how to generate 3-phase 208vac from single-phase 208vac motor 2

[archaicaudio]

can i wire a 3phase motor to a single phase 208vac circuit and generate an ac output of 208vac 3-phase?

 

[waross]

Google rotary converter. But, if single phase 208 volts is available, you must be close to a three phase source. This is a standard three phase voltage but not a standard single phase voltage. In apartment buildings, single phase services for individual occupancies may be connected across two phases of a three phase service. This will give 208 volts, but it is better described as two phases of a 120/208 V three phase supply than as 208 V single phase.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

You can also use a VFD that will accept single phase input and provide three phase output.

 

[waross]

Good solution, David.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Marke]

Hello archaicaudio

Correct me if I am wrong, but the way that I read your question is that you have 208 volt single phase available and you also have a 208 volt three phase motor available and you wish to use that to convert the single phase to three phase.

You can do this provided that you have the motor spinning at the correct speed. Once the motor is up to speed, it will continue to run at full speed on a single phase supply provided that the load is not high.
If a three phase motor is operating at full speed and has two phases applied (essentially single phase across two terminals) it will regenerate the third phase out the third terminal.
If you were to couple the three phase motor to a single phase motor, and connect the 208 volt across two terminals on the three phase motor, you would have a single to three phase rotary converter.
Is that what you were looking for??

Best regards,

Mark Empson
L M Photonics Ltd

 

[ControlThis]

Waross, as far as what I've seen, most 208vac single phase sevice to residential buildings is split phase not coming from 2 phases but one. The secondary coil in the transformer on the pole is split in the middle by the neutral. Neutral to one end of the coil would give you the 120 and end of coil to other end of coil would give you a diffeerenc in potential of roughly 208v depending on how far the transformer is from the house. Maybe in some instances they just grab 2 phases, but this is what I've seen.

 

[davidbeach]

Nope, 208 is from two windings, 120V to neutral from each "hot". Single winding with neutral at the midpoint is 240V with 120V to neutral from each "hot".

 

[roydm]

There is also a way of running 3 phase motor from single phase using a capacitor however I would go with David, much better solution. I did this for a friend who wanted to run 3 phase tools in his hobby shop from 240 single phase. He used a VFD large enough for the largest load figuring he could only use one thing at a time. Each of his tools has it's own O/L protection, contactor etc.
Roy

 

[waross]

Hi ControlThis,
If you see 208 volts on single phase, call the utility and complain. Single phase is 120 volts and two phase or three phase is 208 volts.
But, large apartment buildings and commercial buildings have three phase services.
200 amps is often the limit for single phase services but you may find the odd 400 amp single phase service. Finding single phase 400 amp service equipment may be a challenge.
Commercial buildings will have three phase sub panels but apartments often have single phase sub panels running on two phases at 120/208 vols. You will encounter this one day, and you will now be able to recognize and understand the system when you see it.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ControlThis]

With all due respect, the 208 in most homes is single phase. If you looked at each of the sine waves, the phases would not be offset in time. You can have 208 with single phase. Hence the reason you see capacitors on the side of most appliance motors in the home to create the phase shift that will let these motors run at the 208 single phase voltage.

In this split phase case, maybe it's going through 2 separate windings, but in series they act like one.

 

[davidbeach]

208V in "most homes" is a severe voltage sag. Residential service for "most homes" is 240V. 208V single phase for residential service is generally limited to large multi-dwelling projects where service from the utility is 208Y/120V and then each unit gets two phases, A-B, B-C, or C-A. These are from two of the three secondary windings. Certainly, when you look at the voltage from A-B (or the others) you will see a single sine wave, in that regard it is single phase. But if you look at the two voltages to neutral you will find that they are not 180[°] from each other. In the case of 240V single phase service, when you look at the two phase voltages to neutral you will find them to be 180[°] from each other.

 

[waross]

Well said DavidBeach.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[archaicaudio]

thanks for tips and info-after reading about rotary converters-i think i'll buy me a fancy store bought one-instead of rigging up something that will fry my techron 3kw rms audio amps that i picked up. maybe a 6kw or 7.5kw should work for the pair of mono amps. i'll put a wanted to buy ad on wash. dc area craigslist for a good used one.

 

[cswilson]

Building on davidbeach's points, to see how the 208VAC comes from two legs of a 3-phase supply with 120VAC in each leg, it is best to crunch the math. The voltage between two legs can be described as:

V = 120sin(Theta) - 120sin(Theta - 2Pi/3)
= 120sin(Theta) - 120[sin(Theta)cos(2Pi/3)-cos(Theta)sin(2Pi/3)
= 120sin(Theta) + 60sin(Theta) + 104cos(Theta)
= 180sin(Theta) + 104cos(Theta)
= 208sin(Theta - Pi/3)

Curt Wilson
Delta Tau Data Systems

 

[LionelHutz]

I see, you are most likely powering a 3-phase rectifier in the amplifier. There is a company in Canada called Mirus that makes a 1Q3 Lineator which takes in 1-phase power and will provide 3 phases of output and it is designed to feed a rectifier. Unfortunately, these things are pricy.

 

[waross]

You may consider looking for a high power single phase power supply the feed the DC supply of these amps. It may be easier and cheaper to convert the amps to single phase than to supply three phase.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Depending on what you're driving with the amps you could well be able to just run them on single phase and get away with it.

If you're only looking for 1.5kW RMS for an example.

Keith Cress
kcress -

http://www.flaminsystems.com