motor into a generator??

[mikec56]

Does anyone know if a typical 3 phase, 4 pole, induction motor that is rotated at 1500 rpm, can be used as a 3 phase generator producing nice sinusoidal waveform?

We would rotate this "generating" motor with another motor powered by a VFD. The equipment we are ultimately testing requires 50hz, 3 phase power. I'm sure we will have some issues to deal with regarding the output voltage level and efficiency of the system.

Our objective here is to build a 50hz generator from in-house surplus electrical pieces and 60hz service instead of renting the diesel units.

Thanks for your help. mikec56

 

[oldfieldguy]

Induction motors driven at higher than their synchronous speeds will function as induction generators, but they will require reactive power from the system to which they're connected.

Since you're apparently asking that your motor function as a stand-alone generator, I don't think this is going to work, since your motor have no source of excitation.

old field guy

 

[jraef]

There are ways of making an induction motor function as an "island mode" generator using residual magnetism in the motor along with capacitors to supply the reactive power. Just be aware that not all motors will retain enough residual magnetism and sometimes require some assistance to get going. You can use Google to find papers and websites on "self exciting induction generators" for more information.

 

[itsmoked]

If you are only running motors in your tests perhaps your VFD alone could solve the problem.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[aolalde]

To make an induction motor work as a generator you need a syncronous field matching the line frequency, and the rotor speed higher due to the required slip.

The normal way to achieve those conditions is by connecting the induction machine to the 3-phase line ( exciter) and over-speed the rotor with the mechanical power source, returning electric power to the line. Only then, a capacitor bank connected in parallel could help to provide the required reactive power (excitation). If now, you disconnect that generator from the line voltage, the generated voltage will decay to a few volts (millivolts?) in a matter of seconds.

 

[davidbeach]

Only then, a capacitor bank connected in parallel could help to provide the required reactive power (excitation). If now, you disconnect that generator from the line voltage, the generated voltage will decay to a few volts (millivolts?) in a matter of seconds.

Nope, not if you have sufficient capacitive support. True statement for an induction machine without reactive support, but not true of an induction machine with sufficient support.

 

[waross]

I have done that a couple of times in the lab.
First time, induction motor driven over speed by a DC motor, with a load bank in parallel with the motor.
A watt meter confirmed that we were supplying power back to the line. The swith to the supply was opened leaving the motor, wattmeter and load bank islanded. The motor continued to supply real power to the load bank with no decay. There were no capacitors in the circuit.
Second try, The match of induction motor to DC motor was not good. The power available out of the DC motor was fairly small compared to the rating of the induction motor. We were not able to duplicate the results.
Many old textbooks stated that the motor contribution to a fault was for only a few cycles. Correct, but the reason that it is correct is that with a fault on the system being back fed by the induction motors, the motors are now induction generators supplying current to a fault. The energy extracted to supply the fault acts to quickly slow the motor to the point that the out put is negligible. That is, the reason for the fast decay is the fast decelleration of the motor rather than loss of excitation.
If the speed is maintained and a load is connected such that current continues to flow in the motor, many induction motors will continue to generate with no decay or litle decay of voltage. With capacitors in the circuit it is possible to maintain the voltage without decay and theoretically even see a rise in voltage when disconnected from the line.
respectfully

 

[aolalde]

I would love to see a real explanation from Davidbeach, Waross or other to the following question:
How is the frequency sustained if the line reference is lost? . As far as I understand, an induction generator needs rotor speed slip from the synchronous field to generate, isn’t it?.

 

[ozmosis]

mikec56
It sounds like you wish to create a rotary frequency converter. This cannot be done by spinning a standard induction motor in the way you have described. A useful dscription of a rotary frequency converter can be found on Wiki:

http://en.wikipedia.org/wiki/Rotary_converter

 

[CJCPE]

Lots of information can be found by Google searching "self excited induction generator." Note "excited" not "exciting" and "generator" singular not "generators" plural yields the most results. Some of the available material covers the problems of operating in the "island mode" or "isolated" without a connection to another power source. As is often the case, some material is available only to IEEE members or is otherwise not available at no charge. As jraef, davidbeach and waross suggest, AC can be generated by spinning a standard induction motor with capacitors connected.

 

[davidbeach]

If one were to get an induction generator started (probably requires a line connection) and is then able to keep it running without a line connection there will not be any frequency stability. The electrical frequency will be less than the synchronous frequency of the rotor speed by the slip and slip will be dependent on the load.

 

[CJCPE]

According to what I have read, a simple induction generator can be started with capacitors connected but without a load connected provided that there is sufficient residual magnetism. The frequency and voltage will be subject to variation with load changes. The load also influences the required value of capacitors to maintain self excitation and rated output voltage. If the generator is overloaded, it will lose excitation and quit generating. If there is not sufficient residual magnetism to establish self excitation, some kind of DC injection “flashing” scheme can be used.

Depending on the detailed requirements of this application, building an induction generator system from available components might be worth trying. Adding a sine filter to the output of an available VFD might be a better alternative.

 

[davidbeach]

provided that there is sufficient residual magnetism

That's one of those things that happens when it is undesirable and doesn't happen when you want it too. Check out the works of some guy named Murphy.

 

[waross]

Frequency will vary depending on the slip.
1800 RPM corresponds to 60 Hz.
An induction motor at full load may run at 1750 RPM. That corresponds to 58.3 Hz. A drop of less than 3%.
The frequency does vary with loading, yes, but not much.
respectfully

 

[aolalde]

Hello Waross.
Please consider we are talking of a "generator" and what is desired is to create 50 HZ from 60 HZ power supply. A 4 poles machine should turn around 1.02x1500 rpm ~ 1530 rpm.

But if you do not have a reference line already with steady 50 HZ, how the induction generator will keep the 50 HZ adding only capacitors? Residual magnetism? That is for synchronous machines.

Mikec56. For nice sinus wave (and effective frequency and voltage control) use a "Brushless synchronous generator", rotating at 1500 rpm driven by your electric motor-VFD array. The voltage regulator will control the required field excitation (Voltage) and the VFD will keep the frequency (speed).

 

[waross]

Hello aolalde;
The frequency depends on both the speed and the slip of both driving and driven motors. If you set the no-load speed 3% high you will get a frequency of about 3% low at full load.
The capacitors affect the voltage, not the frequency.
I agree with you completely that a synchronous generator is a much better solution.
The value of the time spent trying to get the system up and running with an induction generator will probably be more than the difference in price between a synchronous generator and an induction generator.
respectfully

 

[rbulsara]

Mikec56:

I am confused. VFD is the frequency coverter. Why can't you use the VFD as 50hz source? How nice the waveform has to be? What is being tested?

How big (HP/KVA)is the motor/generator requirement?

 

[waross]

Hi rbulsara;
I have been wondering that also. Thanks for asking.
respectfully

 

[itsmoked]

And I said this 7 days ago!!!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

... nice sinusoidal waveform...

I'd say that was why. Not everything likes having a PWM pseudo sine wave.