If you are getting a VFD, watch out for the following:
- Get the VFD sized properly based on the motor's rated current (look at the motor name plate). If the VFD is rated for operation at different input/output voltages, the current rating is still the same, no matter the voltage (i.e. a VFD rated for 10A at 480V is still rated for 10A at 240V).
- If the VFD you are buying is rated for operation with a single-phase source, you are good to go. If it is only rated for operation with a three-phase source, you will have to get a VFD rated for 173% more output current or you may blow out the input rectifiers.
- If your budget can handle it, get the next larger VFD to have a bit more margin. Many are built down to a price and have very little margin.
- Install the VFD where metal dust and shavings cannot get into it, otherwise, it can be permanently damaged. The same thing would happen to the Jaguar's of several years ago but VFD's store much more energy and can sound like a shotgun blast when they die. If you can see through slots in the VFD's enclosure and see circuit boards and other components, it can be damaged this way. A good place to mount a VFD would be up high on a wall above the mill. Most VFD's have inputs so that you can connect start and stop switches and mount those next to your mill. If you must mount it where metal dust/shavings can fall on it, install it in a sealed metal box with a stirring fan. The box needs to be large enough to dissipate the heat generated by the VFD. The VFD vendor should be able to help you select a box of appropriate size (based on the VFD's heat output, the dimensions of the box and the sides that are exposed to the air). If they cannot help you, PM me and I will get the formulas from our Applications Department.
- A motor does not have to be VFD-rated to be used with a VFD. VFD's generate high voltage spikes that speed up the break down the insulation in a motor over many years of continuous use. VFD rated motors are manufactured with stronger/thicker insulation to resist this and are usually a bit more expensive and may be a bit harder to get. A mill for an FRC team will probably not see enough use for this to really matter. Many of our customers (my employer is a VFD manufacturer) do not use VFD-rated motors.
If you are setting up a home shop where you usually have only one person operating only one piece of equipment in the shop at a time, you may want to consider putting an appropriate in-line receptacle on the VFD output. You can then convert some other pieces of equipment to run on the same VFD and control the speed with a 10-turn pot. Drill presses run on a VFD are really sweet to use. No more swapping the belt around when going from a 1/8" bit to a 1" bit.