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diesel generator for motor

diesel generator for motor

diesel generator for motor

(OP)
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 !!

 

RE: diesel generator for motor

Quote:

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.

 

RE: diesel generator for motor

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

RE: diesel generator for motor

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.  

RE: diesel generator for motor

Done this a lot have you???

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

RE: diesel generator for motor

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

RE: diesel generator for motor

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.)

RE: diesel generator for motor

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

RE: diesel generator for motor

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.

RE: diesel generator for motor

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.

RE: diesel generator for motor

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

RE: diesel generator for motor

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

RE: diesel generator for motor

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.

RE: diesel generator for motor

"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

RE: diesel generator for motor

(OP)
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.

RE: diesel generator for motor

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

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

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

RE: diesel generator for motor

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.
 

RE: diesel generator for motor

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

RE: diesel generator for motor

I'll get my coat.

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

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

RE: diesel generator for motor

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.

RE: diesel generator for motor

I probably should have left the room when invited, but I felt like I was on the verge of getting something from the discussion.

How does that rule of thumb work when there are multiple motors?  For example, could I run five 10 kW motors off a 65 kW genset if all the motors had VFD and there was no way to start more than one at a time (all are manual start, one guy has to start them all, and they are 1/4 mile apart).

David

RE: diesel generator for motor

You can stay Scotty. Under Frequency Roll Off and volt/hertz limiter are similar enough to not matter. On the small machines the UFRO gets a lot more use than the volt/hertz limiter on the big ones.
Wayne, Under Funded, Really Optimistic really gives Under Frequency Roll Off a run for it's money.
You are always welcome David. That is an interesting suggestion.
The quick answer is yes it will work. A more accurate answer may be, I don't know.
If I may share an anecdote that may have some relevance;
I was on a project years ago when forbidden frequencies for pump drives were not well understood. We had some 60 HP centrifugal pumps with VFDs under PID control. At one particular speed setting, the pressure pulse reflections from the first 90, about 50 feet away coincided with the impellor vane period and the pressure pulses started to reinforce. The end result was a piece of casting about the size of my hand being blown out of the pump body. Twice. Lesson learned about forbidden frequencies.
What has that to do with generators? There may be a combinations of motor speeds and or loadings that causes the non-linear current draw of VFDs to reinforce with a bad interaction with the AVR.
I don't know, but were I faced with the installation that you suggest, I would tread cautiously.
Good question!!

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

RE: diesel generator for motor

David - You'd likely be getting close to 60kW of load on the generator assuming all motors running at full load and I would be worried about the harmonics of the VFD rectifier causing issues with the generator more than the generator being capable of starting the motors. There is also the issue of the VFD harmonics possibly messing up the regulator unless the regulator has the appropriate filtering in it. You should be able to set the VFD so that any single motor would not draw much over 10kW as it starts.
 

RE: diesel generator for motor

I have to use 10 kW motors because that is the smallest motor the manufacturer will put on this application.  Actual load is closer to 3 kW.  I think that in real life it is going to be a lightly loaded system, but it would be irresponsible to do the analysis based on that (I have a meeting with a EE next month and I'll drop it all in his lap, I'm just trying to understand enough to recognize smoke being blown up my pant legs).

Bill, that "forbidden frequency" discussion is new to me.  Has it been discussed anywhere you could point me to?  I have a bunch of folks that want to use centrifugal pumps for an application that I think should be PD (hugely variable head and capacity requirements, order-of-magnitude changes happen in under a second and can happen dozens of times in a minute), if I start chasing huge changes with a centrifugal pump I find myself in surge pretty early, but I didn't consider that there were frequencies that I had to avoid as well.

David

RE: diesel generator for motor

Hi David. It has to do with the speed of the pump which is closely related to frequency.
As you know, as each impeller vane passes the outlet it generates a pulse or pressure peak. The timing of these pulses will depend on the number of vanes on the impeller and the speed of rotation. These pressure pulses will be reflected back from the first sharp bend in the piping. When the transit time of the pulses is equal to or a small multiple of the vane timing the pressure pulses will reinforce and dangerous peak pressures may build up in the pump housing and the discharge piping. Problems are easy to anticipate knowing the speed of a pressure pulse in the fluid in question. Look for VFDs that have provision for a forbidden or do not run parameter. The VFD will hold just below the forbidden speed until the control parameter increases enough that the speed setting will jump to a speed just above the forbidden speed.
Jeff (jraef) can add a lot more here.
As for the generator size;
If the 10 kW motor sizing is power in rather than mechanical power out, and the starts are staggered you should be fine.
The issue with generator sizing has to do with the interaction between the Automatic Voltage Regulator on the gen-set and the non-linear current demands of the VFD front ends.
I don't know but Jeff may be able to add comments here; should the loading of several motors result in similar wave forms adding there may be stability issues with the AVR.
I would add a disclaimer to the contract mentioning possible stability issues and holding the designers responsible for the overall performance of the system.
Please share with us, as much as you are able, the results of this project.
Hi Lionel. Your concern is my concern also.  Not capacity or overloading but possible interaction with the AVR and the resulting instability.

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

RE: diesel generator for motor

Thank you.  Maybe this will help.  It is a downhole application with probably 80 stages of centrifugal pump.  I will introduce this "forbidden frequency" into the discussion and then duck.  My client REALLY wants to use centrifugal pumps (there is a manufacturer of this kind of pump in South Africa and he's trying to minimize his imports as much as possible).  I was concerned about the applicability of them even before I heard of this twist.

David

RE: diesel generator for motor

It has to do with the pressure variations as the impellor vanes pass the discharge outlet.
For downhole, I am thinking that the pressure pulse from one stage (the last) won't be as severe as from a single stage pump.
Keep us posted.

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

RE: diesel generator for motor

We had a 1200 hp motor, huge inertia and 17 second run up time at 4.5 full load current.  No problems starting on two generators each 2 MW, 2.5 MVA (total 5 MVA) supplying the power.  I tried splitting the board and starting on one generator but the generator tripped on overload after 15 seconds by which time the motor was almost up to speed.  Pretty sure that, if necessary, the motor could have been started on one generator with the overload setting increased.  The above comments about power are relevant.  The stating current is low power factor except for the last one or two seconds and the generator inertia should see it through this.

RE: diesel generator for motor

Quote:

Look for VFDs that have provision for a forbidden or do not run parameter.

I don't think I've seen a VFD that doesn't offer this lately.

Keith Cress
kcress - http://www.flaminsystems.com

RE: diesel generator for motor

Oh, and to continue with the resonances, VFD users may need to avoid all sorts.  Fan blade/housings/ducts, machine resonance frequencies, and even regions where a system just doesn't perform well.

Keith Cress
kcress - http://www.flaminsystems.com

RE: diesel generator for motor

Thanks for the update Keith. I haven't been around VFD apps for a few years.
Zdas04; Another issue with down hole applications is long leads and the need for sine wave filters and/or inverter rated motors.

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

RE: diesel generator for motor

The science of very long downhole cables is pretty mature.  The industry has had endless problems associated with cables over the last 50-60 years, but the number of cable-related pump issues seems to have fallen off dramatically over the last decade or so.  Pump manufacturers look like they've solved the worst of the recurring issues.

David

RE: diesel generator for motor

Inverters introduce a whole new set of issues and problems. Non VFD pump cables do not have to withstand high voltage high frequency pulses. The voltage may increase with cable length.

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

RE: diesel generator for motor

We see it all the time on submersible motors.  VFD's and long cable lengths are not the best of buddies.  It is not that the cables get damaged, but more of what happens to the motors.  If a VFD is to be used on a submersible or any motor with long leads, an output is filter is definitly required.  The filter used will depend on many factors, but a true sine wave filter is the best option, with a dV/dt an ok second.  
 

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