View Full Version : VFD minimum frequency setting
duke of earl
12-30-2006, 10:23 PM
22 hertz is the magic number I was taught for motor cooling in regards to minimum frequency. Anyone have a different view on this?
12-30-2006, 11:00 PM
The number I remember is 25%, I'll have to check some notes to be sure.
Which would be 15Hz, if you are 60Hz.
What are you controlling? AHU, pump, cooling tower?
12-30-2006, 11:28 PM
22hz is usually the minimum for DX units, Just so you have enough air flow to keep from flooding back, etc. I have set Chilled water coil unit fans
down to a 10 hz minimum. If you go much below 10 hz on some motors the amps oscilate and motor gets a little jerky.
Usually 22hz is plenty low for a supply fan, usually I will only put the minimums below that on exhaust fans if controlling building static and they move to much air at 22hz.
As far as motor cooling dictating minimum speed, that shouldn't be an issue.
The amp draw at that speed is really low and you still are moving air across the motor. I have seen Return/exhaust fans run for days/weeks at 6 hz. before.
Pumps I usually set to run a 30% or 18hz min. Most pumps don't move much fluid below 30 % anyway.
Some gear drive cooling towers have minimum speed limit due to gear lube requirements, check with tower Manuf.
12-30-2006, 11:29 PM
22 sounds right, but you should be able to adjust this factory setting, even though thats is not recommeded
01-01-2007, 02:25 AM
I recently went through Danfoss training, their drives can go as low as 6 HZ as long as the motor is rated for drive duty.
01-01-2007, 10:17 AM
I think most of would agree, without knowing the application of this motor/drive there is no "magic" number.
01-01-2007, 11:57 PM
the reason for the minimum setting is for motor cooling and lubrication. the minimum setting is going to be determined by the motor manufacturer and the application.
i see 22 hz quite a bit.
01-02-2007, 12:36 PM
I have seen older Mitsubishi VFD only go to 30 hz, and they were amp cut out on the high side
the newer gear has more flexibility, but as stated its about the motor, you cant run an general ops motor on low hz, the winding heat up , the bearings dont lube
01-02-2007, 02:35 PM
Trane Mseries and Tseries units have a minimum factory setting of 20Hz when they changed from SquareD to Danfoss Drives. The even 20Hz seemed a better default.
01-04-2007, 05:42 PM
I once worked on a 100 ton DX, VAV AHU supply fan and separate Return/Exhaust fan setup. We converted the Ret/Exh fan to building static control. After a couple of Ret/Exh fan VFD "over voltage" trips, I figured out the supply fan was wind-milling the Ret/Exh fan causing it to become a generator. This created a hi bus voltage the VFD didn't like. Even though it was a VFD rated motor, we had to raise the minimum HZ to 25.
duke of earl
01-04-2007, 07:48 PM
Thanks for the input. You have shed some new light on the subject that will help all who read this.
01-04-2007, 11:02 PM
Recommend you look at the actual amperage of the motor relative to the nameplate running amps. We've done many vsd hvac fan applications and its amazing the variability of percent full load. My other concern would be the insulation rating of the motor - B or F ? The liklihood of overheating failure is greater on an older motor. I wouldnt go below 30hz if a new motor wasnt in my budget.
01-21-2007, 03:38 PM
Rule of thumb:
RTU Blower 15 Hz
Cooling Tower Belt Driven 10 Hz
Cooling Tower Shaft Driven 20 Hz
Of course there are exceptions to every rule!
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