synchronous motor as generator 3

My old generator has burned up in a hydroelectric plant. I am planning on using a synchronous motor as a generator replacement. What are the issues/concerns with using a synchronous motor as a generator? Also, do you simply run it opposite rotation to make a generator instead of a motor? I know this is how it works for induction motors as generators.

 

[nbucska]

The rotor of the 3 phase sync. motor is late relative to the rotating magnetic vector and so is the back-EMF generated. If you speed it up, when the rotor is exactly lined up, the current is zero. If the rotor is advanced,
it supplies current.

<nbucska@pcperipherals.com>

 

[xnuke]

I cannot answer your first question regarding issues/concerns.

If by saying &quot;do you simply run it opposite rotation to make a generator instead of a motor? &quot; you mean to swap two of the three phases, it won't work. You need to provide a prime mover (I assume it will be a turbine from your hydroelectric reference) and rotor excitation, along with speed and voltage regulation.

As to your final statement, induction motors cannot be used as generators since there is no connection to the rotor for field excitation, and even if there were, I doubt the rotor windings/squirrel cage would work well to provide an excitation field.

Hopefully, someone with more experience in this type of job can comment further, because I've never had to do anything like this. Any takers?

 

[resqcapt19]

Can't you generate power using an induction motor connected in parallel with the utility and driven by a prime mover above the slip speed?
don(resqcapt19)

 

[rhatcher]

An induction motor will generate electricity when driven by a prime mover at speeds greater than synchronous speed. The excitation for the rotor is provided by the stator, much the same as in induction motor operation. The down side to this is that this type of generator cannot be used for a black start. Specifically, the excitation is induced in the rotor by the stator voltage. Unless the stator is already energized by an external voltage source, no current will be produced no matter how fast you spin the rotor. As well, if the generator is in service and the external voltage source is removed, the output will decay to zero within a few cycles.

I have never considered induction generators past this level of thought, but I do know that based on the fact that the excitation is induced, the dynamics of operation for this type of generator are certainly much different than those of a synchronous generator. My gut feeling is that an induction generator will probably behave more as a current source than a voltage source.

 

[xnuke]

Some thoughts on the above two postings:
Thank you for clarifying how an induction motor can be used as a generator. I understand the principles of generation enough to know that if excitation is provided to the stator and the rotor is turned by a prime mover, generation occurs. However, this raises a few points:

Do you put in a rotor winding that is three phases and apply DC excitation to the stator?
You'd need to reconfigure the machine to provide electrical connections to the rotor.

Synchronous generators typically connect the stator windings to the bus. Slip rings and brushes are used to provide excitation to rotors of synchronous generators, but I don't think brush current capacity would be able to handle as much current as the stator connections if you generated the voltage on the rotor in an induction motor. Besides, adding this many brushes would greatly increase the maintenance periodicity due to brush wear.

It seems very impractical to me.

Any more thoughts?

 

[jbartos]

Suggestions:
1. A synchronous motor can generate electricity. Normally, the synchronous motor is used to supply capacitive kVAR.
2. A synchronous motor is essentially designed to propel shaft load.
3. A generator is essentially designed to generate electricity.
4. The constructional features of motors do not differ from those of generators of comparable rating and speed, except that the salient-pole design is the most common, and that (unlike generator) the machine is almost always required to be self-starting, using the pole-face cage as a starting winding.
5. There is no essential difference between the stators of polyphase synchronous and induction motors of comparable rating.

 

[chimp]

CONTACT brook hansen THEY PRODUCE WHAT YOU REQUIRE

 

[electricpete]

nbucksa was right as usual. Since his comments were brief, I would like emphasize his implied but unstated point... that there is no reversal of leads or rotation when a syncronous (or induction) machine is switched between motor and generator operation.

xnuke - there are no rotor mods required to operate an induction motor as a generator. As rhatcher said... the excitation is supplied to the rotor through the air gap.

If you draw the motor torque speed curve, you'll see it hits zero as slip goes to zero. Well... the machine torque speed curve does not stop there... it extends straight through zero with the same slope. If slip is negative (speed greater than syncronous), then Torque becomes &quot;negative&quot;. Since the Torque reverses direction (compared to motor mode) but the direction of rotation does not change rotation, it should be easy to see that the direction of mechanical power flow (P = torque*Speed*2PI)changes from motor mode to generator mode.

The way I look at induction motor and induction generator are the same:

The stator field behaves like permanent magnet rotating at 3600 rpm.

If rotor is rotating at syncrounous speed, then there is no relative motion between rotor and stator field => no rotor current induced.

As the rotor speed deviates from syncronous speed, the induced currents will be in such a direction to produce a torque which tries to reduce the relative motion. (in fact, if you introduce varying magnetic field to any shorted coil, a current will flow in attempt to oppose rate of change of flux, and resulting force is also in a direction to oppose rate of change of flux. Since the stator field is &quot;dc&quot; in it's own syncronous reference frame, just substitute &quot;relative motion&quot; for &quot;rate of change of flux&quot; to determine comparable behavior for motor).

If the rotor speed is below syncronous, the electromagnetic torque is in a direction to decrease relative motion which is a direction to speed up the rotor. Since electromagnetic torque is in the same direction of rotor rotation, the electromagnetic torque is delivering power TO the rotor/shaft.

If the rotor speed is above syncronous, the electromagnetic torque is in a direction to decrease relative motion which is a direction to decrease speed of the rotor. Since electromagnetic torque is in the same opposite direction of rotor rotation, the em torque is taking power FROM the rotor/shaft.

 

[gordonl]

How is an induction generator started? Do you close in the breaker and motor up close to synchronous speed and then apply torque to go slightly over synchronous speed to start generating?

Could you get away without speed feedback and a governor on a system like this, only use overcurrents? (Assuming were tied to a utility system)

Just Curious

 

[electricpete]

There's a lot of info on induction generators at:

http://www.windpower.dk/tour/wtrb/async.htm

One quote which partly addresses the Gordon's question:
&quot;An asynchronous generator is different [from syncronous], because it requires the stator to be magnetised from the grid before it works.
You can run an asynchronous generator in a stand alone system, however, if it is provided with capacitors which supply the necessary magnetisation current. It also requires that there be some remanence in the rotor iron, i.e. some leftover magnetism when you start the turbine. Otherwise you will need a battery and power electronics, or a small diesel generator to start the system&quot;

 

[electricpete]

More info on starting:

(Just a guess... don't try this at home).

My ASSUMPTION would be that IF grid power is available, then bring the machine up to syncronous speed with the turbine (breaker open). Then close the breaker and there should be no torque. Gradually increase speed above syncronous with the turbine to pick up load.

Part of the trick would be determining when the machine is at sync speed since we can't use a sync check if there is no voltage on the machine with breaker open. Perhaps a hand-held strobotach? I'm guessing you'd want to err on the side of slightly high frequency, if anything, so you don't motorize your prime mover.

Once again... just a guess.

 

[busbar]

Links to support e-pete's information:

http://www.usmotors.com/products/profacts/1-204.htm

http://www.nrel.gov/wind/ab_selfex_1.html

http://www.zetatalk.com/energy/tengy10o.htm

 

[naru]

tell me about theswitching characteristiccs of IGBT's

 

[electricpete]

Naru - my recommendation would be to start a new thread with a descriptive subject. There may be knowledgeable IGBT folks not looking at this thread on syncronous motor/generators.

 

[jbartos]

Suggestion to nbucska (Electrical) Sep 24, 2001 marked by ///\\\.
The rotor of the 3 phase sync. motor is late relative to the rotating magnetic vector and so is the back-EMF generated.
///Please, could you clarify this in terms of 3 phase synchronous motor versus 3 phase induction motor?\\ If you speed it up, when the rotor is exactly lined up, the current is zero.
///Please, how does this principle work if you have the 3-phase permanent magnet synchronous motor?\\ If the rotor is advanced,
it supplies current.
///Please, clarify this for the 3ph permanent magnet synchronous motor.\\

 

[electricpete]

jbartos
I'll invite Nick to answer your question also

But in the meantime, here's my view.

Power transferred accross an inductance XL is given
as P = |V1||V2|/|XL| * sin (delta) where delta is the angle between V2 and V2.

In the case of syncronous motor or generator, V1 is &quot;internal&quot; stator voltage resulting from rotor field, V2 is stator voltage, including effects of armature reaction. The angle between the two fields will be proportional to the power transfer. Rotor field leads stator field for generator. Stator field leads rotor field for motor.

 

[EKTORAS]

Dear friends,

in the following mmf stands for magnetomotive force.

In the case of a synchronous generator/motor, rotor and rotor field (mmf) travel with exactly the same rotational speed (w) with the stator field (mmf). Power transfer depends on the angle delta (ä) between the two mmf vectors.
If the rotor mmf is in advance at delta degrees the machine is a gen. If rotor mmf lags delta degrees the stator mmf, the machine is a motor. The greater this angle is the more power is produced or absorbed.
As far as reactive power is concerned both rotor and stator can contribute in magnetising the air gap no matter if the machine is a motor or a generator. This is because the rotor carries an emf form external source(dc supply). So the synch. motor/gen can be made to either produce or absorb reactive power.

In the case of the induction machine rotor does nor rotate at the same rotational speed with the stator field (mmf).
The rotor speed lags s*w rev/min (motor case) or it leads (s*w) rev/min the stator field (mmf) (generator case). However the rotor field (mmf) does rotate at the same speed with the stator field (mmf) but it lags or leads again in a certain power angle delta. This is because the rotor emf is produced by the speed difference (s) and the corresponding mmf rotates at s*w rev/min but it is also carried by the rotor which rotates at w(r), so in effect its speed is
w(r)+s*w=w (synchronous speed) . The reason we never refer to the anlge delta is that here we prefer the slip (s).
Only the stator is capable of magnetising the air gap and so reactive power is always absorbed in both the motor and the generator case.

The above mentioned mean:

-Induction machine does not require synchronisation to be connected.
-If there is no speed difference no emf is induced in the rotor and the current is zero and of course no mmf is produced.
-If we rotate the rotor of an induction machine by a prime mover close to synch speed and then close the breaker we minimise the transient current duration.
-The induction machine has to be connected to a qrid where synchronous machines operate and provide the reference synchronous speed above or below which the induction machine generates or absorbs power, and the necessary reactive power to magnetise the gap of the induction machine.
-If the induction machine is connected to no grid and with only capacitors at the terminals, it can operate as generator, but god only knows what the frequency and the voltage of the produced power will be.
-If the connection to the grid stops (due to a fault or whatever) the magnetisation of the air gap collapses and generation ceases.


To go back at the original question of gold11 because I don't think it has been answered:

CAN WE INDISCRIMINATIVELY USE A MOTOR AS A GENERATOR?

In other words to get an existing machine manufactured to work as a motor and use it as a generator.

I think that we have to ask the manufacturer/vendor.

For example,

In the case of syncrhonous machines a motor may have a different operating chart that does not favour our intentions when we need a generator. Overheating of the rotor or the stator may occur when operating beyond a certain point.

In the case of the induction machines the level of saturation may become greater when we operate as a generator a machine designed to work as a motor. This is because the air gap voltage will be greater than the terminal voltage. The result will be higher magnetising current drawn and higher iron losses.

 

[jbartos]

In my posting on October 7, 2001, I mentioned the synchronous machines with permanent magnets attached to the rotor. These machines are heavily researched because of their simplicity and with the intent of a foreseen lower cost, the higher reliability and availability. However, many of the above statements and explanations pertain to rotating machines that have rotor coils.