starts per hour 5

[BRIS]

My field is civil/hydraulic engineering. I am looking for advice on the frequency of motor starts for pump sets.

I have a series of irrigation pump stations which will run continuously for 9 months per year. The pumps are fixed speed and will cycle on and off. I need to size the pump sumps to limit the cycle time. Normally I would allow no more than 8 starts per day but I am being pressed to reduce the sump sizes. The client's electrical engineer is suggesting as many as 10 starts per hour. The client also wants a long design life and minimum maintenance which would not seem to be compatible with 10 starts per hour.

Motor sizes vary from 20 kW (27 hp) to 600 kW (800 hp). Motors are air cooled. Ambient temperature may be 40 c (105 f).

Brian

 

[jOmega]

Hi GGOSS,

you said:

"This is valid however you should note soft starters are capable of sustaining any duty cycle if they are selected correctly. For example we have many installed that are doing upwards of 300 operations per hour."

Your point is well taken...with some reservation and that is the law of diminishing return.

Yes, the Soft Starter can be oversized to tolerate a higher number of starts per hour. For example, a 50 hp unit with a limit of six starts per hour or once every 10 minutes... could be oversized to say 200 HP ... and then you could start that 50 HP motor as often as you like.

But at what cost ? and how many starts/hour can the 50 HP motor tolerate ?

Certainly the 50 HP Soft Starter is a less costly unit than the 200 HP unit. Agreed ? And I'm sure you realize that you have to provide, at additional cost, motor branch circuit protection (O/L etc) for the motor because you cannot use the BCP in the oversized Soft Starter since it is sized for a much larger motor..... Agreed ?

Now, suppose that instead of a 200 HP Soft Starter, a VFD were to be used. It could be sized at 50 HP and be a lot less expensive to the user than the 200 HP Soft Starter.

And by not having to oversize the motor to accommodate the higher number of starts/hour, the user realizes additional cost savings.

That, GGOSS is what I mean by "the law of diminishing return".

But to your thought, no argument! It can be done.
The only question to be answered is the one of cost/benefit ratio.

Next, not being familiar with IEC60947-4-2, could you provide some amplification to your comment about mfgrs. having to supply product based upon user specified duty requirements. It would seem that the mfgrs. would be required to provide custom designed products above and beyond their standard designed catalog listed products.
Is this so ? From a manufacturing view point, that would be a nightmare.... It costs a lot of money to custom design product for every customer's individual application needs.

Hope you can provide additional insight as I'm over budget this month on ordering specs.
Thanks. ..



jO

 

[GGOSS]

Hello JOmega,

Your points are well taken and in fact fully supported. I too firmly believe VFD's are a far better option in cases of high starts per hour.

For more information re soft starter ratings to IEC60947-4-2, please view my post of July 14, 2003 in thread237-59356 "Ratings of Soft Starters". Needless to say, many other posts in that thread also provide very good information.

Should you require any additional information and/or clarification, please do not hesitate to ask.

Regards,
GGGOSS

 

[jOmega]

GGOSS:

Much appreciated. That's one of the best threads I've seen in the forum.

By-the-by.... have you noticed that SS mfgrs are going to smaller footprint (sized) units by providing an auto-bypass that transfers the motor to the mains when the acceleration is over ? In this way, the SS serves only to accel (or decel) the motor.... and generates no heat in the devices while running in bypass.

I would expect that this technique will also increase the number of starts/hour when the recommended time between starts is observed. I believe that some have also increased the air flow thru the heatsink as well to improve the duty cycle.

Thanks again.

jO

 

[PUMPDESIGNER]

jOmega - I have heard (read not documented)that at least one VFD manufacturer does not like to see the load switched to ACL after motor is up to speed. Complaint was front end bus could be damaged.

Anyone else about this?

PUMPDESIGNER

 

[TheDOG]

BRIS;

I would suggest using outlet damping as a viable alternative in this situation. It will save you dollars (just not as much as VFD) and is easily maintainable.

 

[GGOSS]

Hello jOmega,

Many thanks for the kind words.

Soft starters with inbuilt bypass contactors and/or rated for use with external bypass contactors are also covered by the standard. A utilisation category (AC53b) has been established accordingly. However there are a few issues worth noting. As these are not relevant to this thread, I will start another (or write an FAQ) over the next couple of days.

Regards,
GGOSS

 

[BRIS]

Thanks for the response.

What is outlet damping?

I have now modelled my hydraulic system and I am happy that fixed speed pumps with on off control work well. From a hydraulic/efficiency point of view there will be no advantage in using VFD. I have also got the maximum number of starts at no more than 3 times per hour.

This takes me back to my initial problem of number of starts per hour.
The comment that: “VFD's are a far better option in cases of high starts per hour" I understand to mean that VFD will be more robust than Soft start for frequent starts but from other posts I also read that VFD will be significantly more expensive. Maintenance (or lack of it is a key factor) hence I am taking the advice that VFD is not the solution.

So I think my solution is soft start. Although I now need to question why we are using soft start rather than simple Star Delta starting which must be both more robust and less expensive.


Brian

 

[Marke]

Just to add my comments on the effect of the use of soft starters.
1. The use of a soft starter will not thermally increase the number of starts. This is because the power dissipated in the motor during start is a function of the full speed kinetic energy of the load plus an additional amount related to the time taken to overcome the work component of the load. This is the same for all means of starters. If the start torque is limited too much, the start gets extended excessivly and the power dissipated in the motor is increased.
2. The use of the soft starter will reduce the electrical disturbances on the supply and this may allow the starter to be operated more frequently.
3. The use of the soft starter will reduce the transient loading on the supply, and the mechanical shock loading on the motor and driven load. This will reduce the stresses on the machnine and can reduce the frequency of maintenance on the equipment.

To summarize, if the number of starts are restricted by the motor thermal rating, then the soft starter will not increase the number of starts per hour. If the starting frequency is limited by the disturbances on the electrical supply (Flicker regulations and the like) then the use of the soft starter may provide a useful gain. If the starting frequency is limited by the mechanical degradation at each start, then the soft starter will provide a useful extension of equipment life, or increase in start frequency.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jOmega]

GGOSS:

Great. Will be watching for new thread or FAQ.

Kind regards,
jO

 

[jOmega]

PUMPDESIGNER

You Said.....
"I have heard (read not documented)that at least one VFD manufacturer does not like to see the load switched to ACL after motor is up to speed. Complaint was front end bus could be damaged."

I'm not sure how the "front end" of the VFD gets damaged if the load is suddenly disconnected from it. To me, it would be no different than giving the VFD a STOP command that immediately turns-off the output IGBTs (Coast-to-Stop mode).

But I do know that there are VFDs that are not designed to tolerate the interruption of power flow from VFD to motor under load. There are transients that occur as the contactor between the VFD and the Motor opens...that can damage the output GBTs.

Some manufactures will allow opening the power flow to the motor under load, with the addition of 100 microHenries of inductance between the VFD output and the line side of the contactor. (or a 1-1/2% reactor with gapped core can also used).

There are a few manufacturers that claim they can tolerate the interruption of power flow; Danfoss VLT 4000, 5000,6000, & 8000 drives.... to mention one...In fact, while witness testing some 600 HP drives being factory tested about a year and a half ago... I observed that part of their test routine is to open the output between drive and motor.... and..... to reclose the motor on a running inverter without damage. I was impressed. I've seen other mfgrs. drives take a hit on such a test.

Ok.... them's the words on VFDs and interruption of output power flow to the motor ....i.e., taking the motor up to rated frequency and then disconnecting it from the VFD and transferring it to ACL in asynchronous fashion. There is however a concern about how much current the motor will draw when it is closed in on the line power...remember the rotating field inside the motor isn't synchronized to the line before the transfer.... ergo....aysnchronous....

Usually there's a timer....that gives a bit of time before the motor is closed in to the line power... and in this way, allows the flux field in the motor to decay before closing in.

Now to the case of the SoftStarter.... This is a different case...

Consider that the bypass contactor is a three-pole contactor, the contacts of which short across each inverse-parallel set of SCRs.... In this way, the field in the motor is synchronized to the line because it was directly supplied from the line thru the SCRs while soft starting.
No change to phase sequence occurs.

With no current flowing thru the SCRs.... they cease to conduct. If they don't conduct.... no power thru them...ergo...no heat ....

So, using a bypass contactor to connect the motor to the line IS different for a VFD than it is for a soft starter.

Ok ?

jO

 

[PUMPDESIGNER]

jOmega - Thank you for your considered response to my question. I saved the file, will quote you from now on.
Just kidding.
Thought you were correct on everything you stated.

PUMPDESIGNER

 

[jOmega]

BRIS:
Your comments:

"The comment that: “VFD's are a far better option in cases of high starts per hour" I understand to mean that VFD will be more robust than Soft start for frequent starts but from other posts I also read that VFD will be significantly more expensive. Maintenance (or lack of it is a key factor) hence I am taking the advice that VFD is not the solution."

1. The advantage of using a VFD when you have a high duty cycle (frequent start-stop sequence per hour] is not because of the robustness of the VFD.

2. The advantage is that motor and pump are not subjected to anywheres near the stress (both electrical and mechanical) that DOL or Softstarting imparts to the motor and pump.

3. Consider that if you set the current limit on the VFD to 100% ... that the motor can start and accelerate at 100% current as often as you like, all day, all week, all year, and not exceed its thermal design rating.

4. In addition, the pump is not subjected to the shock starting torque that DOL or Softstarting subjects it to.

Now, wouldn't you say that that was much friendlier to both the motor, and the pump ?

As to the argument of cost differential, it's a one-time cost and in addition, there can be energy savings realized with it as well.

Balance this against the cost of recurring maintenance, reduced life of pump and motor, and downtime.

Still think the VFD is more expensive ? long term ?????

 

[BRIS]

Jomega

Thanks for the clear arguments

VFD will also be friendlier to the pipelines which are likely to be fibreglass (GRP) and will be weakened by fatigue caused by hydraulic transients from frequent starting.

BUT, the pump stations are in a undeveloped region where maintenance skills are basically blacksmith technology and limited to little more than swinging a big hammer.

What I see in existing pump stations is cabinet doors permanently open, automatic trips by-passed, instrumentation disconnected etc. (replacing fuses with 6 inch nails is a real possibility). Supply voltage fluctuations vary from 13 to 16 kv. Robustness in my situation means it has to be robust against unskilled and ham fisted operators and maintenance personnel. The staff do manage to keep basic technology working, in a fashion, but rarely as the original design engineer intended.

Hence my concern over VFD - I think I should also be concerned over soft start?

Brian

 

[jraef]

BRIS,

Even with all the fascinating and well thought material above, I don't see what I consider to be a very important point addressed. You mentioned early on that you were going from a 3 pump system to a 2 pump system, hence the need for more starts/hr. There is a flaw in the concept that perhaps your client is missing. No Redundancy. If you have "blacksmith technology" skill levels in the maintenance people, wouldn't it benefit the owner to make sure the system has a modicum of backup capacity? I would think that in your corner of the world, reliability of irrigation water is critical (not that it isn't everywhere actually). Prudent design theory would have a system wherein 2 pumps can carry the entire load, but 3 are installed and alternated for even wear-and-tear. That way the system can still operate fully when one pump is down for "big hammer" maintenance. My first approach would be to sell the old design concept back to the client. Ppoint out the losses that will occurr when (not if) one pump dies at a critical time. We all know that it WILL happen.

As to SS vs VFD, I can offer my 2 cents worth as well. We sell RVSS starters to the portable rock crushing industry all over N. America and there are no electricians at the sites for maintenance. Just "Bubba" the crusher operator who is a step up from ditch digger. RVSSs work fine in that environ, and reliability is such that they rarely need to open the box and look oinside, thus avoiding trouble.

Although VFDs are used successfully in that industry as well, your concerns are valid not only for maintenance but also for their being a focal point for curiosity. More often than not the problems I must solve in the field with VFDs at sites with unsophisticated operators are due to someone "tweaking" the settings because they THINK they know what they are doing. It can be a real nightmare.

By the way, the reason VFDs can start the motor more often than RVSS has nothing to do with robustness (robustosity?). It is simply because the motor can be accelerated to full speed without exceeding FLA because of the ability to alter frequency. Less heat at acceleration = more thermal capacity remaining in the motor = more starts/hr. If anything, the additional components required to accomplish this would decrease the "robustosity" as comparison to the RVSS.

Good luck.



Quando Omni Flunkus Moritati

 

[jOmega]

Bris:

I understand well your concern about shock to the piping that is attributed with both DOL starting and stopping. While SoftStarters can mimimize the shock somewhat, it is far more gentler to utilize a VFD with a "Pipe Fill" mode that controls the fill rate of the pipe as a function of pressure build and with -up...

Several VFD manufactures have incorporated such control in their designs for the water industry. I know the Danfoss VLT 8000 has it and a New Zealand VFD manufacture (PDL) I believe also has it. I wouldn't be surprised if Grundfos also has it.

As to the "tweaking" problem, many drive manufacturers have removable keypads capability which elimiates the tweaking. Some VFDs also have a Lock-out mode so that parameter adjustment requires a password be entered before gaining access to the parameter adjustment capability.

Also, consider that with "communications" capability and proper software, the drive can be set-up with a lap-top computer interfaced to the VFD. This then impedes the "tweaking" by those that have nothing better to do than to push buttons and twist knobs.... and the Base Setup parameters can be maintained on the hard drive of the Laptop... as well as copies of the set-up after adjustment... etc.

And don't overlook the fact that Soft Starters are very "tweakable" too...



Pipe Fill Mode functions to eliminate the "hammer" that occurs on both starting and stopping the pump by controlling the pressure build-up and build down so as to prevent "shock" occurrences.

jOmega