Constant Torque range
Constant Torque range
(OP)
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.
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.





RE: Constant Torque range
Motors driven by VFDs are constant torque / variable power up to base speed. Above base speed, the motor will operate in what is called "constant power" mode, meaning that you will not be able to attain more than 355kW, so if you do the math, if the speed increases, the torque will need to decrease. The decrease is at a different slope than the power increase, so at only 10% above base speed, your torque loss will not be much, maybe around 5%.
So in your case, I would look first as to whether you really need every NM of torque at that 10% above base speed operation. The amount of torque loss may be so little as to be insignificant for your application. If it is too much, you can look at increasing the motor size. Adding a belt drive will result in power loss through the mechanical transmission, so when you are running below base speed, you may no longer get full power.
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RE: Constant Torque range
Di I have this right? The maximum power out of a 355 kW motor at rated voltage is 355 kW.
So at 10% above base speed, at the final driven element, does it matter whether the overspeed is achieved with a belt or with an overfrequency VFD?
Isn't your torque still based on power divided by the final RPM?
If full torque is needed at 110% speed, is it a valid solution to check the voltage rating of the VFD and if the voltage rating is high enough add a couple of autotransformers to boost the voltage 10% or 15%?
Then you could maintain the V/H ratio at above base speed and develop over 355 kW.
Am I missing something here?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Constant Torque range
However anything that avoids the permanent waste of belts should be considered.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Constant Torque range
I think you need to take a few steps back. You have already advised that the motor/drive is 355kW.
The torque capability will be in the region of 2100NM x ratio of the gearbox less efficiency loss of the gearbox. Helical gearboxes are quite efficient so the losses will be minimal.
Work this one out and determine if you have adequate torque at full speed (half speed will be ok as jraef points out, torque will be constant assuming the G150 is sized correctly for constant torque).
At 10% above base speed the torque capability will be approx 1800NM x ratio of gearbox. You need to determine if this is enough. You also need to determine and clarify that the calculated torque is less that the rated torque of the gearbox otherwise this will be the limit of torque output.
Remember, all I've spoken about is torque capability and not required torque. You need to determine this for your application and ensure the drive system is capable.
Siemens have a pretty good software program called SIZER (in my day it used to be called PFAD) that can do all these calculations and print out a graph showing your torque curve. I would ask your supplier to do this for you.
Now that Flender is part of the Siemens group, get them to underwrite it as the motor will be a Siemens motor too.
What is the application btw?
RE: Constant Torque range
I understand that we are talking about a VFD drive. Looking at jraef's diagram we see that the torque remains constant until the voltage reaches rated voltage at rated frequency.
Also I understand that the torque is constant as long as the proper V/H ratio is maintained.
Above rated voltage and frequency, the V/H ratio drops off and so also does the torque drop off.
If the voltage rating of the VFD is adequate to accept a higher voltage, I am suggesting/asking about boosting the input voltage to the VFD 10% so the V/H ratio may be maintained to 110% frequency.
IF THE VFD is suitable, we would have 110% voltage, and 110% frequency and speed.
The current and torque would be the same.
There may be some parallels to running a 50 Hz motor on 60 Hz at 60/50 times the rated voltage. Same torque, more voltage, horsepower and speed.
I am sure that this has been discussed before, here. I am asking for clarification of my understanding.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Constant Torque range
Sure, you can play some of those tricks mentioned about raising the voltage and frequency above the motor rating to acheive constant torque above base speed. The motor's mechanical, electrical, and magnetic behavior may accommodate this. However, unless this motor never sees a 40 degree C ambient temperature, the motor's thermal behavior will not be acceptable. Or, to put it another way, the motor will likely overheat. This is particularly true of IEC motors which do not usually have any service factor above 1.0 to reach into.
Bottom line: The laws of physics apply even when you don't want them to. I would be very careful about reaching into the ether on this and hoping you survive unscathed.
RE: Constant Torque range
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RE: Constant Torque range
But as DickDV says, since the temperature rise will be proportional to I^^2 and the increased cooling at 110% speed (for a shaft mounted fan) may not be enough to cool the motor, there may be thermal limitations. If you have a separately driven cooling fan, it would be worse since that fan would have been designed for the 100% rated speed and 100% HP of the main motor.
RE: Constant Torque range
Say you have a 100kW, 1200rpm motor. Keeping the V/Hz constant, the same motor is rated;
50kW at 600rpm
100kW at 1200rpm
200kw at 2400rpm
Well, at least in theory anyways. The capability of the motor rotor and motor bearings to handle the higher speed has to be investigated and the cooling fan may become less efficient at moving air above a certain speed. 10% over speed should not be a problem though.
I don't know where you are in the world but use a supply voltage that is 10% higher than the motor voltage. If you are in 380VAC, 50hz area then use a 240VAC rated motor. Then, you can program the VFD to go to 55hz @ 264VAC, keeping the V/Hz ratio constant. Then, you will get rated torque up to 110% speed.
RE: Constant Torque range
Torque, current and heating stay the same.
Frequency, voltage, speed and HP increase 10%.
At a higher frequency the motor is happy with a higher voltage, it's the V/Hz that counts.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Constant Torque range
But Dick, Jeff and Edison all seem to think the motor can't run at +10% speed even keeping the V/Hz constant.
Not period! If the V/hz is kept constant then the motor is capable of 390.5kW at 10% over speed.
RE: Constant Torque range
RE: Constant Torque range
But how about promising to report the results back to us even if they are embarrassing.
Is that a deal? (What was that about the laws of physics?)
RE: Constant Torque range
RE: Constant Torque range
Same torque, same V/Hz, same current, same I2R, 120% speed, 120% HP and 120% voltage.
I have used this conversion in the field a number of times, both from 50 Hz to 60 Hz and from 60 Hz to 50 Hz without any embarrassment.
The first time I had to do this conversion I exercised due diligence and had my calculations checked and verified by the head engineer at a large motor rewind shop.
Now, if it works at 120% why won't it work at 110%?
And if I need a voltage adjustment for one machine, I put in a couple of small auto-transformers and adjust the voltage for just that machine. I have done this many times to either increase or decrease voltage to one machine or motor and have never had a problem.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Constant Torque range
Keith Cress
kcress - http://www.flaminsystems.com
RE: Constant Torque range
Tackling this from another direction, assuming the same efficiency, wouldn't the losses be higher at higher speed and hence the temperature rise be higher ?
In your example, assume 90% efficiency
At 380 V, 50 Hz, the losses would be 0.5 HP.
At 456 V, 60 Hz, the losses would be 0.6 HP.
RE: Constant Torque range
RE: Constant Torque range
No, the OP isn't participating.
Yes, the OP's question has been answered. (days ago)
Keith Cress
kcress - http://www.flaminsystems.com
RE: Constant Torque range
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.
RE: Constant Torque range
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?
RE: Constant Torque range
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
RE: Constant Torque range
what
Keith Cress
kcress - http://www.flaminsystems.com
RE: Constant Torque range
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: VFD overfrequency )
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
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RE: Constant Torque range
RE: Constant Torque range
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.
RE: Constant Torque range
Sorry about the flames. I respect and agree with you 99% of the time.
Yours
Bill
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Constant Torque range
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!!!
RE: Constant Torque range
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
RE: Constant Torque range
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Constant Torque range
IRL; Thanks for the comeback and DO let us know how it all goes together.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Constant Torque range
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?
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.