3 phase motors run on single phase as a production norm

I'm not a motor wiz but I did build a rotary phase converter years ago that had many good comments on this forum and it functioned very well until I was in a shop with 3 phase delivered.
Last year I bought a cold saw blade sharpener off the bay from a guy who been using it for years (30+?)....
He was retiring and when it was listed it was mentioned it needed some work.
Being the only bidder I won it.
It turned out a couple wires were loose and a capacitor was toast.

This summers project is to clean up the machine, rewire and re-switch with components that are available if need be.
Everything looks to be unobtanium.
So in making a drawing of the existing wiring diagram, it looked surprisingly like what I remembered of a phase converter start circuit.
Upon removing the motor connections cap sure enough a delta and wye diagram.
They are using a start capacitor with a momentary manual switch to get it going and a run capacitor to "generate"
The sharpener motor is probably 1/2 hp and the load minimal.

Question
Is this or was this normal way of building industrial machinery when single phase is required?
This was not a retrofit, it was delivered as a 110v single phase machine

motor plate pic

Normal? What's that?

Yes, some people did that / do that. You lose about 1/2 of the motor's capacity in the process but for some small machine applications, it makes sense to just start with a 3 phase motor twice as big as you need and rig up a start system for it. The thing is that the motor runs in a severe current imbalance state so it will have what are called "negative sequence currents" circulating in the rotor that produce counter rotating torque, meaning torque that is OPPOSING the torque that the motor is creating. That's why you have to de-rate the motor; it is "fighting itself" the entire time you are using it. So if you load it up to the nameplate rating of the motor, it over heats very quickly to the point of damage.

Delta Wye does offer starting at low input currents, but the design is not just to enable a run 3ph situation on a start 1ph. I have several d/Y motors, and they work just fine on true 3hp of the correct voltage.

With that, I ran the little K.O.Lee T&C on single phase start with drop out caps for a number of years until a VFD was fitted. The motor frame was large for the motor capacity, and when was the last time 1/4 hp was needed to dust off an end mill? Inertia will carry the wheel through the load just fine.

Sure the motor is constantly doing the work of manufacturing it's own third leg. Big deal.

For <$200 you can fit a VFD as well. ;-)

On a much, much larger scale, the 10hp blower on my grain bin's dryer is a 3ph motor. It was built, however, from the git-go, to be powered by a single-phase overhead farm feeder. The magic box on the side of the blower is full of capacitors.

It is referred to as a 'static converter', but yes, that is not uncommon, especially at low power levels. As Cal noted, inertia does most of the work.

And an aside note, Kaiserslautern is 'sister city' to Davenport, Iowa, and sponsors many foriegn exchange kids, as my family did. From what I understand, the presence of the US army base there gave K-town made it advantageous for local postwar businesses to manufacture and market to US industrial markets... so they did. Building small tools using existing parts sources, and adapting them in-factory, made lots of sense, and plenty of dollars, so they did.

First, the usual purpose of a delta/wye connection is for soft start. Instead of using a transformer to start on a reduced voltage, you connect the motor in a way (wye) that sets it up as a higher voltage motor, which draws less starting current than it would in the delta connection. After it gets up to speed, you switch to the full voltage connection. On the 100 horse type grinders I work on, the load meter goes right up to 100% during start, not higher, then when it reaches near full speed, switches to delta and settles down to normal operation. Without the change, the starting surge would be much higher.

All of that has nothing to do with running a three phase motor on single phase.

Most small single phase motors have a starting winding connected to a capacitor and centrifugal switch. Once it is switched out, the motor runs on single phase, like a single cylinder gasoline engine getting a kick every turn and coasting the rest of the time.

Some static converters do the same thing on a three phase motor, using only one phase's winding for running. Because, unlike a single phase motor, the three phase one is not optimized for this service, it will not deliver full power.

You can generate full three phase voltage at one motor load with a run capacitor to shift the phase of the line voltage to the voltage and phase angle of a normal third phase. At that load only, the motor will think it is running on a standard three phase line. I ran a vertical mill that way for years, using a current sensing relay to keep the start capacitor in the circuit when the motor was drawing a high current and switching it out, leaving the run capacitor in the circuit, for normal running. Adding an idler motor makes the run capacitor work over a wider range. If you had a variable capacitor that would automatically adjust for the load, there would be no need for an idler. Constructing a variable capacitor that big is not practical but I have used a Variac to reflect a variable capacitor and adjusted it for any load.

This is all so simple, but I know of no other electrical subject that suffers from as much mumbo jumbo and folklore.

Bill

DaveKamp said:

On a much, much larger scale, the 10hp blower on my grain bin's dryer is a 3ph motor. It was built, however, from the git-go, to be powered by a single-phase overhead farm feeder. The magic box on the side of the blower is full of capacitors.

It is referred to as a 'static converter', but yes, that is not uncommon, especially at low power levels. As Cal noted, inertia does most of the work.

Click to expand...

Yup, was troubleshooting neighbors grain dryer, it has a 15 hp single phase motor.
Pushed the start button, and rohwer.... rohwer..... rohwer....., up it wound.....and kept winding up.....

I swear it took a full minute before it came up to full speed, really had me wondering
how the start winding & start caps could take that long of a start cycle.

I once repaired a heater for what must have been a huge aluminum annealing oven. The heater was an edge wound ribbon around eight inches in diameter and 4+ feet long. The customer said that some engineer had miscalculated the motor load and he had to let the temperature come up before he could turn the circulating fans on because the denser room temperature air would overload the motors.

Bill