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50/60 Hz Motor Selection
8

50/60 Hz Motor Selection

50/60 Hz Motor Selection

(OP)
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

RE: 50/60 Hz Motor Selection

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

You say 200 KW. At what speed ?

Muthu
www.edison.co.in

RE: 50/60 Hz Motor Selection

Quote (roydm):

      
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...

Quote:

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...

Quote:

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.

Quote:

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.

Quote:

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: Eng-Tips.com Forum Policies  

RE: 50/60 Hz Motor Selection

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

RE: 50/60 Hz Motor Selection

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!
 

RE: 50/60 Hz Motor Selection

(OP)
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
 

RE: 50/60 Hz Motor Selection

(OP)
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

RE: 50/60 Hz Motor Selection

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
www.edison.co.in

RE: 50/60 Hz Motor Selection

(OP)
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

RE: 50/60 Hz Motor Selection

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
www.edison.co.in

RE: 50/60 Hz Motor Selection

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
www.cvoes.com

RE: 50/60 Hz Motor Selection

2

Quote:

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.  
 

RE: 50/60 Hz Motor Selection

(OP)
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
                  

RE: 50/60 Hz Motor Selection

(OP)
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
 

RE: 50/60 Hz Motor Selection

Quote (roydm):

"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: Eng-Tips.com Forum Policies  

RE: 50/60 Hz Motor Selection

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

RE: 50/60 Hz Motor Selection

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
www.edison.co.in

RE: 50/60 Hz Motor Selection

There is no biger iron losses ???

RE: 50/60 Hz Motor Selection

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

RE: 50/60 Hz Motor Selection

Iron losses occur each time the magnetic flux reverses. If the frequency increases then the number of flux reversals per unit time increases linearly with frequency, and the iron losses therefore increase linearly with frequency assuming flux density remains constant. The V/Hz flux density question is related to core saturation, which is a slightly different topic.
  

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

RE: 50/60 Hz Motor Selection

ScottyUK,

Your story is true for hysteresis losses, but eddy current losses are quadratic depend with frequency. So totaly you have something between linear and quadratic.

Milovan Milosevic.
 

RE: 50/60 Hz Motor Selection

Milovan - Yes, yo do have a point about iron loss increasing with the frequency (from linear up to power of 1.4, IIRR).

But that equation is valid for up to core saturation point only. Beyond saturation, the iron loss shoots up due to flux harmonics. And hence, the requirement that V/Hz be constant. Since the iron loss is a small component (up to the saturation point) in the motor losses, an increase of iron loss due to frequency will be off-set due to inherent increased cooling air flow/turbulence at the higher rotor speed.

Muthu
www.edison.co.in

RE: 50/60 Hz Motor Selection

Edison123,

I agree with you that iron losses are small part of total motor losses. I just wanted to hear from some expert for motor design that this is no big deal increasing iron losses for  20-30 %.

Thanks,

Milovan Milosevic

RE: 50/60 Hz Motor Selection

roydm;

The NEMA specs really should not be hard to meet. I was thinking part 31, but both can apply - 30 is for general purpose motors that can be used with VFD's and 31 is for definite purpose inverter duty motors. 31 has some extra requirements which can be important. In any case, if the supplier feels his motors are not capable of meeting these specs then either he doesn't know what his motors are capable of or he knows he's building a very poor motor. Both cases are bad from a users point of view.

On the other hand, I have seen cases where motors from manufacturers that have a decent reputation fail on a VFD too...


Milivan;

I'd say you have a valid point if the motor is being operated right at it's limit. The iron losses do increase and the power required to turn the fan will increase (or it should increase depending on fan design??). So, a motor that can output 200hp at 60Hz probably is not capable of outputting 233Hp at 70Hz.

That is why I recommended just picking a 60Hz motor - it eliminates possible issues like this.


 

RE: 50/60 Hz Motor Selection

(OP)
Gentlemen
A few final questions

We feel fairly confident but would just like to pass it by you gurus in case we missed something.

Attached is a graph showing how we think the Horsepower, Voltage and Torque relate with the curve (A) not (B)

We assume the horsepower keeps going up as long as there is enough voltage to maintain the V/Hz relationship or does it flatten off at 50 Hz?

 Since the line voltage is 400 and the motor is 380 will the voltage at the VFD terminals continue to rise past 50 Hz or does entering the motor nameplate data (380 V @ 50 Hz) mean that at 50 Hz the terminal voltage is already 400?

As our desired operating speed is 3213 rpm with 363 ft lb of torque, will we have enough torque at the frequency required to give us 3213 rpm. How does the slip frequency factor into this?

Provided the HP doesn't drop after 50 Hz we should be OK, is it reasonable to expect constant HP between 50 & 60 Hz?

Note: As I pointed out earlier, the motor ramps up to speed (3213) and stays there 24/7

Thanks in Advance
Roy Matson
Noram Engineering

RE: 50/60 Hz Motor Selection

When you run the motor above 50Hz the curves will not continue as you have graphed. In theory, your graphs are correct.

The available shaft HP = HP produced by rotor minus the bearing, windage and fan losses. So, spinning the motor faster in the "constant HP region" will result in the available HP actually going down and not remaining constant.

Even running with a constant V/Hz I doubt the available HP will increase at the same rate as the over frequency.

We played with running it in the so-called constant HP region on a 5HP motor with an open shaft. We could not run the 60Hz motor above about 75Hz because the motor would "stall" and quit accelerating. So, theory may say one thing but in reality it's not necessarily true. In that experiment, there was 0HP available at 75Hz, not the 5HP that theory suggests.

I thought there was a thread a while back which addressed this in more detail but I can't find it now.
 

RE: 50/60 Hz Motor Selection

Yes you are correct and as in your graph up to so-called field weakening point (380V@50Hz), motor will work in the constant torque area and the power will increase in proportion of the speed. After this point, motor will work in constant power area and produced maximum torque will decrease in square of the frequency (e.g. (50/70)^2 x Tmax).

Entering the motor nameplate 380V@50Hz will set the field weakening point, meaning that the voltage on the motor terminals is in principle 380V at 50 Hz, but the modulation and voltage losses in the chokes etc. inside the converter will decrease the voltage of the fundamental wave ie. producing the torque roughly saying appr. 5...8%. Therefore many motor manufacturers will give some 90 % of the nominal torque at the field weakening point. In your case 250 HP (185 kW) close to 165 kW, what is the same as in your application, 165 kW@3213 rpm.

Yes you should have enough torque. I suppose that the maximum torque of your motor is somewhere 2.5 x Tn (nominal torque) and if taking into account the freq. and assumed voltage drop, the torque would like ((360/380)^2 x (50/54)^2) x 2.5 = 0.77 x 2.5 = 1.9xTn. Even with the maximum torque of 2xTn, the maximum would be 1.5 x Tn. As your line voltage is 400V, I would set the field weakening point like this, 400V@53Hz, giving 380V@50Hz (minus the voltage losses)and 400V@52.6Hz and above that.

Yes, it is reasonable to expect to have constant HP between 50 to 60 Hz.

LionelHutz, sorry but I don't follow you. Why the 60 Hz motor should not run at no-load with 75 Hz ?
 

RE: 50/60 Hz Motor Selection

Quote:

Why the 60 Hz motor should not run at no-load with 75 Hz ?

You'd have to ask the motor. It refused to spin faster than about 2250rpm when increasing only the frequency.

I don't follow why you are calculating anything using 2.5 x Tn. This is the torque the motor could produce before stalling but while it is overloaded. You can not expect to run the motor continually above Tn.

I also don't follow your voltage arguement. The voltage drops that occur between the line voltage and the motor terminals is the reason the line voltage is 400VAC and the motor is rated for 380VAC. There is 400VAC available to ensure voltage drops in equipment and wires do not drop the voltage below 380VAC.  

That squared ratio jogged my memory and I figured out which thread I was thinking about before.

thread237-247210: Maximum speed a motor can be run at

There is a frequency ratio squared theory as posted by jraef. This theory goes contrary to the constant HP theory but personally, my instincts tell me it is correct.

There is also a nice curve posted by ScottyUK which shows the torque is actually not reduced linearly with the overspeed but rather follows an exponential type curve. The chart using simple speed ratios to calculate the torque yet the torque is not a straight line. This curve was produced using the constant HP theory.

In the end, this is the calculation I would have done using the squared theory...
Rated torque is 438 ft-lbs.
The overspeed requires 53.5Hz.
(50/53.5)^2 x 438 = 382 ft-lbs

Using the constant HP theory gives 409 ft-lbs instead.

Of course, neither of the above account for the fan requiring more torque at the higher speed leading to annother reduction in the motor torque output capability. I expect the real torque capability is less than both of the above numbers...

I see another interesting (or maybe not so interesting) thing that may be an issue. The motor is listed as Class 1 Div 2 on that pdf - I don't believe you can install a motor in a Class 1 Div 2 enviroment and then apply a VFD. There might be a rule that it is allowed if the motor has been tested with the VFD. Hopefully, someone with more knowledge can comment on this.
 

RE: 50/60 Hz Motor Selection

Lionel,  I don't know why your motor acted the way it did but I assure you that the vast majority of motors will go to at least 25% and most to 50% overspeed with very constant horsepower.  In fact, I routinely design machine power trains so the motor runs at 75-90Hz at full speed and have never been unsuccessful doing so.

And, that is to be expected if you can believe the published data from the motor manufacturers.  Most publish torque-speed curves that go to 90hz and constant hp is right there to see.

Many inverter-duty motors have nameplate data that spec's constant hp up to 120hz.  And, yes, that can be trusted too.

RE: 50/60 Hz Motor Selection

Do you have some literature and sample curves from a motor manufacturer which address this?

RE: 50/60 Hz Motor Selection

Well, running a motor test laboratory more than 25 years in one of the biggest motor manufacturers in Europe, this is my experience where theory and practise are daily met. Regarding the voltage, I understood so that the line voltage is 400V and the motor is rated to 380V, so if there 400V available on VFD terminals, why not to use as there is voltage drop in the VFD itself ?

RE: 50/60 Hz Motor Selection

I use the curves from the Reliance/Baldor motor catalog.  I don't have a current version in electronic form to send you.  I suspect your local Baldor distributor would be able to email you one.

RE: 50/60 Hz Motor Selection

Thanks elmotor. Could you clarify something. I believe your calculation was to show the new breakdown torque or peak torque, correct? How would you calculate the continous available running torque at the higher speed?

The theory I know and have proven to myself says that the available torque approximately reduces by the square of the voltage reduction. At 2x rated speed or base speed, the motor is effectively receiving 1/2 of its rated voltage. Squaring 50% means the available torque is 25% of rated. A 380VAC, 50Hz motor overspeed to 100Hz is expecting 760VAC, but still only has 380VAC applied. I have never read an explanation of why a VFD can maintain constant HP above the base speed, only "because it does".

I have seen some typial Baldor special VFD motor curves showing over speed but they didn't seem correct to me. The main issue I had was their curve showed the current was constant from 0Hz to 2x+ the base or rated frequency. That doesn't make any sense to me since the current should drop above the base frequeny. This data was shown for the special purpose Baldor blower cooled or non vented VFD motors. I have never seen data like that for standard induction motors.
 

RE: 50/60 Hz Motor Selection

Lionel, when the motor reaches base speed, the voltage would normally be right at the nameplate value.  The frequency would also be right at the nameplate value (the definition of base speed).  At its nameplate hp load, the current also should be at nameplate amps.

When the motor is driven overspeed into the field-weakened range, the voltage can go no higher so it stays the same.  The frequency continues up to whatever higher limit is set.

Since the torque has fallen off by the inverse ratio of overspeed, and hp = T x rpm/5250, the hp will remain constant.  If the output power is constant, then the input power must remain at least constant too.  Since the voltage hasn't changed, at the nameplate hp load the input amps would also not be changed.

Now, we both know that the motor is largely an inductor so it seems wrong that the current doesn't fall off as the frequency rises.  I'm sure someone else can offer a more technical explanation than I but, I believe that the increased frequency causes the current vector that represents the magnetization current to drop as expected.  It must be then that the torque-producing current vector increases to keep the total current the same.  I can't say that I understand why but it must be true.

Someone want to explain why it takes more current to produce less torque as the motor goes over its base frequency?  (Keep it basic enough for us ordinary folk, please!)

At some point in the overspeed range, the torque starts falling faster than the overspeed ratio.  I suppose this is due to all kinds of losses including windage but mostly magnetic losses.  When that happens, the hp also starts to fall and the motor probably isn't very useful above that point.

The motor's peak torque (the breakdown torque) drops as the square of the overspeed ratio.  So, if a motor has 200% peak torque at base speed, at double speed it would have none since the continuous torque has fallen to 50% nameplate and so has the overload torque.  This fact has caused a lot of disappointment when using inverter-duty motors deep into the overspeed range since it is often overlooked.

RE: 50/60 Hz Motor Selection

Quote:

Someone want to explain why it takes more current to produce less torque as the motor goes over its base frequency?  (Keep it basic enough for us ordinary folk, please!)

I don't believe it does.  You've reached constant hp, it does not require more energy coming in than going out.  You said this yourself just above.

As the motor spins faster the windage and fan loads will go up causing some loss in shaft output.  Perhaps this is what you're seeing?

Keith Cress
kcress - http://www.flaminsystems.com

RE: 50/60 Hz Motor Selection

(OP)
DickDV wrote
"Now, we both know that the motor is largely an inductor so it seems wrong that the current doesn't fall off as the frequency rises"

Would be true for motor under load, I believe the inductive reactance drops resulting in a PF change.
Interesting discussion.
Roy

RE: 50/60 Hz Motor Selection

Keith, I know it doesn't take more current overall but, the torque-producing vector must be getting larger if the magnetizing vector is getting smaller and the total is holding constant.

What is happening to make the torque-producing vector get larger as the frequency goes up?

RE: 50/60 Hz Motor Selection

(OP)
This may be a silly question but if you know you want to run a motor slightly overspeed why would you enter the nameplate voltage so it maxes out at synchronous, why not enter nameplate + 10% if you want to run it 10% over.
I know you will run out of voltage perhaps before you reach 10% over but why artificially limit the voltage at nameplate.

RE: 50/60 Hz Motor Selection

Dick,

Torque current rises as slip increases in the weak field area? Just thinking aloud, brain ain't fully awake yet. sleeping2
  

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RE: 50/60 Hz Motor Selection

Quote (DickDV):

Since the torque has fallen off by the inverse ratio of overspeed, and hp = T x rpm/5250, the hp will remain constant.

I was looking for the explanation of this...

Quote (DickDV):

Someone want to explain why it takes more current to produce less torque as the motor goes over its base frequency?  (Keep it basic enough for us ordinary folk, please!)

This one is easy now I think about it again, if the HP or output power is constant then the input power should be constant (assuming losses stay constant). So, above base speed the voltage and current remain constant which produces a constant output power.

Quote (ScottyUk):

Torque current rises as slip increases in the weak field area? Just thinking aloud, brain ain't fully awake yet.

I'd say Scotty hinted at the answer. The available torque when operating at twice the rated speed (at rated slip) has dropped to 1/4 of rated. In other words, if the motor was running at rated slip and 2x base frequency then the current and voltage would both be 50% of rated and the output torque would be 25% of rated. However, operate at about 2X rated slip and the current should go back up to 100% of rated with a voltage of 50% of rated giving an output torque that is 50% of rated.

So, it appears the constant HP theory relies on the motor having a breakdown torque that is at least 200% of rated torque.

Realize that the fan load could go up to 225% at 150% speed and 400% at 200% speed. I'm guessing the windage increases similarily and bearing lossing maybe increase linearly? Add these to the mix and the HP curve would drop drop off as the speed increases, not remain constant. The special VFD duty Baldor motors are either fanless or blower cooled which eliminates the fan load problem. I think the same applies to the Marathon BlackMAX motor line. So, eliminate the fan and the HP may drop but it wouldn't be a huge drop off.

In the end, the OP is looking at a fairly small overspeed with an oversized motor. I do't believe he will have any problems.
 

RE: 50/60 Hz Motor Selection

Quote (roydm):

This may be a silly question but if you know you want to run a motor slightly overspeed why would you enter the nameplate voltage so it maxes out at synchronous, why not enter nameplate + 10% if you want to run it 10% over.I know you will run out of voltage perhaps before you reach 10% over but why artificially limit the voltage at nameplate.

Yes, you could put in the motor data as 380V, 52.8Hz for a 360V, 50Hz motor. This would extend the operation using the correct V/Hz ratio to 52.8Hz.

Class 1 Div 2 - Are you using this in a classified area? I recall that a  Class 1 Div 2 motor loses that rating when operated with a VFD so I'd recommend you check into this further.
 

RE: 50/60 Hz Motor Selection

(OP)
Once again, thanks for the valuable input.
Lionel - yes we are aware that VFDs can cause overheating, thanks for the reminder though.
The versions of NEC (1999) and CEC (2002) that I have don't call for a motor to be explosion proof for Class 1 Div 2 (actually our area is Class 1 Zone 2 T1 (450°C)) hopefully the motor won't get that hot. The CEC points out the potential harmonic overheating problem.
Both codes say you can use open type motors for Class 1 Div 2, but of course we will use TEFC. The pump is pumping Sulphuric Acid.
I think its very important to specify that the fan and or the coupling gaurd is non sparking.
I recall reading somewhere that the VFD and motor should be matched but that's pretty hard to do since we are providing the motor, the client is providing the VFD.
Ninety % of our work is Class 1 Div 2 or Zone 2

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