Variable Frequency Drives...how do they work?

[mastercnc]

Hello all,

I'm in process of gathering information on Variable Frequency Drives and contollers.
I've found a controller at:
Bob Campbell Designs
If you have any info on this or VFDs let me know.

Thanks in advance.

[balsaman]

They work by varying the frequency of the AC current which in turn varies the speed of the AC motor. An AC motors speed is directly related to, and a multiple of, the AC frequency (a little less actually, because they "slip" a little under load, for example 1750 rpm, which is 1800 rpm minus a little slippage at full load). Household AC lines are 60 hz in north america. A VFD can change that to between 5 hz and 120 hz typically, so you can get from near 150 rpm to ~3500 from that same motor. You may need a special motor to go that slow and not over heat.

Eric

[Al_The_Man]

VFD's are used with 3 phase induction motors, and have various features depending on what you want to use it for, some have configurable (low voltage) inputs and outputs useful for integrating into a CNC environment. e.g. zero speed & up to speed indication.
If you are looking at the lower end types WEG has a good range or Mitsubishi for the top end type.
BTW Eric, it is physically imposible for a common induction motor to run at the supplied frequency i.e. 3600 or 1800 because if the rotor ever reached 'synchronous' speed the conductors in the rotor would no longer be moving through the rotating field, they would be in step. there would no longer be a magnetic field induced into the rotor. So they end up running just below synchronism due to this(slip).
Synchronous speed is only possible with a synchronous motor, which is another subject.
Al

[mastercnc]

Thanks for the feedback...so much to learn!

So basicly The motor will maintain constant torque with a variable speed, and at higher than normal speeds the torque drops off dramaticly.
Some VFD I've seen have single phase inputs with three phase outputs, how is that done without a phase converter?

M

[Al_The_Man]

Originally posted by mastercnc Thanks for the feedback...so much to learn! Some VFD I've seen have single phase inputs with three phase outputs, how is that done without a phase converter? M

The VFD takes the AC single phase or 3 ph and converts it to DC then produces a three phase variable frequency voltage.
Some manufacturers derate their VFD's on 1ph , I have found that you can go to about two thirds the HP rating on 1ph, more if you do not usually use motor at full load current.
Some have self tuning features to automatically set the VFD parameters to the characteristics of the motor. Both WEG and Mitsubishi do this.
Al