Thanks for the input, guys. I’m new to this(obviously) but I’ve been reading up on VFD’s. So basically it would eliminate the need for a transformer and contactors, and give me wiring connections for estop, pilot light, jog button?
It would also give me speed control so I could adjust the speed while the machine is running, as opposed to the gearbox which requires the spindle to be stopped to adjust speeds? What do people normally do, just leave the gearbox in a middle gear and use the vfd?
I‘m sure there are multiple threads covering all this, I should probably just keep reading…
Yes indeed, it would give you all of that, and all of it can be signaled by low safe voltage inputs, normally to a switch that closes to GND 0V.
Yes - the VFD will provide isolation, of a special sort, and depending on the quality, will have the same degree of safety relay type of protection.
About safety latching relays
The concerns expressed by
@TorontoBuilder are valid, and there are folk who would have a safety relay latched power contactor upstream of everything, but when a VFD is present, the interlocked contactor set would no longer be the device in control of start-stop-reverse, and is no longer needed. An interlocked reversing control set of contactors basically prevents what would happen if you slammed a forward driving lathe straight into reverse, (on the control switch). When VFD is in control, reverse becomes a "software signaled request". If the machine happens to be running, the VFD should brake the drive to a smooth halt, then accelerate up to the the demanded speed, now in the other direction. It does this by direct control of the phases of power offered to the motor windings. The best of them have a connection to add an overrun energy absorbing resistor to get the full deceleration function. These may not be needed on 2kW lathe motors, but I used them always. They are essential on 10kW or 15kW motors. Beyond that, one needs drives that can return energy to the grid.
To be clear regarding safety relay arrangements. These are special separate relays where every contact that would open to OFF state in response to E-stop are in fact two contacts in series, in case one fails stuck on. Every contact that is supposed to go to ON state is in fact two contacts in parallel, just in case one of them fails. They contain various other interlocked electronics that have guaranteed safety operation by design. They are also expensive!
I am not up to speed on the current "safety relay replacement" specifications of commonly available VFDs such as we members would buy, but for some years now, all the motor control devices I have used in motor servo applications had an E-stop input that guaranteed removal of energy to the same standard as safety relays, and makers claim that upstream safety relays are no longer necessary by design. Do look closely at what exactly any VFD you contemplate using actually does.
Usage with gearboxes.
Some VFDs, and all servomotors can deliver full torque even when the shaft is stopped, without it being a short-circuit motor horror. That said, every VFD has a range of sweet, efficient operation. It can have the motor slow right down, and it can have it go at speeds beyond those possible with 60Hz AC. Neither is is great idea. If you use the lathe gearbox, geared down to go real slow, the additional capability of the VFD to deliver quite high motor torque, with massive field slip, while also going slow, can break things. A (sort of) analogy might be putting a jeep into lowest gear, loading up until the wheels won't slip, then driving it to push against a wall. The engine might not stall!. For this, a VFD has torque limiting and current limit parameters, but there is still value in having a belt that will slip, or one of those torque-slip clutches somewhere in line.
Going the other way, with gearbox set to go speedy, then ask the VFD to rev up the motor to it's new maximum, you might have a light load, and discover there are speeds that might be a problem for the spindle bearings.
So - most folk who use VFDs have a favourite gearbox setting that they cann use most times, and get everything else from VFD speed controls, and they have to figure SFM cutting rates from the scaled RPM. That said, we all quickly find out that it's a good idea to do slow running cuts using the gearbox, and allow the VFD to have the motor run faster, so getting the great torque.
My apologies for letting the post get a bit long. I know we have had many posts on VFDs before, but this stuff is what I think I have distilled out of them. A good VFD is an investment, and I think what you get is very much better than a basic latched switched contactor setup. It does cost more than contactors and relays, but there might not be much gain in it if one has full safety relays with interlocking reverse.
