Can I run a motor into the SF on an extended basis?

[KernOily]

Dumb mechancal engineer question here guys. I am sizing a new installation for a boiler feed pump. Pumps are split-case multistage centrifugal. The bhp req'd is right on the edge of 1500 and will be slightly over 1500 at times. Motor will be 1.15 SF. We are arguing over what the nominal motor HP should be: 1500 or 1750. This will be a 4160V motor on a VSD.

The question is, can I run a 1500 hp nominal motor into the SF on an extended basis, or is this a no-no?

Thanks!

Thanks!
Pete

 

[waross]

Hello 74Elsinore
Yes but, there are lots of buts.
You didn't say how far into the SF you will be running, or for how long.
I don't know much about boilers but I was under the impression that when a boiler wanted feed water, it wanted it now, and didn't accept excuses. From an electrical point of view, if i was asked to size a mission critical application like a boiler feed pump, I would tend to go with the bigger motor.
However, let the forum know the extra information and wait for a few more responses.

 

[edison123]

I agree with waross. No sense cutting corners with a critical motor like BFP. I would definitely oversize the motor.

SF comes with a rider that the service life of motor will be reduced but by how much is a million dollar question.


* Basically, I would like a full-time job on part-time basis *

 

[aolalde]

At 1.15 SF the temperature rise will increase more than 10 °C, That means the life expectancy of a motor will be at least cut in half.
In theory the motor can run continuously at 1.15 SF but the pay is reduced life. Analize the investment vs expected life to make a decition.

 

[ScottyUK]

You're using a VSD which will arguably reduce the insulation life anyway unless special measures are taken - why further reduce by running it above nominal rating? The maintenance crew will curse you forever if the marginally sized motor becomes troublesome. The smell of a badly performing project hangs around a long time - important if you are looking for future work at the site, or are a resident engineer.

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I don't suffer from insanity. I enjoy it...

 

[NEWFIEBOY]

Is there a down side to going with the larger motor and vsd such as cost, availability or installation concerns? If not why take a chance on the smaller motor?

 

[DickDV]

Not a good risk, especially on an inverter. The general rule is that there is no service factor when on inverter power and its a good one.

Further, how would you set up the drive parameters. You would have to misrepresent the motor FLA to get the service factor and then the drive will not calculate motor temperature correctly.

One major exception to all of the above---if you are measuring motor temperature directly thru the use of thermal switches, rtd's etc. then I would expect some overcapacity of the motor to be possible especially if you control the ambient temperature to less than 40 degrees C.

In that case, the amount of continuous overload capacity is largely determined by the ambient temperature--the cooler the ambient, the larger the overload capacity. With a medium voltage motor of this size, I would expect that direct temperature measurement is being done.

 

[UKpete]

74Elsinor, there are some similarities here with another very recent thread that may be of interest to you:
thread237-150374
This question about SF comes up quite frequently and is always a bit difficult to answer because of the imponderables, so the advice is invariably to play-safe.

Be clear about what is meant by rated current. When a new motor design is type-tested by the manufacturer it receives a continuous load run at rated current and rated speed until temperatures stabilize, to confirm that the winding temperature is below that (by an undisclosed safety margin) permitted for the class of insulation at the specified ambient temperature. It is most likely done on a sine-wave supply. One obvious reason for the safety margin is that motors of exactly the same build quite frequently give differences of up to say 10°C on test.

How that relates to your application is the big question; for instance, what do you mean by "extended", and how predictable is the application process.

The penalty of exceeding the permitted winding temperature is often quoted as halving the insulation lifetime for every 10°C ABOVE the permitted temperature. It's very approximate but it's all we have to go on.

It is certainly true that you can operate a motor above its continuous rating for short periods without shortening its life, it depends on the ambient temperature, the speed (if cooling is related to this), the RMS current calculated over the complete duty cycle (only valid if this period is short relative to the thermal time constant of the motor as seen by the winding), the emergency duty requirements. And probably other factors that I have overlooked.

 

[UKpete]

I should have added: your question was given in terms of the motor rated HP, strictly speaking it should be on the motor current - that is what you should be measuring and comparing to the nameplate rated current. The temperature of the motor is more closely related to the current than the HP.

Neither the rated current or rated HP apply unless you are at rated speed and voltage.

 

[DickDV]

UKPete, love that word "imponderables"!! We should use it more often around here. Maybe elsewhere too!

 

[UKpete]

Well I did look it up just to make sure it was the right word.

While I'm on, and regarding SF, I have since checked MG-1 and it does describe it in terms of an allowed POWER up-rating as correctly described in the original question; i.e. an SF of 1.15 allows you to run at 1.15 x the rated shaft POWER (at rated voltage and rated speed). Not current.

But if you are looking at a motor already installed that you want to push a bit beyond its rating, the SF could be directly applied to the rated current, as this is easy to measure and it's more closely related to heating effect (especially on a variable speed application).

In rail traction (where I was once) a significant amount of effort has to be put into determining the required motor rating for a particular application as there are so many variables. The use of an RMS rating was essential - because the motor operating point is changing all the time it was necessary to calculate current step-by-step for a journey and obtain a single value of RMS current for the whole journey. This became the rated current and was valid because generally the thermal time constant of the motor was long relative to the time between station stops. So you could quite happily run for reasonable periods well above the rated current without shortening the life of the insulation.

Why do I mention this? Well, if you have a tricky though well defined duty cycle, you could use the same principle.