Vary the output voltage of a generator to control an AC Motor?? 1

[WJB340]

Can someone much smarter than I give me insight on this. Is it possible to vary only the output voltage of a 300KW generator to control a 150HP submersible pump with high initial torque requirements. They are using a soft start currently and someone questioned why it could not be done by varying the output voltage of the generator only. Any expert opinion on this would be appreciated.
Thanks

 

[ScottyUK]

Is it possible? Yes. Is it simple? It depends!

Most small generators have a self-contained AVR which gives a limited range of adjustment. Outside that range you can manually control the terminal voltage using a variable DC source, and then transfer to the AVR once the generator output is near full voltage. This isn't too hard to arrange and is similar to the schemes used on larger machines where an external DC source is used as a supply for the field, which then transfers to a supply derived from the main output once the output reaches a sufficient level. Blocking diodes and/or contactors prevent the two sources interacting with each other.

If you need high torque then a VFD will provide far better torque from low speed right up to operating speed than a variable voltage scheme can manage, plus it would use COTS equipment rather than custom-engineering your generator controls.

 

[iceworm]

Sure. It would be a bit trickey for the controls - especially if you wanted a relatively constant current.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[WJB340]

Thank you for your response on this. This particular generator uses a DVR2000E regulator that can be controlled many different ways. However, in this application they are using the AVR but is also has a FCR, PF and VAR regulation option depending on the processor programming done initially on the regulator. This all came up because right now they are using a soft start to control the start of the motor but were looking to reduce this cost and have is run straight from the generator. I'm reluctant to test this simply because of the cost of the motor and generator if something were to go wrong. What would a motor setup for 460 operation of this size do when a 240V 60hz signal is applied and then slowly increased to 460V? I assume it would need more current to run it in this scenario. The reason I stat 240-460 is because I think that is all I can get this controller to adjust using the programming on it.

 

[ScottyUK]

At the lower end of the voltage range you have available I suspect the motor will stall if the starting torque is high, resulting in the rotor getting hot quite quickly as it relies upon internal air movement generated by the rotor to reject heat to the stator and casing. To achieve the soft start you're aiming for you would need control of frequency via the governor in addition to the voltage via varying the field, which would essentially convert your generator into a variable frequency drive from the pre-electronic age. Most generators aren't designed to do this type of thing and although it is certainly possible to get the set to run at a much lower speed it is unlikely to be simple and will attract substantial cost to design and debug it.

A VFD might actually work out by making the overall system cheaper because you may be able to use a smaller generator than you're presently using with the soft start.

 

[WJB340]

Ok, makes sense. Thank you for your insight on this.

 

[ScottyUK]

I'm hoping jraef, marke or waross might see this thread - a wealth of expertise on drives and generators spread between those three.

 

[waross]

The simpler the AVR the better chance you have that this will work.
An induction motor has an optimum Volts per Hertz ratio.
Too high and the motor iron saturates and the motor fails quickly.
Too low and torque is lost as an inverse square function.
A soft start limits the starting current by limiting the effective voltage but does so at the expense of torque and increased starting times. Even with the reduced starting current, the increased starting time adds heat to the motor.
A VFD functions by reducing the voltage and the torque in such a way as to maintain the optimum V/Hz ratio. Full load torque without over current at any speed up to rated speed.
Your generator AVR should have a feature called Under-Frequency-Roll-Off. If the generator frequency/speed drops more than about 3 Hz, he AVR should reduce the voltage in proportion to the reduce frequency so as to maintain the proper V/Hz ratio.
If you start the motor Direc-On-line, it will probably slow the generator down. The AVR should reduce the voltage correspondingly to maintain the V/Hz ratio. The combination of reduced frequency and reduced voltage is probably easier on the motor than soft starting. It also avoids the possibility of complications caused by unwanted interaction between the soft start and the AVR.
Another option is to connect the motor to the generator before starting the generator. This method will probably give the best starting performance.
Both methods may require modification of existing controls.
Under frequency trips may have to be defeated.
You may have to use a black start capable UPS or inverter to supply control and AVR power until the set is up to speed.
You may have better performance if you have a diesel tech cut back the dead rack setting and reduce the maximum KW of the gen set.
The problem will be to find a diesel tech who understands what you need.
As an example of V?Hz ratios:
Your motor rated at 460 volts and 60 Hz has a V/Hz ratio of 460V/60Hz or 7.67 Volts per Hz.
At 230 Volts the motor will perform much better and have the same torque as at 460 Volts if the frequency is cut to 30 Hz.
30 Hz x 7.67 Hz/Volt = 230 Volts. Note: it is common practice to set generators at 480 Volts to run 460 Volt motors. The no load frequency is commonly 61.8 Hz.


Bill
--------------------
"Why not the best?"
Jimmy Carter