[TheForge] Fwd: Shop tools & layount (3-phase converter)
Ralph Sproul
[email protected]
Mon Jan 19 08:45:00 2004
Charles, Thanks for writing this reply to the converter topic.
I have one question. On the Static converters your mentioning the size of
the capacitors for starting and running loads. Does one have to "tune" the
third leg of a rotary phase converter with capacitors?
I'm asking as the store bought 7 1/2 HP rotary phase converter that was
included in my Iron worker package deal has a bank of capacitors in
it.........is this just for starting? or is this to bring the manufactured
leg up to acceptable voltages?.......or both?
I'm asking as I want to make a larger converter to run a 10 HP motor on a
self contained hammer......and I'm willing to give it a go with scrounged
parts and would like to know if I can do a bit better with some proper
capacitors added to the third line.
Ralph
----- Original Message -----
From: <[email protected]>
To: <[email protected]>
Sent: Sunday, January 18, 2004 9:37 PM
Subject: Re: [TheForge] Fwd: Shop tools & layount (3-phase converter)
> A couple of points.
>
> First, regarding pulling extra wire for three phase, not a bad idea if
> you really think you will use it, but it will be more expensive. Also,
> if you are building under a permit, the electrical inspector can get a
> little excited about extra unconnected wires in your boxes. If you are
> not an electrician and you explain it is for possible three phase later
> they will likely get even more excited. All depends on the inspector
> and the code you are operating under. I would suggest if this option
> interests you, wire your shop for 220, two hots, neutral and a ground
> for a select number of outlets. This is not the standard 20amp 220
> plug though. This will meet code and can be used later for three phase.
>
> Regarding three phase, the first thing you need to look at is the
> voltage for the three phase motors you are looking at. They typically
> are for 208 or 480. You will not have much luck with 480 volt motors on
> 220, you want to pick up only 208 volt motors. Next thing to look at
> is the winding type. There are Y wound and Delta wound. Delta wound
> are purported to work better as rotary phase convertors and to run
> better on "converted" single phase power.
>
> Next you have a variety of ways to "convert" single phase to three
> phase, all of them using inductors to shift current. None of them (
> with the exception of VFD) will run a three phase motor at its full
> efficiency, and generally they will be less efficient that a comparable
> quality single phase motor. Three phase motors are much more efficient
> than single phase motors, but only when powered by real three phase
> power. I know someone here has stated that a three phase motor
> running off of a rotary converter drew less current than the single
> phase that it replaced, but that is probably due to the fact that since
> three phase motors are almost always intended for industry, they tend to
> be better made and more efficient to begin with and you probably aren't
> pulling anywhere near nameplate hp. A comparable quality single phase
> motor will use less power running on single phase for the same torque/hp
> produced. The reason they are not going to run as well is because the
> power factor and phase separation is messed up. There are a lot of
> disadvantages to running three phase motors on single phase. The only
> advantages are: You can bump stop three phase motors, and often
> inductrial equipment comes with integral three phase motors that can't
> be easily replaced with standard single phase.
>
> Here are the ways of getting three phase motors to work.
>
> 1) Static Converter, which is made by capacitors to shift the incoming
> current and create an additional phase. This is is essentially the
> same method used for starting a typical single phase induction motor.
> Some single phase motors also use a "run" capacitor, which exactly
> mimics a static three phase converter. These are not as popular
> because they are a little more difficult to get set up, and it takes
> time to balance the size of capacitors to the load, particularily if the
> load varies. Finally, they won't allow you to pull full load
> continously, genrally recommending maximum of 75% of nameplate. I
> expect that this is the result of the capacitors screwing up the power
> factor, resulting in higher current draw. The advantage of them is
> they are compact and can be wired into the machine switch, so using the
> machine is as if it were ordinary single phase. This is better for air
> compressors and other equipment that runs automatically. I have seen a
> lot of different recommendations for sizing capacitors, but the first
> book I pulled down recommends 28mfd per HP for the start capacitor and
> 60 mfd per HP for the run capacitor.
>
> 2) Autotransformer. Not very popular outside of industry, but does a
> great job of simulating three phase. It basically uses capacitors to
> shift current ahead for one leg and an autoformer to shift it back for
> another leg. This gives three phases unlike the static converter and
> rotary converter that only create two phases.
>
> 3) Rotary Converter. This is the easiest. Basically, you use a three
> phase motor to generate a second phase. The generated second phase is
> generally weaker, and power factor is an issue, but it is usually
> sufficient and if you have multiple motors on the circuit, they pwer
> factor will be improved. You will still not get nameplate HP from the
> motors. To build a rotary phase convertor, you start with a delta wound
> three phase motor to act as the converter. Wire two of its motor leads
> to your 220 in. and run wires from these to the motor you want to
> run. The third power lead is run directly to the load motor. At this
> point, if you were to apply power to the converter it would just hum and
> eventually burn out, nuch like a single phase with a bad start capacitor
> or centrifigal switch. There are a couple of ways to fix address
> this. One, you can belt up a single phase motor to spin the converter
> on start up. This is my favorite way, since it results in the lowest
> starting current. I have actually used the start switch in the single
> phase motor to hold open a TPDT relay for the converter. When the start
> switch on the pony motor kicks out, the power kicks in on the converter
> motor and off for the pony motor. The second way to start it is with
> a rope, or if it is near the floor, spinning it with your foot.
> Remember to always spin it the same way, or the controls on your
> machines hooked to it will be reversed. Finally, you can put a start
> capacitor on it to start it that gets switched out when it is up to
> speed. This will have the highest starting current of the three.
> Once it is running, it works by inducing a current in the third winding
> using the residual magnetism in the rotor from the the other two
> windings. This necessarily means that the current and voltage in the
> third winding will be less than the other two. You can improve things
> by monitoring it with an amp meter, and adding a capacitor or two to
> balance the currents. This does not produce true three phase, it
> produces quasi unbalanced two phase with a hinky power factor.
>
> 4) VFD - Variable Frequency Device. This is a solid state device that
> uses high power semiconducters to create a very accurate three phase.
> It is also capable of controlling the shape of the waveform created as
> well as the frequency. These two capabilities combined allows it to
> control the speed of the motor exceptionally well. Once very expensive,
> they have considerably dropped in price. They generally are installed
> on a single machine due to the speed control capability. These make
> three phase motors an absolute joy. It is probable that they will
> result in a shift of many products to three phase motors under the
> covers. For example, using a VFD you could build a gearless washing
> machine.
>
> I have written up quite a bit on this and intend to publish it someday
> in my Blacksmiths Omnibus, otherwise known as "What you need to know now
> that you've made your hook in your barbecue forge" along with sections
> on pneumatics, hydraulics and basic stress analysis among other
things.....
>
>
> Charles
>
>
>
> Kenny O. wrote:
>
> >
> >
> >
> >
> > is there a way to change a three phase motor to be used with 220 ...
> > I see them much cheaper at the tool zoo? I also saw this thing
> > called a phase-a-matic which cost $150.00 . Its claim is to be a
> > "static phase converter". Has any one used these things or
> > attempted to make there own?
> > Thanks
> > Kenny o
> >
> >> On Friday, January 9, 2004, at 05:23 AM, Chuck Robinson wrote:
> >
> >
> > Bettina:
> > I would suggest that when you wire your shop , you pull an extra
> > wire for 3
> > phase power and just leave it un-connected in your electrical
> > boxes....
> >
> >
> >
> >
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