variac vs. vfd

A

anvilarms

Joined
Nov 5, 2003
Messages
212
:confused: :confused:
ok i am still lost ;although some things have been cleared up. what is the differnce between a variac and a vfd. what situation is best for which device?
i really am electronically challenged.
 
My 2 Cents-Simplified
as i understand it...

Varic
Cheap, simple, low power application, no load
ie ceiling fan
running low voltage to slow a motor can cause excessive current, stalling and smoke.


VFD
Expensive, must be matched to the motor for HP
Special AC motors are available for VFD use ( $$$ )
Dosen't lose Horsepower on low speed
Good for Grinders

Don't be cheap, greater HP ratings mean greater cost.
Too small means expensive smoke, or frustrating power cutouts from overload protection.
Get the sealed enclosures

http://www.beel.ca
http://www.vfd.ca
http://www.driveswarehouse.com
http://www.drivesys.com/
http://www.thomasnet.com/products/drives-variable-speed-24420200-1.html
http://oee.nrcan.gc.ca/industrial/equipment/vfd/vfd-video.cfm?attr=24

plus others

Steve
 
In a nutshell, Variacs (Variable AC) are for adjusting the AC voltage.
VFD's vary the frequency, thus changing the on off duty cycle from 60 times per second to other ratios.
A variac is great for a resistive load - light bulb, a soldering iron, or a PM motor. It will not work on most larger motors or electronic devises.
For a grinder or shop equipment only a VFD and a matching motor will work (or go with a DC motor)
Stacy
 
Prices on VFD's are coming down. If you keep an eye on ebay you can find one for under $100. I picked one up for $100 that will run up to a 3HP motor. It takes 220 single phase and converts to 3 phase. You lose about 20% but on the 1hp motor on my disc and the 3hp on my mill I have no problems. THe control is excellent.. The only problem is the motor gives off a high pitched noise that some cannot stand. I have grown accustom to it. Also 3-ph motors are cheaper than comperable single phase. Ebay once again will save some $$. Whae I set up for my VFD I was able to pick up a 1hp and two 1.5hp delivered for under $150. one was brand new in factory box,never had a wire hooked to it. The other 2 were washdown motors from a carwash valued at over $600 each.

Good Luck
Chuck
 
Chuck

If your VFD is only rated to be connected to a 3-phase input, you have to derate the maximum output power to 57% of the rated output power. Some VFD's are designed to allow them to put out full rated power regardless of whether they are connected to a single-phase or 3-phase supply. They generally cost a bit more due to the beefier rectifiers used.

The high pitched noise is due to the PWM action (http://en.wikipedia.org/wiki/Pulse-width_modulation) of the VFD. DC motor controllers can emit high pitched noise for the same reason. Different brands/models of VFD will operate at different switching frequencies. Different motors will give off different amounts of noise depending on the switching frequency of the VFD. If your motor or VFD is mounted on a large panel, the panel may be acting as a speaker cone. Adding some mass and stiffness to the panel may help reduce the noise emitted (I just had to do something like this at work a few months ago). The best thing to do is to wear the hearing protection that you should be wearing anyways.

Good score on the washdown motors. You should never have to worry about metal dust getting into those. I am still trying to get some prototype washdown motors from work.

Phil
 
Phil,

I am not sure if the input is only rated for 3ph power. The instructions I have so how to set it up for either 1ph o r3ph. I do know that I am getting plenty of power out for all the motors I use it on. My mill has enough power to take a .1 inch full diameter cut with a 1 inch single point carbide cutter. I cannot stop my 9 inch disc unless I am down below 20hz and I really have to push hard to get it to stop. I do not have nearly the power from a 1hp DC motor. I am sold on VFD's I will never buy another DC motor or controller again. I did score on those washdown motors. Both are Baldors and the third was a 1.5hp Leeson. All were in southern Oregon and the shipping was minimal.

Chuck
 
Chuck

The VFD that you bought may be one of those that does not need to be derated when connected to a single-phase input. In any event, as long as you are getting enough power out of your motor and the VFD does not trip when under load, you have a VFD with "sufficient" ratings.

It is inherent that 3-phase motors do not lose torque at low RPM's and DC motors do.

Phil
 
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