vac, and I believe from size that 1500W is doable,
Power: 1500 W / 120 Vac = 12.5 A
V per T: 120V / 103 Turns = 1.16 Vac per T

Question 1:
After reading the HvWiki and talking to others, I have wanted to take the core out of saturation, by increasing the primary turns. So, do I just increase the count by 10% or some other number? Or should I measure the input current while adding turns?

Question 2:
Now that I have removed the magnetic shunts, and increased the turns for the primary, will ballasting be easier/different? (By theory im thinking I should stilll need the ballast)

More Math:
If I want 12.5 amps at 120v then,
120 Vac / 12.5A = 9.6 Ohms total impedance (ohms + reactance)
(valid if the secondary is pulling about 1500W)

If I assume 0.3 Amps magnitizing current, then:
120 Vac / 0.3 Amps = 400 Ohms total impedance (ohms + reactance)
(valid at no load on secondary)

Logical Conclusion ?:
Should I choose to increase the primary turns such that 12.5 amps can still pass through that reactance, with the difference between that and 9.6 ohms being made up with the ballast? I just dont want the Volts per turn to drop much below 1.0 or
0.9 Vac.

EDIT:
I appologize if previous threads dealt with these matters in detail, using our search function before posting this, I found nothing useful enuogh, but I will remind all that our search function is basically crap.

Secondary and shunts removed.

MOT in a vice and the spool I will use to lengthen the primary.

Beer bottle for scale.

Dimensions 3.45" x 4.14" x 2.4"

haxor5354

Sat Mar 26 2011, 05:44AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

does transformer steel have a magnetic flux limit like ferrite?

Patrick

Sat Mar 26 2011, 06:08AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

haxor5354 wrote ...

does transformer steel have a magnetic flux limit like ferrite?

Yes, it does, but steel is higher. Ferrite is about 0.3T, steel is like 1.3 -1.7T, under common conditions. If I remember right.

Dr. Dark Current

Sat Mar 26 2011, 10:13AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Your MOT could have magnitizing current of ~4 Amps and a no-load input of ~100 Watts. It will get VERY hot operating even at no load.
If you want to use a MOT for continuous work, you should add a fan or increase the primary turns by at least 20 percent.
As to your question 2, if you need a low internal resistance power supply you should not use any ballast.

Marko

Sat Mar 26 2011, 04:42PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

If you find ading more primary turns troubling, you could use a buck transformer in series with your input. A transformer of 100-150Va with 12V output or so can be conencted to mains and it's secondary in series with supply to your mot, in anti-phase so it reduces the voltage on it. It might be easier but not more cost and space efficient, though.

Marko

klugesmith

Sat Mar 26 2011, 05:16PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

I applaud Marko's suggestion of using a buck transformer to reduce the volts/turn, magnetizing current, and core loss. You would look for a 120 to 12V or 24V transformer with -secondary- current rating at least as high as your intended primary current in the MOT. Ubiquitous for low-voltage lighting systems.

Why bother reducing the volts/turn, unless you want to run for more than 10 or 15 minutes without a cooling fan?

You can calculate the peak flux density (B) directly from
1) sinusoidal RMS volts per turn [edit] per hertz.
2) cross-sectional area of the core - the center leg should be twice the area of the outside legs.

Write back if you need a reference, or want your numbers checked.
A few months ago I reported on an XRT with peak flux of over 1.8 T, which is exceptionally high for steel cores except those for intermittent use. MOT's might be similar.

[edit] p.s. I recently investigated the behavior of transformers near saturation, with a resistive ballast in series. You can actually get significantly more RMS volts per turn for a given RMS magnetizing current, as the voltage waveform distorts (in the direction of being peak-y rather than square-ish). No time to elaborate today.

Patrick

Sat Mar 26 2011, 07:21PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

This was exactly the response I needed:

Dr. Kilovolt wrote ...

Your MOT could have magnitizing current of ~4 Amps and a no-load input of ~100 Watts. It will get VERY hot operating even at no load.
If you want to use a MOT for continuous work, you should add a fan or increase the primary turns by at least 20 percent.
As to your question 2, if you need a low internal resistance power supply you should not use any ballast.

Well OK, the 4 amp thing at no load is a real downer.

I removed the shunts to make it function more ideally. I will use resistive ballast on the primary for test/evaluation and proving. Then Ill switch to inductive ballasting on the primary or secondary.

It will be inside a well cooled case, but I was hoping to keep them cooler for 10 -15 minute runs. I will add 25% on the primary's turns.

I was planning on this type of usage: 15 mins on, 30 mins off, for several hours at 80% of max capacity with continuous cooling.
Im not sure what the heat load would be, ill have to run some tests.

Nah

Sat Mar 26 2011, 09:08PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

I added around 15 turns to mine and had a load of 100 watts at 10 volts. It didn't even get warm.

Patrick

Sat Mar 26 2011, 10:09PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Nah wrote ...

I added around 15 turns to mine and had a load of 100 watts at 10 volts. It didn't even get warm.

How long did you run it?

Dr. Dark Current

Sat Mar 26 2011, 11:14PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The core can take up to 3 hours to get up to temp.

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