Best way to run two fans on one Variac?

[papaduc]

Need to run two extraction fans on the one variac. Amp-wise I'm ok with a total of 1.6 from the two fans (the variac is 2amp)

Can I hook up a small two plug adaptor to the variac and then plug the fan/s into that? Or do I have to wire them both directly into the transformer, and why?

Thanks.

 

[RM - aquagrower]

One thing to concider is that many fans draw more than the rated amps upon start up.

A rating of 300VA means that at 100volt, the unit can sustain at maximum 3amp. If at 120 volt the unit can sustain at 2.5amp
Note that most device have surge (when first turn on) much higher than operating amp rating.
E.g, motor with 3 amp rating may have over 5 amp surge, and you will need 5 amp variac

Taken from amazon.

That said, I don't know that the variac you have can handle what you want to do with it. Ya may need to get another one.

 

[rives]

One thing to concider is that many fans draw more than the rated amps upon start up.



Taken from amazon.

That said, I don't know that the variac you have can handle what you want to do with it. Ya may need to get another one.

Nailed it. It isn't unusual for electric motors to draw 300% current while accelerating up to speed. Motors running on low voltage (the Variac) will accelerate slower than at full voltage, and pull higher current as a result of the lower voltage.

 

[papaduc]

Thanks for the replies. I've already set it up now and it's working fine.

With regards to the draw, they're rated 0.6 & 1amp max. I'm pretty sure a 5 or 6 amp variac would be massive overkill for two extraction fans. I've been advised by qualified sparks that the 2amp is plenty for my needs so I think I'm cool in that respect. Nothing else will be running on the extension.

 

[Chunkypigs]

I have plugged a power strip into my variac and run 5 fans before (5 amp unit)
works fine.

 

[papaduc]

Yeah man I'm running them now no problems.

 

[imnotcrazy]

Your "qualified sparkies" do NOT understand proper motor theory if they suggested a variac for control of motor speed to begin with.

Let me explain:

Motors are rated in HORSEPOWER which is translated directly from wattage ( 746W / HP)

Regardless of the voltage you feed the motor it tries to pull the same wattage to produce the torque necessary to rotate.

By dropping the voltage, the motor's inherent design begins to draw more current then the motor windings were ever intended to handle. Eventually your fan and or underrated variac will fail (shorted windings potentially a fire). It may work for a while but eventually the motor WILL fail.

There are only certain types of motors that you CAN use a variac to control the speed because these type (shaded pole) of motors inherently inhibit current flow to a maximum level.

Fantech, Vortex, can-Fan etc motors are P.S.C. (Permanent Split Capacitor) type motors which DO NOT limit the current and will eventually fail due to overheating the windings

On these type of motors you want to use a Diac-Triac type speed control which limits the Voltage AND CURRENT to the motor

 

[imnotcrazy]

P.S. : This has been covered here before

https://www.icmag.com/ic/showthread.php?t=117969

 

[imnotcrazy]

BUMP

BTW, the people who don't believe this..... you are doing a lot of other growers a favor....when your filter fans fail....

The low hanging fruit are the ones who get picked

 

[papaduc]

In the thread in which you say it's covered, the fella had a faulty variac. It wasn't the use of a variac itself which caused him problems.

From what I've been told, running the fan at 50% or over will negate the problems/risks you talk about. Only when it's reduced to 40% or lower could it cause a problem. That's what I've been told anyway.

I've read quite a bit of conflicting advice on this matter and in one article someone expressed the same opinions as you and someone dismissed most of the concerns, citing a variac controller as one of the better ways to control the speed of your fans and far better than some of the more commonly advertised items. I'm not saying who is right or wrong, but it seems to be the majority of qualified opinion says use them but use them correctly.

 

[the gnome]

hi imnotcrazy,

I use the cheaper diac-triac $20 on my inlines with no probs.
I bought a can 12" maxfan on a smokin deal for a future project.
its pretty loud so i hooked it to the diac-triac controller and it worked but the fan made a noticable humming sounds as the speed was reduced,
which i understand isn't good for the motor.
is a mixed flo fan motor the type that needs to be powered by a variac for safe variable speed operation?

From what I've been told, running the fan at 50% or over will negate the problems/risks you talk about. Only when it's reduced to 40% or lower could it cause a problem. That's what I've been told anyway.

correct, you should never run your fans at 50% or less regardless of the type controller used.
for one thing, the fan motor has to be able to cool itself and does this by running at a proper speed.
the lower the speed and the motor isn't able to cool it self as efficiently and will over heat at some point
with possible fire danger there.
2nd...? I really don't know another reason... maybe someone that does will chime in.

personally imo if you have to run it anywhere nears 50% you should just resize accordingly to a smaller size fan.

 

[imnotcrazy]

In Pontiac's thread, where another person mentioned S&P fans are specified for speed control with a Variac, those must be motors of the Shaded Pole design (or the manufacturer themselves do not understand motor theory). These motors limit input current to a maximum level and therefore, a variac can be used. Before doing so, I would CONFIRM they actually do have shaded pole type motors as I am NOT familiar with these fans.

The reason the Variac failed on Pontiac in the other thread was because the variac is essentially a single winding, adjustable transformer. MOST variacs are only fused on the "Primary" side of the winding. When reducing the output voltage on the "Secondary" side of the variac it is still pulling current at 120Vac on the primary side yet the windings are only rated for the specified current. In this case, the "primary side" is typically pulling a normal current but the ramping of motor input current can exceed the variac or motor insulation rating. This is basic OHM'S LAW; Power = Volts x I (current in amps)

Furthermore, the inherent design of a P.S.C. type motor will continue to ramp current trying to return back to full speed. The only reason a variac gives any control of speed is because you are overloading the motor at a reduced input voltage. The motor will continue to pull higher and higher currents in an attempt to reach full speed. At some point the maximum current of the windings and/or variac will be reached and there WILL be a short circuit.

You read conflicting advice because each type of motor requires a different method to control their speed in an electrically safe manner.

As far as the Can Max fans go,I am also not familiar with these fans and would consult with the manufacturer as to the proper method.

As a rule of thumb :

Permanent Split Capacitor Type Motor : Diac - Triac type speed control

Shaded Pole and Brush type (corded power tools) Motors : Variac

 

[rives]

Imnotcrazy is absolutely correct. My electrical background is from industrial plants, and feeding low voltage to a motor is to be avoided at all costs for most motors in order to attain anywhere near their rated life. This is reflected in virtually all aspects of electrical design - when designing the infrastructure and calculating voltage drops, industrial motor circuits are specified to stay under a 3% drop, whereas residential circuits are allowed to drop 5%.

The relationship between low voltage and high full load amperage is nearly linear for most motors. Conventional motors will see an 8-10% increase in full load amperage for a 10% reduction in voltage, and nearly a 20% reduction in both starting and running torque. Compared to conventional motors, these fans can more easily withstand an increase in heat because the motor is suspended in the airstream, but they were designed to use the nominal airflow to cool them and the increased heat will affect their lifespan. The disproportionate drop in starting torque also makes it pretty easy to dial the voltage down to the point that the motor has inadequate torque to restart after a power failure, subsequently burning the motor up.

As INC touched on, there are exceptions. I believe that CanFan has one product line that they specifically recommend using a variac for speed control. That is the exception, not the rule, and you should check with your fan manufacturer for compatibility.

 

[the gnome]

As INC touched on, there are exceptions. I believe that CanFan has one product line that they specifically recommend using a variac for speed control. That is the exception, not the rule, and you should check with your fan manufacturer for compatibility.

i looked on canfans site, in the max fan mixed flo series they have
6" ---->20" fans, but only the 12" like i have and the 8" can be speed controlled and are required to/and be only used by the variac to control motor speed.
wonder why only the 8and 12inchers can be speed controlled?


great info rives and INC
were lucky to have guys like you on the boards!


ok, found this in the FAQ at the CF site
FAQ
FANS:
What makes a Max-Fan better?
- Extremely energy efficient
- Lower life time cost
- Light Weight
- Airtight housing
- Easy installation
- Quiet
- Internal parts optimized for aerodynamic efficiency
Do Can-Fans and Max-Fan use brushless motors?
Yes. The Can-Fans and Max-Fans use a brushless motor in their operation.
Are your fans Speed Controllable?
Yes. Both the Can-Fan and the Max-Fan line of fans are speed controllable, DO NOT SET THE SPEED OR MOTOR CONTROL BELOW 40% OPERATING SPEED TO AVOID OVERHEATING THE FAN.
The Max-Fan 12" is not conventionally speed controlled, The Max-Fan 12" requires the Can-Trol Speed control.
This control provides absolutely no motor hum when controlling the fan, it also gives you the option to control the voltage entering your fan, with an easy to use dial you can go from 1v to 130V with a turn of your hand, no matter what voltage you run, your fan will not hum!
Why did my fan shut off?
Can-Fans and Max-Fans are thermally protected. DO NOT SET THE SPEED OR MOTOR CONTROL BELOW 40% OPERATING SPEED TO AVOID OVERHEATING THE FAN.

 

[imnotcrazy]

"You must spread some reputation around before giving it to Rives again"

Kudos my electrical brethren!!

 

[papaduc]

That's good info, thanks. It seems based on the manufacturers website and what I've been told previously that there's no problem running my fans as long as they're running above 50%. They'll never be as low as that anyway and are more likely to run about 70% or thereabouts.