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#66500 - 06/06/06 09:11 AM VFD on Single Phase Unit
rad74ss Offline

Registered: 04/19/04
Posts: 154
Loc: Pryor, OK USA
I had a request from a customer to put VFD's on their condenser fan motors. This is a 208/1/60 HVAC unit.

I have never encountered this before. Would I have to put a phase converter on then a VFD. How does phase conversion affect the amps per leg? Would that cause problems with the VFD?

I would prefer to put a variable speed control on the first fan on each circuit and stage the rest as on/off but I am not sure if I convince everyone between me and the end user that that is the way to go.

Any thoughts or expertise would be appreciated.

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#66501 - 06/06/06 09:19 AM Re: VFD on Single Phase Unit
Peter Salegna Offline

Registered: 05/10/06
Posts: 19
Loc: norwich,conn, USA
Have they given you a reason for the vfd on the condensor. This is a residential customer I'm assuming.

#66502 - 06/06/06 09:52 AM Re: VFD on Single Phase Unit
rad74ss Offline

Registered: 04/19/04
Posts: 154
Loc: Pryor, OK USA
No reason for the VFD's as of yet. These units are large and are for very, very large residences. I think that since money is no object they use VFD's as a selling point instead of relying on the on/off staging. Right now there are two fans per circuit on this unit. I haven't heard if they want a VFD for each fan or one VFD with two fans.

A P66 speed control on the lead fan should give them what I 'think' they want and it is for single phase fans. It may just be a case of the sales to customer link not realizing the difference between variable frequency and variable speed.

#66503 - 06/06/06 10:28 AM Re: VFD on Single Phase Unit
resqcapt19 Offline

Registered: 11/10/00
Posts: 2209
Loc: IL
Single phase VFDs are available. Here is the first one that came up on a Google search.

#66504 - 06/06/06 01:21 PM Re: VFD on Single Phase Unit
Bob Offline

Registered: 02/05/02
Posts: 182
Loc: Mobile, AL, USA
Most VFD driven A/C and Heatpump units come with VFD as part of the unit. You need to verify that the units will operate properly with an add on a VFD. You can damage the motors and compressors if they are not designed to run this way.

[This message has been edited by Bob (edited 06-06-2006).]

#66505 - 06/06/06 05:15 PM Re: VFD on Single Phase Unit
renosteinke Offline
Cat Servant

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
If they want VFD's, might as well go all the way, and switch to three-phase motors. The VFD is a good way to 'create" three phase from a single phase service.

#66506 - 06/06/06 06:58 PM Re: VFD on Single Phase Unit
jay8 Offline

Registered: 07/28/05
Posts: 183
Loc: Vancouver, BC
I wouldnt do any field modification like that on a new unit.The entire system would need some control modifications. As was mentioned, you need to discuss with the manufacturer or you could end up with a lot of grief. If anything the VFD would be on the compressor, rather than the condensing side, why would you want to not fully condense the refrigerant? Typical would beto somehow modulate both evaporator and condensor.


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