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Compensating For An Undersized Generator

04/13/2017 1:39 PM

Hi all,

I have for a few months now been looking into a problem for customer of mine that is a property manager at a large remote vacation townhome complex of about 50 homes. The problem he came to me with was the capacitors on the motors attached to the vertical sewage pumps were blowing while trying start under generator power. I am a generator technician and have some familiarity with pumps and systems but not any in depth knowledge of their controls or options. Now the generator is a 17kw single phase, 60hz, air cooled, Generac generator with an output amperage of 141.7/70.8amps. The motors, which are not used together, one is a backup are 5 hp, 1750rpm, 230volt one FLA is 20.1 the other is 23. I know a couple things (or so I think) right off the bat. 1.) The generator is undersized and should be at least a 25kw which would solve this whole problem BUT.....my customer has upsized this generator once already per the guidance of the electrical contractor he was previously using and at all costs does not want to go back to the Homeowners Association for second time and tell them that the generator the have spent a lot of money on is to small. 2.) The capaciors are blowing because they are on to long and over heating due to them starving for power to perform while the heavily loaded generator comes up to proper outputs.

Now my questions.......

1.) Can the motors be changed to motors that dont use capacitors

or

2.) Is there a way to(i have only heard of this) double or triple up the capacitors being used on each motor.

or

3.) Is there a devise that is more or less opposite of a soft start like a "hard start"

I know all of this is just to avoid changing out the generator but my customer is really in a tough spot. If changing the generator out is what is most reliable and effective than that's what he needs to do but I am trying to exhaust all options first.

FYI. The pump pushes sewage up about 60' in elevation and during very busy weekends, like memorial day weekend coming up the tank these pumps are on activates the float switch about every hour and a half.

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#1

Re: Compensating For An Undersized Generator

04/13/2017 1:54 PM

17kw for 50 homes? Surely you jest...50 families can't afford a few grand to keep their toilets from backing up? Put in outhouses...

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#3
In reply to #1

Re: Compensating For An Undersized Generator

04/13/2017 2:44 PM

I once lived in a house down in the Carribean that had a 3 KW generator. I could use a hot plate or flush the toilet, but not at the same time!

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#2

Re: Compensating For An Undersized Generator

04/13/2017 2:16 PM

Maybe if you had some type of clutch between the motor and pump that would engage when the motor got up to speed, the motor could get up to speed under no load.

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#4
In reply to #2

Re: Compensating For An Undersized Generator

04/13/2017 2:51 PM

Maybe he could install a 4-speed transmission....haha

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#5

Re: Compensating For An Undersized Generator

04/13/2017 6:26 PM

You might investigating the use of a VFD (Variable Frequency Drive) and take advantage of its soft start capability, it will ramp up the speed of your motor without the large inrush current.

The other alternative is to put a valving arrangement in the piping so that the motor starts unloaded instead of against the 60ft+ head of poop.

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#6
In reply to #5

Re: Compensating For An Undersized Generator

04/13/2017 7:00 PM

I wonder what issues might be present with variable frequency and starting capacitors applied together.

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#8
In reply to #6

Re: Compensating For An Undersized Generator

04/13/2017 7:28 PM

If the motor is single phase, I believe it needs a starting cap to have any starting torque, whether driven by the line or a VFD.

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#9
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Re: Compensating For An Undersized Generator

04/13/2017 7:59 PM

Indeed. That is precisely what raises the question.

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#7
In reply to #5

Re: Compensating For An Undersized Generator

04/13/2017 7:05 PM

You know what they say about 'it' running downhill.

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#10

Re: Compensating For An Undersized Generator

04/13/2017 8:49 PM

Sounds like someone didn't do their homework from the get-go. Bit the bullet and tell the HOA their genset is undersized and they'll be money ahead to fix it now before they start replacing motors because of low voltage burn outs.

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#11

Re: Compensating For An Undersized Generator

04/13/2017 10:15 PM

The most straightforward way to compensate for an undersized generator is to acquire a larger generator that is not undersized. Just sayin'

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#12
In reply to #11

Re: Compensating For An Undersized Generator

04/13/2017 10:27 PM

Maybe a hybrid approach with an inverter and battery bank to support those surges at start-up? But I also suspect a larger generator is still probably cheaper than an inverter kicker.

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#13
In reply to #12

Re: Compensating For An Undersized Generator

04/13/2017 10:42 PM

Same here. Just solve the problem and be done with it. Maybe get more somewhat more generating capacity than they actually need at the moment, to accommodate the Future (which always comes soon enough).

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#14

Re: Compensating For An Undersized Generator

04/13/2017 11:08 PM

It never pays in the long run to cut corners in the beginning

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#15

Re: Compensating For An Undersized Generator

04/14/2017 9:58 AM

Load sheddings maybe the best option without incurring additional cost?

Scheduled and selective power outages everytime the pump starts may buy the manager some temporary solution... This inconvenience may also provide needed hints for the owners!

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#16

Re: Compensating For An Undersized Generator

04/14/2017 11:28 AM

In agreement, here, with most of what's been said... [OP] states:

"The capaciors are blowing because they are on too long and overheating due to them starving for power to perform while the heavily loaded generator comes up to proper outputs." Most probably correct.

"No", you will never get sufficient starting torque going to a "capacitor-less" design...

Rixter was spot-on, that motors in such service require a starting cap to deliver sufficient starting torque. (unless, of course, you switch to a sufficiently-sized 3 phase setup, which "we hear" ain't gonna happen.)

If you truly are "a generator technician" with some pump experience, surely you have seen an occasional a/c system being trouble-shot (?)

In my younger days, I spent a number of years working on every imaginable "electrical spinner upper" (and a few "plungerators and clutches" to boot). So, this comes from a tad bit of experience. I never saw a cap-start unit that said "Never replace the black cylinder-thingy with anything but the exact same brand and rating."

The start caps provided from the factory are generally "middle-of-the-road" rating, for what the windings can utilize effectively.

You asked whether you might "double or triple up the capacitors being used".

Well ... I swapped-out quite a few start caps (for over/under rated ones), with only one "grunt" (from the motor) that I can recall.

Back to my comment about observing an a/c system being trouble-shot.

WHAT , if anything , have YOU tried , yet ...?

Got any a/c buddies who might loan/rent you a compressor kickstarter...?

Cheaper ones are out there... and... you're going to wind-up either adding or replacing SOMETHING (obviously).

I doubt that anybody reading THIS thread will have faced the EXACT SAME scenario in which you now find yourself, and have the exact-best solution.

Let the owners know what you wish to try ("It's probably never been done before, so the outcome can't be guaranteed... but it's SOMETHING, to try...")

And don't forget to come back and share your results! Best of luck to you, on this hallowed day.

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#17

Re: Compensating For An Undersized Generator

04/14/2017 1:31 PM

You might be able to find motors with start windings, centrifugal switch kicks them out when up to speed. These motors will have higher starting torque capability than capacitor start motors, but will also have somewhat higher starting current.

Still the loss in starting torque with low voltage at capacitors is the downfall of this, it is a spiral that is difficult to get out of, the lower the voltage, much lower the starting torque.

A 17kW should be marginally able to get those motors spinning, one at a time, especially speed load curve of centrifugal pump. Perhaps your voltage regulator could be adjusted to be faster, check wire connections, check voltage at motor when you start, anything that drops voltage will kill it...

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#18
In reply to #17

Re: Compensating For An Undersized Generator

04/14/2017 2:01 PM

uh... a brief REVIEW of the basics before posting, can save (others) a lot of "tail-chasing" ...

{I recently made the same sort of mistake, in another 'arena', jumping-the-gun whilest being too tired}

["Double-checking" {the link} here, before posting; I see that even a SEEMINGLY well-done site can contain "issues"...

At the site linked above, discussion appears to go well until the reader gets to:

"...in case of capacitor start capacitors run induction motor there is no centrifugal switch so, the capacitor remains in the circuit and helps to improve the power factor and the running conditions of single phase induction motor."

Doh! What it MEANT to say is that the switch - which keeps the START cap in series with the Start winding, until it gets "up-to-speed" - will throw such that the Start cap is taken OUT of series with the (Start) winding and the RUN cap is placed in series with the START winding. [There is no capacitor in series with the Main/Run windings.]

[Apologies, but I am not going to "nit-pik" the remainder of that site right now. The first part clarifies the error in the 1st paragraph of post 17... and, enuf-for-now.]

Cheers ~

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#19
In reply to #18

Re: Compensating For An Undersized Generator

04/14/2017 2:15 PM

Providing you can keep the voltage up, perhaps. If you can't keep the voltage up, then the torque is not there...

It's all in the design of the actual motor, and the operating conditions, not necessarily the generalized statements of one web site to take as the final say...

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#20
In reply to #19

Re: Compensating For An Undersized Generator

04/14/2017 2:26 PM

? ok ... help me ... I appear to be missing the logic ...

It is agreed that the expensive solution ((to the thread, here)) of replacing the generators with higher ampacity, "keeping the voltage up" in spite of the voltage drop that occurs when a pump is starting, will likely solve his issue.

However, I am not sure that I understand how it will help by switching FROM "capacitor start" / "high starting torque" motors TO "split-phase" / "moderate starting torque" motors...

Is there some OTHER kind, described by "motors with start windings, centrifugal switch kicks them out when up to speed" ... ?

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#21
In reply to #20

Re: Compensating For An Undersized Generator

04/14/2017 3:03 PM

Capacitor start motors require the voltage to stay up to develop the torque, if the voltage sags, the current supplied drops to the square.

As for torque, your old washing machine motors were not capacitor motors, plenty of starting torque for constant torque load, designed for the job. Not all motors with start windings are moderate torque, torque compared to capacitor start motors. You can design it to what is needed.

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#23
In reply to #21

Re: Compensating For An Undersized Generator

04/14/2017 4:16 PM

Just add my 2 cents in your discussions......

It is the current that lags the voltage in this situation....having the pump as an inductive load. The motor tends to draw more power during start-up, in the process pulling more current which will then cause the current to lag the voltage even further!

The capacitor is there to correct that lag, as much as possible to be in-phase with the voltage for maximum power transfer!

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#24
In reply to #23

Re: Compensating For An Undersized Generator

04/14/2017 4:50 PM

[[ A personal thanks for providing that reply. I was going to wait until I could scan, and include, page 456 from my Hindmarsh.]]

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#25
In reply to #23

Re: Compensating For An Undersized Generator

04/14/2017 5:20 PM

If you can't hold rated voltage on your capacitor, you won't get design current from it. As it appears that the voltage available at the motor is too low to accelerate it in a timely fashion, you can't rely on capacitors to generate the current needed. As I said before, the current the capacitor can supply drops to the square of the voltage, so it is a downward spiral.

Try something that is not as dependent on line voltage, and assuming a centrifugal pump(?) doesn't need high starting torque, pick a technology with lower starting current, allowing the voltage to rise some.

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#22

Re: Compensating For An Undersized Generator

04/14/2017 3:14 PM

I think a large part of you problem is having way too small of generator powering way too many other things on top of it trying to start the 5 HP motor.

I used to work for a local business that carried Generac power units and after having been around them I came to rather like them. Especially their stand alone and heavy portable units which I suspect your 17 KW unit is likely in that category.

If so it should have plenty of reserve power to start a 5 HP single phase motor being most of the 15 - 20 KW rated units have some rather impressive 20 - 30+ KW surge capacity reserves built into them.

Generac 17 KW generator. ~26 KW surge.

I don't know what sort vacation townhome facilities you are running but I can't imagine ruining 50 units off a single 17 KW generator. In my books that would call for a 50 - 100 KW (minimum) multiphase unit.

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#26
In reply to #22

Re: Compensating For An Undersized Generator

04/14/2017 5:22 PM

I think all he's trying to do is to start the one pump... That generator should do it, even with a fixed voltage regulator.

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#27
In reply to #26

Re: Compensating For An Undersized Generator

04/14/2017 7:24 PM

IT should do it just fine but as of yet no secondary load values or line size and distances from the generator have been given so there is no way to know what else is drawing power off he generator when the pump starts plus how much of a line voltage drop the pump and generator is fighting to overcome on startup on top of that undefined base load.

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#28
In reply to #27

Re: Compensating For An Undersized Generator

04/14/2017 9:45 PM

Which could indeed be the root of the problem.... The kW & governor should take care of the real power needs to start, but the reactive needs, could be a whole 'nother story.

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#30
In reply to #28

Re: Compensating For An Undersized Generator

04/15/2017 7:13 AM

I'm not looking at it as reactive power loads but the whole 50 other homes it powers all drawing some as yet to be dined load as well.

For all we know they put a constant 15 KW load on the unit before the pump ever does anything.

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#31
In reply to #30

Re: Compensating For An Undersized Generator

04/15/2017 8:03 AM

True, and factoring that unknown into the equation, the OP's only interim solution available at this point is a distributed Load shedding!

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#29

Re: Compensating For An Undersized Generator

04/14/2017 11:07 PM

I'll reiterate what I said in #10 post, "Sounds like someone didn't do their homework from the get-go. Bit the bullet and tell the HOA their gen-set is undersized and they'll be money ahead to fix it now before ...." Cutting corners won't work in this situation

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#32

Re: Compensating For An Undersized Generator

04/18/2017 8:36 AM

Sorry Guys,

I was away for the Easter weekend. I've read all the responses and it doesn't seem like there is a real good solution to this problem other than the obvious, which is upsizing the generator. That's what my customer is going to have to come to terms with and get this taken care of. I did see where someone suggested increasing the size of the capacitor and maybe i'll give that a try for awhile first but it doesn't feel like a long term solution to me.

Thanks for everyone's help on this I will keep you posted as to what we finally do.

FYI... The generator is only backing up the pump and two lights inside of the pump house.

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#33
In reply to #32

Re: Compensating For An Undersized Generator

04/18/2017 9:32 AM

Check everything that can affect your voltage, as mentioned by others earlier. The generator real power should be adequate, as well as reactive power. I think your voltage is sagging so much that you can't develop the magnetic field strength in the stator to pull the rotor around.

Measure voltage at the motor when you try to start, and see what you have. See if you can tweak anything on the voltage regulator, like a Boost setting.

A bigger generator set with a cheap voltage regulator could still give you trouble.

You were on to something when you asked about different motors, some motor designs are not as dependent on good starting voltage as others. Your pump may not need the available starting torque to get started, so a lower starting current motor could also fix it. More capacitors will help to compensate for insufficient voltage at the motor, as the current from the capacitors (you need to develop torque in your starting winding) drops to the square of the voltage, or try a motor that doesn't use capacitors to get started. You need to watch out that the voltage doesn't go too high once the motor is started, if the extra capacitors stay in after the start.

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#34
In reply to #32

Re: Compensating For An Undersized Generator

04/18/2017 9:51 AM

"FYI... The generator is only backing up the pump and two lights inside of the pump house."

No wonder your friend - the manager is pissed!

Apparently this pump house has been the only load, and This will now be the 3rd time that generator will need to be replaced and be upgraded?

For "friendship's sake" hopefully this time you'll sized it properly.....

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#35

Re: Compensating For An Undersized Generator

04/18/2017 11:08 AM

Having been personally involved with Generac's and a condo's sewage pumping station, it's possible that you're looking at the wrong end of the problem. The condo had idiots that would throw anything that would fit (and some things that wouldn't) down the drain, resulting in fouling of the pump and its inability to start even under utility power.

A sewage expert redesigned the sump with a macerating ejector pumping system that effectively ground up the waste before it reached the impeller. It worked fine after that.

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Andrew Westman (2); Brave Sir Robin (2); D-Power (1); dj95401 (3); ndt-tom (4); RAMConsult (2); Rixter (3); rwilliams (7); SolarEagle (2); tcmtech (3); Tornado (2); vsar (4)

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