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Running my 3HP 3-PHASE MOTOR from household  AC
using capacitors. 

 Jerry Nelson, 2009

WARNING: These methods involve lethal voltages (lethal means you die).  The wiring discussed probably violates code.  I take no responsibility for your pain, injury, fines, and loss of life. /WARNING

I have a 3-phase, 3 HP motor.  SHAFT:   1 1/8” diameter, a common size.   3/8” parallel (square-rod) shaft key.  My house has only the usual 2-phase wiring.  Now what?  


To run a  3-PHASE MOTOR from 2-phase AC, you first connect the line across 2 phases from the motor.  The empty phase needs to be fed via capacitors from BOTH the 1-phase lines.  This is not a 120deg phase shift from each line because they are 180 deg to begin with, so the ideal capacitance values will not be equal, but it's easy to find values that work.   Equal capacitors would be balanced, and yield no starting torque.   To keep it simple, you can  energize the third phase from just a single capacitor just for starting, and run (with less power) with just two phases energized and the capacitor disconnected.  


GETTING ROTATION STARTED:  Our AC (Alternating Current) alternates so that a terminal is positive and then flips to negative half a cycle later (one-half of 1/60th second later).  If you make an electromagnet to turn a motor using this current, the magnetic pole will be "North" and flip to "South" 1/120th of a second later.  Motors work by taking the part that rotates and pulling (attracting) it around in a circle with a magnetic field.  But a single magnetic field that only reverses is only good for pulling the motor rotor up one moment and down the next, not round and round.  This is why all our motors have some kind of complexity to get them started -- a capacitor inside the bump on top of a "capacitor start motor", or a special starting winding that you can hear being switched in and out as the motor gets up past the starting speed.   

RUNNING ON FLIP-FLOP 2-PHASE:  Once up to speed, a conventional home-AC (2-phase)  motor will keep running.  When one cycle is finished, one rotation is finished, and the basic speed is thus 3,600 rpm (60 cycles or rotations/second x 60 secs/min).  When running at speed, the moving part chases the magnet at 12:00 o'clock that has the north pole, overshoots and starts around towards 6 pm (the bottom position), getting a strong pull to "hurry up" and get to 6 PM because -- presto! -- now the north pole is there.  That pull adds torque (force) to the rotation, and the torque-adding tug will be repeated again at 12 o'clock (top) if the rotor doesn't hurry.  But, with no tricks to get it started in the first place, any AC motor rotor will point (lock) straight at one pole and sit there buzzing until someone turns it off, or the electromagnets' copper windings burn out. Most motors have four poles, not two like this example, and so their synchronous speed is 1800 rpm, not 3600.  Little electric clock motors work this way.  Bigger, fractional-horsepower induction motors -- induced magnetism in the rotor, not real magnets -- have "slip" and turn at around 1725 rpm, not synchronously at 1800 rpm.  The poles of the induced magnet are spread out to reduce vibration at the higher powers involved, but strict synchrony is lost. 

THREE-PHASE:  Three-phase power comes on three separate wires, plus the 4th common return ("ground", "neutral").  Each is reversing 60 times a second (60 Hz AC), but the magic is that each reaches its peak 1/3 of a cycle (1/3rd of the way around a circle) later than the one before.  The rotor of a three-phase motor will be pulled around the circle -- no matter where it is sitting, and regardless of whether it is started or not.  So:  3-phase motors have tremendous starting torque.  A two-phase motor can be stopped and restarted in the opposite direction, but a three-phase motor can be commanded to reverse while running.  It is easy to implement electronic braking (e.g., by shorting the windings through diodes).  

Bottom line: with woodworking machinery, 3-phase motors offer the ability to stop quickly, check the work, and restart quickly.  They have more torque at any speed and always restart at once if stalled.  This is a klutz-friendly motor. 


4-5x motor HP for one line
For my 3HP motor, this is 3 *4 = 12uFD.  And then,
10-15x HP for the other AC line-> 30uFD

USING PAIRS OF CAPS ON AC:  Use pairs of capacitors back-to-back (not head-to-tail).  All electrolytics have a positive pole because they only work on DC (the current always goes one way).   So the back-to-back connection of plus/minus--minus/plus  means that one capacitor is always connected the right way as the AC current alternates its polarity.  That way, one capacitor can always fill up to whatever its rated capacity (capacitance) is, and resist further current flow, while the other would have been screwed by that direction of current-voltage pressure, if the partner hadn't stopped it.  In short, two back-to-back, ordinary (i.e., DC) electrolytic capacitors make one AC capacitor.

For the motor, the capacitors are optimal when 1) vibration is minimal, and 2) the voltage between all phases is most nearly equal.   It is possible that the optimal amount of capacitance changes with motor load -- I don't know yet.  This is the best “el cheapo” way to run a single 3-phase motor – max HP, minimum buzz and vibration. 

SAFE VOLTAGE RATING: The capacitor’s rated voltage for 220 VAC lines should be at least 1.4*220 = 308V; figure 330V.  There are great capacitors in old electron microscopes -- hospitals and universities throw them out, their voltages are very high.  If your capacitor voltages are too low for your motor, use them in pairs.  Two identical capacitors in series (plus/minus--plus/minus)  can take twice their rated voltage but give half the rated capacitance.  (And now, to deal with protecting DC-loving electrolytic-type capacitors from AC, you need to make a pair of capacitors connected in opposite polarities.  So if you used a pair of capacitors to double the voltage, then you need a second pair to deal with AC's two polarities.  Your stack becomes:  plus/minus--plus/minus connected to minus/plus--minus-plus.) 

Parallel connections sum capacitances, but the lowest rated voltage (the weakest link on the chain) is the one that has to be good enough.   

If a voltage surge (e.g., flicking the motor on and off a couple times while running) exceeds a capacitor’s rating, the capacitor sparks inside.  If the spark leaves a conduction path,  the current traversing this path will heat the capacitor until it explodes.  Most capacitors of these high capacitance values will be electrolytic types (not oil-bath or solid polypropylene, whatever).  An exploding electrolytic capacitor is **very** messy.  They need to live in boxes.  Really, this is horrible.   

High-quality electrolytics last 30 years.  As electrolytics age (“dry out”), their capacitance may drop—if you don’t have a capacitance meter, the motor’s vibration will tell you the capacitance you expected isn't all there.  You can add some more capacitors in parallel to top things back up again.  Old capacitors can also “leak” more current, which makes them warm, which dries them out faster.  Not a big deal – once you know that the capacitor can has no leakage voltage on it, you can feel the temperature with your hand a couple times a year.  If it ever feels different, order a new one. 

In this dual capacitor method, the motor will start by itself, but slowly.  A greater imbalance between the motor windings would produce a greater starting torque, but then it doesn't run so quietly.  You can temporarily connect additional capacitance to the larger of the two capacitors, or temporarily disconnect the smaller one for starting.


For simplicity (fewer capacitors) and for less vibration -- but less HP -- start on 3 phases but run on two.   .

 Temporarily connect the empty phase through its capacitors to either side of the line (depending on desired rotation direction) for starting, and disconnect once running.

 I had luck with 950/2=475uFd for my 3-phase 3HP motor.  The rule of thumb  is:  at least 80 uFd per HP for starting.  So, for 3 HP, that’s least 240 uFD.  Obviously capacitance value is not critical--we are not tuning for minimum vibration (optimal phase shift) here, we just want a good few seconds of kick to get going.  


A 3-phase motor can be run without any capacitors on just 2 phases once you get it started.  Do whatever you want to start the motor spinning.  Above, I discussed a starting capacitor temporarily connected to the third (empty) phase, but some home workshop people use a spare run-of-the-mill second motor to initially spin up a big 3-phase motor before turning on its power. 

In these home shop setups, once started, this first motor typically acts as a 2-to-3-phase (rotary-type)  converter for all the other (3-phase) motors in the shop.  The other motors are no bigger than 2/3rds the HP of the rotary conversion motor (2 out of 3 phases energized; 2/3rds power available).  Only  one shop machine is run at a time.  People do this for two reasons:  once you have real 3-phase current from the conversion motor, all the other 3-phase motors on the shop tools have enormous starting torque, and it’s easy to install switches that give electrical braking.  If you are doing demanding turning on a big wood lathe, this translates into easily stopping to look carefully at the work, then instantly starting back up again to continue the cut.  The second reason is that people give these motors away free because no one can figure out how to run them.  Sometimes it's easier to collect free motors than free capacitors -- whatever works for you. 

My favorite installation was a Canadian who kept both the big conversion motor and the little 1/4HP kicker motor that started it every morning inside a Styrofoam-lined wooden box.  The ¼ HP 110VAC motor was on a hinged board with a loose pulley belt.  To spin up the big motor,  he just pulled the board with the small motor until the belt was tight, got the big motor turning, then  applied the main power and dropped the board (and turned of the aux motor).   The shop was not centrally heated, so a 100-watt light bulb and a lid for the box completed the setup—everything was always warm enough to go first thing in the morning.  Wired around the walls, this motor served as the rotary converter for 3 other (smaller) motors in the shop.  Putting a pulley on it for the kicker motor was no problem, because the motor didn’t have to be connected to any machine – it was just the rotary converter for the rest of the shop.  



Home for this website, such as it is
A nicer website, mostly travel pictures

Rev 22May09
Rev 31May2018, Found a photo of my dear old motor.
                         OMG, I got the phase and frequency wrong -- fixed that. 
                         Explain basic RPM is 3600, 4 poles divide it down to 1800,
                         blurred pole structure lets 1800 rpm motor slip out of synchrony to 17hundred-something.    
                         Still more errors? 
                         Did you use one of these for a wood-turning lathe and make anything pretty?