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3 phase generator from 3 phase motor?
- Thread starter dznit
- Start date
- Replies 26
- Views 11,472
What KIND of 3 phase motor?
An induction motor is one thing, and is not much like a generator, although it CAN generate, either when line-connected, OR independently.
A synchronous motor is essentially a generator operating as a motor, and so the differences are much less. And it obviously can generate
An induction motor is one thing, and is not much like a generator, although it CAN generate, either when line-connected, OR independently.
A synchronous motor is essentially a generator operating as a motor, and so the differences are much less. And it obviously can generate
peterh5322
Diamond
- Joined
- Dec 15, 2002
- Location
- Monterey Bay, California
Induction machine:
Can motor up to its full nameplate rating.
Can "generate", after a fashion, up to a percentage of its nameplate rating.
Always presents a lagging (i.e., inductive) load, and this load increases with the machine's rating.
Doesn't require an exciter, and there is no provision for one.
Operates with "slip" [ * ] , and there is no power transmitted unless there is slip.
Poor transient performance (but this is usually not a concern in a motor).
Synchronous machine:
Can motor up to its full nameplate rating.
Can generate up to its full nameplate rating.
Presents a lagging, leading (i.e., capacitive) or neither leading nor lagging (i.e., resistive) load.
Amount of lead or lag is continually and easily adjustable simply by varying the machine's excitation.
Operates with no "slip", and slip, should it occur, indicates a loss of synchronism, and can be catastrophic in a large generator.
Excellent transient performance up to the point of loss of stability (equal area criteria).
[ * ] "Slip" is the difference between the speed of the magnetic field in the stator (called the synchronous speed), and the speed of the induced field in the rotor. In an induction machine, the shaft, when motoring, turns at the speed of the induced field.
Can motor up to its full nameplate rating.
Can "generate", after a fashion, up to a percentage of its nameplate rating.
Always presents a lagging (i.e., inductive) load, and this load increases with the machine's rating.
Doesn't require an exciter, and there is no provision for one.
Operates with "slip" [ * ] , and there is no power transmitted unless there is slip.
Poor transient performance (but this is usually not a concern in a motor).
Synchronous machine:
Can motor up to its full nameplate rating.
Can generate up to its full nameplate rating.
Presents a lagging, leading (i.e., capacitive) or neither leading nor lagging (i.e., resistive) load.
Amount of lead or lag is continually and easily adjustable simply by varying the machine's excitation.
Operates with no "slip", and slip, should it occur, indicates a loss of synchronism, and can be catastrophic in a large generator.
Excellent transient performance up to the point of loss of stability (equal area criteria).
[ * ] "Slip" is the difference between the speed of the magnetic field in the stator (called the synchronous speed), and the speed of the induced field in the rotor. In an induction machine, the shaft, when motoring, turns at the speed of the induced field.
bdx
Cast Iron
- Joined
- Apr 3, 2007
- Location
- N. Ireland
3 phase engine driven generators will normally have a DC field.
Wind powered 3 phase generators are normally Synchronous machine
Mark
Wind powered 3 phase generators are normally Synchronous machine
Mark
peterh5322
Diamond
- Joined
- Dec 15, 2002
- Location
- Monterey Bay, California
Engine driven generators with dc fields are usually constructed as alternators, but these are still synchronous machines.
A synchronous machine does not have to have a dc field, or even a field at all, as there are configurations of armatures and solid rotors which will run at synchronous speed.
However, only a synchronous machine which has an adjustable field may operate as an inductor, a capacitor, or as a resistor.
A synchronous machine does not have to have a dc field, or even a field at all, as there are configurations of armatures and solid rotors which will run at synchronous speed.
However, only a synchronous machine which has an adjustable field may operate as an inductor, a capacitor, or as a resistor.
bdx
Cast Iron
- Joined
- Apr 3, 2007
- Location
- N. Ireland
Opps i stand corrected on the wind powered 3 phase generators are normally Asynchronous machine. Typo...lol
The reactive power generated by a synchronous machine can be adjusted by varying the field current.
And what? normally....
dznit - basically yes depending upon chosen motor type.
Mark
The reactive power generated by a synchronous machine can be adjusted by varying the field current.
And what? normally....
dznit - basically yes depending upon chosen motor type.
Mark
HelicalCut
Stainless
- Joined
- Apr 9, 2007
- Location
- Melbourne Australia
dznit if it is a portable unit forget the induction motor, these generate very poorly if at all in stand alone situations. Also you will be lucky to find a synchronous motor that small they do exist but I have never seen one, 3 phase alternators are much more common than small (5 -20 hp) synchronous motors.
bdx wind generators are often synchronous, either sychronised to line frequency or free running with some form of invertor to create line frequency. Permanent magnet units are synchronous.
bdx wind generators are often synchronous, either sychronised to line frequency or free running with some form of invertor to create line frequency. Permanent magnet units are synchronous.
peterh5322
Diamond
- Joined
- Dec 15, 2002
- Location
- Monterey Bay, California
"... either sychronised to line frequency ..."
Placing a synchronous machine across the line automatically ensures synchronism, as the machine will exchange "synchronizing power" with the bus until it is precisely on-frequency.
Thereafter, the phase of the machine will vary slightly, but the frequency will be spot-on.
Unless there is a slight phase difference, there can be no power interchange.
"Permanent magnet units are synchronous"
A permanent magnet is simulating a wound field, but as a permanent magnet field cannot be adjusted, it is not possible to achieve the same flexibility as with a wound field.
Placing a synchronous machine across the line automatically ensures synchronism, as the machine will exchange "synchronizing power" with the bus until it is precisely on-frequency.
Thereafter, the phase of the machine will vary slightly, but the frequency will be spot-on.
Unless there is a slight phase difference, there can be no power interchange.
"Permanent magnet units are synchronous"
A permanent magnet is simulating a wound field, but as a permanent magnet field cannot be adjusted, it is not possible to achieve the same flexibility as with a wound field.
I don't disagree on avoiding them, particularly if there are large inrush loads.
But they are VERY EXTENSIVELY used in remote locations for water power electric in South America. generally for lighting, which is low inrush for any individual light.
High inrush will cause the field to drop, and stop generation.
jim rozen
Diamond
- Joined
- Feb 26, 2004
- Location
- peekskill, NY
Hmm, so the installation would be, a remote
area with a water wheel or pelton wheel,
coupled to an induction motor rigged as a
generator?
Sounds like a very grass-roots solution.
Jim
area with a water wheel or pelton wheel,
coupled to an induction motor rigged as a
generator?
Sounds like a very grass-roots solution.
Jim
jim rozen
Diamond
- Joined
- Feb 26, 2004
- Location
- peekskill, NY
Do they do the trick with a capacitor, to
get some excitation to build up?
Jim
get some excitation to build up?
Jim
The capacitor is a "load" that will do it, but any load will.
Th real trick is residual magnetism in the coils, which is required to get rotor current and cause it to "build up".
After that, load current produces rotor current, which induces more load current until it has "built up". That's the deal the capacitors do, they provide a "wattless" (reactive) local load that provides a minimum load current and so maintains output. They also cause a phase shift (leading current) that can assist in generation.
Somewhere I have a link to a paper that the government sponsored which analyzes the whole deal. I don't remember the whole analysis.
Th real trick is residual magnetism in the coils, which is required to get rotor current and cause it to "build up".
After that, load current produces rotor current, which induces more load current until it has "built up". That's the deal the capacitors do, they provide a "wattless" (reactive) local load that provides a minimum load current and so maintains output. They also cause a phase shift (leading current) that can assist in generation.
Somewhere I have a link to a paper that the government sponsored which analyzes the whole deal. I don't remember the whole analysis.
Jraef
Titanium
- Joined
- Aug 10, 2004
- Location
- San Francisco Bay Area
The trick to that however is the residual magnetism, which is not universal in motor designs and in fact is somewhat undesirable when used as a motor. So picking up a standard induction motor off the shelf somewhere may or may not work as an induction generator in an island mode (island means not connected to a grid). You pretty much have to test it out first. And if that "load" which is supplying the reactive current ever goes away, i.e. someone shuts off something they shouldn't have, you can loose generation.
Still, one of my long term projects here at home is to rig up a little 1/2HP 230V 3 phase motor I have laying around into being a wind generator, probably using a Savonius turbine. If I had my druthers though, I'd like to try an AeroTurbine!
http://www.youtube.com/watch?v=iPuWSBYMWqg
Still, one of my long term projects here at home is to rig up a little 1/2HP 230V 3 phase motor I have laying around into being a wind generator, probably using a Savonius turbine. If I had my druthers though, I'd like to try an AeroTurbine!
http://www.youtube.com/watch?v=iPuWSBYMWqg
HelicalCut
Stainless
- Joined
- Apr 9, 2007
- Location
- Melbourne Australia
The point I was making was that the generators output frequency is locked to shaft speed or vice versa unlike an induction generator which has slip. One of the fun things for me when I was a kid was pulling the switch when the 3 lights went out to put a genset on line
JST thanks for the info on the induction generators, I was never sure if there was enough residual magnetism in an induction motor to get it to generate reliably and the comment about the capacitors answered any question I may have had about a load. Frequency stability would be interesting unless the governer was controlled by frequency rather than shaft speed. I suppose a simple feedforward system such as a line current driven solenoid on the governor bar would work OK as well.
Now the cost issue may be a thing in 3rd countries but many generators end up as scrap in Australia and they have the same scrap value per ton as motors (they weigh about 30% more per KVa though) so cost shouldn't be an issue.
All intersting!
So where would this 3 phase traction motor fall: induction / syn or ? (For Ford Ranger EV that has regenerative drive)
http://cgi.ebay.com/ebaymotors/Ford-Siemens-EV-Electric-Vehicle-New-Crate-Motor_W0QQcmdZViewItemQQcategoryZ33615QQihZ013QQitemZ230158790449QQrdZ1QQsspagenameZWDVW
So where would this 3 phase traction motor fall: induction / syn or ? (For Ford Ranger EV that has regenerative drive)
http://cgi.ebay.com/ebaymotors/Ford-Siemens-EV-Electric-Vehicle-New-Crate-Motor_W0QQcmdZViewItemQQcategoryZ33615QQihZ013QQitemZ230158790449QQrdZ1QQsspagenameZWDVW
peterh5322
Diamond
- Joined
- Dec 15, 2002
- Location
- Monterey Bay, California
"The point I was making was that the generators output frequency is locked to shaft speed or vice versa"
It's locked to the system frequency.
If the shaft speed is below the system frequency, then the system will force the generator to increase its shaft speed, or the system frequency will fall.
If the shaft speed is above the system frequency, then the system frequency will rise, or the system will force the generator to reduce its shaft speed.
It is possible to control the system frequency of a very large system, such as the Western System, by using a single unit at Hoover.
It's locked to the system frequency.
If the shaft speed is below the system frequency, then the system will force the generator to increase its shaft speed, or the system frequency will fall.
If the shaft speed is above the system frequency, then the system frequency will rise, or the system will force the generator to reduce its shaft speed.
It is possible to control the system frequency of a very large system, such as the Western System, by using a single unit at Hoover.
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