Motor starting current and genset size 1

[Munequip]

We have had a problem at a sanitary lift station and would appreciate any comments or suggestions.

The station is equipped with two (2) 28.2 HP (21.0 kw)submersible pumps and a 80 kw diesel generator with an 200A automatic transfer switch. The generator is sized ot operate two motors on a staggered start sequence.

The pumps operate fine when connected to utility power. When the pumps are operated via the stand by generator, the across line motor starter will "chatter" briefly (less than 1 second).

The submersible pump motor has a full load amp rating of 32.3 amps at 480 volts, and operates at 3450 RPM. The published locked rotor current for the motor is 245 amps. The NEMA code rating for the motor is "H".

During operation from the utility power, the in rush current has been measured between 380 amps and 420 amps (depending on leg and test). This was measured with a Fluke model 453(?).

When the generator attempts to start one of the pump motors, the control voltage will drop from 120 volts to ~ 75 volts. The control transformer is 5 kva. Two different transformers have been installed, with the second unit incorporating taps to try to provide some adjustment. The only items requiring control power at this point are the coils of the motor starter. There have been two different manufacturers of motor starters installed in the control panel

The submersible pump manufacturer states that the inrush current is with in an acceptable limit of 10 to 15 times FLA. They also state that since the units operate on utility power the issue is with the gen set.

The generator has had the voltage regulator changed, and the voltage has been adjusted to provide between 460 volts to 502 volts. The same results have occured. During the last round of testing , one of the units stopped "chattering", but the other unit has not (for no apparent reason or adjustment).

We would appreciate any type of response as we are currently baffled.

 

[rbulsara]

Review FAQ faq237-766

 

[dpc]

The starter contactor can drop out at anything less than 85% rated voltage. At 75% control voltage, I suspect your contactor is just having a tough time pulling in. The voltage regulator/generator system cannot react fast enough to avoid this transient voltage dip. The fact that one contactor seems OK and the other has problems could be due to manufacturing tolerances, or could indicate one contactor is harder to pull in than the other.

Simplest solution is to put the contactor control power on a separate UPS system to isolate it from the line voltage. This would be easy to test to see if a separate source of 120V solves the problem.

I don't think the problem is in the motor.

You might need to get a EE out there to see what is going on.

 

[itsmoked]

Hi Munequip,

I see this a lot. You would also end up with the second pump
chattering after the generator is loaded with the first even worse.

dpc is correct about UPSing the contactors.

I believe the best solution though is to put soft starters in. They aren't very expensive and you would never shock load your generators again. They limit the motor starting current to whatever you set them at (within reason).

We had the same problem when a compressor started. All sorts of controls would drop out due to the sudden brief voltage sag.

Very annoying when the VCR spits out its tape.. Anyway the S.S. toally cured all the problems.

 

[jraef]

I agree with itsmked, solid state soft starters are the simplest readilly available solution.

That said, there is something in your post that bothers me a little.

The submersible pump manufacturer states that the inrush current is with in an acceptable limit of 10 to 15 times FLA.

10x to 15x is an acceptable limit? These had better be some really good energy efficient motor designs, because that is NOT generally considered acceptable, should be closer to 6X FLA for a design B motor. And why would someone use a high efficiency motor on a short duty cycle application like a sewage lift station anyway? You are asking for trouble IMHO.

Be that as it may, you already have the motors and now need to deal with the issue. Add a couple of simple retrofit soft starters.

Side issue for your future reference: you mentioned changing out the control transformer twice. If you did not use a 'control power transformer', that can be a problem, because standard distribution transformers are not specifically designed to handle the inrush transient of an AC coil as CPTs are. That means you must use a much larger transformer to avoid voltage drop. In this case however, a 5kVA transformer for 2 x 30HP contactors was serious enough overkill to make up for that.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[GGOSS]

Many soft starters monitor the frequency of the incoming supply and will trip if that goes outside of the manufacturers specified range. They are therefore not always the best proposition for starting loads fed from a generator.

As the load come on the prime mover of the generator may slow down which in turn reduces the generators output frequency. If the alternator section does not have a sufficient overload capacity or the governor does not act to increase fuel supply to the prime mover quickly and smootly enough, there is a possibility that the soft starter will see an 'out of frequency range' condition and trip accordingly.

Most soft starters have a frequency tolerance of +/- 3Hz, some +/- 10Hz, others disregard frequency altogether once they enter start mode.

As indicated by others above, the contactor chatter issue would normally be voltage related. A soft starter may act to minimise voltage disturbance, but may trip if the generator is undersized or poorly maintained.

Regards,
GGOSS

 

[itsmoked]

Yow GGOSS you are absolutely correct! I had that same problem once... Had to then change the governor to a fast one...

Munequip do look for the widest freq soft starter u can find if you go that route.

I wish the S.S. makers would get their heads outta the sand on that one.

 

[GGOSS]

itsmoked,

Some of them have got their heads outta the sand, but as there is no room for commercial stuff here, I will refrain from mentioning names.

One other possible solution for Munequip is to change the AVR (automatic voltage regulator). To assess the possibility for improved performance (better voltage regulation), discussions should be had with the manufacturer of the gen-set.

Regards,
GGOSS

 

[rbulsara]

My two cents:

Most of above repsonses are good..for cost and ease to implementation, try this:

1. Provide a separate source of control power, like dpc said, via small UPS fed by the gen power. UPS battery will hold the power for 10 seconds. This appears to be cheapest.

2. Fit the generator with a digital voltage regulator, for faster response.

3. Add capacitor to the motor, to bring starting current down and hence the voltage drop.

4. Use soft starter...best but most expensive..

5. See if you can find a motor starter coil (contactor) which work on lower voltage..that is one with a wider range of voltage..

 

[weh3]

If your generator cannot handle the starters, then it cannot handle the motors. It is possible that phasor voltage difference between the source and the motor is greater than if the motor is stopped.

Why does it matter whether the coils are held in during a changeover?

Why don't you put a delay in your ATS so that the motors can spin down and the generator can spin up?

Then you can bring them back up on soft starts. Your 100 kVA generator ought to do OK as long as it doesn't have to deal with a motor inrush.

William

 

[rbulsara]

What i meant in my last post is that try the soultions in that sequence and stop where it works.

weh3:

you are not reading the question right, its not during the change over..I think OP is talking about starting motors from standstill but fed from the generator. Coils are chattering during the start..not during the change over..there is no mention of ATS in the question.

 

[Munequip]

rbulsara is correct in that the problem occurs when starting the motors from a standstill and the power is being supply by the genset.

The genset manufacturer has been to the site on several occasions, and to date, have been very repsonsive. Unfotunately, they have changed most of the items that are easilly changed, and adjusted the voltage, and the problem still occurs. They have now stated that there is nothing they can do, and that the genset is sized properly for the load, so it must be a problem in the motors.

The motor manufacturer states that they do not have a prbome in the motor. So we are in the middle trying to figure out a resolution.

As for the comment from jraef regarding the current, it is my understanding that there are three "levels" (simply term from a Civil Engineer) for the current. The full load current of the motor, the starting current of the motor (which is ~ 6x FLA), and the in rush current of the motor (which is 10 to 15x the FLA depending on which reference item is used).

Any clarification to this item would also be appreciate.

 

[rbulsara]

starting current is same as the inrush current. The maximum inrush or starting current will be equal to the LRA (locked rotor current). 6x FLA is most common value. 7X at the most.

If you have a motor with LRA of 10 or 15 times the FLA, that will be very unusual. So it is possible that you have very different motor than other installations.

The motor should have a code letter (not the insulation level) like G or H on its nameplate. NEC has the multipier assigned to those code letter which will give you maximum kVA or LRA. If you use that multipier with the FLA you get the LRA.

 

[itsmoked]

GGOSS; What S.S. maker makes an S.S. that either doesn't care about incoming frequency or allows the greatest diviation?

I have no idea why they all insist on providing frequency diviation detection functions with softstart. It's rather like going to the auto dealer and finding every vehicle comes with an anchor on fifty feet of chain.

 

[dpc]

A couple of comments on motor starting current:

Submersible motors are notorious for having high inrush, low power factors, low efficiency, and all sorts of other unusual "features". These motor generally do not have to comply with NEMA MG-1.

It is important to distinguish between locked rotor current and transient inrush current. They are related but not the same. Locked rotor current is the steady-state symmetrical ac current drawn during locked-rotor conditions. Transient inrush current is the total asymmetrical current drawn at the instant the motor is energized and can include a significant dc transient component in addition to the symmetrical locked rotor current. The 10x to 15x value is probably the transient inrush current including the worst-case dc offset which is a function of the motor X/R ratio and the source voltage phase angle when energizing. 10x doesn't sound high at all for this value.

15x is high for a general-purpose motor, but maybe not for a submersible motor.

I agree that a solid-state reduced-voltage starter is an option, but there can be interaction with the generator during starting since the solid-state starter is a potent source of harmonics during starting.

 

[jraef]

WARNING! TANGENTIAL DISCUSSION!

I have no idea why they all insist on providing frequency diviation detection functions with softstart.

I'll butt in here. "They" don't all need to do this, but many do as GGOSS pointed out. That is mainly because of their phase angle sensing circuit. In order to know when to fire the SCRs within the phase angle, you need to know what it is to begin with. You can't measure zero, so you look at the cross over point between two phases, i.e. where the 1/2 wave voltage of A-C falling = B-C rising.

A common and inexpensive sensing system looks at just one cross-over point and determines (through timing) the firing point of all six SCRs from that one point in a phase-locked-loop circuit. This works absolutely fine on utility power, but if your frequency starts drifting out of range, this timing system gets messed up and begins firing the SCRs at the wrong time.

The first design to use this system came out of the South African minig industry years ago, and they were very successfull all over the world because they were simple and inexpensive. Their success created several spinoffs and copycats. Some of those spinoffs got bought up by other larger companies in the industry consolidation that happened a decade or so ago. Unfortunately all carried with them the same basic flaw, leading to a rash of soft starters that had problems running behind generators. Ahhh... the pitfalls of reverse engineering or buying your way into a technology!

A few manufacturers have fixed this problem, but because it involves adding to the component count many have not bothered, chosing instead to just shut the starter down whenever the frequency drifts too far.

There are however other firing angle sensing methods out there that started earlier or later than that one, and spinoffs or copycats of those systems don't have that same problem.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[itsmoked]

I see... Thanks for the history lesson. I do fully understand the basic functioning of the SS. If I was building one it would seem easy to base the coming cycle's control on the present zero crossing without bothering to look at the actual frequency. It just seems so logical to use SSs on generators, specifically to allow smaller more efficeint generators start larger motors. sigh...

 

[GGOSS]

Rbulsara - most High Efficiency motors will have a Locked Rotor Current of greater than 7.5 x FLC. I have just completed a start performance appraisal for a customer, his motor had an LRC of 8.7 x FLC. I have also experienced motors with LRC values of up to 11 x FLC.

In addition most HF motors will have an LRT value of around 120 - 140%. That means elevated starting currents; even under reduced voltage conditions, which of-course makes it all the more difficult to start machines that are gen-set supplied.

It is far less common these days to find a motor with an LRC value of less than say 6.5 x FLC than it was 5 years ago.

Itsmoked - I won't post manufacturers names here, just not comfortable in doing that. I can give you a tip however. All soft starters will measure frequency at the pre-start phase of operation and it must be within specified range during that time. Most lock into that during the soft start, run and soft stopping phases of operation and any drift beyond specified range will result a trip condition. There are some soft starters on the market that provide a frequency tolerance during soft starting and soft stopping that is wider than the frequency tolerance during pre-start and run phases of operation. There are some soft starters that ignore frequency or dynamically recalibrate themselves during the soft starting and soft stopping phases and those (to my way of thinking) provide real advantages in gen-set supplied installations. As stated above I will not mention names here, but with this information available to you, you you are now armed to ask all the 'right' questions of your soft start suppliers.

jraef - good input as usual. When purchasing soft starters it's certainly a case of you get what you pay for.

Regards,
GGOSS.

 

[DoggyDog]

I'm a bit surprised that no-one has suggested the trusty Autotransformer starter? It has the added advantage of the transformer effect (ie. VA in = VA out), and it will not trip on any supply frequency issues.

An Autotransformer starter will (generally) provide more torque at lower current than an Electronic Soft Starter will.


Sometimes, simple is best!

 

[jraef]

DoggyDog,
You are correct in theory, however look again at the HP size. 28HP at 460V? I challenge you to find an RVAT starter that small, and in the larger sizes where they are available, solid state soft starters have been less expensive for years now.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more." Nikola Tesla
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