Using a 3-Phase Synchronous Motor as a Generator

hello sir i think u first clear u are talking about synchronous motor or induction motor . in case of induction and synchronous if u change the direction of rotation of rotor or make output more as compared to input our motor work as generator. suppose in power sector when our input fails or pumped storage plants then our altertnatir act as motor and take supply from grid. so it is possible

Hi Chid.rohit,

Thanks for your comment. I was thinking induction motor and thier synchorous speed.

yes a 3 phase induction motor can be used as a 3 phase generator, by having it connected to the source of power that would run the motor , with good power factor
and over speeding the shaft above name plate rpm. the synchronous motor has a,
rotor with 2, 4, 6, 8, or more poles excited by varying dc voltage, can be used for a 3
phase generator when driven by a suitable source of mechancial power, and applying dc
voltage to the rotor.not to be confused by a 3 phase wound rotor when used with resistor banks to vary the shaft speed

Yes - there is problem with question - are you talking about a 3 phase synchronous motor or a 3 phase induction motor - they are different things.

The 3 phase induction motor has a solid rotor, inducing current into equally spaced 3 phase stator poles causes a rotating magnetic field. The rotor is influenced by the rotating magnetic field and rotates to try and catch up with the rotating field. It will never catch up with the rotating field and therefore it is not synchronous motor.

If the rotor is turned without input power to the stator, then power will be realised at the stator terminals but the rotor cannot be excited and therefore the output is limited.

In a synchronous motor, the rotor is wound with 3 phase windings with connection to the motor connection box via slip rings on the rotor shaft. In a motoring condition the rotor connections would be taken to speed or torque control in terms of resistance banks etc.

If the rotor connections were shorted out at the terminals and rotor shaft turned without input power to the stator then again an output would be available from the stator terminals. The output would be further affected by inputting three phase current into the rotor windings, which will create a rotating magnetic field which will induce current in the stator windings, but it will not be a synchronous generator.

A true synchronous generator has a DC electromagnet built into a solid rotor with DC current varied into the rotor (field excitation). This controls the stator output and apart from being a synchronous machine it can, and often does run at leading power factor.

Note my comments are in Bold Italics..

Boy, you are pretty mixed up with your knowledge of motor operation, You need to go back to the text books. .

Lets set one rule straight. In order to generate an EMF into a wire, YOU MUST HAVE a magnetic field and IT MUST be moving in relation to the wire.

Conversely, In order to create a magnetic field about a wire you MUST have a CURRENT in the wire. A voltage alone will not do it.

The 3 phase induction motor has a solid rotor, inducing current into equally spaced 3 phase stator poles causes a rotating magnetic field. Wrong, it's the other way around. Short circuit currents are induced in the rotor by the transformer action from the incoming line current.

The rotor is influenced by the rotating magnetic field and rotates to try and catch up with the rotating field. It will never catch up with the rotating field and therefore it is not synchronous motor. There is a phase angle between the incoming stator current and the short circuit current induced in the rotor!.

If the rotor is turned without input power to the stator, then power will be realised at the stator terminals. Where do you think the magnetic field is coming from? The terminals are open, there can be NO CURRENT and no magnetic field.

but the rotor cannot be excited and therefore the output is limited. There is NO OUTPUT.

In a synchronous motor, the rotor is wound with 3 phase windings with connection to the motor connection box via slip rings on the rotor shaft. In a motoring condition the rotor connections would be taken to speed or torque control in terms of resistance banks etc.

Are you starting the synchronous motor as an induction motor by short circuiting the terminals during starting. It CANNOT RUN AT SYNCHRONOUS SPEED unless there are defined magnetic poles on the rotor either from a permenant magnet of an external source of DC current. (Gods Law)

If the rotor connections were shorted out at the terminals and rotor shaft turned without input power to the stator then again an output would be available from the stator terminals. Again, where is the current coming from to create the magnetic field

The output would be further affected by inputting three phase current into the rotor windings, which will create a rotating magnetic field which will induce current in the stator windings, but it will not be a synchronous generator.

A true synchronous generator has a DC electromagnet built into a solid rotor with DC current varied into the rotor (field excitation). This controls the stator output and apart from being a synchronous machine it can, and often does run at leading power factor. This is the only true statement you have made…..

Very Well Put!

I even used the UNDRERSCORE!!!

This discussion did not even broach the subject of VOLTAGE CONTROL.

If you want to make power, get a generator.

Thank god there are still some knowledgeable electrical (engineers??) out there - we mechanical guys need them to make the root 3 sparkly stuff so we can do real engineering on our big machines.

In terms of the original question and subsequent answers - there are two solutions.

1. Let them get on with it (stand well back and don't touch anything metal)

2. Buy one from China - £99 gbp for a 2.3kw single phase

Let us all not forget - one can't expect Brilliance from a 2W bulb.

It is perfectly feasible to run a synchronous motor as a synchronous generator.

The only thing you must care is about the capability curve, which relates reactive to active power. This curves show actuation limits for the operation of a synchronous machine, in the four quadrants say, forward or revers operation.

Care must also be taken to exitation as in one case it will provide the adequate output voltage and therefore, the corresponding reactive power.

For more information, please refer to specific bibliography that may be found in any university.

Kiond regards

Hello, guys.

Of cause, 3 phase inductor motors may work as a generator. In this situation two requirements have to be completed:

1. Any external mechanic power source rotates the shaft of the motor

2. The reliable supply of capacitance reactive power has to be connected to motor outputs for generating excitation magnetic field. This supply may be the bank of capacitors (AC 3 phase ).

3.Then, with disconnected load one has to excitate the motor/generator. This would be complete by the weak residual magnetic flux remain in the motor armature. After the motor/generator will produce voltage the load may be connected to the terminals.

4. The question is the voltage regulation and efficiency of this arrangement. But this theme may be found in special literature/

That is correct and very well spoken (written)

Did you ever look at the crazy waveform of an induction motor excited in this manner?

I once tried it with a 75 Hp induction motor belt driven by a 670 GM diesel and then connected it (back-fed) to a 7200 volt distribution circuit Via a 100 KW transformer. (the circuit breaker was open at the substation bus) I blew ALL the LA's (lightning arresters) on the line. Dam near got fired..

There were no customers connected but they finally retired me after 35 years at the power company.

PS the transformer and the line capacitance supplied the unexpected (and required) reactive excitation component.

You are right,

This method may be used as an emergensy stand-alone generator.

Sometimes it is the possible solution only

Why do that when I can sell you a real generator ?

I don't need a generator. I was paid by EPRI to study back-feed parameters when using Induction motors as generators on Windmills for co-generation.

Windmill operators (at that time) believed that if the line went dead, so would the source of excitation to the generator and therefore no reverse power could be fed back to the power grid should the power company source go dead.

This was a wrong assumption and my test data became the basis for the initial publishing of the IEEE standards for windmill co-generation applications.

Ron, I think theorically it is possible.

A synchronous motor will proved both voltage and power factor correction with the correct controls and without being connected to a utility supply. This is typical of standby MG sets. The diesel engine is set for synchronous speed (1800rpm), there is a voltage regulator for the field excitation for the rotor.

An Induction motor connected to a reliable AC source will automatically generate power back into the utility supply when supplied with an "overhauling" torque. To think of it in another way a 4 pole 60Hz induction motor may be rated 100kw at 1760 rpm. Now we note that the synchronous speed is 60HZ x 60/sec x 2/(4 poles) = 1800 rpm. To absorb 100kw from the supply it must slip by 40 rpm. At no load the slip is almost zero. Now if you supply 100kw mechanical power to the shaft it will overspeed by 40 rpm to 1840rpm and deliver that power back to the AC source.

Using a regenerative Variable Frequency Drive (yes, there are a few on the market that don't use DB grids) the induction motor can be used to deliver power to a system without any utility AC supply. This can be done at any speed and also provide power factor compensation. There is a drawback with high harmonic content from the PWM carrier typically at 2 to 4 kHz that needs to be filtered out.

We have used this technique for power dip ride through on paper machine winders and balance the deceleration of the machine against the losses of the winder while supplying the AC to keep everything energized. Gives about 90 seconds of inertial energy and brings the machine to rest in a controlled fashion without loss of product.

Here's a link to a US patent that solves part of the harmonics problem:

United States Patent 4849871
http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=5&f=G&l=50&co1=AND&d=PTXT&s1=Wallingford.INNM.&s2=Errol.INNM.&OS=IN/Wallingford+AND+IN/Errol&RS=IN/Wallingford+AND+IN/Errol

Perhaps we need to look at the question again.

Is it almost possible to convert a 3 phase syn. induction motor into working as a generator?

If the goal is to use a 3 phase motor in place of a generator, I would say sure it is ALMOST possible. But why would you want to?

If the goal is to use the 3 phase motor as a means of co-generation, I would say yes.

Large HP motors directly coupled to a fly wheel are used on Drilling rigs to help control Var and Hz when large load swings are encountered. Some rigs have the capacity to generate 7 MW and have an instantaneous load swing from 1 MW base load to almost full load 7 MW in less that 6 cycles.

The large motor coupled to the fly wheel uses the stored energy to boost the Var and help with Hz decay.

It is ALMOST possible to do anything you can think of.

T.

I guess that none of you have heard of an APU (Auxiliary Power Unit), which is standard equipment on all commercial and military aircraft. APUs are designed to act as both motors and generators, hint the rotor is the magnetic part and the stator is coil. The trick is in the control.

Paulmc

"If engineers did every thing correctly there would be no technicians!"

With out protection relays and an open breaker, every generator will motor or drive the engine that is normally use to drive the Generator. This is called MOTORING.

I have seen a CAT 3306 powered generator tied into a remote grid that ran out of fuel but kept turning. The reason for this was the protection relays where bypassed, resulting in the breaker not opening on reverse power. At this point the generator was a MOTOR, it was doing the work. The engine was still turning but not RUNNING. When fuel was added to the day tank the engine stated running again and pulling its load. At this point the generator turned back into a GENERATOR, the engine was doing the work.

Yes an APU generator is DESIGNED to be used as a STARTER MOTOR and a GENERATOR, but this is not a standard 3Ph MOTOR trying to be a Generator.

Of Course I have heard of APU's but we are talking about synchronous AC devices, which APU's are not.

They NEVER run in parrallel with other AC sources.

The exact answer to your question - yes, the induction motor may work as a generator. For this purpose first three capacitors (AC and not DC capacitor with poles!)with appropriate capacitance have to be connected to the output motor/generator terminals. Second, the motor has to be rotated with the quite rotating velocity, which is more than synchronous velocity for this case. Moreover, the residual magnetic flux of the rotor will excite this generator. After the voltage will appear on terminals the load may be connected to the generator. There isthe problem of voltage regulation and the velocity of the generator has to be observed and controlled.