50/60 Hz Motor Selection 8

[roydm]

I wonder if you motor gurus could give me a few pointers.
We have a project in China (50 Hz) where we are installing a 200 kW pump. We want to run the pump on a VFD somewhere between 3000 and 3600 rpm.
The pump will draw ~ 200 kW right at the point where the client specify changing from 380 V to 10 kV
Q1) Should we be selecting a 60 Hz motor and underspeeding it or a 50 Hz motor and overspeeding?
Q2) Would the 50 Hz option be the same HP motor as the 60 Hz option?
Q3) If you overspeed a 50 Hz motor does the Voltage go over as well or would we set the VFD for 380 Volts at the new top speed?
Q4)Would a 10 kV 200 kW motor be the same frame size as a 380 V motor?
Q5) What is the relevent cost of a 10 kV motor v/s a 380 Volt one?
Thanks in advance for the help.
Regards
Roy

 

[edison123]

10 KV for 200 KW is an overkill and is definitely a cost & size driver.

You say 200 KW. At what speed ?

Muthu

http://www.edison.co.in

 

[jraef]

Q1) Should we be selecting a 60 Hz motor and underspeeding it or a 50 Hz motor and overspeeding?

The frequency is insignificant alone. What matters (as far as the motor is concerned) is the voltage and frequency relationship, known as the V/Hz ratio. A motor designed for 460V 60Hz has a V/Hz ratio of 460/60 or 7.67:1. A motor designed for 380V 50Hz is 380/50 or 7.6:1; virtually the same. So the motor is really no different and changing the speed is not the main problematic issue here; but read on...

Q2) Would the 50 Hz option be the same HP motor as the 60 Hz option?

Your HP requirement needs to be driven by the load. For a pump, load is determined by flow (and/or pressure). Determine what you need, then select a motor that makes that work. If, at the relative speed at 50Hz, the motor power is insufficient to provide the flow and pressure you need compared to a design that used a 60Hz motor, then yes, the motor power would need to be higher; ; but read on...

Q3) If you overspeed a 50 Hz motor does the Voltage go over as well or would we set the VFD for 380 Volts at the new top speed?

As stated above, the V/Hz ratio must be maintained. But when using a VFD, it cannot INCREASE the voltage by itself beyond what it gets from the supply. So if you input 380V, the most you can get out is 380V (nominally). When you get to 380V output at 50Hz, then you keep increasing the speed to 60 Hz, you have no more voltage and the V/hz ratio drops. That equates to having a constant kW operation, meaning that your torque now decreases at that 50Hz point forward. If however you can increase the voltage input to the VFD (say to 460V?), the proper ratio can be maintained up to 60Hz. So at 60Hz, your motor output power (kW) is higher than it would be at 380V 60Hz, but the same if it were 380V 50Hz, just slower. But that too is not the whole story.

In a centrifugal pump system, flow (Q) is directly related to the speed of the pump, but POWER is related to the CUBE of the speed difference. So if, for example, your pump system was designed around a 460V 60Hz motor, running at 50Hz, even if the V/Hz ratio was correct by using 380V at 50Hz, will decrease the flow by the speed difference. 50/60 = 83.3% speed, so flow will drop to .833 of what it would be at 60Hz. If you now speed that motor to 60Hz to get the flow back, but you cannot keep the V/hz ratio correct, the POWER required to do this would increase by the cube of the speed increase, so 120%³ or 173%. Therefore in order to get the same flow with a motor running over speed, your pump power would need to almost double. This answers your question directly, but it is not necessarily the way to go about this. By simply adding a transformer ahead of a VFD, you can run the system at it's original 60Hz design parameters.

Q4)Would a 10 kV 200 kW motor be the same frame size as a 380 V motor?

I'm not sure anyone makes 200kW 10kV motors. If they do, it could be smaller, but more likely it will be a larger standard MV motor frame with less winding material.

Q5) What is the relevent cost of a 10 kV motor v/s a 380 Volt one?

MV motors tend to be only a little more or even less for an equivalent power rating, but a 10kV VFD would be 20X the price! One reason is, there really is little relationship to the power rating and the cost to manufacture a MV drive, the expensive issue is in dealing with the voltage level. So for all intents, a 200kW 10kV drive is likely to be a 2000kW 10kV drive, just de-rated to 200kW. I know that for a long time, the smallest 5kV VFD you could buy was 360A, which equated to about 2000HP (1500kW), however most manufacturers would gladly put a nameplate on it telling you it was only a 300HP drive for no extra charge. I suspect the same to be true at 10kV and I would expect to pay about US$250,000.00 minimum (maybe more) for a 10kV VFD, no matter how small.



"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

A couple of options.
If this is a centrifugal pump, check and see if a slightly larger diameter impeller is available for the pump. A larger diameter impeller will let you develop the head you need at slightly below 3000 RPM and you may use a standard 50 Hz motor directly.
Re the V/Hz ratio. Many 380V motors are star connected and may be reconnected as a delta motor for 220 Volts. The V/Hz ratio will now be 4.4 V/Hz. You may now go up to almost 5200 RPM at 86.6 Hz and 380 Volts.
In your instance, a frequency of about 62Hz at 273 Volts will drive the motor at very close to 3600 RPM.
The 200 kW motor will be capable of developing about 248 kW
A 175 kW motor will be capable of developing about 217 kW at this frequency and voltage.
A 150 kW motor will develop 200 kW at around 67 Hz and 293 Volts.
You may want to reduce the diameter of the impeller at these speeds.
The cheapest solution by far is to go with a larger diameter impeller to develop the head and then restrict the flow slightly to reduce the load to 200 kW.
The next best bang for the buck may be to use a 380 Volt VFD to overdrive a 220 Volt connected motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Consider the spares situation in China: is it easier to obtain a replacement NEMA motor or an IEC motor? Or are there so many copies around that it doesn't matter? {smile]

I'm surprised there isn't an intermediate voltage at the site. If you'll excuse the UK voltage references, 200kW is roughly where I'd expect to jump from 400V to 3.3kV or 6.6kV, but I wouldn't go to 11kV until roughly 2MW or so. There's a number of good reasons to keep below 10kV, for example the lack of PD below this voltage and the availability of reasonably standard motors. No one makes 'standard' 11kV motors.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[roydm]

Thank-you gentlmen for your prompt response.
Jraef, You bought up several points I never would have considered.
So it seems like a low voltage motor is the only reasonable way to go, I don't think we will have too much trouble convincing the client that he needs to make an exception to his 200 kW limit.

Waross, We already have the largest impeller size for the selected pump so the only option is to up the speed to around 3,600. Interesting solution to change from Star to Delta.

Scotty, I don't think 3.3 is an option at this site, at least they didn't mention it.

Thanks again
Roy

 

[roydm]

The mechanical engineer who is responsible for purchasing the pump has asked me to run this by you.

"I have a centrifugal pump complete with a VFD application that requires 380/3/50 Hz motor. It is a critical pump for the process. The operating speed will be somewhere between 3,000 and 3,200 rpm (ie overspeed).
I specified a VFD rated motor according to NEMA MG-1 part 30 & 31. The vendor said that this MG-1 requirement is unneccesary and expensive and a VFD rated motor.
Q/ How do you specify a "VFD rated" motor?
Would this be sufficient or inferior?
Comments?
Also is slowing down a 60 Hz motor a better option as opposed to speeding up the 50 Hz motor?
The line frequency is 50 Hz"

Thanks in advance

Roy Matson

 

[edison123]

Bill (waross) has answered your question already. If you're using a VFD, yes, you need to buy a VFD duty motor. The OEM will know what it means. You don't know need to specify about VFD duty except the running speeds and the corresponding loads.

Muthu

http://www.edison.co.in

 

[roydm]

Edison123, Sorry, I missed the last couple of words the sentence should have read
"The vendor said MG-1 requirement is unneccesary and expensive and a VFD rated motor is sufficient"
I guess we are trying to figure out what's so special about a motor that meets MG-1 part 30 & 31
Re reading Waross's reply.
A larger impeller is not available
He recomends overdriving a 50 Hz motor connected for 220, am I correct.
Thanks
Roy

 

[edison123]

I'll let the MG1 specialists answer that question though IIRR, MG 1 is the just basic motor standard.

Yes, Bill is advising you to use 220 V, 50 Hz motor so that you can overspeed it to 60 Hz. This is because that at 60 Hz, you need 1.2 x 380 V to maintain V/Hz and your drive won't output more than 380 V.

Muthu

http://www.edison.co.in

 

[dpc]

Unless you are referencing some standard, just saying "VFD Duty" is pretty meaningless - a little like specifying "energy-efficient".

NEMA MG-1 has specific requirements for "inverter-duty" motors.

MG-1 is the standard motor spec used in the US. It is not especially difficult to meet, since NEMA is controlled by the manufacturers themselves.

In China, I have no idea.



David Castor

http://www.cvoes.com

 

[LionelHutz]

Also is slowing down a 60 Hz motor a better option as opposed to speeding up the 50 Hz motor?

First of all, get a curve of the HP vs Speed requirements for the pump, up to 3600rpm.

Second, figure out what motor will work as follows.

To begin with, here is the difference between both motors assuming both motors are rated for 380VAC.

A 200kW, 50Hz, 380V motor will produce 200kW at 50Hz. Below 50Hz, the power reduces linearly with the speed or frequency. Above 50Hz, the kW will be constant and the torque will drop off. However, I'd expect the power to drop above 50Hz and not actually be 200kW at 60Hz.

A 200kW, 60Hz, 380V motor will produce 200kW at 60Hz. Below 60Hz, the power will reduce linearly with the speed or frequency. At 50Hz, this motor is 167kW.

What this means. This pump likely has a power curve which is the cube of the speed. So, pick the motor which has a kW rating to match the pump requirements at the maximum operating speed. In other words, figure out what the power requirement is at 60Hz and then buy a 60Hz motor that meets or exceeds this requirement. In this manner, the required pump power will drop quicker than the motor capacity and you will be safe.

As for MG-1, part 31 - this has some requirements for building inverter specific motors. If you do not specify any requirements then asking for a "Inverter Duty" motor means nothing.

 

[roydm]

Thankyou Lionel,
I suspect the vendor in Korea doesn't want to meet MG-1 part 31 that's why he is trying to accept "VFD rated motor", we were quoted C$50,000 for a motor meeting MG-1
The pump will run 24/7 at a fixed speed (fixed flow rate actualy) We used to specify fixed speed pumps and throttle back to get the correct flow but since using VFDs we have found they last much longer (years instead of months), also the downstream pipework is glass lined so starting with a ramp is much more gentle than DOL.
Thank you all for your input, it will help us make an expensive decision.
Regards
Roy

 

[edison123]

A few links of VFD duty motors and their application.

http://www.usmotors.com/Datasheets/PDF_PDS/pds811-215.pdf

http://www.ab.com/drives/techpapers/InstallationConsiderations01.pdf

I agree with Lionel on the motor power rating. You need to have the pump power requirements on hand across the speed range you intend to run it.

Muthu

http://www.edison.co.in

 

[roydm]

Edison123,
The informative paper you linked from ab gives a simple explanation of the difference between part 30 & 31.
Page 4 spells out why you need part 31 or in the very least a reactor for 480V drive.
I am unfamiliar with the line terminator network but that seems to be the Cadilac solution.
Thankyou again for the valuable help.
Thanks to Lionel also.
Regards
Roy

 

[jraef]

"The vendor said MG-1 requirement is unneccesary and expensive and a VFD rated motor is sufficient"
I guess we are trying to figure out what's so special about a motor that meets MG-1 part 30 & 31

In trying to second guess why your vendor would make this statement, I'll offer this;

The reason why NEMA added the "part 30 and 31" to their specs is because in the early days of "inverter duty" motors, motor vendors were allowed to say anything they wanted about their products being "inverter duty" without having to meet a specification or test standard. In other words it was a largely undefined term. I remember one manufacturer (who shall remain nameless but who also used to sell welders under the same name) that sold a version of their motors which they called "inverter duty" but were nothing more than a nameplate that lowered the HP rating. In other words, if you wanted a 5HP inverter duty motor, they gave you a 7.5HP standard motor with a nameplate that said 5HP (then charged you the price of a 10HP). So the term inverter duty became essentially meaningless. IEEE attempted to force a standard and failed for the most part, then NEMA tried to as well by implementation of these addenda to the basic MG-1 spec.

The reality is, the market changed. The major manufacturers of motor magnet wire have much better insulation available and if purchased in enough volume, I'd even bet costs the motor mfr close to the same as older style wire, so it behooves them to avoid having inventory of both. Consequently if a major manufacturer made a piece of junk and tried to pawn it off as a special design, they had no repeat customers. That's not to say junk s still not available, but reputable manufacturers are all making good products now and I would imagine it difficult to find a major mfr's product offering labeled as "inverter duty" that would not meet or even exceed the requirements of part 31.

So with that in mind, I find it curious that your vendor would go out of his way to specifically say that you don't need it. Makes me think he is unaware that his motor vendor already does, of he has found some knock-off clone or bottom feeder supplier making cheap motors that allows him to make more profit, so he has to convince you you don't need the better quality.

My advice is to make him give you detailed specs on the motor in advance and check with the motor mfr as to what it performs to.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[Masbibe]

Question for Warros and Edison123,

Can you just like that supply motor with 273 V and 62 Hz that is originally designed for 220 V and 50 Hz (both connections are delta).
What about iron losses thats now are bigger.
Can you be sure that this is not enough to damage the iron.

Milovan Milosevic

 

[edison123]

As long as V/Hz ratio is constant, the flux density is constant and so no damage. For a given machine winding, the flux density is directly proportional to the voltage (until saturation anyway) and inversely proportional to the frequency.

Of course at higher speeds, one needs to check the rotor for the ability to take higher mechanical stresses.

Muthu

http://www.edison.co.in

 

[Masbibe]

There is no biger iron losses ???

 

[Masbibe]

As I know iron losses can be divide in 2 groups: hysteresis and eddy current losses. Equations are below:
Ph = Kh × f × B^n
Pec = Kec * f^2 * B^2
According to this you will have bigger iron losses.
What is your opinion ?

Milovan Milosevic