|
Alan,
It has to do with efficiency, cost, complexity, etc. It was not a casual decision. If one uses a delta configuration, instead of a Y, then you only need three leads, not four. Evan |
why 3-phase power?
Alan,
It has to do with efficiency, cost, complexity, etc. It was not a casual decision. If one uses a delta configuration, instead of a Y, then you only need three leads, not four. Evan |
"K9SQG" wrote in message ... Alan, It has to do with efficiency, cost, complexity, etc. It was not a casual decision. If one uses a delta configuration, instead of a Y, then you only need three leads, not four. Unless you are tapping for 120 on a 240 Delta or 240 on a 480 Delta, in which case a center tap of any of the phases becomes Neutral for a split phase Edison circuit. |
"K9SQG" wrote in message ... Alan, It has to do with efficiency, cost, complexity, etc. It was not a casual decision. If one uses a delta configuration, instead of a Y, then you only need three leads, not four. Unless you are tapping for 120 on a 240 Delta or 240 on a 480 Delta, in which case a center tap of any of the phases becomes Neutral for a split phase Edison circuit. |
On 31 Dec 2003 00:48:51 GMT, K9SQG wrote:
It has to do with efficiency, cost, complexity, etc. It was not a casual decision. If one uses a delta configuration, instead of a Y, then you only need three leads, not four. IIRC (it's been 50 years since I studied this) a Y-Y circuit has problems with in-phase circulating third harmonic currents flowing on the neutral. -- 73 de K2ASP - Phil Kane P.E. |
On 31 Dec 2003 00:48:51 GMT, K9SQG wrote:
It has to do with efficiency, cost, complexity, etc. It was not a casual decision. If one uses a delta configuration, instead of a Y, then you only need three leads, not four. IIRC (it's been 50 years since I studied this) a Y-Y circuit has problems with in-phase circulating third harmonic currents flowing on the neutral. -- 73 de K2ASP - Phil Kane P.E. |
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Bill wrote in message ...
wrote: In sci.electronics.basics Rob Paisley wrote: | For larger horsepower motors, Three Phases means that they will | start with no external mechanical assistance. And, as I understand it, they can then "stand on one leg" although they will be a bit "shaky". You can also reverse the rotation by swapping two of the phases. If one phase is blown you can start it by giving it a spin in the right direction, 'without' getting your hand caught Bill Baka I am not sure if a 3 phase motor would even rotate unloaded on 2 phases but it certainly would not develop a usable amount of horsepower. I would suspect that running on 1 phase would be out of the question. I do know that if a loaded 3 phase motor blows a fuse it will almost always blow a fuse in one of the other lines due to an overload. Due to the way the control power for the motor is connected the starter will then open the circuit if it did not already do so when the first fuse failed. Also, there is a number of 1.57 being bandied about in this thread for the relative horse power of a 3 phase versus single motor for a given current. This number should be 1.73 (The square root of 3. Rob. |
Bill wrote in message ...
wrote: In sci.electronics.basics Rob Paisley wrote: | For larger horsepower motors, Three Phases means that they will | start with no external mechanical assistance. And, as I understand it, they can then "stand on one leg" although they will be a bit "shaky". You can also reverse the rotation by swapping two of the phases. If one phase is blown you can start it by giving it a spin in the right direction, 'without' getting your hand caught Bill Baka I am not sure if a 3 phase motor would even rotate unloaded on 2 phases but it certainly would not develop a usable amount of horsepower. I would suspect that running on 1 phase would be out of the question. I do know that if a loaded 3 phase motor blows a fuse it will almost always blow a fuse in one of the other lines due to an overload. Due to the way the control power for the motor is connected the starter will then open the circuit if it did not already do so when the first fuse failed. Also, there is a number of 1.57 being bandied about in this thread for the relative horse power of a 3 phase versus single motor for a given current. This number should be 1.73 (The square root of 3. Rob. |
How would you hook up a 3-phase motor to run on 2 phases?
How would you hook up a 3 phase motor to run on one phase? I don't care whether they work or not, I just want to see the wiring diagram. Thanks in advance. "Rob Paisley" wrote in message om... Bill wrote in message ... wrote: In sci.electronics.basics Rob Paisley wrote: | For larger horsepower motors, Three Phases means that they will | start with no external mechanical assistance. And, as I understand it, they can then "stand on one leg" although they will be a bit "shaky". You can also reverse the rotation by swapping two of the phases. If one phase is blown you can start it by giving it a spin in the right direction, 'without' getting your hand caught Bill Baka I am not sure if a 3 phase motor would even rotate unloaded on 2 phases but it certainly would not develop a usable amount of horsepower. I would suspect that running on 1 phase would be out of the question. I do know that if a loaded 3 phase motor blows a fuse it will almost always blow a fuse in one of the other lines due to an overload. Due to the way the control power for the motor is connected the starter will then open the circuit if it did not already do so when the first fuse failed. Also, there is a number of 1.57 being bandied about in this thread for the relative horse power of a 3 phase versus single motor for a given current. This number should be 1.73 (The square root of 3. Rob. |
How would you hook up a 3-phase motor to run on 2 phases?
How would you hook up a 3 phase motor to run on one phase? I don't care whether they work or not, I just want to see the wiring diagram. Thanks in advance. "Rob Paisley" wrote in message om... Bill wrote in message ... wrote: In sci.electronics.basics Rob Paisley wrote: | For larger horsepower motors, Three Phases means that they will | start with no external mechanical assistance. And, as I understand it, they can then "stand on one leg" although they will be a bit "shaky". You can also reverse the rotation by swapping two of the phases. If one phase is blown you can start it by giving it a spin in the right direction, 'without' getting your hand caught Bill Baka I am not sure if a 3 phase motor would even rotate unloaded on 2 phases but it certainly would not develop a usable amount of horsepower. I would suspect that running on 1 phase would be out of the question. I do know that if a loaded 3 phase motor blows a fuse it will almost always blow a fuse in one of the other lines due to an overload. Due to the way the control power for the motor is connected the starter will then open the circuit if it did not already do so when the first fuse failed. Also, there is a number of 1.57 being bandied about in this thread for the relative horse power of a 3 phase versus single motor for a given current. This number should be 1.73 (The square root of 3. Rob. |
On Sat, 3 Jan 2004 09:30:19 -0500, the renowned "BFoelsch"
wrote: How would you hook up a 3-phase motor to run on 2 phases? A 3-phase motor has 3 wires. If you break one wire, it's running on single phase, not two. Break two wires and it tends to just sit there. How would you hook up a 3 phase motor to run on one phase? I don't care whether they work or not, I just want to see the wiring diagram. Single phase applied to two of the three wires. It works better with capacitors to the third (otherwise unconnected) wire. The motor won't start like this, you either need to give it a spin or use another (starting) capacitor and some switching. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com |
On Sat, 3 Jan 2004 09:30:19 -0500, the renowned "BFoelsch"
wrote: How would you hook up a 3-phase motor to run on 2 phases? A 3-phase motor has 3 wires. If you break one wire, it's running on single phase, not two. Break two wires and it tends to just sit there. How would you hook up a 3 phase motor to run on one phase? I don't care whether they work or not, I just want to see the wiring diagram. Single phase applied to two of the three wires. It works better with capacitors to the third (otherwise unconnected) wire. The motor won't start like this, you either need to give it a spin or use another (starting) capacitor and some switching. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com |
Aww, you sprung my trap!
The previous poster was expostulating on running the motor on 3, 2 and 1 phase. I was waiting for him to show me the difference between the 2 phase and single phase connections! Same basic concept as phase rotation in a 3 phase system. You and I think there are only two, but there are really many; ABC, BAC, CBA, ACB etc.etc. (Facetious mode turned OFF) I spent many years in an old manufacturing city in the Northeast, where we had, in common use on a daily basis, AC (25 and 60 Hz), DC, every possible permutation of 3 phase, 2 phase 3,4 and 5 wire, and everything in between. My absolute favorite was the 6 phase double delta. Picture a straight 220 (YES, 220, not 240) delta connection. Now, center tap all the transformers, and these midpoints give you another delta at 110 for the lighting load. Of course, the whole thing is ungrounded. Yes, all the light switches in the building were 2 pole. Yes, the small motors were 110 volt 3 phase. This system gives you 2 three-phase voltages out of one set of transformers, The power factor transforms at unity, unlike many connections. Only thing is, it is very had to protect the transformers unless you use 6 primary fuses. Years ago, maintenance electricians had to be aware of a lot of varied systems. Today, life is a whole lot simpler. Thanks in advance. "Spehro Pefhany" wrote in message ... On Sat, 3 Jan 2004 09:30:19 -0500, the renowned "BFoelsch" wrote: How would you hook up a 3-phase motor to run on 2 phases? A 3-phase motor has 3 wires. If you break one wire, it's running on single phase, not two. Break two wires and it tends to just sit there. How would you hook up a 3 phase motor to run on one phase? I don't care whether they work or not, I just want to see the wiring diagram. Single phase applied to two of the three wires. It works better with capacitors to the third (otherwise unconnected) wire. The motor won't start like this, you either need to give it a spin or use another (starting) capacitor and some switching. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com |
Aww, you sprung my trap!
The previous poster was expostulating on running the motor on 3, 2 and 1 phase. I was waiting for him to show me the difference between the 2 phase and single phase connections! Same basic concept as phase rotation in a 3 phase system. You and I think there are only two, but there are really many; ABC, BAC, CBA, ACB etc.etc. (Facetious mode turned OFF) I spent many years in an old manufacturing city in the Northeast, where we had, in common use on a daily basis, AC (25 and 60 Hz), DC, every possible permutation of 3 phase, 2 phase 3,4 and 5 wire, and everything in between. My absolute favorite was the 6 phase double delta. Picture a straight 220 (YES, 220, not 240) delta connection. Now, center tap all the transformers, and these midpoints give you another delta at 110 for the lighting load. Of course, the whole thing is ungrounded. Yes, all the light switches in the building were 2 pole. Yes, the small motors were 110 volt 3 phase. This system gives you 2 three-phase voltages out of one set of transformers, The power factor transforms at unity, unlike many connections. Only thing is, it is very had to protect the transformers unless you use 6 primary fuses. Years ago, maintenance electricians had to be aware of a lot of varied systems. Today, life is a whole lot simpler. Thanks in advance. "Spehro Pefhany" wrote in message ... On Sat, 3 Jan 2004 09:30:19 -0500, the renowned "BFoelsch" wrote: How would you hook up a 3-phase motor to run on 2 phases? A 3-phase motor has 3 wires. If you break one wire, it's running on single phase, not two. Break two wires and it tends to just sit there. How would you hook up a 3 phase motor to run on one phase? I don't care whether they work or not, I just want to see the wiring diagram. Single phase applied to two of the three wires. It works better with capacitors to the third (otherwise unconnected) wire. The motor won't start like this, you either need to give it a spin or use another (starting) capacitor and some switching. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com |
Oops, should have said that this connection needed 6 SECONDARY fuses.
Sorry about that. "BFoelsch" wrote in message ... Aww, you sprung my trap! The previous poster was expostulating on running the motor on 3, 2 and 1 phase. I was waiting for him to show me the difference between the 2 phase and single phase connections! Same basic concept as phase rotation in a 3 phase system. You and I think there are only two, but there are really many; ABC, BAC, CBA, ACB etc.etc. (Facetious mode turned OFF) I spent many years in an old manufacturing city in the Northeast, where we had, in common use on a daily basis, AC (25 and 60 Hz), DC, every possible permutation of 3 phase, 2 phase 3,4 and 5 wire, and everything in between. My absolute favorite was the 6 phase double delta. Picture a straight 220 (YES, 220, not 240) delta connection. Now, center tap all the transformers, and these midpoints give you another delta at 110 for the lighting load. Of course, the whole thing is ungrounded. Yes, all the light switches in the building were 2 pole. Yes, the small motors were 110 volt 3 phase. This system gives you 2 three-phase voltages out of one set of transformers, The power factor transforms at unity, unlike many connections. Only thing is, it is very had to protect the transformers unless you use 6 primary fuses. Years ago, maintenance electricians had to be aware of a lot of varied systems. Today, life is a whole lot simpler. Thanks in advance. "Spehro Pefhany" wrote in message ... On Sat, 3 Jan 2004 09:30:19 -0500, the renowned "BFoelsch" wrote: How would you hook up a 3-phase motor to run on 2 phases? A 3-phase motor has 3 wires. If you break one wire, it's running on single phase, not two. Break two wires and it tends to just sit there. How would you hook up a 3 phase motor to run on one phase? I don't care whether they work or not, I just want to see the wiring diagram. Single phase applied to two of the three wires. It works better with capacitors to the third (otherwise unconnected) wire. The motor won't start like this, you either need to give it a spin or use another (starting) capacitor and some switching. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com |
Oops, should have said that this connection needed 6 SECONDARY fuses.
Sorry about that. "BFoelsch" wrote in message ... Aww, you sprung my trap! The previous poster was expostulating on running the motor on 3, 2 and 1 phase. I was waiting for him to show me the difference between the 2 phase and single phase connections! Same basic concept as phase rotation in a 3 phase system. You and I think there are only two, but there are really many; ABC, BAC, CBA, ACB etc.etc. (Facetious mode turned OFF) I spent many years in an old manufacturing city in the Northeast, where we had, in common use on a daily basis, AC (25 and 60 Hz), DC, every possible permutation of 3 phase, 2 phase 3,4 and 5 wire, and everything in between. My absolute favorite was the 6 phase double delta. Picture a straight 220 (YES, 220, not 240) delta connection. Now, center tap all the transformers, and these midpoints give you another delta at 110 for the lighting load. Of course, the whole thing is ungrounded. Yes, all the light switches in the building were 2 pole. Yes, the small motors were 110 volt 3 phase. This system gives you 2 three-phase voltages out of one set of transformers, The power factor transforms at unity, unlike many connections. Only thing is, it is very had to protect the transformers unless you use 6 primary fuses. Years ago, maintenance electricians had to be aware of a lot of varied systems. Today, life is a whole lot simpler. Thanks in advance. "Spehro Pefhany" wrote in message ... On Sat, 3 Jan 2004 09:30:19 -0500, the renowned "BFoelsch" wrote: How would you hook up a 3-phase motor to run on 2 phases? A 3-phase motor has 3 wires. If you break one wire, it's running on single phase, not two. Break two wires and it tends to just sit there. How would you hook up a 3 phase motor to run on one phase? I don't care whether they work or not, I just want to see the wiring diagram. Single phase applied to two of the three wires. It works better with capacitors to the third (otherwise unconnected) wire. The motor won't start like this, you either need to give it a spin or use another (starting) capacitor and some switching. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com |
On Sat, 3 Jan 2004 10:34:35 -0500, "BFoelsch"
wrote: My absolute favorite was the 6 phase double delta. Picture a straight 220 (YES, 220, not 240) delta connection. Now, center tap all the transformers, and these midpoints give you another delta at 110 for the lighting load. Of course, the whole thing is ungrounded. Yes, all the light switches in the building were 2 pole. Yes, the small motors were 110 volt 3 phase. This system gives you 2 three-phase voltages out of one set of transformers, The power factor transforms at unity, unlike many connections. Only thing is, it is very had to protect the transformers unless you use 6 primary fuses. Wow. If it was delta, then 110 3-phase would require 6 wires (or five if you cheat and share one corner) so the 110 3-phase motor would need three isolated windings and most likely a 6-pole on/off switch. Was it really like that? Might it have been wired wye with one central common/neutral? That seems more logical, but then maybe it wasn't very logical. Years ago, maintenance electricians had to be aware of a lot of varied systems. Today, life is a whole lot simpler. The thing that always amazes me is that buildings are wired in the most bizarre ways, and no documentation is left behind. If you call in an electrician, they figure it out somehow, kluge it some more, and leave. John |
On Sat, 3 Jan 2004 10:34:35 -0500, "BFoelsch"
wrote: My absolute favorite was the 6 phase double delta. Picture a straight 220 (YES, 220, not 240) delta connection. Now, center tap all the transformers, and these midpoints give you another delta at 110 for the lighting load. Of course, the whole thing is ungrounded. Yes, all the light switches in the building were 2 pole. Yes, the small motors were 110 volt 3 phase. This system gives you 2 three-phase voltages out of one set of transformers, The power factor transforms at unity, unlike many connections. Only thing is, it is very had to protect the transformers unless you use 6 primary fuses. Wow. If it was delta, then 110 3-phase would require 6 wires (or five if you cheat and share one corner) so the 110 3-phase motor would need three isolated windings and most likely a 6-pole on/off switch. Was it really like that? Might it have been wired wye with one central common/neutral? That seems more logical, but then maybe it wasn't very logical. Years ago, maintenance electricians had to be aware of a lot of varied systems. Today, life is a whole lot simpler. The thing that always amazes me is that buildings are wired in the most bizarre ways, and no documentation is left behind. If you call in an electrician, they figure it out somehow, kluge it some more, and leave. John |
"John Larkin" wrote in message ... Wow. If it was delta, then 110 3-phase would require 6 wires (or five if you cheat and share one corner) so the 110 3-phase motor would need three isolated windings and most likely a 6-pole on/off switch. Was it really like that? Nope. Draw a picture of a Delta, center-tap each winding, and the three center taps form another delta at half the voltage. Used a perfectly standard (for 1922) 3 phase, 3 wire, 110v, 25Hz motor. The 110 volt service made no connection to the corners of the 220 delta. The 110 delta was fed only from the center taps. |
"John Larkin" wrote in message ... Wow. If it was delta, then 110 3-phase would require 6 wires (or five if you cheat and share one corner) so the 110 3-phase motor would need three isolated windings and most likely a 6-pole on/off switch. Was it really like that? Nope. Draw a picture of a Delta, center-tap each winding, and the three center taps form another delta at half the voltage. Used a perfectly standard (for 1922) 3 phase, 3 wire, 110v, 25Hz motor. The 110 volt service made no connection to the corners of the 220 delta. The 110 delta was fed only from the center taps. |
On Sat, 3 Jan 2004 14:10:54 -0500, "BFoelsch"
wrote: "John Larkin" wrote in message ... Wow. If it was delta, then 110 3-phase would require 6 wires (or five if you cheat and share one corner) so the 110 3-phase motor would need three isolated windings and most likely a 6-pole on/off switch. Was it really like that? Nope. Draw a picture of a Delta, center-tap each winding, and the three center taps form another delta at half the voltage. Used a perfectly standard (for 1922) 3 phase, 3 wire, 110v, 25Hz motor. Oh, yeah, OK. New Orleans is mostly below sea level, and the Mississippi runs along most of it and peaks way above in the spring. Up to fairly recently, New Orleans was drained by ancient pumps with ancient switchgear, running on 25 Hz, and all the generators that made the 25 Hz were (of course) in the city, below sea level too. John |
On Sat, 3 Jan 2004 14:10:54 -0500, "BFoelsch"
wrote: "John Larkin" wrote in message ... Wow. If it was delta, then 110 3-phase would require 6 wires (or five if you cheat and share one corner) so the 110 3-phase motor would need three isolated windings and most likely a 6-pole on/off switch. Was it really like that? Nope. Draw a picture of a Delta, center-tap each winding, and the three center taps form another delta at half the voltage. Used a perfectly standard (for 1922) 3 phase, 3 wire, 110v, 25Hz motor. Oh, yeah, OK. New Orleans is mostly below sea level, and the Mississippi runs along most of it and peaks way above in the spring. Up to fairly recently, New Orleans was drained by ancient pumps with ancient switchgear, running on 25 Hz, and all the generators that made the 25 Hz were (of course) in the city, below sea level too. John |
Clever, clever, delta, New Orleans.........................
And who said there's no wit here! Yeah, 25 Hz has kind of pooped out recently. It is still generated on the Canadian side of Niagara Falls, but it is all converted to 60. Across the river in Buffalo, the utility still provides 25 Hz to about 50 customers, but they have announced its termination as of 2007. There are a few isolated paper mills that make their own, I believe that Con Edison makes a little, and there is one isolated pocket in Iowa, but that's all I know of that's left. I remember when 60, 50 and 25 cycle motors were all stocked items! Thanks in advance. "John Larkin" wrote in message ... On Sat, 3 Jan 2004 14:10:54 -0500, "BFoelsch" wrote: "John Larkin" wrote in message ... Wow. If it was delta, then 110 3-phase would require 6 wires (or five if you cheat and share one corner) so the 110 3-phase motor would need three isolated windings and most likely a 6-pole on/off switch. Was it really like that? Nope. Draw a picture of a Delta, center-tap each winding, and the three center taps form another delta at half the voltage. Used a perfectly standard (for 1922) 3 phase, 3 wire, 110v, 25Hz motor. Oh, yeah, OK. New Orleans is mostly below sea level, and the Mississippi runs along most of it and peaks way above in the spring. Up to fairly recently, New Orleans was drained by ancient pumps with ancient switchgear, running on 25 Hz, and all the generators that made the 25 Hz were (of course) in the city, below sea level too. John |
Clever, clever, delta, New Orleans.........................
And who said there's no wit here! Yeah, 25 Hz has kind of pooped out recently. It is still generated on the Canadian side of Niagara Falls, but it is all converted to 60. Across the river in Buffalo, the utility still provides 25 Hz to about 50 customers, but they have announced its termination as of 2007. There are a few isolated paper mills that make their own, I believe that Con Edison makes a little, and there is one isolated pocket in Iowa, but that's all I know of that's left. I remember when 60, 50 and 25 cycle motors were all stocked items! Thanks in advance. "John Larkin" wrote in message ... On Sat, 3 Jan 2004 14:10:54 -0500, "BFoelsch" wrote: "John Larkin" wrote in message ... Wow. If it was delta, then 110 3-phase would require 6 wires (or five if you cheat and share one corner) so the 110 3-phase motor would need three isolated windings and most likely a 6-pole on/off switch. Was it really like that? Nope. Draw a picture of a Delta, center-tap each winding, and the three center taps form another delta at half the voltage. Used a perfectly standard (for 1922) 3 phase, 3 wire, 110v, 25Hz motor. Oh, yeah, OK. New Orleans is mostly below sea level, and the Mississippi runs along most of it and peaks way above in the spring. Up to fairly recently, New Orleans was drained by ancient pumps with ancient switchgear, running on 25 Hz, and all the generators that made the 25 Hz were (of course) in the city, below sea level too. John |
I read in sci.electronics.design that John Larkin jjlarkin@highlandSNIP
techTHISnologyPLEASE.com wrote (in ke7evvo4mvalsj6km5ja2u4di666ge0g49@ 4ax.com) about 'why 3-phase power?', on Sat, 3 Jan 2004: Up to fairly recently, New Orleans was drained by ancient pumps with ancient switchgear, running on 25 Hz, and all the generators that made the 25 Hz were (of course) in the city, below sea level too. Naturally: 25 Hz is an unusually LOW frequency. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! |
I read in sci.electronics.design that John Larkin jjlarkin@highlandSNIP
techTHISnologyPLEASE.com wrote (in ke7evvo4mvalsj6km5ja2u4di666ge0g49@ 4ax.com) about 'why 3-phase power?', on Sat, 3 Jan 2004: Up to fairly recently, New Orleans was drained by ancient pumps with ancient switchgear, running on 25 Hz, and all the generators that made the 25 Hz were (of course) in the city, below sea level too. Naturally: 25 Hz is an unusually LOW frequency. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! |
Rob Paisley wrote:
Bill wrote in message ... wrote: In sci.electronics.basics Rob Paisley wrote: | For larger horsepower motors, Three Phases means that they will | start with no external mechanical assistance. And, as I understand it, they can then "stand on one leg" although they will be a bit "shaky". You can also reverse the rotation by swapping two of the phases. If one phase is blown you can start it by giving it a spin in the right direction, 'without' getting your hand caught Bill Baka I am not sure if a 3 phase motor would even rotate unloaded on 2 phases but it certainly would not develop a usable amount of horsepower. I would suspect that running on 1 phase would be out of the question. I do know that if a loaded 3 phase motor blows a fuse it will almost always blow a fuse in one of the other lines due to an overload. Due to the way the control power for the motor is connected the starter will then open the circuit if it did not already do so when the first fuse failed. Also, there is a number of 1.57 being bandied about in this thread for the relative horse power of a 3 phase versus single motor for a given current. This number should be 1.73 (The square root of 3. Rob. The motor I replaced was on an air conditioning blower and would rotate if given a boost before turning on the power, but as you said not with enough power. It just would not start without the third phase. The fix was to replace a 3/4 horse motor with a 1 horse motor in the same NEMA frame size. It was a 480 volt unit so we did not play much with it and just tossed it. Seems us engineer types prefer 5 volt stuff. Bill Baka |
Rob Paisley wrote:
Bill wrote in message ... wrote: In sci.electronics.basics Rob Paisley wrote: | For larger horsepower motors, Three Phases means that they will | start with no external mechanical assistance. And, as I understand it, they can then "stand on one leg" although they will be a bit "shaky". You can also reverse the rotation by swapping two of the phases. If one phase is blown you can start it by giving it a spin in the right direction, 'without' getting your hand caught Bill Baka I am not sure if a 3 phase motor would even rotate unloaded on 2 phases but it certainly would not develop a usable amount of horsepower. I would suspect that running on 1 phase would be out of the question. I do know that if a loaded 3 phase motor blows a fuse it will almost always blow a fuse in one of the other lines due to an overload. Due to the way the control power for the motor is connected the starter will then open the circuit if it did not already do so when the first fuse failed. Also, there is a number of 1.57 being bandied about in this thread for the relative horse power of a 3 phase versus single motor for a given current. This number should be 1.73 (The square root of 3. Rob. The motor I replaced was on an air conditioning blower and would rotate if given a boost before turning on the power, but as you said not with enough power. It just would not start without the third phase. The fix was to replace a 3/4 horse motor with a 1 horse motor in the same NEMA frame size. It was a 480 volt unit so we did not play much with it and just tossed it. Seems us engineer types prefer 5 volt stuff. Bill Baka |
In sci.electronics.basics Spehro Pefhany wrote:
| On Sat, 3 Jan 2004 09:30:19 -0500, the renowned "BFoelsch" | wrote: | |How would you hook up a 3-phase motor to run on 2 phases? | | A 3-phase motor has 3 wires. If you break one wire, it's running on | single phase, not two. Break two wires and it tends to just sit there. 2 phase would not be the result of a failure of power source, but it could happen if the internal windings/circuit on one of the phases opened up. I've never seen that happen but I can't say it's impossible. -- ----------------------------------------------------------------------------- | Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ | | (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ | ----------------------------------------------------------------------------- |
In sci.electronics.basics Spehro Pefhany wrote:
| On Sat, 3 Jan 2004 09:30:19 -0500, the renowned "BFoelsch" | wrote: | |How would you hook up a 3-phase motor to run on 2 phases? | | A 3-phase motor has 3 wires. If you break one wire, it's running on | single phase, not two. Break two wires and it tends to just sit there. 2 phase would not be the result of a failure of power source, but it could happen if the internal windings/circuit on one of the phases opened up. I've never seen that happen but I can't say it's impossible. -- ----------------------------------------------------------------------------- | Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ | | (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ | ----------------------------------------------------------------------------- |
Why is three phase used?
Several people mentined the reason (motors) , but didn't go further. To get a motor to turn you have to generate a magnetic field that "turns". With three phase this can be done. The relative voltage/cirrent between two phases (lines) can each drive a winding and the result is a rotating magnetic field inside the motor. This will then "drag" the rotor around. With the single phase motor, this rotation must be made some other way. With the capacitor start/run motors, the cap provides some phase shift on a second winding giving a pseudo two phase supply and therefore some rotation in the field. The cap-start motors do then run on one phase when the cap is switched out. Thus the comments on "running a three phase motor on 2 phases (actually this is just one phase when one of the three lines is cut -- the "three phases" needed for the motor are not the three lines, but the phase between each TWO line-pairs...of which there are three pairs) . Shaded pole motors use a part of the field core which is separated from the main core.. You will see a single turn of large copper wire wound around a small section of the core. This "shorted turn" causes a lag in the field in that section of the core and a resulting rotation of the composite field. These are usually in small blowers like typically found in bathroom fans. Go to the hardweare store and look at one of these motors. Steve K;9;D:C:I .. |
Why is three phase used?
Several people mentined the reason (motors) , but didn't go further. To get a motor to turn you have to generate a magnetic field that "turns". With three phase this can be done. The relative voltage/cirrent between two phases (lines) can each drive a winding and the result is a rotating magnetic field inside the motor. This will then "drag" the rotor around. With the single phase motor, this rotation must be made some other way. With the capacitor start/run motors, the cap provides some phase shift on a second winding giving a pseudo two phase supply and therefore some rotation in the field. The cap-start motors do then run on one phase when the cap is switched out. Thus the comments on "running a three phase motor on 2 phases (actually this is just one phase when one of the three lines is cut -- the "three phases" needed for the motor are not the three lines, but the phase between each TWO line-pairs...of which there are three pairs) . Shaded pole motors use a part of the field core which is separated from the main core.. You will see a single turn of large copper wire wound around a small section of the core. This "shorted turn" causes a lag in the field in that section of the core and a resulting rotation of the composite field. These are usually in small blowers like typically found in bathroom fans. Go to the hardweare store and look at one of these motors. Steve K;9;D:C:I .. |
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