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Anonymous Poster #1

Torques of Variable Frequency Drives

10/04/2014 4:45 PM

Hi,

If I have to use VFDs with 50 kW induction motors to raise or lower gates of a barrage in irrigation system, is it constant torque drive or a variable torque drive?

Regs,

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#1

Re: Torques of Variable Frequency Drives

10/04/2014 5:43 PM

Can't you look up anything for yourself??? Is the load constant? ...or is it variable??

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#2

Re: Torques of Variable Frequency Drives

10/04/2014 6:22 PM

Yes.

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#3

Re: Torques of Variable Frequency Drives

10/04/2014 7:04 PM

Constant. Why do you "have to" use a VFD?

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Anonymous Poster #1
#4
In reply to #3

Re: Torques of Variable Frequency Drives

10/04/2014 9:25 PM

Is it not variable torque because when there is more flow of water, then we have to pull with greater torque, and when the water level leaves the gate as the gates are raised - it has to be constant torque.

Any other way to control it i.e. apart from VFD?

Regs,

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#5
In reply to #4

Re: Torques of Variable Frequency Drives

10/04/2014 9:35 PM

That is not what variable torque means in this context. Why do you have to control the speed of the gate movement?

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#6

Re: Torques of Variable Frequency Drives

10/05/2014 3:47 AM

Torque is what the load presents to the motor, Mildred, not the other way round. If you set the drive for constant torque then the speed will vary. Is that what you want?

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Anonymous Poster #1
#7
In reply to #6

Re: Torques of Variable Frequency Drives

10/05/2014 5:37 AM

Gate blocks flow of water for a while.

Then gate is raised up slowly. There is additional torque presented to motor because part of gate is still submerged.

After a while no part of gate is touching water. Then if gate is further raised up, no

extra resistance is presented to the motor. Then constant torque is presented to the motor.

But it appears that we should choose a VFD with constant torque characteristics. Why?

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#8
In reply to #7

Re: Torques of Variable Frequency Drives

10/05/2014 6:03 AM

To repeat, that is not what variable torque means in this context.

Fans and centrifugal pumps present a torque load that varies with speed. At low speeds, the VFD can furnish low torque; i.e., this is an easier duty.

Conveyors, lifting devices, and positive-displacement pumps present the same torque requirement at all speeds, hence are constant-torque. The VFD must furnish full torque at low speeds as well as high; i.e., a heavier duty.

Why are you trying to vary speed, anyway?

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#21
In reply to #8

Re: Torques of Variable Frequency Drives

10/06/2014 9:41 AM

Maybe he is not varying the speed but using the VFD as a "soft start". (???)

Not being very familiar with the actual applied use, I offer this suggestion. A VFD can actually be cheaper than a soft start and gives the same effect with the ramp up feature on the speed. I've seen this done with large water pumps that are intended to run full tilt boogie once they get going, to avoid the massive in rush at start up and the huge torque hit on the impeller.

And then there is the 50Hz motor on a 60Hz system, or vice versa - again a VFD is needed. The original post did say he has to use them.

The OP needs to define why he needs the VFD - it may well be necessary.

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#10
In reply to #7

Re: Torques of Variable Frequency Drives

10/05/2014 7:52 AM

Use a MagDrive, motor runs same speed but loadspeed can be varied according to requirements with some inputs and outputs.

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#9

Re: Torques of Variable Frequency Drives

10/05/2014 6:10 AM

Tornado is correct.

You don't need a VFD.

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#11

Re: Torques of Variable Frequency Drives

10/05/2014 9:41 AM

Whether or not you need speed control is a separate matter, but as the others have said, this will require a CT drive.

The only difference is in the sizing of the VFD with regard to overload capability. Because the nature of a VT load, such as a centrifugal pump or fan, is that you CANNOT overload it if sized for peak conditions, the VFD need not be sized to be able to deliver that extra current needed during a temporary overload condition. Temporary overloads occur when there is a step change in load that requires re-acceleration, where the new load slows the rotor, increases the slip and the added slip demands more current to produce more torque in order to get back to speed. In a centrifugal load, you cannot change load that fast, so the VFD components will never need to supply that extra current and can therefore be de-rated a little.

Your load will not be like that, it may require peak torque, ie Break Down Torque, from the motor at the worst case, when the gate is all the way down and water pressure against it is at maximum. BDT can be as high as 200% on a Design B motor, which means the VFD components must deliver 200% current, even if only for a few seconds. That is how a CT drive is sized. A VT drive will be sized to never be expected to need to deliver more than 150% current for one to three seconds at most, so it would go into current limit to protect itself and fail to accelerate the motor, failing in the application.

The simple way to look at it is this: is your application a CENTRIFUGAL (also called "quadratic") pump or fan? If no, then it is ALWAYS a Constant Torque application for the VFD.

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Anonymous Poster #1
#12
In reply to #11

Re: Torques of Variable Frequency Drives

10/05/2014 5:04 PM

Will a magdrive or magnetic drive give high efficiency than VFD? Is it cost effective than variable speed drive?

Regs,

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#13
In reply to #12

Re: Torques of Variable Frequency Drives

10/05/2014 5:11 PM

Do you even know what a mag-drive pump is?

Please go away and read

Pump Fundamentals - Pumps! That's what it's about

Come back with intelligent questions!

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Anonymous Poster #1
#14
In reply to #13

Re: Torques of Variable Frequency Drives

10/06/2014 3:06 AM

mag drive pump seems to be water pump. We are not talking about water pump at the moment.

Regs,

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Anonymous Poster #1
#15
In reply to #14

Re: Torques of Variable Frequency Drives

10/06/2014 3:17 AM

I think breakdown torque of IM is 250% of full load torque, not just 200%.

Regs,

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#16
In reply to #15

Re: Torques of Variable Frequency Drives

10/06/2014 3:59 AM

If the torque remains constant as the speed increases, then it is called constant torque applications.

It appears to me that lowering and raising of gates is like hoist - a case of constant torque application.

But it may not be a case of constant horsepower application. Maybe higher hp motor is required when the motor has to operate when the gate is submerged.

Regs,

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#17
In reply to #16

Re: Torques of Variable Frequency Drives

10/06/2014 7:33 AM

You're talking nonsense. A VSD is not needed for this application.

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#19
In reply to #15

Re: Torques of Variable Frequency Drives

10/06/2014 7:35 AM

Don't go there, then.

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#18
In reply to #14

Re: Torques of Variable Frequency Drives

10/06/2014 7:34 AM

Ignore it, then.

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#20
In reply to #14

Re: Torques of Variable Frequency Drives

10/06/2014 9:39 AM

You are clueless.

I see why you posted anonymously.

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#22
In reply to #20

Re: Torques of Variable Frequency Drives

10/15/2014 9:44 PM

Is variable speed required?

If YES, use a VFD.

If NO, the starting mode of the motor must allow a high enough torque regarding the load, also include an appropriate safety margin.

Using a sofstarter is probably not a good idea due to the limited starting torque (do the maths to check if it's the case or not). Star/delta has a limited starting torque too. DOL has a high starting torque but stresses the motor and the load.

Where a progressive start, as well as a high starting torque are required it's common to use a VFD even if speed variation is only used during starts and stops.

The term "variable" or "constant" torque drive is misleading as the torque is related to the load. For the drive it's a mainly a question of dimensioning the power circuits, there can be some firmware details too.

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#23

Re: Torques of Variable Frequency Drives

05/23/2021 3:50 PM

As you raise the gate out of the water, it gets heavier as it displaces less water. If there is water against the gate, force on the gate will fall as it opens, reducing friction.

So it is not a constant torque load. If the gate is stuck up with mud or debris, too much torque could damage the mechanism. On the other hand, too little torque could give frequent jams & nuisance.

A VFD suggests you have a variable speed, obviously the torque needed will depend on the speed.

I would expect there is a "get it to move" stage requiring high torque and an opening phase, maybe a steady speed, but falling torque.

To avoid erosion of channels, you may need a speed schedule to avoid violent water flows. Heads on inlet & outlet side may affect optimum lift/time curve.

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#25
In reply to #23

Re: Torques of Variable Frequency Drives

05/24/2021 1:19 AM

7 years later, i doubt anybody cares by now…

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#26
In reply to #25

Re: Torques of Variable Frequency Drives

05/24/2021 9:54 AM

Curiously, whole rafts of new posts are coming up with dates in 2014, including your post #11 on this.......................

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#24

Re: Torques of Variable Frequency Drives

05/23/2021 5:01 PM

Are you a water master or ditch rider? I worked for an irrigation system 50 years ago. People were still shot over water in the USA. Remote head gate and canal control is not a good idea without visual inspection. If you take out a canal, many will be upset and wanting their water. Letting clients turn in and turn out.....is a nightmare.

We used geared hand cranks and check boards. And at times.....chained locks on everything. A slide gate can be opened with a lever.

Do you get much debris in your system? Do you measure out your water? Is this the river feed head gates?

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