Musical microwave oven transformer? / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

Musical microwave oven transformer?

previous 1 2

Move Thread

LAN_403

radiotech

Wed Jul 24 2013, 01:23AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Years ago I had to quickly make a display illuminated with many 4 foot colored fluorescent tubes.
It was a giant color organ.

Three high power amplifiers, 400 watts, each drove transformers blasting the tubes into flashing to
the music. They were 60 HZ transformers Heavy ones.

Ir worked perfectly for the three week of the exhibition but burned up hundreds of lamps. The bass
ones lasted about 6 hours.

I am sure ir could have with oven transformers, which weren't available then.

TorontoDXer

Sat Nov 16 2013, 02:11AM

Registered Member #38848 Joined: Mon Oct 21 2013, 12:19PM
Location:
Posts: 2

That is why they often weld a section of the core laminates together to make sure they don't vibrate and make sound.

Manufacturers do what they can with modern microwave ovens to prevent them from making noise, particularly with the MOT.

Newton Brawn

Sat Nov 16 2013, 05:39AM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

They weld the plates to save time in the assembling,. The result is a cheap transformer with high magnetizing current due the air gap.
Evem with a 0.05 mm gap the core magnetic reluctance results in high inrush current.
BUT as a MOT work schedulle is to work 10 minutes and maybe no more 20 minuts @ 30% SF, it is ok using MOT for experiments.

Shrad

Sat Nov 16 2013, 10:22AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

you can also work at a higher harmonic of the xformer for less heat and still usable power

at 150Hz you will already meet better specs for audio modulation if you rectify the output, as you'll have low ripple

Dr. Dark Current

Sat Nov 16 2013, 10:56AM

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

Newton Brawn: The high magnetizing current is mainly due to core saturation (usually around 2.1T peak). The inductive part of the current is relatively small. Also, I don't see how welding the laminations together would increase the air gap.

klugesmith

Tue Nov 19 2013, 11:36AM

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

DDC's right about the inrush current. The "grunt" is due to core saturaton when the transformer is switched on too close to a zero-crossing time, and happens even with "good" power transformers.
The MOT's "airgap" just increases the steady-state magnetizing current.

Newton Brawn's point is that the core E's and I's are not interleaved. To facilitate assembly, the E's are all stacked facing in the same direction. No -deliberate- airgap, but a small gap due to tolerating some non-flatness and oxide coating where the E and I meet. WIth traditional interleaved construction, the flux approaching each EI junction can migrate to neighboring layers which provide an uninterrupted metal path.

Welding is a cheap way to hold the block of E's and block of I's together, but as DDC said it does not increase the gap. Steel UU cores and round-section "R" cores wound from metal ribbon are often bonded and cured, then cut in half, and their joint surfaces polished smooth & flat as we're used to seeing on ferrite core joints.

MOT's operate at such high flux densities that the permeability of steel is becoming low. THat reduces the relative cost of the "airgap" that must be tolerated. I think they can generally run continuously with forced air cooling, sharing a noisy fan and duct system with the magnetron.

Tony Matt

Sat Nov 23 2013, 12:04AM

Registered Member #3700 Joined: Sat Feb 19 2011, 12:59PM
Location:
Posts: 107

Hi
I believe that the MOT may be used as a modulator transformer, that may wok well in the range of 200~2000 Hz. The turn ratio used to be 1:20, oe impedance ratio 1:400 ohms, or primary 50 ohms, secundary 20000 0hms... maybe delivering 100 - 150watts that can "modulate" a 300W beast.... useful for voice in hallowin party !
In using the MOT as a modulator, do not forget to provide the E and I gap to reduce the dC core saturation.

Newton Brawn

Sat Nov 23 2013, 04:03AM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Dr Dark Current,

I agree that the MOT is designed/constructed to work with high induction level, I have tested M0Ts that work with Bmax =1.75T.
What I have try to say is that the junction of the E pakage with the I pakage (not interlieved) allways result in small, small air gap that contribute to increase the core reluctance >> high magnetizing current. This air gap is due the steel burs that remain in the plates after its shearing.
During the welding cooling the welding contraction reduces gap. But is impossible eliminate the gap.

A typical MOT has a core cross section of 38 x 65mm that results em 0.0025m^2, primary coil with 120 turns, at 120V. 60Hz.

So, the calculed induction is about 1.8T and calculed magnetic flux is 0.0045Wb.

A gap of 0.05mm in a 0.0025m^2 result in a reluctance of 16000At/Wb.
Then considering the flux = 0.0045Wb, the FMM can reach 72At.

This 72At in 120 turn coil, result in about 0.6 A @120 V , or 72VA from the 120V power line.

It should be noted that this is only for one air gap, and for a E I core magnetic circuit we shall consider 1.2A @120V, 144VA from the power line.

I also agree the main magnetizing current is the result of high induction, close to the steel saturation, that can reach 300~500VA from the 120V power line.

That means in the botton line air gap is responsable to 30 ~ 50% of increasing the magnetizing curent.

As Kuglesmit point out, the high induction results also in stell losses (watts) resulting in heating, that a requires forced ventilation if the work requires more than 10 minutes. This happens with good brand welding machines.

Newton Brawn

Sat Nov 23 2013, 02:50PM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Hi Radiotech,

Here some details about connect a 50W poor amplifier in a MOT and a 1.2m (4 feets) 20W fluorescent neon bulb...

A typical MOT, 120/2000V, 600W. >> voltage ratio 2000/120 = 17, impedance ratio = 1:270.

The amplifier is set to work in the range of 100 ~ 2000Hz, delivering 20W to the primary of the MOT.

The secondary high voltage is connected to the neon bulb, that requires 300V to starting glow. After that the bulb voltage is kept to 120V.

To ignite the bulb the amplifier has to deliver 18V to the primary. ( that is 300V at secondary )

We may "gess" that the bulb load is about 720 ohms (bulb glowing).
If so, the reflected load at primary could be 720/270, or 2,6 ohm.

A 1.4 ohm resistor ai conneted in series with the primary to provide a total of 4 ohms load to the amplifier.

So, the amplifier will be " happy " to have a 4 ohms load...

The MOT that was designed to work at 1.7T induction will be fine because the new lower voltage and higher frequency will reduce the MOT induction.

The indution reduction by the lowering voltage will be 18/120 = 0.15 (or 0.255T).
The induction reduction by higher frequency will be 60/100 = 0.6 (or 1.02T)
The induction at MOT with lower voltage and high frequency will be about 0.153T.

Now a question: how about the MOT magnetic shunt? Shall it stay or be removed?

previous 1 2

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob