Generator sizing doubts 1

[eeprom]

Hello,
I'm on a project where we will be using a 410 kw prime diesel generator to run three motors: a 30 Hp, 30 Hp, and 335 Hp. All motors start across the line, and the 335 will start first. The generator can produce 617 peak amps. The big motor has an FLA of 400. I have doubts that the generator will be able to start the big motor. The generator dealer disagrees. Does anyone have some information to convince me that the generator can produce enough current to start the big motor?

thanks

 

[davidbeach]

Why not? What does the generator sizing software say? That big motor is about 60% of the generator capability and as long as it starts, you don't care about voltage dip. So you will have a reduced voltage start.

I'm sure waross will weigh in with more details.

 

[itsmoked]

I believe it will start it. I also expect you to see a large voltage dip. Do you have equipment that won't like that?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

That will depend on the torque requirements of the motor and its load. A motor can often start with voltage down to 50% of rated as long as it is unloaded and provided the generator can hang in there long enough. The torque provided by the motor varies with the square of the voltage. This will be a reduced-voltage start, even with an across-the-line starter.

I would be concerned as well. I'd suggest a motor starting analysis.

David Castor

http://www.cvoes.com

 

[eeprom]

The vendor's generator software says it will start, but if I can't make sense of it with a calculator, then I am leery. The motor will draw between 2000 and 2300 amps on start up, which is almost 4 times the generator's rating. According to the vendor's project load analysis, the voltage dip will be near 30% during start up. Nothing else will be running, so the voltage drop won't matter much. There is the possibility that the 30% drop might drop out the motor contactor, but I doubt it.

I would like to see some documentation which says the generator can provide 400% peak current for 4 seconds. That would make me feel better about this.

 

[davidbeach]

Why? At 70% voltage the motor won't draw 2000-3000A. The motor will take what it can get, but it can't get more than there is. If what it can get is enough, then it starts, otherwise not. If unloaded it should work if the sizing program says it will.

 

[itsmoked]

What are the three motors doing? What is this thing? That would help with deciding.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The last install that I did was pushing the envelope. Cat software said "NO WAY".
The customer owned an unused generator and was not about to buy another. I explained the amount of load curtailing that would have to be done and he said;
"Fine. We can live with that. Go ahead."
It turned out that although we did have to curtail loading, we could handle quite a bit more load than the software allowed.
But you are pushing the envelope as far as motor starting on a gen set.
I find that equating HP to KVA is acceptable for motors on generators. The combined efficiency and PF is often close to 80% and using .746 rather than .8 is a close approximation.
On that basis I would want to see 670 KVA or 536 KW on the gen set.
But all is not lost. Almost all diesel generators in that size range use an Automatic Voltage Regulator (AVR) which incorporates a feature known as Under Frequency Roll Off (UFRO).
When the 335 HP motor starting load hits the generator, the speed and frequency will drop quite a bit. UFRO generally allows about 3 Hz frequency drop and then starts dropping the voltage in proportion to the frequency drop. This gives a constant Volts per Hertz ratio. (With a small 3 Hz. offset).
What we have here is the poor mans VFD. The reduced voltage avoids the high currents expected with DOL starting.
Possible issues.
Trips> Starting current will still be high but not as high as DOL on the grid. You may have to adjust the current trip settings.
Low voltage> You may have to use a UPS to provide control power during heavy starting.
Gen Set duty> If you have a prime rated set, add 10% to the generators KVA and KW ratings and use the higher ratings to predict allowable motor starting loads. For a standby rated set you must use the nameplate ratings.
Frequency> If your set has only basic controls you are probably good to go. If you have a more sophisticated control scheme you may have to disable under frequency tripping.
Bottom line> You project may well be doable, but be guided by itsmoked's comment regarding the big picture. When you push the envelope this much, you have to have all the factors. Being on site is even better.

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

 

[eeprom]

The two smaller motors are conveyors, feeding rocks into and out of a rock crusher. The large motor is a crusher. The crusher will never start loaded. I don't know how long the crusher will take to start, but I am assuming 3-5 seconds max since it is unloaded. The controls are currently set up to force the motors to start and stop in a "proper" sequence, i.e. the downstream devices must start first and stop last. Given this condition, the 335 Hp will have to start with one 30 Hp already running. The 30 Hp conveyor may be loaded, so this removes 40A from the generator's capacity. If I have to, I can set up a timer which allows the crusher to start first, as long as the downstream conveyor comes on within 1 minute of the crusher starting.

However this thing starts, that generator is going to produce a big groan on start up.

 

[X49]

Try modelling the genset in the software using a reduced voltage starter. The peak voltage dip should drop considerably. Adding a soft starter may be more economical than increasing the size of the genset, especially if it may have some side benefits for the crusher.

Schneider recommends a current limit of 400% FLA for their solid state soft starts in crusher applications, so try using this number in your software. Make sure if you use a solid state starter that you include an automatic bypass to reduce harmonics on the generator.

 

[davidbeach]

I'd not use a soft starter in this application. The generator and the soft starter will not play nicely together. With the voltage sag, speed sag, and the UFRO working together the generator will be a very nice soft starter all by itself.

 

[dpc]

I agree with David on this - the soft starter may cause more issues than it solves.

Since the motor always starts unloaded, I suspect it will start OK.

You do need to give some thought to the motor starter, if there is one. The contactor can start to dropout or chatter around 85% voltage on the control power. I suspect your voltage at the starter will be less than that initially.

David Castor

http://www.cvoes.com

 

[X49]

david,

Any specific reasons why a soft start would cause problems? I was under the impression that as long as it is a permanent magnet generator there should be no problems with the voltage regulator circuits.

 

[waross]

I missed the note that this generator will be prime rated. Consider it as a 451 KW generator for motor starting.

With the voltage sag, speed sag, and the UFRO working together the generator will be a very nice soft starter all by itself.

That's what I was trying to say. David used a lot less words.
You may consider going one step further and closing the starter for the large motor before the generator is started. You will need a source for control power to do this. A UPS for just the crusher controls should work well.

By all means, don't try to start the large motor with a 30 HP motor already running.


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

 

[davidbeach]

The generator (the AVR in particular) won't do well with the highly distorted load that soft starter will produce. The only reason you can get away with them on commercial power is that they are so very small compared to the total amount of generation.

I like the idea of connecting the motor prior to starting the generator. If the motor is unloaded and the field is applied gradually, so there is a longish voltage ramp, you could get very little "inrush".

 

[racookpe1978]

How are you (your customer) going to ensure the rock crusher will "always" start unloaded?

Seems that there is a (not often but significant!) number of times that rocks will be in the crusher (after an emergency shutdown, jam, generator trip, crusher fault) and can't be hand-removed before re-starting.

 

[X49]

"The generator (the AVR in particular) won't do well with the highly distorted load that soft starter will produce."

Not trying to get off topic, but I don't agree with this as a blanket statement. Gensets with three phase averaging AVR's (as opposed to single phase half-wave regulators) are commonly used in applications with high harmonics to prevent issues. Also, with a bypass on the soft starter, harmonic heating of the windings would not be an issue.

I would like to see any reports to the contrary, as I will be commissioning one such system soon and would like to be prepared for potential headaches.

 

[waross]

Most, (all?), of the crushers that I have seen depend on the inertia of the flywheel to crush the larger rocks. A rock that can not be removed by hand will probably have to be removed by machinery of some kind before the crusher can be started.

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

 

[eeprom]

The crusher has some sort of clutch system which allows the motor to disengage from the crusher wheels.

 

[LionelHutz]

I haven't had problems with soft-starters on generators as long as the generator uses a suitable regulator. Typically, an RMS measuring regulator works very well. I've seen issues with poor regulators, in one case I recall the voltage would drop almost 100V as soon as the soft-starter began to draw current.