Constant Torque range 3

[IRL]

Hi,

I am looking for a bit of help with a new application we are installing in work,this is my first visit to this site so sorry if this has been asked before. I am from a mechanical back round and this is my first job in engineering. We are looking at installing a 355Kw 1485 rpm Motor with direct drive through a Flender helical gearbox. The machine will work at full speed most of the time but on occasion it will need to work at half speed and it may even need to work at 10% over base speed. my question is this, will we have the full motor torque at these speeds, the Inverter being supplied is a SIEMENS 355Kw constant torque drive from the Sinamics G150 range. We were thinking of using a belt drive between the motor and gearbox and changing the pulley diameter when a speed change is required but if the inverter drive will give us full torque we can eliminate this. Thanks in advance.

 

[LionelHutz]

Well, I'm trying to tell the original poster than he can keep the same torque and get about 10% more HP if he ensures the VFD will maintain the V/Hz ratio up to 10% over speed.


Dick - Can you explain why it's not possible?

Bill - Just Do It? I agree with what you are saying about adding local transformers and bumping the drive input voltage as being a possible solution.

Edison - There is some truth in that, the bearings might use a little bit more HP to turn and the windage losses will go up a bit and the fan will likely be the big one taking more HP to turn so you might not get 60hp, but rather say 59hp. But, using more of the available HP to turn the fan shouldn't cause extra motor heating.

 

[DickDV]

You know, as brutally competitive as the motor business is, you would think there would be tremendous pressure on motor manufacturers to build 355kw motors and then loudly publish how these motors are really 390 or 420 or (who knows!!!) maybe even 500kw motors in disguise. All you need are a few clandestine "tricks" involving drives to make it happen.

Or, maybe, they have appreciation for the laws of physics that exist and have decided that there is good reason not to, despite the heavy breathing of the marketing and financial guys.

Really now, why would they choose to be silent about such a wonderful discovery? If it is real?

 

[waross]

Back off Dick. Check out 50 Hz to 60 Hz conversions, and then instead of sarcasm, explain why a 20% increase in voltage and frequency results in a 20% increase in HP but a 10% increase in frequency and voltage is doomed to failure!!
Search this site for previous discussions on doubling a motors effective HP by supplying a 230 V connected motor with 120 Hz from a 480 Volt VFD on a 480 volt supply.
Yes the laws of physics do work.
We are not just suggesting a 10% voltage increase, we are suggesting both a 10% voltage increase and a 10 % frequency increase. Same V/Hz ratio, same current, same I² losses, same torque. Higher frequency, higher speed, higher voltage.
Same torque times higher speed equals higher HP.
Same current times higher voltage equals higher VA.
Laws of physics.
But, it's probably time for Keiths "Dead Horse" icon on this thread.

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

 

[itsmoked]

I know

not

what

you

speak

of kind sir.






Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Too many if's and's and but's in here.

Bill,
Yes, in theory it is possible, it's done all the time. I have many times strapped a 230/460V motor for 230V, then fed it with a a 460V drive in order to double the speed without losing torque. The current does of course double, meaning the VFD is twice as large, but the motor being strapped for 230V, you have more winding to spread the I²R losses out into. Never had a problem with that. I used to think I had to double the size of the motor when I did this, but a while ago I was straightened out by our very own Gunnar (Skogsgurra) and have discovered he was right; I was over doing it for nothing it turns out. (by the way, we have discussed this all in the past, thread237-207798 )

BUT (and it's a BIG BUTT), the voltage issue can loom large in a case like this one where you are just boosting the working voltage. IF you are using a 480V drive and motor, in order to get 10% more out of it you have to increase the VFD input voltage to 528V. That means a 575V drive, because a 480V drive is typically NOT capable of more than 500V output. The transistors to do that are in a different class. Same for the diodes (or SCRs) on the front end. 480V needs only 1200PIV, 528V needs 1400PIV rated devices and they may or may not have them. So it isn't as easy as it seems. In the example you used however, it could indeed work, because a 380V drive is usually the same as a 480V drive, so the 456V input will be well within range.

The temperature does go up however, because remember, at the original 50Hz base speed your line voltage, and therefore your DC bus voltage, is higher. That means a higher switching frequency in the PWM pattern to keep the RMS down to 380V, so your switching losses in the drive and motor will increase. Add that to your other losses mentioned above and DickDV is right, the motor heats up faster. Still, 110% is probably fine. Your analogy to running a 50Hz motor at 60Hz and a higher voltage is correct. You get more HP out of it.

DickDV,
Why don't motor mfrs jump all over this? They do, but only when they have to! In other words, they WANT to sell a bigger motor, and most of the time it's a better idea anyway. But if someone knows these issues and the motor mfr wants their business, they will agree to this use. I see it all the time.



"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

 

[jraef]

By the way, isn't it humorous how we sometimes lose the OPs in these discussions? We are a sorry bunch of propeller heads sometimes...

 

[ozmosis]

I thought it might all end up in tears...
This is what happens when we talk kW and not Amps.
What is kW? sqrt(3) * kV * Amps * pf
The Siemens VFD the OP was referring to (the Siemens Sinamics G150) does not actually have a rating of 355kW, it is either 315kW or 400kW (380-480Volts). But hey, we now talk about a voltage range of 380-480Vac. What is the 315kW based on? well, it must be 380Vac otherwise they wouldn't get the "kW's" if the supply voltage was 380V. So on this basis, yes, if the supply voltage was 480V and the constants (root3, amps and PF) remain constant then the only thing that can move with increased voltage is kW and this will move up accordingly.
Now we come to the motor and the point Dick is making. If the motor is already purchased and is rated for 355kW, then this is all you will get out of it. If you increase the voltage then the current rating goes down (not a constant) as it is designed to develop a specific rated torque. So in the case of IRL, you wouldn't get more out of it, even increasing the voltage.
The only way you would get something more is if the VFD remains, you check the amp rating of the VFD and look toward a bigger motor. This is, I think where you are coming from Bill and also where Dick is coming from.

Having said this, we still need the OP to advise his actual needs compared to what he's purchased. It is quite possible that it can be achieved without doing anything.

 

[waross]

DickDV;
Sorry about the flames. I respect and agree with you 99% of the time.
Yours
Bill

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

 

[DickDV]

Gentlemen, you will please note in my first post in this thread that I agreed that the motors mechanicals, electricals, and magnetics will permit extending the constant torque range upward in speed. I am well aware that this is done sometimes with dual voltage rated motors and have, in fact, done that myself.

I am only claiming that the losses in a motor increase and therefore the thermal behavior of the motor deteriorates when this is done and that will usually prevent the motor from being fully loaded at those speeds. Experience has proven this to be true.

jraef also brings up another valid point regarding motor insulation. In PWM power environments where motor insulation is already stressed to the max, deliberately designing a system that uses up all of the insulation's safety margin on voltage is high risk, for sure.

At this point I've pretty much said my piece and invoke the "dead horse principle" suggested by waross.

And, finally, those moticons (or whatever they call them) offered by itsmoked are most excellent and have made the whole thread worthwhile regardless of everything else that was said. Thanks, itsmoked!!!

 

[IRL]

I would like to thank you all for your very informative and entertaining replies on this matter.

We will be doing the install in April and i will let you guys know how it goes, the designers do not exactly know the required power for this application, but we are buying the system at a special price to help with R&D for the Austrian builder of this machine.


IRL

 

[jraef]

Hey, we didn't drive him away after all!


"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

 

[itsmoked]

My pleasure DickDV.

IRL; Thanks for the comeback and DO let us know how it all goes together.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

That means a higher switching frequency in the PWM pattern to keep the RMS down to 380V, so your switching losses in the drive and motor will increase.

I don't follow where the higher PWM frequency comes from. If a drive is set to 6kHz for the carrier then the PWM frequency is 6kHz. The same would have to be true for running a motor at 1/2 speed then, wouldn't it?

jraef also brings up another valid point regarding motor insulation. In PWM power environments where motor insulation is already stressed to the max, deliberately designing a system that uses up all of the insulation's safety margin on voltage is high risk, for sure.

I've looked inside some motors and I don't really see the voltage stress increasing if you use a motor that is dual-voltage rated and apply the higher PWM voltage to the lower motor connection, compared to using the higher voltage connection. The voltage change can be either via series/parallel windings or Y-D connection.