A VFD controlled blower

**pjt** · 27th October 2014, 01:32 AM

I am in the process of setting up a blower fan to supply some cooling to the main bedroom in the house, the house has been added to in the past and as a result the main bedroom has no natural ventillation so at this time of year it can be quite uncomfortable, it's not dust collection per se but there's relevence I think; air movement, VFD control, power consumed etc.. sorry if it gets a bit detailed/nerdy.

The blower in question....
fan wheel.jpg drive side.jpg blower fan.jpg

It (and another) was an auction pickup, not sure but they may be out of the roof mounted evaporative style air conditioner, the plan is to set it up outside and take outside air into the bedroom via some ducting, I'm going to experiment with some thermal mass and water evaporation for some additional cooling.
This wheel type of fan with the forward facing blade design is good for moving bulk volume of air and for overcoming ducting resistance, I did put a piece of ply up to the outlet to block it off which took a bit of effort, I must remember to do a static test at least!!! this fan would be good for ventillation of the shed as well and that is what I will most likely do with the other one I have.

It will be speed controlled via a VFD and a temperature controller, um, all once again gleaned from an auction...
wiring.jpg
The temp controller (lower left in pic) has a digital input and two 0 to 10vdc analog outputs, I just use one that goes to the VFD, (temp above setpoint=speed increase) for the shed the simplest is to just use an adjustable pot. to give a variable output.
Unfortunately it's a bit of a mishmash of voltages, 5vdc from the VFD, 12vdc for the voltage switch, 24vac for the temp controller, 24vdc and 240vac for the relay/contactor coils and 415vac for the fan motor but we got there, a bit more labelling and it will be done!
For additional control I want the fan to turn off when below SP and after a time, this was accomplished with a timer, if above SP the fan turns back on which was done via a voltage switch...
voltage switch.jpg
Jaycar came to the rescue here, it's a kit designed for use in cars to turn a fan on or such dependent on a voltage from a sensor, I am using it here to give me a start signal for the VFD at just above SP (0.3v from the temp controller). The other bit of electronics on the left is a 12vdc regulated power supply and a delay on timer (about 7 secs) which gives the temp controller time to turn on before the voltage switch turns on.

After a few test runs and a few measurements...
data from tests.jpg
At this point all I have done is run the fan as pictured above, the data is taken straight from the screen of the VFD, I then plotted a few graphs, ended up a little messy but I wanted get a visual idea of performance.
Basically I increased speed from 10 to 70Hz by 10Hz increments and then read off the variables as per the table. Unfortunately I don't have anyway of measuring the airflow other than to say it's a nice breeze at 10Hz and at 70Hz it's a fair gale.

One of the VFD setup programmes covers the current draw, the motor nameplate gives 3.6 amps which is the maximum allowable, I can also set the start It2 current and the It2 corner Hz, (t=time) with these three values I can adjust the current trip value at any speedxtime, this sort of programme setting can be good for a motor trip well before the motor lets the blue smoke out,
the current draw is an important number to be aware of especially if we are driving our DC motors at more than the usual Hz.

The power consumed is rather interesting, the table/graph shows as speed increases there is more power consumed per increment, something we know from the fan laws! the kw line looks a little too straight at the end tho, I suspect it might be to do with there being only 1 decimal point on the readout, the %load looks a bit better, I reckon another 10Hz would be about the limit.

Another interesting point is that the motor is rated at 1.5kw, at 70Hz the readout gives 2kw, this extra 0.5kw is most likely due to the fact the motor is not drawing full current and the higher speed above the rated 50Hz, if it was on a larger fan the motor would have reached it's limit before 70Hz. Before I purchased this motor I had a 2pole 2.2kw motor on it as a trial, it did run it but not a good combination, 25Hz was max current draw at 1.1kw from screen readout for this motor, I liken this to driving up a hill foot flat in top gear at 60km.
The motor is a 6pole 955rpm with about 2.2 to 1 drive ratio which gives a fan shaft speed of about 570rpm (from memory) at 70 Hz, there's probably a little more speed in the fan before any big inefficiencies start to arise but I doubt I'll need to run it any faster.

Time for bed

Pete

**BobL** · 27th October 2014, 11:03 AM

Interesting project Pete.

Originally Posted by pjt

Another interesting point is that the motor is rated at 1.5kw, at 70Hz the readout gives 2kw, this extra 0.5kw is most likely due to the fact the motor is not drawing full current and the higher speed above the rated 50Hz, if it was on a larger fan the motor would have reached it's limit before 70Hz.

The max power a 3P motor can output above its rated frequency is usually constant for a some 10's of Hz before it starts to trail downwards due to frictional and other forces, AND the max power is typically 25 - 50% above its nominal rating but it would opt be wise to run it continuously at these outputs.

At 50Hz the power required to turn your fan is 1.2 kW which is less that the rating (as you say the motor does not need to draw its full current rating) to achieve this.
At 60 Hz is can turn the fan at about the current rating - that's about the most I would run it continuously.
At 70 Hz its 30% over rating - if you look carefully the power curve is starting to kick away from the line between 50 - 60Hz suggesting the max power is being approached. I would not recommend running it continuously at this frequency.
It would be interesting to try 75 and 80 Hz and see what happens - I would suggest a soft start just in case it does something unwarranted.

The other factor to consider is the fan efficiency. No fan can be run at higher and higher speeds without incurring an increasing loss of efficiency, this is where the fan starts to cavitate and move less air than it should be moving. its not all/nothing feature but if this starts to happen the power use will not be as much as one might expect and might even drop. Try restricting the airflow at 50Hz and watch the power requirement.

The other thing about operating fans at higher speeds is noise especially where bedrooms are concerned.

**BobL** · 27th October 2014, 07:11 PM

Pete, here is the power consumption curve for a nominally 0.25HP fan running on a 1HP VFD.
The curve is not quite the same as yours as it includes the power consumed by the VFD.
Nevertheless the overall shape is the same as yours and like yours it even starts to kick over @ 70Hz.

A VFD controlled blower-powercurve-jpg

**pjt** · 30th November 2014, 02:13 AM

As usual things are taking longer than first thought, one thing leads to another but it is progressing, I have been making ducting and installing it, enclosing the space that will house the blower and an a multitude of fiddly bits, here's a few pics of progress
ducting to outside.jpg duct to inside.jpg duct entry.jpg fan being lifted up.jpg
First pic shows the ducting going thru the wall to outside, I made up two s shaped sections to lift the duct up higher closer to the floor just so I have a little bit more head room, I didn't fancy hitting my head every time I walked underneath it.
Second pic shows the bend with a central vane, this helps turn the air around the corner, it will also assist with dividing the air that will flow into 2 rooms, bit of a change of plans there.
Third pic shows the duct entry and enclosed space which is underneath the old stove recess and the last is lifting the blower into place, not the most elegant lift I have ever done but we got there.

Pete

**John Samuel** · 6th December 2014, 04:17 PM

Pete,

When do you start your new career as a HVAC designer/installer?

**pjt** · 7th December 2014, 12:24 AM

Originally Posted by John Samuel

Pete,

When do you start your new career as a HVAC designer/installer?

Hi John,
With a bit of luck and with the gods smiling I am on my last career change, the odd project like this is good to do.
How did you go with the storm?

Pete

**John Samuel** · 7th December 2014, 02:05 AM

Originally Posted by pjt

Hi John,
With a bit of luck and with the gods smiling I am on my last career change, the odd project like this is good to do.
How did you go with the storm?

Pete

The storm gave us a good shake up, but fortunately the only real damage was stripped trees. Put my truck in the shed just as the first hail fell ... very lucky ... some of it was as big as a fist.

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