Application of VFD for AHU/HVAC

An AHU for HVAC is the easiest of all VFD applications, anything will do fine. I happen to dislike Schneider VFDs, but that is an admitted bias based on some bad experiences I have had with them. But if you insist on using them, the cheapest one will be fine for that.

One thing you need for a temperature control system is closed loop PID control. If the drive has it built-in, you can use it. But that was precisely the problem I had with Schneider drives, the programming for their PID loop control is atrocious, I NEVER got it to work correctly and I know what I am doing. If you have not done PID loop control programming before, I strongly suggest getting a separate PID loop controller and just let it provide an analog signal to the drive that the drive can follow. That way, you get the added benefit that if anything does happen with the drive later, you can replace it with any off-the-shelf drive quickly. If you use the built-in PID control, you will have to learn and reprogram any new drive, or buy the EXACT same one you have.

And if you have no clue what I mean about PID control, this is something you had best turn over to someone who does.

Thanks Jraef,

No I am bit concerned after your feedback of Schneider, Let me ask you straight question:

1. Do you remember which series you used , they claim their new series(ATV21 and ATV61) has the best PID control...,

2. Did you have difficulty in setting up PID only, right? or there are some more issues with Schneider VFDs? As of now, my customer wants to use only control the speed in steps, do you see any potential problem in using Schneider..

Regards,

Manish

I used the ATV71, it is the same basic instruction set used in the 61, except the 71 has a little more. The PID function would be the same.

Best? No way. To me the "best" is one that I can set up and implement in 20 minutes or less. I spent over 5 hours on the phone with Schneider Tech Support trying to get one working once, we finally gave up.

The latest evolution of VFD technology is called the HELP System. The technology was developed and patented in Asia (2008) by a major power company. This system is driven by an Algorithm controlled Smart Microprocessor that replaces PID/PLC controls to optimize VFD functions. This turn-key retrofit system is for existing motors and HVAC equipment and does not require any programming or changes. Additional product information is found at http://thetechnologyconsortium.com/

Hi Manish,

If all you want to do is "...control the speed of motor for AHU based on the air temperature" using a VFD then do it the easy and straight-forward way without programming, etc.

Your VFD should have a 0-10 VDC analog input channel. Buy an inexpensive wall mounted thermostat with analog output and connect it to an input of the VFD. Then select the minimum and maximum output frequency range on the VFD and the desired air temperature on the thermostat and you're in business. It's that simple!

The Eberle model KLR-E 517 7801 Air Conditioning Thermostat was made for this application. It runs on 24 VAC and has two 0-10 VDC outputs: one for heating and one for cooling. It comes with a built-in temperature sensor or you can use an optional external sensor if you wish to control from return air duct temperature instead of room temperature.

Hope this helps you.

Zvi

Manesh,

Go with the ATV21, which I hope is actually an ATV212, the latest series. The ATV32 is for process control and is a constant torque unit. HVAC applications are variable torque. If you need the enclosed, stand-alone drive, I would recommend the Altivar 212 S-Flex. This unit is in the class of the most commonly-applied HVAC VFD's. I used several on two Corps of Engineers projects this spring (and they are very picky about what they use.) Zvi's idea about the simple analog thermostat is correct, although you'll also need a contact to actually start and stop the drive (which is very likely available on the thermostat.) Good luck!