From: liz@poppyrecords.invalid.invalid   
      
   Lasse Langwadt  wrote:   
      
   > On 12/20/25 21:13, john larkin wrote:   
   > > On Fri, 19 Dec 2025 22:48:02 +0000, liz@poppyrecords.invalid.invalid   
   > > (Liz Tuddenham) wrote:   
   > >   
   > >> john larkin  wrote:   
   > >>   
   > >> [...]   
   > >>>   
   > >>> Resistors are the best way to dump power.   
   > >>   
   > >> I once had to make a set of replacement resistors for the starter unit   
   > >> of a 5 kW blower motor that took quite a while to run up to speed   
   > >> because of its high-inertia load.  I had some slate bars about 2" x 2.5"   
   > >> and 18" long cut to order by a slate quarry; then hand-wound them with   
   > >> resistance wire .   
   > >>   
   > >> The original wire had broken into short pieces when the unit was   
   > >> destroyed by arcing but there was one piece long enough for me to   
   > >> calculate its resistance using a four-terminal measurement.  When I   
   > >> contacted the wire manufacturers, they supplied the exact replacement   
   > >> which was still in production despite the unit having been made in 1919.   
   > >>   
   > >> The windings were interesting, with several tappings and one section   
   > >> wound two-in-hand to handle a higher current.  The control gear was   
   > >> connected to the air outlet of the blower and, as the motor speed   
   > >> increased, a cast-iron chamber was pressurised.  This caused a leather   
   > >> bellows inside the chamber to collapse at a rate controlled by lead   
   > >> weights hung on a lavatory chain.  The belows pushed a rod which slid a   
   > >> set of brushes over some brass contacts on an ebonite panel and   
   > >> progressively cut out the resistors.   
   > >>   
   > >> The motor was interesting too.  It was a repulsion-start, induction-run   
   > >> motor, which is quite rare nowadays but was popular in 1919, before   
   > >> reliable starting capacitors were available.  It needed a major rebuild,   
   > >> including skimming the face commutator.  The shaft was so long it   
   > >> overhung the tailstock end of my lathe, so we had to support it on   
   > >> wooden 'V' block bearings and feed it with a constant supply of oil.   
   > >>   
   > >> A very challenging and satisfying job.   
   > >   
   > > Sounds like fun.   
   > >   
   > > I had to dump about 8 KW once. We used a garbage can full of water and   
   > > a coil of copper wire. Had to refill it now and then when it boiled.   
   > >   
   > > Plastic garbage cans get wobbly when they are full of boiling water.   
   > >   
   > > A PCB with etched or dremeled traces might be a good dummy load, with   
   > > forced air or water cooling.   
   > >   
   > > There are HVAC things, nichrome duct heaters with fans, that are about   
   > > the cheapest dummy load you can buy.   
   > >   
   >   
   > Some Haas CNC machines use standard stove coils as spindle brake resistors   
      
   I had to repair one of their mills which had a huge wire-wound load-dump   
   resistor in a cage on top of the electronics cabinet.  The fault showed   
   up as 'electronics overheating'.   
      
   When I went to open the cabinet I nearly burnt my hand - the resistor   
   was almost red hot.  It turns out that the switching transistor had   
   failed short-circuit and the resistor was permanently connected across   
   the power supply.   
      
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)