diesel generator for motor 1

[godel123]

hi

a 50 KW asynchronous motor needs a 7 to 10 times that power to start(direct start) .

if we need to use a diesel generator we will need to 450 to 600 KVA !!

 

[dpc]

a 50 KW asynchronous motor needs a 7 to 10 times that power to start(direct start)

Not quite. If started at full voltage it will draw perhaps 6 or 7 times its rated CURRENT while accelerating. Current does not equal power.

Depending on the load being driven, most motors will start at far less than full voltage and draw less than the full locked rotor amps.

If you need to use a diesel generator, talk with a diesel generator supplier and they can estimate the size of the generator required to start your motor.

 

[waross]

A rule of thumb that has served me well numerous times.
Other shared loads, allow three times the motor full load current for generator sizing.
No other shared loads or other loads willing to accept severe voltage dips when the motor starts, allow 2.5 times the motor full load current for generator sizing.
The motor is direct connected to the generator and started with the generator. allow 2 times the motor full load current for generator sizing. You may need a separate source, possibly a UPS to supply the controls and the AVR.
For prime rated sets you may use the overload rated generator current rating.

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

 

[iop995]

Depending of load type, may be used a soft starter and limited motor current to maxim allowed by generator. No need for oversized of generator.

 

[waross]

Done this a lot have you???

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

 

[davidbeach]

I'd be far more inclined to believe Bill's numbers than the nonsence posted by iop995.

 

[dpc]

But keep in mind that Bill is referring to the kVA and full load current rating of the generator, not the kW rating of the prime mover. (I think.)

 

[waross]

Hi dpc
To keep it simple, full load current of the motor versus rated full load (or overload in the case of a prime rated set) current of the generator. An overpowered generator does have some advantages but I have had single phase generators that needed 250% to start A/C units and the voltage dips and frequency dips were noticeable and beyond accepted limits.
What does single phase have to do with it? Single phase sets are often Re-rated three phase sets. You lose 1/3 of your KVA capacity but the prime mover still has the same KW capacity. Example.
A three phase 50 KVA set will have a 40 KW rating.
When this set is reconnected and re-rated for single phase it will be 33 KVA but the prime mover will not be altered and will be capable of 40 KW. As a result single phase sets often have 20% excess prime mover power.
AVR issues are expected if the generator does not have adequate capacity. Generally 200% capacity for soft start or VFD.

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

 

[iop995]

Intial question was if a 50kW IM need a 450-600kVA genset for direct start. 50kW for IM mean nominal output, not necessary power feed from source. Maybe my post was not so clear and maybe "a nonsence"... Depending of load type (maxim power, time to start-up, J, process requirments) may be used a soft-starter and if load accept, nay setup maxim IM current to maxim overload Genset current to avoid oversizing. But this is true only in above conditions. Anyway, depending of load conditions, maxim start-up IM current (and time) will impose a rate of Genset oversizing.

 

[davidbeach]

Large (compared to generator size) soft starts and generator AVRs generally don't play nicely together. Buying the high end of both can make it work, but by then DOL starting and a generator sized per Bill's recommendations will be both less expensive and more robust.

 

[godel123]

the smallest generator will be whan using VFD, and 200% is its rate :
yes?no?

 

[waross]

Yes. That will be the safest installation.
There are some techniques to direct connect the motor to the generator and start them both together. The generator prime mover is then sized for the maximum motor overload plus a safety factor. This approach must be applied carefully and the generator set will often be a special build, not an off the shelf machine. A separate source for control power is also required. While other loads may be connected they may have to be held off until the motor is up to speed. Definitely not recommended for such a small machine.


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

 

[burnt2x]

My take is that your engine driven generator must be greater than the driven motor. There is no upper limit because that usually depends on how deep the pockets of the owners are.

But I size it at 2.5 times the size of the driven motor.

 

[waross]

"There is no upper limit because that usually depends on how deep the pockets of the owners are. "
That applies not only to the initial costs but the running costs. I have seen quite a few installations, many of them residential, where the cost of fuel to keep the set running when there is little or no load is much greater than the cost of the fuel to supply the needed KWHrs.
That is a ratio that is often usable Burnt.
I prefer 3 times. I have been forced to use 2.5 times and the voltage drop when a large motor starts often causes issues with other loads. I got involved with a number of sets that someone else had seriously under-sized. It was a matter of trial and error as to how many loads (mostly air conditioners) could be started. There was a lot of trial and error involved and some inadvertent damage before we found the maximum usable load on each set. After a lot of first hand trial and error experience, I prefer to spec 3 times, but will go as low as 2.5 in special situations.
Interestingly; after these installations had been running successfully for 10 or 15 years I came across some gen-set sizing software from a major supplier. I ran the numbers on a number of sets that had been running well for years and the software said that most of them did not have sufficient capacity to operate.
Bottom line, my rules of thumb are less than Cat recommendations.
Go less than my rules of thumb at your peril.
Do a lot of research using VFDs or soft-starts on a generator.

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

 

[godel123]

when talking about 2.5 times its "watts", but we know that "vars" are 6 times at start (direct start).
the case of a transformer that have to give watss and vars is also the same.

 

[ScottyUK]

"Bottom line, my rules of thumb are less than Cat recommendations."

That's because Cat are in the business of selling generators!


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[LionelHutz]

I'm with Bill. A typical soft-start application requires a generator kVA sized to a minimum of 2 times the kW of the motor. It can be taken lower on certain loads if you know it'll be a very easy start (such as a bow thruster on a boat) but that is risky.

I don't think I've ever seen an application where the genset was just sized to match the motor, say 60kW genset for this 50kW motor example, and was successful with a soft-start or ATL.

 

[waross]

I was talking about current not Watts but watt the heck.
When a motor that is a large part of a generator load is started there are several dynamic issues in play. It is not as simple as arguing Watts and VARs.
NOTHING is fixed. Not frequency, not impedances not voltage not the response of the prime mover to the increased load, not the current, not even the voltage set point of the AVR. (Anyone who does not know the meaning of UFRO please leave the room. You have no place in this discussion.)
Those of us who have actually done this in the field find the simple suggestions, well, Simple!!


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

 

[ScottyUK]

I'll get my coat.

It's a volt/hertz limiter in my world.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[wayne440]

I submit that in addition to the standard meaning, UFRO can also equate to Under Funded, Really Optimistic in the case of a few way too small gensets I have seen.