rabler
Cast Iron
- Joined
- May 25, 2020
- Location
- Rural S.W. Indiana
I set up a VFD for use on one of my machines and thought I would go over some things as there are often questions of the "What should I do to power this?" variety. This will be a multi-part post going from "why a VFD" to details of circuits and programming the particular VFD I used.
I purchased an old lathe that had a 3HP, 3 phase motor. No 3 phase in my shop, nor is it available on the power line near my farm. Didn't even want to ask the local power co-op what it would cost at their end, on my end would have meant burying new power lines, new outdoor service panel, etc. That left powering the machine either with a phase converter, a VFD, or install a single phase 3HP motor.
Single phase motors need starting capacitors and associated internal hardware to switch those in/out. Also, single phase motors are somewhat like piston engines, they don't turn with continuous torque, as the rotor goes around they push at some angles and coast through other angles. A 3 phase motor runs with continuous torque through the entire rotor rotation (as long as it's running on real, balanced 3phase). For many machine applications this may help with smooth finishes. It also means less wear on gear trains. 3 phase motors also have the option of using a VFD for speed control. Single phase motors basically need the momentum of the rotor to coast through the dead spots, so using a VFD to slow down a single phase motor doesn't work well. The start capacitors get past this on start-up, but they'll burn out quickly if used for any length of time. So generally 3-phase motors are preferable on both counts. (Yes, this is gross generalization of a more technical issue).
There are some trade offs between VFD's and phase converters. In phase converter's favor, they can power multiple motors, and are fairly simple to set up and use. They are also larger, and noisier. VFD's are small, high-tech, need to be configured, and offer the ability to get infinitely variable speeds out of a motor (within some range). Phase converters and VFD's use some power of their own, more so with phase converters. VFD's can actually save energy by running a motor at reduced power, but this is a consideration for running equipment long periods, like a heating/cooling/ventilation system on a large building. Most machine shops are not going to see any power saving benefit from a VFD.
If you end up with one 3-phase machine there is a good chance you'll get another one some where down the road. So phase converters, typically a rotary phase converter, is the suggested solution. There are some potential gotcha's with rotary phase converters, notably the wild leg isn't as well regulated. This means that anything on your 3-phase equipment that is using a single phase to power some control electronics, DRO, etc, should not use the wild leg. Alternately, better, more expensive (rotary) phase converters have the better regulation on the generated (wild) leg.
An aside, but commonly confusing issue. If you have 240VAC single phase, it is indeed 1 phase not two phases, even though there are two "hot" wires L1 and L2. Three phase has three hot wires L1, L2, L3. Basically a "phase" is a connection between two wires. So L1 to L2 is 1 phase, L1 to L3 is a second phase, and L2 to L3 is a third phase. Obviously you only have one of those options without L3. True, you can split 240VAC into 2 phases of 120VAC. For a motor, 2 phases (180 out of phase) still has the pulsing torque, so there is no benefit to making a two phase motor. Residential power uses those two separate phases to get more total current available at 120VAC on the same size feed wire to the house, and offer 240VAC for larger appliances. While higher voltages increase the shock hazard, you get twice the total power for the same wire size. This is especially advantageous in motor windings, finer windings mean a smaller, cheaper motor (less copper).
So, using a rotary phase converter, you need to make sure any equipment plugged into it uses L1-L2 (assuming L3 is the generated/wild leg) for any non-motor electronics. You can probably get away with running an incandescent light bulb on one of the other phases, after all, a blown out light bulb isn't that expensive to replace.
I purchased an old lathe that had a 3HP, 3 phase motor. No 3 phase in my shop, nor is it available on the power line near my farm. Didn't even want to ask the local power co-op what it would cost at their end, on my end would have meant burying new power lines, new outdoor service panel, etc. That left powering the machine either with a phase converter, a VFD, or install a single phase 3HP motor.
Single phase motors need starting capacitors and associated internal hardware to switch those in/out. Also, single phase motors are somewhat like piston engines, they don't turn with continuous torque, as the rotor goes around they push at some angles and coast through other angles. A 3 phase motor runs with continuous torque through the entire rotor rotation (as long as it's running on real, balanced 3phase). For many machine applications this may help with smooth finishes. It also means less wear on gear trains. 3 phase motors also have the option of using a VFD for speed control. Single phase motors basically need the momentum of the rotor to coast through the dead spots, so using a VFD to slow down a single phase motor doesn't work well. The start capacitors get past this on start-up, but they'll burn out quickly if used for any length of time. So generally 3-phase motors are preferable on both counts. (Yes, this is gross generalization of a more technical issue).
There are some trade offs between VFD's and phase converters. In phase converter's favor, they can power multiple motors, and are fairly simple to set up and use. They are also larger, and noisier. VFD's are small, high-tech, need to be configured, and offer the ability to get infinitely variable speeds out of a motor (within some range). Phase converters and VFD's use some power of their own, more so with phase converters. VFD's can actually save energy by running a motor at reduced power, but this is a consideration for running equipment long periods, like a heating/cooling/ventilation system on a large building. Most machine shops are not going to see any power saving benefit from a VFD.
If you end up with one 3-phase machine there is a good chance you'll get another one some where down the road. So phase converters, typically a rotary phase converter, is the suggested solution. There are some potential gotcha's with rotary phase converters, notably the wild leg isn't as well regulated. This means that anything on your 3-phase equipment that is using a single phase to power some control electronics, DRO, etc, should not use the wild leg. Alternately, better, more expensive (rotary) phase converters have the better regulation on the generated (wild) leg.
An aside, but commonly confusing issue. If you have 240VAC single phase, it is indeed 1 phase not two phases, even though there are two "hot" wires L1 and L2. Three phase has three hot wires L1, L2, L3. Basically a "phase" is a connection between two wires. So L1 to L2 is 1 phase, L1 to L3 is a second phase, and L2 to L3 is a third phase. Obviously you only have one of those options without L3. True, you can split 240VAC into 2 phases of 120VAC. For a motor, 2 phases (180 out of phase) still has the pulsing torque, so there is no benefit to making a two phase motor. Residential power uses those two separate phases to get more total current available at 120VAC on the same size feed wire to the house, and offer 240VAC for larger appliances. While higher voltages increase the shock hazard, you get twice the total power for the same wire size. This is especially advantageous in motor windings, finer windings mean a smaller, cheaper motor (less copper).
So, using a rotary phase converter, you need to make sure any equipment plugged into it uses L1-L2 (assuming L3 is the generated/wild leg) for any non-motor electronics. You can probably get away with running an incandescent light bulb on one of the other phases, after all, a blown out light bulb isn't that expensive to replace.
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