Subject to the only alteration to the ventilation system being alteration of
the fan speed within its designed perameters:
Volume varies directly as speed.
Pressure varies as the square of the speed.
Power varies as the cube of the speed.
Are you suggesting that fixed fan blades morph to a different shape in use?
I am aware that attack angle of some duct type propellor aerofoil fan blades
can be changed (whilst in motion? - not sure). This type of fan usually has
a characteristic curve for each attack angle. Different centrifugal fan
types (forward curve, backward curve, paddle blade,etc., and configurations
inbetween) have impellor profiles suitable for its proposed use and each has
its own unique characteristic curve but all still obey the fan laws.
My statement referred to the effect of speed change only without any other
change to the installed system. I am not suggesting that an upward speed
change beyond the fan design parameters and/or system parameters is
desirable or safe.
On Wed, 18 Nov 2015 21:11:25 +0000, gremlin_95 wrote:
As others have pointed out, there might be a cooling issue at very low
speeds but in this case, the torque demanded by the fan will drop
considerably compared to the max torque at full speed (which should be
comfortably within the motor's maximum torque rating anyway - current
demand correlates very closely to torque loading).
Presumably the variable frequency inverter is a purpose made speed
controller in which case the output voltage will also vary in direct
proportion to the frequency. Provided that the torque stays below the
motor's maximum limit when run at the highest speed setting (highest
frequency and applied voltage from the speed controller) there shouldn't
be any problem assuming you don't take the upper frequency/voltage to an
extreme, risking insulation failure of the motor windings.
I'd get the contractor to confirm that the 'inverter' is actually a
speed controller with the characteristics I've just described. If the
output voltage varies in direct proportion to the frequency then you're
"good to go".
If, when you appraise your 'consultant' of the speed controller, he
still rejects the specification with no further justification as to this
decision, sack him and get a qualified consultant who *does* understand
the basics of frequency/voltage converter speed control.
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