The motor is the first thing to check on any old machine, and the most common surprise. Schools, factories and pattern shops bought a lot of these machines with three-phase motors, because that's what industrial buildings have. Most houses don't. A home shop usually has single-phase 220 V service with three wires, and that's why so many good three-phase machines come up for sale. A three-phase machine can still be a good buy. You just need to know what it'll take to run it.
How to tell which motor you have
Read the nameplate. It will say single phase or 3 phase (sometimes 1 PH or 3 PH), along with the voltage, horsepower and speed. If the plate is gone or unreadable, look at the motor. Three-phase motors don't have a starting capacitor (the hump on the side of many single-phase motors), and four wires coming into the motor is a giveaway. Ask the seller for a photo of the plate and of the motor from the side.
Many makers offered both. Powermatic sold the Model 66 with single-phase and three-phase motors, Delta offered the 37-315 jointer in single and three-phase packages, and Oliver's No. 88 saw came standard with a 5 HP three-phase motor at 230 or 460 V. So two identical-looking machines can need very different power.
Option 1: a static converter
A static converter uses capacitors to get a three-phase motor started, and then the motor runs on single-phase power. It's the simplest box, but it comes with a cost in power. VintageMachinery.org says you get roughly 70 to 80 percent of the motor's normal power, Phase-A-Matic says wye-wound motors run at about two-thirds of rated horsepower and delta-wound motors at about half, and American Rotary says statics run one motor at a time. Fine Woodworking adds that they have trouble starting hard loads like big band saws and dust collectors and can overheat a motor. Popular Woodworking's take is that a static converter will rob you of a third of your horsepower.
A static converter can be fine on a lightly loaded machine with plenty of motor. On a table saw or planer that you'll push, it's the weakest choice.
Option 2: a rotary phase converter
A rotary converter adds a three-phase idler motor that spins on single-phase power and generates the third leg. The machine then runs at all or nearly all of its rated power, and a big enough rotary can run several machines of different sizes. The catch is sizing. Phase-A-Matic says the idler needs to be at least 50 percent larger than the largest motor you'll run, and American Rotary suggests adding more capacity again for hard-starting loads. Voltage balance matters too. A 5 percent imbalance between the legs calls for a large cut in the load a motor can carry.
If you're building a shop full of three-phase machines, like an Oliver saw, a Powermatic jointer and a Powermatic 1200 drill press, a rotary converter is the usual answer.
Option 3: a VFD
A variable-frequency drive turns AC into DC and back into three-phase AC at whatever frequency you set, and many can take single-phase power in. That makes it a phase converter and a speed control in one box. VintageMachinery.org rates VFDs best for smaller motors and particularly good on lathes and drill presses, where changing speed electronically is a real bonus. They also give a soft start and, with the right setup, fast braking.
The drive makers set a few rules in their manuals, and owners who've wired these machines add one more.
- Single-phase input means derating. AutomationDirect's drives accept single-phase input on their 230 V models with derating, so buy a drive rated well above the motor.
- Use a 230 V-class drive. AutomationDirect's manual says its 460 and 575 V models must not be fed single-phase power. A dual-voltage 230/460 V motor therefore gets connected for its 230 V arrangement, as shown on its plate.
- Take the starter out. Never put anything between the VFD and the motor other than wire. A magnetic starter or switch in that line has to go, and you start and stop the machine from the drive's own controls.
- Don't switch the input to run it. The drive maker's manual says not to start and stop the drive by turning its input power on and off.
Option 4: swap the motor
Many of these machines take an ordinary motor on a belt, so a single-phase motor of the same speed and frame can replace a three-phase one. Delta's 1961 catalog, for instance, sold the same Unisaw with a 1-1/2 HP single-phase motor or a 2 HP three-phase one. Machines with the motor built onto the arbor, like the big Olivers, can't be swapped this way, so for those it's a converter or a VFD.
Single-phase motors: 120 or 240 V
Most single-phase shop motors are dual-voltage and can be wired for 115/120 V or 230/240 V. The usual scheme puts the two run windings (T1 and T2, T3 and T4) in parallel for the low voltage and in series for the high voltage, with the start winding across T5 and T8. Older motors may label their leads differently, so follow the diagram on the motor itself.
Running on 240 V doesn't cost less. Ten amps at 110 V is the same power as 5 amps at 220 V. What 240 V does is cut the voltage drop on a long run of wire, so the motor reaches speed faster. The one firm rule is that the supply has to match how the motor is wired, because a motor on low voltage can't deliver its rated power without drawing more current than it should.
What this means when you're buying
- Get the motor plate before you think about price.
- Single-phase at 120 V plugs in. Single-phase at 240 V needs a 240 V circuit.
- Three-phase on a belt drive can take a VFD, a rotary converter or a new motor.
- Three-phase built onto the arbor needs a VFD or a rotary converter sized for it.
- A machine with two big motors, like the Oliver 260 double-arbor saw, needs a converter big enough for both, or a drive on each.
Work out what powering it will cost before you agree a price, and a three-phase machine stops being a gamble. Each model page's shop check says which motors that machine was sold with, and our guide to checking a used machine covers the rest.
