Can a 3 phase motor be somehow used as a generator?

Ignoramus26083 fired this volley in news:If6dnStz8dp0vRXPnZ2dnUVZ snipped-for-privacy@giganews.com:

Short answer, "No." Not unless it was a permanent-magnet motor or had a wound armature fed through slip rings or a commutator. There has to be something to excite the fields initially. Common AC motors use reverse- EMF in the armature to repel the fields, but the armature gets its excitation from the field windings.

So-called "universal" motors can act as generators, so long as they either retain some residual magnetism in their fields, or have a DC supply to provide excitation until they can supply their own through a rectifier.

Lloyd

Better answer, depends on the motor design.

Technically any motor can act as a generator, but efficiency will likely not be optimal.

Locomotives use their traction motors as generators to energize dynamic braking systems for example. If the motors and resistors were 100% efficient, they could stop a train without using brakes, but that isn't the case. They do generate quite a bit of energy though, which is dissipated as heat.

As you say above, something must excite the fields.

Pretty much any standard 3 phase induction motor can be used as a generator, provided that it has a load with a leading power factor (normally the controller will connect as much capacitance as required for this) or is paralleled with the utility line and driven above synchronous speed. Control is a bit tricky when not line connected, but a search on induction motor generator should lead you to information on how to do it. A wound rotor or permanent magnets are not required, unless you want to be able to generate power without using a complex controller.

You can find a fairly complete mathematical analysis in the chapter on induction generators in "Principles of Alternating-Current Machinery" by Lawrence, 4th edition, 1953.

Sorry, that was the 2nd edition.

On 17/11/2013 2:18 AM, depucca wrote:

Synchronous AC motors work well as generators- and are often used that way in industry-having the advantage of var control. Their excitation comes from the DC field. Induction motors also can be used as motors-however an external excitation source is needed- preferably the power grid. In Some places (e.g Scotland), in remote areas, small streams drive induction generators- once up to speed, they are connected to the grid and feed power into the grid (drawing reactive from the grid). Capacitors can be used once running and the grid connection could be removed- but voltage stability is a problem. An extreme case that I know of was where a farmer with a single phase supply had a large induction motor which he connected mechanically to a smaller single phase motor and excited one phase with the result that he could generate 3 phase which he used to drive another 3 phase motor. It worked well and was cheap because the polyphase motors were surplus units. Sure the energy all came from the single phase supply but it was a lot less expensive than getting 3 phase in a remote area or buying a large single phase machine (and not a great deal less efficient). Nowadays there are electronic drives that do the job better for less money but then, many weird and wonderful arrangements and machines were used. Nightcrawler's reference is quite dated but includes material not generally taught these days (less emphasis, except in machine design texts, on some factors, and circle diagrams went out when or before electronic calculators came in, and transient response was generally ignored). It brings back some memories as much of the same was done in the early 50's. Approaches in texts after the 50's have changed considerably- emphasizing that essentially a motor and a generator are the same except for the direction of the energy flow and taking into account the various advances in control and power electronics.

Induction motors - that are designed to be motors - make lousy generators, regardless of how well 'tuned' their capacitive excitations circuits are, if they're used for any varying loads, or - for the most part - on any inductive loads.

They're about useless for anything but purely resistive loads like incandescent lights or resistive-limited battery charging.

Lloyd

IIRC when I was in high school or even a bit before, our class visited a Consolidated Edison power plant in Manhattan. The were using turbine driven alternators. At that time, a much smaller machine on the shaft to provide dc excitation for the alternator's armature. Years later, with what I think was the 1965 East Coast blackout, external auxiliary power seemed to have taken over.

I remember an IEEE trip to San san Onofre. They were rethinking reliance of offsite auxiliary power. They were thinking of diesel driven generation rather than attaching dc machine onto alternator shafts.

Why didn't he just use a rotating phase converter? A three-phase motor is often the cheap solution. These are often used by hobbyists to make use of surplus three-phase power tools.

In proper circumstances they work well- such situations would be similar to the Scottish application where advantage can be taken of small streams, using a turbine and induction motor. The motor can be run up as a motor-then the turbine gates are opened(or in reverse order). No synchronizing or voltage control needed. In such a remote application, it is simpler and cheaper than a synchronous machine. True- they must have a source of excitation and need to draw reactive from the grid or grid + capacitors and in the latter case varying loads are a problem. They cannot supply reactive to any load, nor can they (without some other source of reactive) operate if the grid connection goes down.

In early days, and external DC machine was commonly used but later on it was mounted on the alternator shaft - in some cases there were two DC machines- one supplying the alternator field and the second smaller machine supplying the field of the larger machine. Common in hydro plants such as those at Shipshaw, Kemano, Grand Coulee, etc. In later years, such things as magamps replaced the pilot exciters (faster response) and modern units use fully electronic control- there is feedback from the alternator through scr's supplying a stationary winding which induced AC in a 3 phase winding on the rotor- which feeds the alternator field through a bridge, essentially making it self excited and no commutators/brushes involved. Smaller, cheaper, less maintenance and more responsive than the old exciter/pilot exciter The idea of a separate on site power source for auxiliaries is common

-Grand Coulee has some auxiliary units. Tthese were not used for excitation. Hydro plants do have the advantage that even a good battery bank will supply control needs and allow start up- the rest follows. Thermal plants have higher needs (feedwater pumps, etc) so a hefty source is needed to get a unit up and running. The problem in the 65 blackout was that many thermal plants were isolated and some form of external power (or internal diesel units) was needed to get a unit on line to supply other thermal plants. Hydro plants could quickly be brought on line- although for a while, they were restricted due to the need to get thermal plants up rather than immediately supply other loads.

Essentially a 3 phase induction motor at no load and excited on only one phase is a single to 3 phase converter. This is what I was referring to in the farmers case- I got off topic.

I guess I misread what you posted. It sounded like you had a large motor driving a smaller single-phase motor. Haven't seen that before. I have seen the unloaded three-phase motor used as a phase converter many times.

It sounded to me like the small single phase motor just spun up the three-phase rotary converter. jsw

Yes, I guess that makes more sense. Don?

Yes-I didn't state it clearly, the single phase motor was used only to rotate the larger motor.