Sizing of the rental Genset to start a VFD motor

[HamidEle]

We are doing a commisioning test for a motor(No load). we need to rent a genset to start VFD Motor. This motor is a constant torque motor. VFD and Motor is 2MW. Do we need to rent a genset larger than 2MW in order to start the motor, even for VFD application?

Thanks in advance.

 

[jraef]

One of the beauties of a VFD is in its ability to often accelerate a load without exceeding the motor FLA. But to do so, you are assuming the motor is capable of accelerating it without needing the extra torque that comes with a typical torque speed curve. So in other words, you PROBABLY can.

But to be sure, you need to know all of the load torque-speed curve requirements (including the motor's own) to accelerate, and you know the motor output torque-speed curve data. Only then can you determine if the motor needs to run into the Breakdown Torque part of it's curve in order to do the job. If it does, the VF can do that of course, but it will want extra power from the source. The generator then must be capable of supplying that extra power.

Most generators have what is called a "reserve capacity". So now you take your starting requirements from above and compare them to the reserve capacity of the Genset.


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[ozmosis]

That size of VFD system should typically have some level of harmonic mitigation so be mindful that the harmonic distortion created by the VFD does not upset the voltage regulator on the genset. At no load it should be ok using jraef's "PROBABLY" basis but check the expected harmonic distortion and then advise the genset supplier.

 

[waross]

If you get a "Standby" rated set there is no reserve. A "Prime" rated set will have a 10% reserve capacity.
The same set may be rated at 1 mW prime power, and 1.1 mW standby power.
I would go 2500 kW or 3000 kW for the gen-set.

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

 

[HamidEle]

Thanks, Everybody.
jaraf, We are jsut planning to run it at no load.Motor will have to run into breakdown torque Can you advise how to determine the power requirement to run into breakdown torque?

 

[waross]

If your gen-set is too small and you overload it, it will drop the frequency and if the frequency drops more than a few cycles the voltage will start dropping also. Reduced voltage and frequency may invalidate your test.
Some drives (VFDs) and voltage regulators (AVRs) interact. It is often recommended that the gen set be twice the size of the drive to avoid VFD/AVR interaction.
Consider the cost of sending a rental back and renting a larger set versus just renting a larger set in the first place.
A gen set may be much smaller than a drive and still start an unloaded motor. However, that is not always the case. Your intention to run up to breakdown torque may require a much larger gen set than a "best case" unloaded start.
I will accept jraef's estimation ofthe power required to run up to breakdown torque, but if that power is supplied from a gen set rather than the grid, I suggest that the gen set be 150% to 200% over sized.

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

 

[HamidEle]

Thanks, Waross,
Twice the size of the drive to avoid VFD/AVR interaction?
This means, we should rent 4MW Genset to start the motor?This VFD has a built-in 1000Kvar capacitor bank. I am assuming it will help boost the voltage while the motor is being started.

 

[LionelHutz]

The motor is unloaded = no load connected to the shaft?

I would expect the motor to require about 100kW input power to run at rated speed and maybe about 500kW to 1MW to accelerate depending on the acceleration time.

I have no idea why you expect the motor to "run into the breakdown torque". You'd only require that much torque if you intent to accelerate the motor very quickly or connect a load to the motor.

However, as already posted, depending on the VFD model and the genset there might be interactions and other problems which will require more consideration. Connecting that 1MVAR capacitor bank to the genset will most likely present a problem.

 

[HamidEle]

LionelHutz,
I agree with you on this. For VFD starting, we don't need FLA to start the motor. Probally 1MW VFD may be able to start 2MW motor. We will have to look at genset Reactive curve, Stability curve and the acceptable THD limits.

 

[waross]

I'm sorry, I did not explain well.
A VFD will start a motor with not much more than no-load current.
If your no load current is 20% of running current and you are going to start with 30% of full load current, then that is the figure that I would double, giving a gen set rating of 60% of full load.
BUT, running into breakdown torque will require much more current and the capacitor bank is scary.

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

 

[LionelHutz]

OK, the reason I believe the power draw will be quite low is that a DC buss based VFD only pulls the real power from the line. So, the control power the VFD requires and the real power required to accelerate and keep the motor turning. The VFD will source the reactive power the motor requires from the internal capacitor bank.

Now, I'm not too sure about a current source VFD, it may draw the reactive power from the line...

 

[HamidEle]

That's why we need to take a look at the generator rective capability curve to make sure leading power factor won't be an issue.

 

[jcarrr]

A most important drive parameter for successful operation of an ASD with a generator is one to allow limiting input current of the drive to a setpoint. Set this to the current capacity of the generator, less a little. The drive will control voltage and frequency presented to the load to inforce this policy and the generator can run at capacity for starting the load. The rest of the time it will follow other programming.

My experience does not extend to 2MW, but this allows a 240 Kw generator to start and run a 350 hp motor driving a 220 Kw screw compressor load. If the load spikes above 240 Kw, because a valve opens or some other event, the motor slows until normal conditions return.

 

[waross]

Hello HamidEle;
How is your project progressing. We would be interested to know what size generator was selected and the field results.
Was this a No-Load test, or was a load applied after the motor was started?
Thanks in advance.

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

 

[HamidEle]

Some other company is looking after the whole commisioning and start-up so that I am directly involved. But I was going to suggest them to select 2MW genset. The VFD vendor told me they used the same size of genset to start the same size of VFD and load. However, the capacitive load still exceed maximum reactive capability according to the curve provided by genset rental supplier. I don't how they sucessfully started the compressor at that time. Maybe it is acceptable to run exceeding the reactive capability limit for a short period of time. I will check with the genset rental supplier and let you know.

 

[waross]

Thanks for the update Hamid.

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