@Mcgyver and anyone else who feels inclined to contribute. After a good night of focussing my thoughts, I have taken the liberty (and will accept the beatings to come) of drafting a discussion about using a VFD in the context of the hobby shop. What I'd like to accomplish in so doing is to write something that we can all agree with in order to avoid the debates we often seem to have when we discuss VFDs and their limits. It is written in a positive context because I think we all agree that a VFD is a good solution - albeit with practical limits.
Proposed Discussion:
When a hobbiest encounters a used lathe they often have to deal with motor voltages that don't match their shop electrical supply (eg a 550V 3 phase motor and a shop supply of single phase 230V). The most common ways to deal with these problems are to install an RPS (Rotary Phase Converter), or a VFD (Variable Frequency Drive), and/or a new motor that is compatible with the hobby shop electrical system.
An RPC is an electrical machine that converts single-phase AC power into three-phase AC power. It's used to run three-phase equipment in locations where only single-phase power is available. There are advantages and disadvantages to using an RPC. These have been described elsewhere.
A new motor needs no explanation or discussion.
A VFD is a power electronic device that controls the speed and torque of an AC electric motor by varying the frequency and voltage supplied to the motor. Typically a VFD has a Rectifier stage (AC to DC) that converts the incoming fixed frequency single phase AC power from the mains to a stable DC power using rectifier and filters, and an Inverter stage (DC to AC) that converts the DC power back into three phase AC power with a variable frequency and variable voltage to match the machine motor's requirements. A computer control unit in the VFD precisely controls this output to achieve the desired motor speed and torque. The control unit usually also provides protection features for the motor and the drive itself, such as overcurrent, overvoltage, and overload protection. In this way, a VFD can replace the typical control cabinet and its protection system.
A VFD also has advantages and disadvantages. In many ways, a VFD and a new VFD rated motor are a great solution for the hobbiest who is upgrading an old machine that has a motor that doesn't match the available electrical supply system. However, there are some compromises that the hobbiest needs to be aware of.
One of these is the mathematical relationship between HP, torque, and rpm which directly translate to chip load and cutting forces respectively. Horse power is a unit of work and so is a given volume of chip removal per unit time. Therefore, you can think about chip load or the volume of metal removed in terms of horse power, and you can also think about the force required to cut a chip as a given force or torque applied to the cutting tool.
But it is important to recognize that the VFD, motor, and gear system are limited in terms of the maximums they can provide. If the available torque exceeds the force required to cut a chip with a given depth of cut and feedrate, a chip will be cut. Similarly, if the available horse power exceeds the hp required to remove a given volume of chips in a given amount of time, the work will get done.
Unfortunately, the maximum torque and maximum horsepower available are not infinite. They have very real limits that are related to the hp of the motor, the torque multiplication of the gear train, and the hp/torque relationship capabilities of the VFD.
For most VFDs, there is a curve that defines this hp/torque relationship at various speeds - usually constant torque and declining HP at speeds below the motors rated rpm, and constant HP and declining torque at speeds above the rated rpm. Therefore, at speeds below the rated rpm, the maximum chip removal rate will decline with rpm, and at speeds above the rated rpm the maximum chip size will decline.
For most hobbiests with larger machines, these limits are usually well above the actual chip load and chip size that we normally work with. For hobbiests with smaller machines, the limitations can be very real but not unexpected. Furthermore, our machines express themselves in rather obvious ways when we try to push them beyond their limits. Fundamentally, the HP and Torque of any given power drive system in any given machine will have limits that must be accommodated by the hobbiest.
One of the most likely scenarios to cause problems is a large diameter part in a small machine with a small motor. Large parts require slower speeds to maintain the optimal surface speed for optimal cutting. If the HP required to remove chips at the optimal rate drops below that which can be provided by the motor and VFD, the machining process will begin to fail, and the operator will have to either back off on the chip load by reducing the size of the cut or further reduce the speed.
The other likely scenario is attempting to cut a bigger chip than can be supported by the available torque. Big depths of cut and high feedrates that make big chips, require lots of torque. If the torque required to cut that chip at the optimal size drops below that which can be provided by the motor, machine gearing, and VFD, the machining process will begin to stall, and the operator will have to back off on the chip size by reducing the depth of cut or the feedrate.
Basically, in both scenarios, size matters. One cannot expect a small machine and a small motor to perform as well as a big machine and a big motor.
Gearing allows a machine to operate at slower speeds without reducing chip load as long as that remains below what the hp of the motor is capable of providing. And a VFD allows a bigger cut at slower speeds than what the gearing system alone can provide. But ultimately, chip load and chip size are limited by the size of the machine, it's gearing, its motor, and the characteristics of the VFD if so equipped.
For the small machines that are found in a typical hobby shop, these limits are a fact of life that we all accept on a daily basis. It's also why we all lust for machines that are bigger than the space we have. But then again, we have what we have and we learn to enjoy them for what they can do and we learn to accept what they cannot do.