Make Your Microformer

Description

Begin here to make your Microformer. Below, a step-by-step guide is presented which describes how to build one Microformer. Please take all necessary cautions to prevent injury. The Microformer contains a repurposed microwave oven transformer which has a high secondary voltage (2kV-2.4kV) and formidable real power throughput (1kW). If used inappropriately, significant harm could result. Only those skilled in electrical systems should attempt to build a Microformer.

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 Disclaimer

Our group does not take any responsibility for those who are injured or do not take every precaution necessary to prevent injury. The Microformer is a practical open source idea for rural electricity distribution. It is not a toy. Build responsibly.

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Tools

  • Phillips and standard screwdrivers
  • Hand press
  • Wooden blocks
  • Wood or metal rod
  • Hammer
  • Sand paper
  • Wire strippers
  • Terminal crimpers
  • Grinder
  • Soldering equipment
  • Metal hole punch
  • Hand saw or band saw
  • Rubber mallet

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Materials

  • Microwave oven transformer
  • Metal container (paint can)
  • Construction grade adhesive
  • Nomax paper
  • Flux Compound
  • Magnet wire (primary wire gauge)
  • Primary wire, covered, 600V, oil resistant
  • Secondary wire, covered, 5000V, oil resistant
  • Wire terminals
  • Bushings Option 1:
    • Spark plug
    • Terminal connections (screw-type) – black and red
  • Bushings Option 2:
    • PVC pipe 0.75″
    • PVC pipe 1.5″
    • PVC end caps 0.75″ – QTY 4
    • PVC end caps 1.5″ – QTY 2
  • Silicone
  • Threaded rods or bolts – 4″ long (approx.)
  • Rust resistant paint or spray
  • Paintbrush
  • #10 nuts and bolts – 12 pairs.

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Phase 1: Obtain Your MOT

Step 1: Obtain a discarded microwave

  • Every microwave contains a microwave oven transformer, which is a simple and durable device. Even if the microwave is broken (usually due to an electronics failure), the transformer will be intact (if it’s not physically damaged in any way).

Step 2: Unplug the microwave for at least one minute

  • Microwave ovens use a rectifier and capacitor filter to provide high voltage DC to the magnetron.  The capacitor is capable of holding a lethal charge for an indefinite period of time.  Modern microwaves use capacitors with internal bleed resistors with values around 1MΩ.  Combined with a typical capacitance of 1uF, the bleeder time constant is 1s.  Older ovens may not have a capacitor with bleed resistors and could reside at high voltage for long periods of time.
  • Remove the outer sheet metal enclosure of the oven.  Usually a Phillips or flat blade screw driver will accomplish this.  With the cover removed, any doubt of the voltage of the capacitor may be allayed.  Short the terminals of the capacitor with a screw driver while gripping the insulated handle.  If the capacitor terminals are shrouded and a screwdriver will not make contact, connect a wire to the metal frame of the microwave and use the free end of the wire to touch each of the capacitor terminals.  One capacitor terminal is ground, the other high voltage.  If there is voltage on the capacitor there will be a spark when the terminals are shorted together.

Step 3: Remove the MOT from the microwave

  • It is secured to the oven with sheet metal screws and has primary, secondary, and filament winding wires attached to it. Take care to label polarities of individual wires. Unscrew the bolts and wires to remove the transformer.

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Phase 2: Characterize Your MOT

Step 1: Use the Microformer Parameter Estimation Worksheet to characterize your MOT

  • This process informs the reader of the electrical performance capabilities of the MOT.  MOTs are not designed to operate continuously in an oven, and are made with as little copper as possible.  These characteristics appear in the primary and secondary winding resistances as well as leakage and magnetizing inductances.

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Phase 3: Modify Your MOT

After characterizing the MOT, modifications must be made to enhance its performance.  The magnetic shunts must be removed which decreases the leakage inductance of the MOT while simultaneously making room for additional primary turns.

Step 1: Remove the magnetic shunts

  • The easiest way to accomplish this is by the use of a hand press.  Care should be taken to support the transformer by its core, NOT THE WINDINGS, during the press removal of the shunts.  If a hand press is not available, the transformer may be laid on its side across wooden blocks and the shunts driven out with a wooden or soft metal (brass) rod.  The tertiary filament winding, which consists of 3-4 turns, may also be removed since it no longer serves a purpose.  Usually the tertiary wire gauge is not sufficient to handle primary current thus it cannot be added in series with the primary.

 

Step 2: Add more turns to the primary of the MOT

  • After characterizing the MOT in Step 2, the reader can have a sense of how many turns are necessary to add in series with the primary.  For large transformers, around 1 -1.5 kW, 8 to 10 turns are usually sufficient, where less powerful transformers require around 15 turns.  These numbers are based off of the experience of the authors since the steel’s magnetic properties are not readily available to the builder.  Before adding additional turns a flame retarding electrically insulating paper should be positioned to keep the winding off of the core and isolated from the secondary.  Nomex paper is well suited for this task.  After winding the additional turns, connect them in series with the primary.  The polarity of the connection is extremely important, if connected in the opposite polarity MOT performance will worsen.  The polarity of the winding is determined by the right hand rule.  The flux generated in normal operation by the primary and additional windings should orient in the same direction within the MOT core. Repeat step 2 to confirm performance improvements with the parameter estimation worksheet.  The reader may continue to experiment with number of turns and polarity of the additional windings.

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Phase 4: Make Your Microformer

The MOT enclosure serves three main purposes: First, it shields the transformer from outdoor elements and safely contains high voltage parts. Second, it provides a container for a cooling and insulating dielectric fluid (oil). Finally, it provides a mount for securing the system to a pole or wall. Any conducting metal container can serve as the enclosure for a MOT.  It is important that the enclosure be grounded for safety purposes (shorts, faults, lightning strikes, etc.).  A recycled paint can serves this purpose very well as it is water proof, conducting, and relatively inexpensive.  The following process will outline how to use a paint can for an enclosure. Find a paint can and clean it so there is no rust on the surface, especially on the inside. Failure to remove rust or debris from the inside of the can result in MOT failure.

Step 1: Make sure the MOT fits inside of the metal container obtained for building the Microformer

  • Test fit your MOT inside the paint can.  It should be able to fit easily inside and sit squarely on the bottom.

Step 2: Attach wires to the MOT

  • Use wire with insulation rating of 600V and oil/gasoline resistant for the primary winding and at least a 5000V voltage rating for the secondary high voltage tap.  The high voltage return is ground, and this wire should be attached directly to the core of the MOT.  Cut the wires to a length that allows 3 to 4 inches to stick out of the paint can when the MOT is placed inside it.  This will allow for connections to be made to the insulator bushings later.

Step 3: Fasten the MOT to the inside of the metal container, standing it upright on the bottom

  • To fasten the transformer to the inside of the paint can, the Microformer creators recommend a construction grade adhesive.  Apply adhesive to the bottom of the transformer liberally and place it in the can to cure.  The authors do not recommend bolts or screws because they require through holes to be drilled which are prone to leaking oil over the lifespan of the Microformer.

Step 4: Create electrical connections (called “bushings”) to the lid of the metal enclosure

  • Electrical connections must be made through the lid of the enclosure (paint can), capable of withstanding wet weather conditions at 2kV and above.  There are two options using readily available items the authors have used successfully to realize the electrical through bushings:
  • Bushings Option 1:
    • Recycled spark plug – Find an old spark plug that does not have a resistive element inside it, i.e. the resistance between the spark tip and the porcelain insulated end connection is less than 1 Ohm.  Next cut off the ground tab from the threaded shroud.  Use a grinder to remove the tungsten from the tip of the spark electrode.  Solder a wire in line with the electrode; be sure to use plenty of flux compound.  Slide heat shrink tubing over the solder joint to protect and strengthen the connection.  Be careful, the solder joint is prone to failure if it is mechanically stressed. Tap a piece of aluminum or steel to be a nut for the spark plug threads.  Punch, cut, or drill a hole in the paint can lid and attach spark plug.  Be sure to seal around the base of the plug with silicone on both sides of the lid to make a water tight seal.

  • Bushings Option 2:
    • PVC plumbing parts – The bushings can be realized by PVC plumbing products, namely end caps, pipe sections, and hardware components. Start by cutting holes in the lid to the diameter of the pipe section, 1” or 1.5” diameter pipe works well.   Then, cut three sections of pipe 1.5” long.  Drill a #10 hole through the centers of six end caps.  Place the pipe sections through the hole such that half of the pipe is on either side of the lid.  Place a bead of silicone around the pipe where it meets the lid on both sides.  Now apply primer and glue to the pipe ends and end caps in the procedure prescribed on the side PVC glue bottle.  Simultaneously press the end caps on both sides and with great pressure such that silicone squeezes out at the lid joint and the PVC glue holds everything together.  Wipe off the excess silicone and or glue.  Thread a rod or bolt through the holes drilled in the end caps to attach wires.  Seal around the bolts with silicone where they enter the end caps.  Also, be sure to attach the ground wire to the lid with a simple machine screw.

Step 5: Paint the enclosure to protect against outdoor elements and prevent rust

  • Be sure to mask off the electrical terminals and bushings while painting.

Step 6: Fill the enclosure with oil, right to the top of the MOT

  • The type of oil used is up to the builder.  In most cases mineral oil, un-used motor oil, etc. are used. The oil allows heat to move efficiently from the MOT to the outside of the metal container. Pour the oil to a level just past the top of the MOT, but no higher.

Step 7: Attach the MOT wires to the bushings on the lid

  • Secure the lid by lightly tapping around the edge with a rubber mallet. After securing the lid check the resistance with a multimeter between the terminals to make sure everything is assembled correctly.

Step 8: Enclose the MOT by securing the metal container’s lid to the base

Step 9: Place a completed Microformer Nameplate to the outside of the completed Microformer

  • Take care to prevent this nameplate from deteriorating by laminating it.

Step 10: Bask in the glory of your completed Microformer!

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The Microformer Project – Copyright 2012