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.

 

[jraef]

That VFD will allow full torque output within the normal motor design speed or below. But at 10% above base speed, no AC motor will be able to supply full torque.

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.


"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]

Hi jraef;
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

 

[itsmoked]

Doesn't that invite overloading the magnetic circuit Bill? If this motor is designed near its limit and you raise the voltage.. You are heading for saturation. Or are you thinking 10% is small enough? I think it's still a bit risky to spend the $$ for the boost xfmrs only to find the current sky-rocketing.

However anything that avoids the permanent waste of belts should be considered.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ozmosis]

IRL
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?

 

[waross]

Hi Keith;
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

 

[DickDV]

I would suggest calculating the load kw at maximum speed. Divide by the gearbox efficiency and that is the required input kw to the gearbox. If that is more than 355kw, you need a larger motor, period!

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.

 

[jraef]

Bottom line: The laws of physics apply even when you don't want them to.

Amen to that brother!


"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

 

[edison123]

Agree with Bill that with 110% voltage & 110% speed, the torque will be remain constant.

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.

 

[LionelHutz]

OK, you guys have lost me saying the motor heating goes up if you go above the base speed holding V/Hz constant. The current doesn't increase and the magnetic field in the motor doesn't change. Motor torque stays the same and since HP is proportional to speed, the HP will increase as the speed increases.

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.

 

[waross]

Hi Lionel. I don't think that the motor even has to be changed. This is similar to running a 50 Hz motor on 60 Hz.
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

 

[LionelHutz]

Yes, that's my opinion.

But Dick, Jeff and Edison all seem to think the motor can't run at +10% speed even keeping the V/Hz constant.

I would suggest calculating the load kw at maximum speed. Divide by the gearbox efficiency and that is the required input kw to the gearbox. If that is more than 355kw, you need a larger motor, period!

Not period! If the V/hz is kept constant then the motor is capable of 390.5kW at 10% over speed.

 

[LionelHutz]

I mean they seem to believe the motor can't be run at rated torque or 110% rated kW at 110% speed by keeping the V/Hz constant.

 

[DickDV]

Sigh!!!! OK guys, go ahead and try it at 40 degrees C ambient.

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?)

 

[ozmosis]

This is why it would be useful for the OP to advise their torque requirements and how much of a margin there has been allowed in the calculations. VFD+motor+gearbox will provide a certain torque capability assuming the supply voltage is known. As long as this is > the required torque then there is no problem. The difference between the required and the rating of the system will determine how much more you can get out of the system. Increasing the line voltage will allow a greater kVA but you are also increasing the line voltage to everything else on the same network.

 

[waross]

Ok, a 50 HP motor at 380 Volts 50 Hz will develop 60 HP at 60 Hz and 456 Volts.
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

 

[itsmoked]

I agree. I was neglecting the raised frequency which works to counter any saturation.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

Bill

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.

 

[ozmosis]

are there two questions going on in this thread or is this related to the OP?

 

[itsmoked]

Yes, it's related.
No, the OP isn't participating.
Yes, the OP's question has been answered. (days ago)

Keith Cress
kcress -

http://www.flaminsystems.com