Replies

1. 
   jhausch | Feb 17, 2004 06:32am | #1

   Is the motor running hot?  Can you put a clamp on amp meter on a leg to see what it is pulling?  If not hot and not too much current, I'd say that you are probably OK with the static unit.

   How many HP?  I may be able to hook you up with a Variable Frequency Drive that will allow you to gently start and stop it. 

   Nowadays I can't figure out why anyone looking to run some 3 phase equipment (in a single phase shop) under, say, 3 to 5 hp would do so with anything other than a VFD.  This is unless you like to fiddle with electrical stuff and enjoy the capacitor matching and all that stuff (sometimes those projects are really what we enjoy doin. . .) Or are very thrifty.  If you are talking about spending between 150 and 550, you can have a drive.

1. 
   KenMattie | Feb 17, 2004 05:19pm | #2

   Jim,

   Thanks for the reply.  The jointer seems to be running fine with the static converter.  The motor is 3HP.  I purchased the unit, including the static converter, at a discount because the dealer was having trouble selling it due to the 3-phase power requirement.  I don't enjoy fiddling with electrical stuff.  My concern is for the potential deterioration of the motor over time and a general curiosity about the relationship between the quality of the performance of the machine and the type of converter supplying the power.  In other words, why use a rotary converter at three times the price if the static converter works?

   What is a Variable Frequency Drive?  How is it different from a static or rotary converter?

2. 
   CharlieP | Feb 17, 2004 05:20pm | #3

   I have not worked with a 3 phase power converter in a shop, but in general, static power converters don't give a clean wave.  (Beware - I am an electrical engineer by training.)  What's that mean?

   Power from a rotating generator (including the power from the electric company) gradually increases , then decreases, then goes in the other direction, 60 or 50 or whatever times per second.  If you were to make a graph of the voltage or current (a "microscopic" graph, not the average measured by a meter), you would have a near perfect sine wave (or cosine, depending on where you start).  Your motors expect that kind of power, since they, too, are rotary devices. 

   Power from a static converter uses electonics to generate an alternating wave (or in the case of a 3 phase converter, three separate alternating waves, 120 degrees apart).  But most static converters don't make a perfect sine wave.  The worst case might simply turn the power on in one direction, then on in the other direction - a square wave.  Or the electronic "variable speed" or "dimmer" controls that start out with a sine wave from the power company and then "turn off" the wave part way through each cycle - when they "have enough" 

   What if the power isn't shaped like a nice sine wave?  If you are just heating the filament in a light bulb, nobody cares, even the light bulb.  But if you are running a machine, you might.  Mathematically we can take that funny shaped wave apart into several components.  If the basic wave is 60 Hertz (changes 60 times per second), there will also be a component at 180 Hertz.  and some more at 300 Hertz, and some at 420 hertz, etc (odd multiples of the original frequency).  This extra doesn't help turn a motor, but goes through the motor, generating heat or saturating the magnetic field in an unproductive way.  Will it hurt anything?  How much of this unwanted "noise" is in the converted power? (The amount of noise is proportional to how far the wave is different than the sine wave.)

   So back to real the world...

   How good is the "shape" of the power from a static converter?  Depends on the brand (and cost).  The better units probably give better shaped electricity, thus creating less waste in your electric bill, and less waste heat generated in your motors.  

   Do you care? 

   If you are going to run a motor all day every day, you sure do.  Get a rotary converter (which is simply a single phase motor that turns a three phase generator).  Or move to an industrial area where power is distributed as three phase, rather than the single phase distributed in residential areas.  Because you should be concerned about the cost of the power and the extra heat generated in your motors. 

   If you are going to run a 3 phase motor only occasionally and for short periods of time, use a static converter - costs less, and the extra cost of electricity and the extra stress on the motor aren't that significant for occasional use.

   Now that you know how the clocks are built, you said you just wanted to know the time?

   ________________________
   Charlie Plesums  Austin, Texas
   http://www.plesums.com/wood

1. 
   KenMattie | Feb 17, 2004 05:49pm | #5

   Charlie,

   Thanks, the fog is slowly lifting.  Woodworking is an avocation for me so I don't use the jointer continuously.  The static converter, supplied by the dealer, is made locally in Atlanta by a retired General Electric power systems specialist.  I don't have the capability to assess the quality of the converted power but I am told by the dealer that they and others have had very good service from these units.   Based on your explanation, I think I will use the static converter for a while and see how the jointer performs over time.

1. 
   CharlieP | Feb 17, 2004 06:04pm | #6

   A simple converter isn't that hard to build, so if yours is being built by a retired... in their garage, it is probably a simple unit.  But nothing wrong with garages... I sell the furniture I build there, too.

   As a hobby user, your choice makes good sense, and as another post suggested, I would simply be aware of the temperature of the motor.  (Keep in mind that the inside of the motor will be a lot hotter sooner than the outside, so check a few minutes after you turn it off, in addition to while it is running.)________________________Charlie Plesums  Austin, Texashttp://www.plesums.com/wood

2. 
   TKanzler | Feb 17, 2004 07:13pm | #7

   "I don't have the capability to assess the quality of the converted power ..."

   There really is no power quality issue with a static converter.  In fact, calling it a converter is really a misnomer.  The three-phase motor is running single phase, a condition which is frequently monitored for and prevented in industrial machinery (because if the load is not reduced, the motor will burn up).  The motor is running on the 240V single-phase power coming out of the wall, with no other input from the converter, once it's started.  You only need to converter to get the motor started, which you could also do with a small pony motor, or a rope starter for that matter.  Once the motor is running, the converter is no longer in the loop.

   A typical rotary converter uses a 3-phase motor as a transformer to manufacture the (formerly) missing third leg, and the control box for the idler motor consists of a static converter to get the idler motor spinning.  When you build your own, all you're doing, at its simplest, is building a static converter to start the idler.  Better units use additional capacitors to balance the three legs.

   Unless you're going to work the jointer harder than 2 hp (that's a high rate of wood removal, IMO), I'd suggest just using it as-is.  You'll spend more money buying or building a rotary converter, or buying a VFD, that you will buying a single-phase 2 or 3 hp motor if and when the existing 3-phase unit stops working, which could be decades from now.  Remember also that a jointer works intermittently, with no-load running time between passes (but the cooling air is still flowing), and likely at less than the motor's full load capability when it is cutting.  The motor is continuous duty rated, so it has a fairly easy life to start with.

   In this case, I'd say that you're ahead of the game by doing nothing.Be seeing you...

2. 
   Jamie_Buxton | Feb 17, 2004 07:21pm | #8

   Charlie --

   A "static" converter has no active circuitry in it. It uses a capacitor to phase-shift one of the supply legs to provide the third leg to the machine. A drawback to the static converter is that the size of the capacitor must be matched to the load, so that one adjusted to fit one machine may need to be re-adjusted to fit another. However, it is the least expensive way to make three-phase from single phase. As one of the other posters says, if you've got a static converter already, and the machine is running okay, why mess with it?

   The converter chosen by many now is called a VFD, for Variable Frequency Drive. It is an "inverter" on steroids. It ingests single-phase AC, turns it into DC, and chops that up to produce 3-phase AC to run your machines. It contains lots of active circuitry. One of the convenient things about it is that the output frequency does not have to be 60 Hz. It can be anything, hence the name.

   Bought new, VFDs are now less expensive than rotating converters, and only a little more expensive than statics. They also don't require any adjustment, which is an advantage over statics.

   Because the output waveform of a VFD is assembled by active circuitry, it has the potential for higher harmonics than 60 Hz. However, motors don't really care if that stuff comes at them, and most VFD manufacturers don't spec the harmonic content.

3. 
   rslatm | Feb 17, 2004 05:33pm | #4

   A static simply gives it a third leg to start the machine. As soon as it's running a static cuts out as it has done it's job. You get about 1/3 less power because it's only running on two legs. A rotary gives full power because you get all three legs of power continuously.  A static is fine for hobby folks as you don't notice the power loss unless you are pushing the limits of the motor.  I disagree with Jim as inverters have certain applications and rotaries have certain  applications. Rotary can run multiple machines with different hp ratings with one set up.  The inverter is great to make a machine variable speed  and can actually double the motors rated rpm as well as soft start and a bunch of other things. Inverters will never replace rotary converters. Been dealing with phase conversion for over twenty years and cannot begin to summarize the details in a paragraph. Not an engineer but know who to talk to to get the proper answers.