substitute for variac 1

[DrBiff]

Hey guys I'm a ME so bear with me....I have an AC fan that I have been controlling the rpm with a variac, but I now need to be more precise(digital I suspect). I don't necessarily need it to be computer controlled, just need to fine tune the rpm to achieve an exact pressure drop(relatively).

I see you guys recommend VFDs, but doesn't a variac change the voltage and a VFD change the frequency? Whats my best/cheapest options here? I don't know if this is 1 or 3 phase and honestly I don't even know what effect phase has on the motor. Fill me in....

 

[davidbeach]

You have to know what type of motor you have, all else depends on that.

 

[DrBiff]

Ok how do I find this out? Doesn't say anything on the spec page, is there some indication written on the motor? Will look when I go down to the lab.

What does a variac output? single phase 3-phase?

 

[BobM3]

An AC motor that can be controlled by a variac? I'd guess it's a universal motor. Does the motor have brushes?

 

[sreid]

Fans such as 4 inch muffin fans use single phase shaded pole induction motors. They are also impedance protected meaning that they can be stalled continously with out overheating. The high resistance if the rotor does allow you to change the speed by varing the voltage but without much control.

Speed controlled muffin fans use an internally comutated brushless motor. The speed can be changed by varing the DC voltage to the motor usually through PWM. Some fans have a pulse generator so you can sense the speed of the fan for finer control.

 

[itsmoked]

We just had this whole discussion.

Please read this thread:
thread237-199291

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

To reiterate the salient points from the other similar thread:

Nobody can offer meaningful advice without knowing what your motor is.

There are lots of possibilities, depending on what your motor is.

Some of the possibilities run the risk of damaging your motor if you do not find out what type of motor it is.

So will I need to get out my dead horse post again?

 

[DrBiff]

No come on guys I already read that thread, thats how I know you guys recommended the VFD, I wanted to see if there was a product that I dont know about. I am also interested in how some of this stuff works for future reference, like why would decreasing the frequency have the same effect as the variac. Does the variac change the peak voltage and the VFD has a constant peak voltage but on alters the frequency?

The AC fan plugs straight into the wall, so I guess single phase. What would happen if I hooked up a DC power supply to the fan? nothing? destruction?

 

[electricpete]

See jraef's post 17 Oct 07 19:52




=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[DrBiff]

oh come on! killing me here.....

cant you guys answer any of my questions that are not specifically related to the fan?

 

[jraef]

Sigh... Oh well...

Speed of an AC motor is ALWAYS determined by the supply frequency and the number of poles in the motor winding.

Changing the voltage without changing the frequency reduces the TORQUE of an AC motor by the square of the voltage. So for instance at 60% voltage, the torque is .6 x .6 = 36% of the normal torque. If you have a fixed load, the speed reduces because the motor now has insufficient torque to keep it spinning at full speed. DEPENDING ON THE MOTOR DESIGN, that may or may not work out too well, because MOST AC motors will try to keep running the same speed and pull more current to do it, until the motor burns up.

A variac or rheostat (a.k.a. dimmer) can change the speed on SOME TYPES of motors because they have a type of design that inherently limits the current flow, thus allowing the lower torque output to occur without burning themselves up. These are called Shaded Pole motors and most likely your fan is one of these if it hasn't burned up from your test. Another type is a "Universal Motor" which is really more of a DC motor to which you supply AC. This is what you find in small appliances and portable tools, not likely on a fan.

A VFD changes the voltage AND frequency together at a predetermined ratio, which allows the motor to maintain a constant torque while varying the speed by keeping the ratio the same as it was at full speed and full voltage. But it does so by converting the AC to DC, then using high speed transistors to fire DC pulses in what is called a PWM pattern to the motor to recreate a "pseudo sine wave" that the motor reacts to AS IF it were AC. This works great on 3 phase motors. The problem with most types of single phase motors is that the pulsed DC is incompatible with the design of the motors, and you can damage the motor, the VFD or both. Usually both.

Now go back to what I said earlier. Any further discussion is pointless without knowing exactly which kind of motor you have. If it plugs into the wall, and we ASS-u-me you are in North America somewhere (this is an international forum), then the chances are about 99.9% it is a 1 phase motor. That doesn't tell us enough however.

 

[DrBiff]

ah! beautiful explanation. That is exactly what I wanted to know.

I just received an email from a coworker saying the fans are external rotor shaded-pole motors, so I guess since thats not a PSC I will need to switch to a DC fan right? (I already have a digital DC power supply) or try variable resistors.....

Thanks in advance for all your help guys.

 

[jraef]

You can use a VFD on a PSC motor, it's a bit overkill, but if you need high precision that would work best.

If you just want a way to control speed via electronic means (as opposed to manually turning a pot), i.e. a signal follower, you could use an SSR with a variable voltage output. It would need to be of the type that uses what is called "phase angle control" as opposed to the less expensive "zero cross control", but they can be had relatively inexpensively compared to a VFD. The only drawback is that they are electrically "noisy" so if you have other sensitive electronics on the same power source, you will need filtering. It will also make the motor hum.

If it's cheap manual control you want, buy a fan speed switch like they use on the ceiling mounted paddle fans, which by the way are Shaded Pole motors as well, just big ones. Most of them are 3 speed but the Lutron FS-5E allows you to trim the low speed setting manually.

http://www.lutron.com/rotary/

 

[Marke]

As the motor is an external rotor, shaded pole motor, it is designed to be speed controlled by variable AC voltage.
The rotor is a high resistance rotor designed to operate continuously at a high slip. This is why it is an external rotor. The slip losses are dissipated in the rotor which is in the airflow to keep it cool. If you removed the motor from the fan and tried to control it by variable voltage, you would probably overheat the rotor.
Varying the voltage actually varies the maximum torque of the motor rather than the speed. The speed is determined by the intersect of the motor torque and the load torque. If you want to have accurate speed control, then you will need to incorporate some form of feedback into the system such that the error between the feedback and the setpoint will raise or lower the voltage automatically.
A simple triac based voltage controller is adequate for manual control, but you will need to match that in with a simple PID controller to get accurate control.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[DrBiff]

Man, I love you guys. You've given me many options, now I just need to sit and ponder for awhile, hehe.