Generator sizing for 100 HP motor 2

[Atbalance]

I have the following motor and would like some direction on sizing a generator.
100 HP motor
460V 60 HZ, 129 FLA
PF 82
Nema kVA code H
Typical starting current 832 Amps according to spec sheet.
Direct online start

The motor starts and gets up to full running speed under no load or virtually no load and usually only operates at about 30% load for 20 minutes at a time.

I have gone through some generator sizing programs which tell me the starting kVA is the most important consideration when sizing for starting.

Typical generators we have been using are 250 kVA - 300 kVA with a 200 A breaker which seems quite large considering they are under virtually no load once the pump starts. I don't know the starting kVA of these units.

Is it possible to get a generatro with a high starting kVA and low running kVA closer to the motor limit?

Would it help using a star-delta starter? I understand this is only valuable when the motor reaches synchronous speed before switching to delta. We have a 125 KW motor with a star-delta and it doesn't seem to do any good.

What about an autotransformer or other method to reduce starting load and thereby minimize the generator size required, or is it a wash as far as cost goes?

Would it be worthwhile going with closed transition rather than open transition?

A lot of questions but appreciate guidance.

 

[Marke]

Hello Atbalance

This is a reasonably simple request provided that all the information is available.
The first thing to realise, that not all generators are equal, so an appropriate size of one type of generator is very different to an appropriate size of an alternative.

The genset has two major components, the engine and the alternator. The engine provides KW and the alternator provides Amps (KVA).
Some engines can be equiped with a turbo boost to enable a short term overload of 200 - 300%, other engines are marginally sized for the alternator and have very littl overload capacity.
The alternator can be self excited, where the excitation is derived from the output of the alternator, and others have a permanent magnet generator to provide the excitation energy. Self excited alternators typicaly current limit at around 120% of rated KVA and any further output causes the voltage to crash. PMG excited alternators are commonly capable of 300% current for long enough to start a motor.
The alternator also has an Automatic Voltage Regulator (AVR) and these come in two major flavours, single phase half wave peak reading, and three phase averaging. The three phase averaging provides a more stable output and is less prone to instability from transient loads.

If you select a low cost gen set design, you will probably need to size it for the start current. If you use a better genset design, you can probably select it based on one third of the start current.

In selecting the engine, you need to consider the shaft KW of the motor during start, plus the cable losses, motor copper losses and alternator copper losses during start. (These can be considerable!)

Many of the reduced voltage starters create other problems if you are looking to minimize the size of a genset, i.e. primary resistance and auto tranformer starters have a high KW loss during start and will require an increased engine size. I recommend a soft starter as the best way to minimize start current and start KW to minimize the genset sizing. You could look at

http://www.lmphotonics.com/genset.htm

for more information.

What is the motor starting? this is important as it determines the minimum start current and start time.

My approach is to determine the required run KW and KVA, and then determine the required start KW and start KVA and then look at engine and alternator combinations that provide both of these criteria.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Atbalance]

Mark thanks for the quick reply. The motor is starting a triplex pump. There is normally little or no pressure when starting the pump. There is also a small centrifugal boost pump and a small hydraulic pump that run off the generator but normally don't start at the same time and are low power consumption. I will take these into account as well as cable and other losses mentioned.

Thanks for the advice on the soft starter and will heed wha it says on the website. My only experience is with a star delta and it doesn't seem to make a difference. Could this be because it is switching to delta before the motor is up to speed? I will investigate soft starters further.

FYI. When initially testing this unit we were using a 150 kVA generator and recently could not start it with a 200 kVA generator. As you say, they are not all created equal, so I will find out the differences. I suspect one is PMG and the other isn't. Could be cable length and other losses as well.

 

[Atbalance]

Marke thanks for the quick reply. The motor is starting a triplex pump. There is normally little or no pressure when starting the pump. There is also a small centrifugal boost pump and a small hydraulic pump that run off the generator but normally don't start at the same time and are low power consumption. I will take these into account as well as cable and other losses mentioned.

Thanks for the advice on the soft starter and will heed wha it says on the website. My only experience is with a star delta and it doesn't seem to make a difference. Could this be because it is switching to delta before the motor is up to speed? I will investigate soft starters further.

FYI. When initially testing this unit we were using a 150 kVA generator and recently could not start it with a 200 kVA generator. As you say, they are not all created equal, so I will find out the differences. I suspect one is PMG and the other isn't. Could be cable length and other losses as well.

 

[itsmoked]

There must be a echo around here..

Hi Atbalance.

Star/delta is generally frowned upon around here as it can cause many problems all by itself. During the transition the motor loses synchronism with the network and can then be electrically 180 or some other horrible number of degrees out when the delta closes. This leads to things like a tripped breaker every now and then. And occasional self disassembly of some parts of the system. It's often considered just a cheap legal dodge for 'reduced voltage starter' required by some authorities.

Closed transition or auto-transformer starting is much, much, better, but you have limited options with them and recently they have crossed the price curve with soft starters. Soft starters are less expensive smaller and smarter now. Often SS can provide nice freebie functions like motor overtemp shutdowns, etc.

If your application truly is a low starting torque application that runs partially loaded then a SS is a VERY good solution.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

FYI. When initially testing this unit we were using a 150 kVA generator and recently could not start it with a 200 kVA generator.

First check to see if this generator AVR has Under-Frequency-Roll-Off. If it has a jumper selection for 50/60Hz it probably has UFRO.
Connect the motor directly to the generator and then start the generator. The UFRO feature of the AVR will ramp the voltage up with the frequency as the generator accelerates.
The action mimics a VFD.
Do not start the other motors until the generator is up to speed.
Re: Turbo equipped engines. In cases where the generator is unloaded or lightly loaded prior to stating a large motor the motor induced overload may be passed before the turbo has time to spin up. There are other applications where the turbo is advantageous.
Also, the maximum output from a diesel engine depends on the maximum fuel delivery. This is set at the factory with a load bank.
I have had problems with more than one set being unable to start a load within its capability due to a faulty adjustment of the maximum fuel setting in the fuel pump.
I have used a ratio of 2.5:1 for a dedicated generator and 3:1 if other loads are running.
That is 2.5 0r 3 times motor running KVA for generator KVA.
respectfully

 

[Atbalance]

The echo was me - thought I could edit my own post after it was posted. Guess not! Thanks!

 

[itsmoked]

Yes we all wish regularly, that we could do that.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Atbalance]

I have received feedback from the rental company that the 150KW generator we were using had a permanent magnet generator to DOL start the 100 HP electric motor.

It seems to me that using a permanent magnet generator is the most appropriate since we can significantly reduce the generator size for starting the pump.

Would adding a VFD or softstarter reduce the generator further to make it cost effective? My feeling is that we could reduce the size of the generator further but there would trade off for the cost of the VFD or softstarter and potential problems with them (if any?) I am quite happy with the permanent magnet generator and DOL start though since this would reduce the generator size by 1/2 and keeps the starter simple. I will note that we seldom start the pump with any kind of load and will likely never exceed 80%.

Before I jump into this, I would like to know if there are any disadvantages to using a permanent magnet generator and if going with a softstarter or VFD would make a significant difference.

Thanks.

 

[jraef]

I would be skeptical of that sizing if you were to want to start across the line. Similar to waross, my quick-and-dirty DG sizing rule is DGkW = 2.5 - 3 x motor HP for across the line starting,, 1.5 - 2 x HP for soft starting. Marke's method is more accurate though. But I have my doubts that a 1.5 X HP sized DG is going to cut it without using a soft start of some sort.

A VFD can theoretically reduce the size of the DG to slightly more than the kW rating of the motor (you need a little overhead to cover harmonics issues) but is probably more expensive than just upsizing the DG if you don't really need the variable speed aspect.

 

[waross]

Re:Turbos. Actually, for installations requiring a robust generator, I do not use turbo equipped engines. A turbo is great for steady, heavy loads. The turbo allows a smaller cheaper engine to do the work of a larger engine with less fuel consumption per KWHr. When a turbo equipped machine that is lightly loaded is hit with heavy block loading, the load will often pull the revolutions and frequency down for a few seconds until the turbo "spins up". If you hear a gen-set pull down and see a lot of black smoke out of the stack for a few seconds, this is usually the cause. Until the turbo spins up, it is not delivering enough air for complete combustion of the fuel charge.
A non turbo equipped gen-set of the same KW rating will have a larger engine. It will aspirate enough air for complete combustion under all loadings.
The maximum power/KW out of a diesel depends on the amount of fuel supplied, times the conversion factor.
I have been told by a diesel guru on another forum on this site that the maximum power output from a diesel engine is reached with a slight excess of fuel. I have considered opening up the fuel setting on a genset above the factory setting to take advantage of everything available to start a motor that was oversized for the generator. I haven't done it yet but the trick is in the bag of tricks.
Deliberate over fueling is pointless on a turbo equipped engine. It will already be over fueling. Compared with a non turbo equipped gen set that is also over fueled, the larger, non turbo equipped engine will be putting out more power.
Another factor that mitigates against many turbo equipped machines is the typical lower compression ratio of turbo equipped engines. If you compare two similar engines, one turbo equipped and one naturally aspirated, the turbo machine will actually be less efficient at light loading due to the lower compression ratio. With a heavy block load, the turbo machine may be expected to pull down further than the normally aspirated machine. A few seconds later when the turbo spins up, the performance of the turbo equipped machine will surpass the naturally aspirated machine.
But, for block loading, such as motor starting, on machines that may be lightly loaded initially, there is no substitute for cubic inches.
Turbos are great, but not always.
Respectfully

 

[waross]

Permanent Magnet Generators, (PMGs) help to keep up the voltage and help avoid voltage collapse. The motor needs both Watts and VARs. It needs a lt of VARs when it is starting. The Watts are ultimately supplied by the fuel and are limited by the fuel setting. The VARs are supplied by the generator excitation system and a PMG helps the gen-set keep up its VAR production under heavy overloads.
Yes, this is simplified.
Respectfully

 

[Atbalance]

jraef - I appreciate that you're skeptical as I have an electrician that is as well since he's used to the rule of thumb to size the generator for 3x motor running amps. This is where I see the value of the permanent magnet generator to provide the necessary excitation for the start cycle. I will look at adding in a softstarter as well since the cost is not that high. We have some WIRAC star/delta starters which are performing wel and further reduced the starting current.

 

[waross]

To expand my rule of thumb;
A stand-alone motor generator pair, Minimum 2.5 ratio.
A generator supplying mixed loads, base load plus 3 times motor starting. Going much smaller than this may lead to unacceptable voltage and frequency dips for other loads on the system.
This factor applies when the motor is a significant part of the load.


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

 

[Atbalance]

Would the rule of thumb you are suggesting apply to a genset whether it has a permanent magnet generator as well? We were using a genset that was approximately 2x motor load compared to the next closest size genset size without a permanent magnet generator of 300 KW in order to start the motor.

There doesn't appear to be a problem wih there being enough fuel / engine to give enough watts, but with the generator not providing the initial excitation unless it has at least 2.5x starting load capability when it does not have a permanent magnet generator.

Using the same rule of thumb doesn't make sense to me if there is a permanent magnet generator to overcome the starting current.

 

[waross]

My rule of thumb was developed when working with a group of nine or more undersized generators. An extended family all bought identical diesel generators for standby service. They were all undersized. All of the homes had a greater air conditioning load than could be started sequentially by the sets. To make matters worse, some of the air conditioners were old thermostatically controlled units that tried to start concurrently. Most of the sets would start two ACs and bog on the third. Some would start three and bog on the fourth. I had a couple of cases of overload so severe that the voltage would drop to the point of the contactor dropping out in the transfer switch. This was of course under high current. As the set reved up and the voltage recovered, the transfer switch would close on quite a high current. The action was self limiting after a fashion. It did not take too long before there was not enough metal left on the contacts in the transfer switch to complete the circuit.
But back to the point. I found that base load plus 3 x the starting load of the next motor gave good performance.
Base load plus 2.5 x the starting load of the next motor was do-able but the voltage and frequency dip were annoying.
These were all NON-PMG sets.
Within a few years I was replacing all the sets with larger sets. I used my rule of thumb on all but two sets and all gave good performance.
Exception #1- I know I need 35 KVA to run all my ACs, but my budget says,- "Get a 28 KVA and leave the AC in the guest room off."
Exception #2- I am richer amd more important than my nephew and if he needs a 50 KVA set then I get one also. (Only needed about 40KVA.)
Comments on the PMG on small sets. (2 x motor load)
First, I don't know if the low voltage condition that destroyed the transfer switches was caused by voltage collapse or by the Under Frequency Roll Off (UFRO) action of the Automatic Voltage Regulator (AVR).
But, if an engine does not have enough power to maintain the speed (and frequency) the UFRO feature will kick in and drop the voltage proportionately to the frequency.
As the speed drops, the voltage drops and the load drops. Eventually the load has dropped enough that the speed does not drop any further.
It doesn't take long for this condition to be met.
Now the terminal voltage is set by the UFRO to just 5% above the normal Volts per Hertz ratio. The voltage at the motor will probably be almost at the optimum volts per hertz ratio. As the generator accelerates, it acts as a mechanical VFD and ramps the motor up with it.
A NON-PMG AVR will usually get the job done, but the PMG does a better job of holding the voltage at the optimum level.
The UFRO typically starts acting at 3Hz.below nominal. That's 57 Hz.for a 60 Hz. machine.
Normal V/H ratio at 60 Hz is 240/60 or 4 V/H.
UFRO V/H ratio at 60 Hz is 240/57 or 4.21 V/H.
My hat is off to the folks who conceived and designed the UFRO circuit.


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

 

[itsmoked]

Dang, I learn a lot from you Bill!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

There is another factor that we haven't yet mentioned in regards to marginal starting situations.
Is the set rated as Prime power or Standby Power? In some cases a prime power set may be fitted with options such as an oil cooler or a larger oil cooler and/or a larger oil sump. Engine accessories that reflect the high number of operating hours expected for a prime power set.
Often the identical engine is used, or put another way, the same set may be rated as either prime or standby.
The main difference is the rating.
A set may be rated as "Prime power, 200kW, 10% overload allowed one hour in ten."
For standby use, the same set will be rated as "Standby power, 220kW, no overload allowed."
When we are trying to start motors at a 2:1 sizing ratio, that extra 10% from a prime power set will make a big difference.
A thought has occurred to me. I wouldn't use this idea for a new design but we are often faced with using available equipment. Most AVRs have separate power and sense terminals. It would be interesting to supply the AVR power from a small UPS. It would probably be well to interpose a small 1:1 or 1:2 transformer to smooth out the UPS output.
If the AVR can accept the UPS suply, this should accomplish basically the same result as a PMG.
It works both ways Keith. I have learned a great deal on this site and a lot was from you. Thanks for both the mention and the continuing education.
Yours

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

 

[Atbalance]

Just wanted to keep updating this thread.

We were using a 350 KW generator to start the 74 KW pump motor direct online and were getting significant voltage drops to the point of the UPS engaging in the portable control cabin.

We then changed to a 175 KW generator having a permanent magnet generator and have successfully started the pump more than 40 times without a failure (remember we tried to start with a 200 KW generator that did not have a permanent magnet generator) and the contactor failed. Therefore the permanent magnet generator seems to be the answer in providing the almost immediate excitation necessary to start the motor and limit the drop in voltage. It may also be that there are differences in the fuel line / regulator etc... which we have not compared, but likely do not have as significant an effect in this case.

We are still experiencing some dip on the voltage but not to the extent and duration compared to the 350 KW generation. Some voltage dip is probably unavoidable with the direct online start so in the future we will likely install wye-delta starters or another type of soft starter which are on all of the other pumps.

We will continue to work on this and investigate whether the low voltage condition that destroyed the transfer switches was caused by voltage collapse or by the Under Frequency Roll Off (UFRO) action of the Automatic Voltage Regulator (AVR), but it appears that the PMG AVR is doing the job.

 

[waross]

I would discount the experience with the 350 kW generator.
I strongly suggest that it badly needed help. I had a 60 kW set that would bog down when trying to start a 4.25 kW motor. The set would slow down and dim the lights, badly. It couldn't start the small motor. They rebuilt the diesel engine. All but the governor, that is. The operation didn't change. It just needed the right screw on the governor turned about 6 or 8 turns. Most electricians don't understand diesel engine governors. Many diesel mechanics, even good ones, don't know how to set up the governor on a gen set unless they have worked on gen sets or been trained on them.

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