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Join Date: Feb 2014
Posts: 8

Generator Size Selection for Motor

10/05/2014 5:09 AM

Hi there,

Thanks a lot for sharing knowledge in past, i just not learned the technical bit but also some sarcastic and witty comments. simply amusing not harsh.

Here is the another silly one, i have done math and physics and the theoretically calculations make sense but i am wondering how practical would it be?

" I have a 45kW submersible bore hole pump and trying to reduce/correct the size of the diesel generator.

Motor Technical data:

Manufacturer: Franklin Motors

Rated 45kW/60HP , 6" with 210kg total weight (motor+pump)

Thrust = 2800kg

Full load current = 93.2 Amps operating @ 50Hz

Nominal RPM = 2875

Locked Rotor Amps (Inrush) = 526 Amps.

As per my calculations i can use a 80kVA Genset to run this 45kW motor with Soft-Starter or VFD, however, people with 20/30 years field experience are saying they have never seen a 80kVA Genset running 45kW bore hole pump motor.

Here are my calculations (very simple math )

After considering de-rating factor (temp and all) 80kVA genset = 63kW @ 0.8pf

FLC = 109 Amps

DG Motor Starting Capability@ 30% voltage Dip = 196kVA = 272 Amps

PMG + AVR fitted with 300% short circuit capability.

With a 272 Amps starting capability and Soft-Starter/VFD, Genset should be able to handle 526 Amps Inrush current followed by FLC of motor i.e. 93 amps. If i consider losses up to 10% (11amps , 109-11 =98 amps) Genset should be able to provide enough amps for motor running @ 100 % load.

Due to lack of practical experience, limited resource (a simulator to run and test my cals/findings) i am not able to proceed forward without having some inputs from experienced and knowledgeable people.

Currently, client is using 150kW Genset to run 45kW motor with Soft -Starter. In my feasibility report i found out that 150kW genset is subject to Glazing due to load being less than 40% of Genset total rated capacity.

Please shed some light. If you need more clarification/technical information, feel free to ask. I appreciate your time and knowledge.

Regards,

Straya

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Guru

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#1

Re: GENERATOR SIZE SELECTION FOR MOTOR

10/05/2014 6:07 AM

In a bit over your head, I'd say.

Instead of asking strangers, ring up a gen set manufacturer and ask them what they'd recommend.

Then report back the your client.

Gen set manufacturer will be happy because he thinks he may sell you a gen set.

Client will be happy because he got knowledgeable answer.

You will be happy because client may never know that you were unsure of yourself.

There your are. Happy, happy, happy!

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#3
In reply to #1

Re: GENERATOR SIZE SELECTION FOR MOTOR

10/05/2014 9:19 AM

Thanks Guru!!

I already have data from Genset manufacturer and i did my cals based on data.

As i said i studied and can implement my theoretical knowledge, however, having some practical inputs would be beneficial.

I appreciate your time.

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#5
In reply to #3

Re: GENERATOR SIZE SELECTION FOR MOTOR

10/05/2014 10:05 AM

You could have said earlier.

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#2

Re: GENERATOR SIZE SELECTION FOR MOTOR

10/05/2014 6:37 AM

Your general line of reasoning is just fine.

Is the pump centrifugal (easier to start), or other type (harder to start)?

I agree that the 150kW genset is oversized. The 64kW sounds a bit small. In addition to consulting with the genset mfr, I would consult with the pump/motor mfr to see if 30% voltage dip is acceptable. This combination might work, but I think everybody needs to be coordinated.

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#4
In reply to #2

Re: GENERATOR SIZE SELECTION FOR MOTOR

10/05/2014 9:43 AM

Thanks for the reply Tornado,

Yes Pump is centrifugal. Genset is selected as per 30% voltage dip, however, maximum voltage drop would be around 15-20% during Inrush. Permanent Magnet Generator with AVR would be fitted for additional excitation (or internal self-excitation can be used as well) to improve the voltage dip.

Motor Manufacturer says " Generators must be sized to deliver at least 65% of the rated voltage during starting to ensure adequate starting torque".

Motor manufacturer also recommends 75kW (94kVA) Genset for 45kW motor.

But my real question is when my 63kW (78kVA) Genset is capable of producing amps to run motor at full load and can handle the starting inrush with the help of VFD or Soft starter (Theoretically) then what's stopping it to perform practically?

Why does 63/64kW sound a bit small for 45kW ? Am i missing out on something?

Thanks

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#6
In reply to #4

Re: GENERATOR SIZE SELECTION FOR MOTOR

10/05/2014 11:57 AM

It's common practice to oversize everything a bit for longevity and reliability....and it only makes sense to follow the manufacturers recommendation for liability issues....While your design may indeed work, it also could cause damage with any reduction in capacity, and is less reliable for that reason....The resiliency of a system is part of the design criteria...and adequate safety margins are the key to resiliency....learn this lesson well and you can save yourself some embarrassment in the future, when you have to explain a system failure that was your design....cya

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#7

Re: Generator Size Selection for Motor

10/06/2014 12:01 AM

Dear Straya,

Your calculations and Logic seems to be correct..Your Pump would start ON LOAD and hence may require more inrush current for starting than the calculations.

Secondly when the DG set Dips the Voltage as well as the Frequency dips and hence increases the current requirement..It is something which deals with Transient and Sub Transient Reactances and Time constants of the Alternator

Please visit - http://www.cumminspower.com/www/literature/technicalpapers/PT-7007-SizingGensets-en.pdf

I would suggest to get the DG set on Rent for an hr or so, to try your calculations.

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#8
In reply to #7

Re: Generator Size Selection for Motor

10/06/2014 12:13 AM

GA. That rental idea is an excellent way to check things out with minimal commitment and risk. (Though it may depend on local availability.) Aggreko is one company that rents gensets; I don't know how worldwide they are.

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#9

Re: Generator Size Selection for Motor

10/06/2014 7:15 AM

You haven't considered the overloading capability of motor (on continuous basis unlike that of DG set) and also the internal power consumption of soft starter or VFD.

Further, it may be wise to size the DG set to twice the required power rating to be sure that the DG set has adequate margins without causing excessive carbon formation in the engine (at loads <30% typically).

In one Gas turbine power project, we bought a DG set (for black start purposes) in which the engine sized to required kW rating but the generator kVA rating is much higher to cater to the starting kVA of the GTG starting motor (DOL starting).

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#10

Re: Generator Size Selection for Motor

10/06/2014 8:23 AM

Thanks for the comments and information/knowledge.

I did test a 64kW Genset with Resistive Load bank.

It's a 88kVA DG, prime rating = 64kW. I dumped 60kW load in one step and DG did run nicely.

I dumped 65kW load;

RPM went down to 1350

voltage = 384V

Freq= 48Hz

within 1 or 2 seconds

RPM rev up to 1735

voltage = 424V

freq= 52.1

and DG shut down.

It did took the load and showed the readings. It tried to control the voltage , speed and freq but Load was continuous, hence, after an attempt DG stopped.

However, Motor load will change as motor will be picking up speed. DG is capable of holding 300% of it's rated current.

I will be using a 45kW motor to run it this week. I will share my findings.

Regards,

Straya !!

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#11

Re: Generator Size Selection for Motor

10/06/2014 11:46 AM

Dear Mr.Straya,

This topic has been discussed several times in this forum.

Pl. open the link given below and you will find more discussions.

http://cr4.globalspec.com/search/sitesearch?do=show&srchobjs=t%2Cc&order=asc&fs=12&sort=textmatchrank&query=sizing%20OF%20GENERATOR

DHAYANANDHAN.S

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#12

Re: Generator Size Selection for Motor

10/06/2014 1:51 PM

According to our practice, you need a 360 kVA generator for direct start or 150 kVA with a soft starter (or a VFD - but running at a lower frequency is not recommend for this kind of pump).

best regards Snel

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#13

Re: Generator Size Selection for Motor

10/16/2014 3:28 AM

Your reasoning is fine.Normally motors will accelerate under 80% dip .while 150KVA is high consider some condtions when pump is struck or slush some overloading conditions when 80KVA will be a border line case.I would suggest go for 100KVA

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#14

Re: Generator Size Selection for Motor

05/23/2021 5:08 PM

A 45 kW motor at rated speed/volts will have a stall torque & power twice or more the rated kW input - so it could stall the 63 kW engine - A VFD could limit the demand where a soft-starter will not.. The pump may need the maximum torque possible to unstick it - Be careful that the 63 kW is continuous rating not the 10% intermittent rating or a higher maximum rating with a short life to make a cheap offer.

You are correct that running a diesel engine below ~40% continuous rating will soot it up and cause maintenance problems, fuel/kWh increased.

You do not say if there are any loads other than pump which demand normal frequency, 50/60 Hz. If not, the engine speed can be reduced to keep its load factor up. Governors being electronic makes changing speed easier. As an isolated site, no requirement to match a grid, you have a choice of 50 or 60 Hz or both - VFD can run 60Hz out from 50 Hz in or vice versa.

A trick worth considering is changing the engine speed. Motors, engines, generators and even lots of "domestic" load equipment can be dual frequency [motor contactors can be DC with rectifier] while being a bit bulkier - but lower temperature rise & longer life, better efficiency to compensate at lower speed. The company I worked for made most gensets dual frequency because they did not know which country they would sell to.

You would need to speak to the manufacturers of the motor, engine, generator, VFD etc to understand their limits and interworking. And the client to know all the loads and requirements. Also the experienced borehole operators to know what is really essential & get any actual running data (that 45 kW may actually run at a lot less and be a cause of the problem). In my first job, my boss suspected the loads of new equipment were a lot less than demanded - he pushed & pushed to get an actual measurement on whole equipment - load turned out to be 60% making critical difference for microwave off-grid projects.

67model

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Users who posted comments:

67model (1); Crabtree (1); dhayanandhan (1); lyn (1); nesubra (1); raghun (1); Snel (1); SolarEagle (1); Straya (3); Tornado (2); vishram lele (1)

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