VFD on Motor in HVAC System

[yyj10]

Hi,

I have a question about retrofitting the constant speed motor with VFD. The piping or duct system head loss design (pump or fan selection) is based on the full load pump or fan curve. When we do add VFD on the pump or fan motor, shouldn't we take a look of the pump or fan curve to make sure that the VFD is not ramping down too much to a point that cannot meet the minimum head loss?
Any rule of thumb on the minimum speed of VFD should run in this application? I usually used 30% just based on the motor operations.

Thanks

 

[MintJulep]

Yes you should. Don't forget that the system loss is a function of flow.

 

[KiwiMace]

There is a point where the motor cannot cool itself sufficiently. Also test the insulation resistance of the old motor, and the insulation class on the nameplate or spec. 25 or 30% is usually a safe minimum speed though.

 

[yyj10]

Thanks guys.

To KiwiMace,

You are right, but that is for the motor itself. I am considering slowing down the motor will also change the fan curve, so will need to redo the piping head loss calculation to make sure that the pump or fan is running at the speed that still go to the most critical point in the system. Right?

Thanks

 

[ChasBean1]

yyj10, when reducing flow of a fluid by addition of a variable speed drive, the reduction in friction loss in the pipe or duct can only benefit you – there is no need to have to recalculate head loss or have concern about performance issues. The motor reduction to some minimum value is necessary, as stated by Kiwi, to prevent motor damage.

 

[KiwiMace]

For the fan/pump operation, there are variable speed curves for most manufacturers that will allow you to determine the reduced speed performance. The manufacturers usually put both the curves (impeller size and pump speed) in the catalogs.

There are a few aspects of a real system that will give you a non-zero starting point (open loop static head being the obvious) but in a closed loop your system will generally follow a pump/fan law parabolic "System Curve" back to near enough to (0,0) as you reduce flow, like MintJulep said above.

Where these two curves intersect is your operation point.

 

[cdxx139]

An alternate is to place a low flow bypass (near the differential pressure transmitter that controls the VFD). Usually only a couple of GPM (depending on the motor HP). By always allowing flow, you will never have to worry about overheating the pump if most/all of the control valves closed.

knowledge is power

 

[marcoh]

Interestingly had a discussion today with a motor manufacturer regarding minimum speeds and their recommendation was for 5Hz for a fan or pump controlled by a VSD with the system pressure drop following fan/pump laws (ie power is proportional to speed cubed).

Reasoning was that as the power demand drops, the heat generated by the motor also drops, therefore less overheating is not a big issue.

Motor overheating issues can be had on systems such as conveyors (which require constant torque) of if the system is not controlled as per the cube law.

I was suprpised as I had always assumed that you should not go below 25% as per comment below. Suggest you check minimum speeds with manufacturers first if you need to go to low speeds!

If do have the opportunity to go to say 20% speed, and do have an old motor which cannot go too low, they payback to just replace the old motor with a new motor would probably be pretty good. (20% speed = 0.8% full load power!