Parallel a fan motor speed controller with a thermostat

[Al B. Fuct]

You have just invested a few hundred in a centrif fan, some acoustic damped duct and a carbon filter. However, during lights off, you notice scents wafting around just outside the grow room! What gives?

 

When the lights are off, the temps in the room may not come up high enough to trigger your thermostat running the exhaust blower. Absent the blower running to push scents through the carbon filter, scents will leak where ever there is a tiny air gap in your room construction. Keeping the room at negative pressure causes air to be drawn inward through those little gaps, keeping all air exiting the op going through the carbon filter.

 

 

If you wire a fan motor speed controller in parallel with a thermostat, when the temp setpoint is reached, the fan motor does not shut off- it just slows down to whatever the speed controller is set for, could be 20-30% normal speed. This keeps air moving through the carbon filter at all times, sucking air into the room via any air leaks and also reduces wear on fan motors as the start pulse from zero RPM puts the highest forces on bearings and stresses on field windings.

 

When the temp rises back up to the setpoint, the motor speed controller is bypassed and the blower motor runs at full speed until the setpoint is again reached. The fan will run less often and for shorter intervals on max speed if there is a speed controller keeping it running slowly between thermostat demands.

[bufo marinus]

Mate I love it bloody marvelous and spot on circuit Al. Do you want a job?

[Al B. Fuct]

bufo, thanks much.

 

Yes, I'd LOVE to have a job. Problem is, I'd have to do it from this chair; I don't move around so well. The main reason I run an op is to supply my own med use due to chronic pain from being run over by a drunk.

 

I'm a wordsmith, electronics techo, MIG & TIG welder (steel) and can handle PC support & lite network admin.

 

Hire me, baby.

[Frazz]

Nice Alby , are the fan speed controllers the same as light dimmers , looks similar ... whats the difference both slow the fan down , is that detrimental to fan ?

 

[mull]

Nice work Al B

 

Fraz those switch plates look the same yeah. The dimmer and the fan speed controller have different load capabilities. Its very slight tho about 50w. The fan controller switch plate normally has a fan pictured inside the dial.

[Al B. Fuct]

Thanx, mully

 

Nice Alby , are the fan speed controllers the same as light dimmers , looks similar ... whats the difference both slow the fan down , is that detrimental to fan ?

 

 

Fraz, motor speed controllers and light dimmers are electronically similar but not identical. MSCs will work as incandescent light dimmers but dimmers can't run motors. A $10 dimmer and a $35 MSC both are electronically similar; each can handle about 300W, but the maths for the waveform attack angles of MSCs and dimmers are a bit different. Dimmers are not designed to run inductive loads; they are for plain resistive loads.

 

A dimmer will run a small motor but the motor will tend to stall if set much below full speed, due to the phase angle of the controller's waveform. When the motor stalls, it will overcurrent the field windings and trash the motor.

[Samjana]

Hi.

Cheers for the circuit AL.B

i've had the exact same issue, fans off therefore smells up, came home one day to find a nice 'fresh pot' smell in the house, Had to turn the fans thermostat controller down to have fan running longer during night.

 

My only Qs is

What effects will it have on the Centrifugal fan? The place i got mine from said that it is bad (speed contoller) for the motor as it is made to run at 1 speed. Being a shop i guess they were using scare tactics to sell me the three speed for more $$$. Or maybe they thought i'd use a cheap light dimmer...

 

I'm going to pick up a speed controller tmorrw and give it a test , i gather if it F's the motor then so be it, chalk it down, to OH Well.

[Al B. Fuct]

Hi.

Cheers for the circuit AL.B

i've had the exact same issue, fans off therefore smells up, came home one day to find a nice 'fresh pot' smell in the house, Had to turn the fans thermostat controller down to have fan running longer during night.

 

You can either do that or you may find that a dehumidifier running during lights off may introduce enough heat to trip the thermostat now and again. Still, if the fan is not running continuously, albeit part-time on low speed, scents will leak through air gaps as specified previously.

 

My only Qs is

What effects will it have on the Centrifugal fan? The place i got mine from said that it is bad (speed contoller) for the motor as it is made to run at 1 speed. Being a shop i guess they were using scare tactics to sell me the three speed for more $$. Or maybe they thought i'd use a cheap light dimmer...

 

I'm going to pick up a speed controller tmorrw and give it a test , i gather if it F's the motor then so be it, chalk it down, to OH Well.

 

MSCs could not possibly hurt a motor unless you set it to spin so slow as to allow the motor to stall. You should never have any cause to run a motor at less than 20% speed anyway, though. Even if the motor is allowed to stall, an MSC's current limiting function should prevent the motor's field windings from cooking. If anything, MSCs extend motor life quite significantly by reducing bearing wear.

 

Like you, I think your mates might have been trying to sell you a 3-speed fan.

 

A plain old 300W HPM MSC (as available from Bunnings and other hdwe shops, see lighting dept.) will handle most any centrif- except one I have in my stocks which I like to call "Fanzilla." It's a 250mm Spectrum Customline centrif (model C250H) with a 475W motor that is rated at 420L/sec (890CFM). Will suck-start a Harley. Keep away from small, lightweight children unless you want kidburgers.

[Samjana]

thx Al B.

picked up a variable fan speed contoller today, i'll let you know how it went

[Al B. Fuct]

Cool. You may note a bit of 50Hz hum from the motor when running it on the controller. This is caused by vibration in the motor laminations. Better motors have laminations which are well bolted together and lacquered over. Cheap motors' laminations are normally only lacquered and will be noisier.

 

Also note that even with the MSC set to max, the fan will only run at about 80-90% normal max speed, even with the control all the way on 'fast.'

 

If you need full speed sometimes, you can bypass the MSC as shown with an SPDT switch, included with some wall-plate mounted MSCs like the HPM brand. This is the same position as the thermostat if used.

 

 

If you add a thermostat in parallel with the bypass switch, the bypass will function as a manual override so you can switch the fan on max speed at any time without having to change the thermostat setpoint temp.

 

The switch provided with the panel mounted HPM brand MSCs is a SPDT. The second compression screw wire fitting contact on the SPDT switch supplied with HPM MSCs is hidden by a plastic plug which you must remove with a screwdriver tip. This contact is normally used in '2-way' switch systems where two wall switches (say, on opposite ends of a hallway) control the same light or fan. It's not often used and so is sold with the second contact covered to avoid confusion.