Lowest speed VFD'd into fan motor.

[itsmoked]

I have a condenser fan that I'm controlling with a VFD to maintain head pressure on a refrigeration compressor.

It's a 1-1/2 or 2HP 1725 motor. It's running on an equation that could have it sputtering away at some low speed for hours. It's an axial fan up tight against a radiator (condenser coil).

What do you all suggest would be the lowest speed it should bother running at and still be safe from overheating due to its internal losses and no real air likely being drawn across it?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MikeHalloran]

My WAG is 500 rpm to keep the bearings happy and circulate the merest puff of air for cooling.


Mike Halloran
Pembroke Pines, FL, USA

 

[waross]

Old school was to cycle the fan based on head pressure. That worked quite well for many years.
A suggestion:
Don't try to push the envelope, pick a safe minimum speed. My WAG is about 1/3 speed.
Below that, cycle off and on based on head pressure.
You may even have a net saving in energy if you cycle the VFD off also.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Assuming the motor was NOT designed to be run from an inverter, I agree with Bill that 1/3 is about as low as I would go. Technically we say that a V/Hz drive is good for about a 4:1 speed range (so 25% speed), but that's about the control accuracy, not the motor's thermal capability. As an axial fan, it's pretty much going to be a centrifugal load so at 33% speed the load on the motor is going to drop to about 4%, which even when coupled with the fixed magnetic load of the motor itself, is still going to be low enough that the lack of cooling air over it will not likely cause it to over heat. But is that fan even going to be doing any useful work at that speed?


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

Great. That's what I was after. I agree 4% is useless when compared to thrashing a motor for hours doing what? I'll add that to the control algorithm.

Begs the question, will just the magnetizing current overheat a motor sitting in free air? Say a motor with nothing connected to the shaft having 0.1Hz sent to it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

Don't know about the small motors, but when we do open circuit voltage test of wound rotor motors, the magnetising current and the iron loss warm up the winding pretty quickly in the standstill air.

Muthu

http://www.edison.co.in

 

[waross]

I calculate that 1.33 Hz into a stalled 1760 RPM motor will draw FLC.
Have I got this right?
Example:
FLC = 10 Amps
PF = 0.9
Real current = 9 Amps
Reactive current = 4.36 Amps or 43.6% of FLC.
The current at 0.1 Hz may be expected to be less than FLC current but your applied frequency will pass through 1.33 Hz as it ramps down to 0.1 Hz.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Well, you guys nailed it!!



Had a 16hp refer compressor tripping out on high pressure lock-out leading to trapped lightly-roasted passengers in New Mexico. The high side pressure was racing up to 395psi tripping off the compressor and then taking 35 minutes to coast down to 235psi where lock-in occurred only to race back up to lock-out in about 15 seconds. Turns out I had the condenser fan running frequently down around 5Hz

. This caused the fan to mass-flow at a whopping 0.06% leading directly to the fan motor getting roasty toasty just like the passengers. Luckily it apparently has, to our surprise and saving my butt, thermal overloads in the three-phase motor. This caused the fan motor to disconnect until a long cool down passed. Meanwhile the compressor would have demand put on it start up and go thru its galloping torture because the fan was not running. From my lair it looked (correctly so) like there was NO condenser fan. We incorrectly assumed it was dead.

Last night I rewrote all the control logic making the fan run only and always when the compressor was running instead of just when the pressure was above 190psi. It was discovered the pressure could just hover around 190 ALL THE TIME even when the compressor was not being called for so the condenser fan could be dribbling along roasting at the unreasonably low speed. Also, I put into the control equation that the fan could run at any speed between 150rpm @ 190psi and 1725rpm at 220psi except now if the algorithm asks for less than 600rpm the output is set to 600rpm.

I stuffed that into the system and before turning in put a counter on the number of high pressure lock-outs occurring. Next day before blocking out the compressor until the fan motor could be replaced I looked at the counter.. 0, zero Lock-outs. I watched as 81F Seattle loaded the compressor up with all three zones or about 3tons of refrigeration and held steady at 275psi. This went on for hours. We finally concluded the motor must've had thermal protection in it.

I learned a lot on this one.

Mike I'll remember that from now on.
Bill I failed to realize that the motor is still drawing all it's current even when barely running as the HP is trimmed via only the voltage..
Jeff thanks for the cubed reminder. Makes a bunch of sense now.
Edison; I know now. Thanks.

Thanks again all.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[GroovyGuy]

Howdy itsmoked,
If your load is variable-torque, which it is likely for a centrifugal fan, then the turn-down on the vfd can be as large as 10:1. For a constant-torque load the turndown can be as small as 2:1.
Other than that I would want to see the torque-speed capability curves for the motor.

GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[waross]

I suspect that in this case where there is no auxiliary cooling that the motors ability to reject I²R looses will overrule the speed torque curve.

Bill
--------------------
"Why not the best?"
Jimmy Carter