Selection of System Voltage for 50 Hz Country

first, i'm no electrical but as i understand motors will be used to drive pumps as mentioned here. before you proceed with your plan, consider this: you will have a problem on the the pump performance. changing the frequency means changing the speed. and speed is proportional to the pump capacity (reduced speed, reduced discharge flow) while your power consumption will be greatly decreased (this is good though) since power is proportional to the cube of the speed.

thanks and good luck!

Unless you put a electronic converter, that takes care of frequency AND voltage increase, in the circuit, your 60Hz engine will not perform. Count max. 60%.

Besides this, when built for 60 Hz, it contains less iron and will run hot on 50 Hz.

Change the motors.

The performance of the pumps, i.e. their pressure/flowrate characteristic curve, will move downwards at the lower speed, and this may not be acceptable from the Process Engineer's perspective.

I must say, that's worthy of a GA.

What will be the effect of replacing the impeller with a larger one?.

if you want a increase the impeller you just as well get a bigger pump

Flow is directly proportional to speed, Head (pump output pressure) changes at the square of the speed change, Horsepower required changes at the cube of the speed change. If you can live with the reduced performance of the pump then as JRaef says, operating the 60 Hz motors at 380 Volts 50Hz is fine. I agree that the rest of JRaef post is spot on, good answer.

this might be an off-topic but i beg to disagree with your statement: "In a centrifugal (quadratic) pump, flow follows the cube of the speed change"

....in pumps or fan affinity laws for the same pump size, flow is just proportional with the speed, by formula:

Flow rate,Q1/Q2 = N1/N2,

which means when speed is reduced by 50%, flow rate is also reduced by 50%.

....it is the power consumption that is a function of the cube of the speed or:

Power, P1/P2 = (N1/N2)^3

Thanks!

SO, what is the problem? ? ?
You have a VFD drive . . so set it to provide you with the required voltage and frequency.

That is what >Variable Frequency Drives< are for !!!

"SO, what is the problem? ? ?
You have a VFD drive . . so set it to provide you with the required voltage and frequency.

That is what >Variable Frequency Drives< are for !!!"

Read my post above again.

You cannot feed a VFD with 380V and tell it to output 460V! It can change the Frequency to be anything up to 400Hz (mostly) but the voltage change can only be LOWER than the input, not higher.

If I could make one that could create voltage from thin air, I would be a rich man...

Before you start thinking of using VSD/VFD's check the motor.. it may not be suitable for VFD/VSD operation and also check harmonic levels.

Most modern and quality 3 phase motors will handle VFD's, though there are VFD rated motors. Many VFD rated motors may only be standard motors that have been Horsepower de-rated. Do you mean electrical harmonics, or vibration harmonics? Vibration harmonics for a properly designed 60 Hz operated pump will be fine at 60 Hz, but have a problem at 50 Hz running speed. A properly designed VFD installation should not have electrical harmonic problems.

I mean electrical harmonics....

While what you write is all correct up until you say...A properly designed VFD installation should not have electrical harmonic problems.

I wish that was true... but unfortunately its not. VSD's do cause harmonics, and there are thousands of HMU's (Harmonic Mitigation Units) on the market all over the world, tuneable and fixed, to remove the harmful ones. The problem with harmonics it migrates to other power systems and costs money in lost power and damages other units/motors/electrical equipment on those power systems.

In an ideal world, and there are VSD's that have less than 5% TDH which is IEEE519 recommendations, out there, and they are becoming more prevalent in the market place as users are become smarter and greener.

In my industry, oil production using ESP's, we have motor and cable failures because of harmonics and bad power quality, some due to the generated HD caused by VSD's with no filters and numerous inductions motors at the end of VERY long lengths of cable (sometimes over 2 Kms).

Now by saying "properly designed" you mean it has filters or the drive has acceptable levels of harmonics then yes, but that means the user or purchaser has done his homework, or the supplier has sold the latest piece of kit that meets what will soon be an industry standard.

However not everyone is as clued up as we are, many people do not research alternatives or understand how things work. Thats why they ask questions on CR4

My words were cautionary ones, as these things have to be pointed out.

Of course I was speaking of VFDs with filters (reactors, isolation transformers) installed, and proper installation (cable size and length). Though a properly sized, quality motor will absorb most of the harmonics without problems.

The VFD driven motors of the system will run at the same speed whether the frequency is 60 or 50 Hz. Motors run under the rated speeds at 50 and 60 hz. That is not the issue. But it has more to do with the system voltage/frequecy you end up with.

Option 1: 480 VAC/50 hz

Option 2: 380 VAC/50 hz

I would go with option 2 in which case motors will have to be rated for 380 VAC; Option 1 is awckward combination, (agree?) The other reason is in case of motor replacement is needed you can more easily find 380 VAC, 50 hZ motors in that country that 480 VAC 50 hZ.

I ask, do you know of motors or transformers that are rated for 480 VAC 50 hZ? there will be other motors running without VFD (granted smaller motors ie 2, 3 hp) so again if your system is 480 VAC 50 hz i dont know that motors are rated for these ratings.

Any comments?

The VFD driven motors of the system will run at the same speed whether the frequency is 60 or 50 Hz.

Are you saying that the motor RPM is the same at 50Hz and at 60Hz???

Again,

VFD driven motors will run at the same speed wheter 50 of 60 hz on the utility voltage by virtue of the VFD driving the motor at the set (desired) speed that our specific process requires. The speeds at which the motors are set to run for this process are below the rated motor speed wheter 50 or 60 hz, ie a 4 pole motor rated speed is about 1800 at 60 hz and 1500 rpm at 50 hz but we need that motor to run at 1400 rpm hence the VFD driving it. So whether the frequency if 50 or 60 Hz of the utlity the motor will run below its rated speed via the VFD.

i hope this clarifies.

I think the confusion comes from your statement..The VFD driven motors of the system will run at the same speed whether the frequency is 60 or 50 Hz.

If you had included the word..The VFD driven motors of the system will run at the same speed whether the SUPPLY frequency is 60 or 50 Hz.

then I would have understood it better.. let's put it down to a typo.. thank you for expanding... I two have dyslexic fungers!!

If we are talking 3 phase induction motors, then the motor speed is dependent on the number of electrical magnetic poles the motor has, 2, 4, 6, 8, etc and the power frequency. A 2 pole motor runs at approximately 3600 RPM on 60 Hz power, and approximately 3000 RPM on 50 Hz power. 4 pole - 1800 RPM at 60 Hz, 1500 RPM at 50 Hz, 6 pole - 1200 RPM at 60 Hz, 1000 at 50 Hz, etc.

My apologies...

Thank you for your understanding. Part of the reason why i am also siding with doing 380 VAC motors is becuase in my mind availability of 480 VAC (or 460 VAC i should say) motors, TX etc,s in that country maybe more difficult. BUT i need to do reasearch to see if this is a correct assumption.

NOW let's say that availability of 460 VAC rated equiptment is not a problem, say a 460 VAC rated motor-pump set , it problably will be rated for 60 Hz. it would not damage the motor to run at 50 Hz but it would run slower so...

Have any of you seen motors or TX rated for 460 VAC 50 Hz?

In the UK the supply voltage is 33Kv, 11Kv, 3.3Kv, 415V all 3 phase and 240v single phase all @50Hz.

To alter the voltage you require a transformer. But even USA manufacturers that make UK rated motors in the UK have a Nameplate voltage of 415v 3ph, so using a 480 volt motor 60Hz would require the application of the Affinity Law, or part of it, for power output, RPM and motor volts.

In answer to your last question... No, not a 50Hz.. 460v 60Hz, Yes

Hey everybody, we seem to be forgetting that Anon's pumps in his system are are designed to run at 60 Hz. The system will not work as designed at 50 Hz. If Anon can live with the reduced performance of the system, then ok.

i created some confusion. The pumps are in deed rated from the manufacturer to run 60 Hz. We are in the US so it is only natural to find motors rated at 60 Hz. We drive those pumps with VFD's at speeds lower than the rated speed at 60 Hz.

Pump's aren't specified by Hz, but by a given volume per revolution, max pressure and maximum RPM. If the pump is rated for 3600RPM, then I suppose you could say it's rated for 60Hz, but that's not really the common nomenclature.

If you calculate the lower RPM's for the 50Hz frequency drive, and get a reduced RPM, you'll then need to multiply that reduced RPM by the volume per revolution to get your reduced flow... If you can operate the equipment properly at this reduced flow, and the motors can run stable at the lower frequency, you should be ok.

And remember, pump's don't create pressure...

and that neatly brings us full circle to my post 10, the Laws of Affinity!

Thank you. You are correct. Again i am not using my terms correctly. What i meant to say is that motors (not pumps) are rated for 60 Hz and/or 50 Hz.

and yes, pumps do not create pressure. Correctly if i am wrong again , pumps can overcome certain head pressure in front of them (as oppossed to creating it)

You are correct, Pumps only create flow, it's the downstream components that provide resistance to flow which create the pressure.

But you'll need both the correct pressure AND flow for your equipment to operate properly, and some pumps don't do well if they are being run at a lower speed than they were meant to run. A fixed displacement gear pump would be just fine running at a slower speed, but a fixed displacement vane pump requires a certain RPM to thrust the vanes out using centrifugal force in order to make the seal that will create flow, a vane pump cannot run at a reduced speed. Variable displacement pumps such as a piston pump with a swashplate are designed to operate at a constant speed, and would not do well (in most cases) being driven by a variable speed drive either. Many of your common trash pumps require a minimum RPM to operate correctly as well.

Your specific application may or may not work even if your motor's spin perfectly forever, all depending on the type of pumps you are running. I would hate to see you put a bunch of time and money converting the motors over to run 50hz, and find out the hard way that you needed that extra couple hundred RPM to operate your pumps the way you intended.

Just an FYI, I have no clue what your pumps are, and most likely you'll be just fine, but just in the off chance that Murphy has decided to leave my building and wander off to yours, you should be prepared for his arrival.

Murphy

-R

No, pumps aren't rated at a specific Hz, they are rated at a design condition at a specific RPM. Unless they are designed to operate using a VFD or other variable speed source, then they may be specified different design conditions, at different RPMs.