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Torque/speed curve


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#1 Mick35

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Posted 10 March 2006 - 04:25 AM

Dear Sir,

I have an application for a fan and the manufacturer has sent us the torque/speed curve of the fan. Th latter will be driven by a VSD. My question is as follows : Does the motor or the VSD which is function of the speed/torque curve ?
Many thanks for a reply.
Besides this is a wonderful forum.

Mick

#2 marke

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Posted 10 March 2006 - 04:47 AM

Hello Mick

Welcome to the forum.

You do not need the speed torque curve of the fan to apply a VSD to it. Just connect the VSD and control circuitry and all should be well.
Note: Many people are sold VSDs for pumps and fans on the grounds that they will save energy.
They will only save energy if they spend a significant period of time operating at less than rated speed.

Best regards,

#3 Mick35

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Posted 10 March 2006 - 04:52 AM

Thanks Marke for your quick reply. If I am right the VSD will cope with whatever torque/speed curve of the fan. However, I believe that equally important is the torque/speed curve of the motor to prevent overloading of the motor.
Am I right /

Mick

#4 marke

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Posted 10 March 2006 - 05:13 AM

Hello Mick

If you are operating at rated speed and bleow, there will not be a problem, howver, if you are operating above rated speed, then yes you can overload the motor very easily.

Best regards,

#5 Mick35

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Posted 10 March 2006 - 05:40 AM

Thks again Marke, bear me if I`m stupid. I have got one 600 k W electric motor rated at 750 rpm, the maximum torque obviously occurs at the rated speed, which means that the maximum torque at 750 rpm will be 7642 Nm. Now looking at the speed torque curve of the fan, I need to ascertain that the maximum torque that could be provided that is 7642 Nm is within the range of the curve. Right ?
Thks Marke again for your time.
Mick

#6 marke

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Posted 10 March 2006 - 07:32 AM

Hello Mick

As you slow the fan down, the torque required by the fan will drop very quickly, but the torque of the motor will stay the same, so provided that the torque of the fan at rated speed is less than the torque of the motor, it will definitely be OK at all speeds less than rated speed.

Best regards,

#7 jOmega

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Posted 17 March 2006 - 09:00 AM

Hi Mark,

I would offer that your postulate holds true only in-so-far-as the VFD is set-up for Constant V/Hz.

Many VFD's today offer the ability to select either Constant V/Hz for Constant Torque Applications ----- or a square-law V/Hz pattern that reduces as the speed is reduced ... for Variable Torque loads such as fans and centrifugal pumps.

The VFD allows the user to adjust the fan operation to the system curve which, translates to the provided speed/torque curve as stated, without the need for louvers and dampers; ergo, operating more efficiently and reaping the consequent energy savings therefrom.

There is one 'gotcha' to be mindful of; that is, when a new motor and VFD are provided as replacements for a much older, fixed speed motor that was part of a system where the fan was belted to the motor and the requisite fan rpm was obtained by selection of the sheave ratio between the motor and fan.

The newer, more efficient motors of today typically have a higher base speed than their older cousins of yesterday. As a consequence, operating the newer motor of today at rated voltage and frequency can cause the fan to overload the motor because of the consequent higher operating rpm. In this situation, the VFD easily allows the user to trim back or limit the motor speed so it does not operate the fan beyond the system design point.




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