You already have excellent answers but hey more info never hurts.
Everyone, I am curious about VFDs.
How do they work?
Short form: The convert AC voltage into DC voltage and then convert that DC voltage into variable frequency AC voltage.
Do I need to buy a special motor to use with a VFD?
No, it does need to be a three phase motor though. That isn't a bad thing because good quality three phase motors are often cheaper than single phase motors.
That being said inverter rated motors have insulation that can withstand high voltage transients that can be seen with these drives. For light users of machine tools that particular feature isn't of much use. Some inverter rated motors supposedly maintain torque over a wider range of operating speeds but I sometimes think that the manufactures are playing funny with the numbers.
What if I have a machine with a three phase motor? I need a three phase VFD?
All VFDs I know about are three phase drives. Don't sweat this though, you actually want a three phase motor in many applications. The VFD drive, if you purchase the right one, converts single phase to three phase no problem.
What brand gives the best results?
That is a tough one! We use various brands at work and few have ever failed. Sized properly and installed as recommended they are very reliable these days.
As you know I have an 11 x 36 Rockwell lathe I am attempting to restore and the variable drive mechanism for it was ruined in a fire. I hope to use a VFD to run the lathe and provide me with safety mechanisms such as an E-Stop.
Thanks much,
"CX"
Well a VFD can help with some of that but it shouldn't be seen as a solution to an E-Stop situation. For one thing in an E-Stop situation the drive may be shut off or disconnected. You might want to consult with an engineer if the lathe is going into manufacturing or even a job shop with one employee, but if an E-Stop function is a requirement you will likely need a clutch brake assembly.
A VFD can give you a fast ramp down in normal operation though which is a very very good thing to have. You can also moderate ramp up which can greatly reduce stress on the machine that might come from an across the line starter. In any event there is a lot of good associated with VFDs, just don't think of them as an E-Stop solution.
In any event somebody already pointed out that VFDs are not perfect, the big problem being the loss of torque at low speeds. You will not get away from the need to use a gear box or belt reduction. What you should do though is some sensible engineering to fit the right motor to the machine. Often this means a 4 pole motor (1800 RPM). You will need to know the motors maximum rotor speed which is not its nameplate rated 60HZ speed. Most motor rotors can handle at least 3600 RPM so you want a VFD drive that can run that 4 pole motor at 120 HZ.
If you follow me so far we have a 1800 RPM motor being driven by the VFD to run at 3600 RPM. In this example that becomes your maximum RPM design speed. At this point you can figure out a belt ratio that will give you the safe operating speed for the spindle and any chucks you may have. This is a metal lathe so that will likely be well below the 3600 RPMs. In any event what we have done here is to make use as much as possible the constant horse power range of the motor. At the motors rated speed of 1800 RPM your belt reduction will have the spindle running at well under that speed, possibly a 2:1 reduction, meaning 900 RPM. That puts your motors rated horse power into a very nice RPM point for a lathe this size. You will loose torque below that speed but it won't be too bad down to around 500 RPM.
In any event I hope you see this as an example and not a well worked out solution. A lathe that might be used for bar work with collets may need a different solution as would a lathe that can handle different RPMs max for the supplied chucks. The point is if you do a little leg work you will likely be able to get very good results. As such reading up on three phase motors and VFDs should help you with understanding what you need to do. Also realize that the numbers above basically have rolled off the top of my head. For example many AC induction motors have rotors rated for more that 3600 RPM but you need to know what your specific machine is rated for. Also VFDs have operating ranges that vary with model, some can go up to 400 HZ others top out at 120HZ.
I hoping this helps and doesn't add confusion, the concepts are pretty straight forward, it might take a better explanation than I just delivered though.
Sent from my iPad using Model Engines
