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How to use a three phase machine with single phase power

Three phase tools with only Single phase power available? Problem Solved!

My job in this life is supplying people with electric motors. One of the most frequent asked questions by my customer’s is that they want to use their heavy duty or commercial grade tools such as: table saws, drills, milling machines and lathes to work in their home garages.

Since the tool industry mainly manufactures these commercial grade tools to run on three-phase electrical power. The homeowner is stumped to figure out how to convert these machines so they will work on residential based single-phase power source. Now some of the smaller motors can be easily converted to a single-phase motor. The main problem arises when the motor is built specific for the machine or if the motor is larger than 3 horsepower. This becomes very expensive or unavailable in a stock part number type electric motor to fit the machine’s design.

Now I am very familiar with static and rotary phase converters and they have been around for a long time. The static converter is cheap and works ok, but it only works on small hp motors and in most cases it is common to see loses of 1/3 of the motor’s horsepower. In my industry they call that de-rating the motor/machines capability. Since I am a follower of Tim Allen the tool man. Power is a good thing. Who wants a 3HP saw or drill that only puts out 2HP of work? Not Me!

Now the second option is the Rotary Phase Converter. This is a good option and this type of power conversion has been used for many years. I am not getting to involved on how this devise works but basically you are running an oversized 3-phase motor on input power of 1-phase power and taking the generated power from the third leg to create 3-phase. Now the drawbacks are several things. One: this setup can be expensive depending on the horsepower needed. Two: the phase converter is heavy and needs to be shipped to you by truck freight. This also adds a hefty cost to it. Third: this design requires a secondary sub-panel to distribute the 3-phase power safely: Fourth: the rotary phase converter has a chance for a miss-start (when the motor does not rotate) and draws a huge amount of amperage upon start up. This makes your demand meter peg costing you more money for each amp-hour you use. With the cost of electric power these days every penny counts. Fifth: this design also needs to be de-rated because the third leg of power is not as strong or as balanced with thee other two legs of power. So in most cases I have seen power losses of ¼ of the total horsepower. Again who needs this? Sixth: this is my least favorite. You have a noisy electric motor running all day long in your garage and depending on the size of the motor it can be pretty noisy. One other thing, even when you are not using your tools the rotary-phase-converter is still consuming electric power. So referring this design as technically equivalent to a black and white TV. Now don’t get me wrong. In some cases this is still the best way for some applications.

Now I am finally getting to what I want to talk about. “The VFD” Variable frequency drive.

Now this technology is not new but with advancements of microchips and computer technology. The variable frequency drive has become much more affordable to use this type of devise on a homeowner’s budget. Now let me tell you the drawbacks of this devise before we get into all the good things that it can do. This devise is only designed to work with electric motors. Electric transformers, relays and other electronics DO NOT like VFD’s! So if you want to use a VFD on your machine you must isolate it from the other electronics. You should only use it to operate your motor. This is not the end of the world. Variable frequency drives have many low voltage inputs/outputs to control your electric motor’s performance and work well with controlling the speed of your motor and CNC (computer numeric control) machines. You just cannot solely power the CNC machine by the VFD.

This is how the VFD basically works. The devise inputs the single phase AC (Alternating current) power and converts it to DC (Direct current) and stores it in a DC BUS. A BUS is basically a reserve supply of DC power stored in capacitors rated for the application. Upon demand, the microprocessor chops up the DC current into three individual synchronized Pulse-With-Modulated frequencies to simulate a three-phase power source. Electric motors work great in this environment and depending on the individual design of the motor you can get a wide range of speed control out of this devise, but again electric transformers and electronics DO NOT WORK in this environment and are most certain to be destroyed if applied. I have personally smoked a few braking resistors learning about this technology.

Now most people I find do not want to know how the technology works, they just want it to work. So here are some of the benefits.

The first benefit is Full Rated Power of your machine and overload protection of using your original electric motor! So you save all that time not needing to remove your original electric motor.

Second benefit is that the variable frequency drive is compact and light and in most cases cost less than the replacement with a single-phase electric motor. So you say “So What or this is starting to sound cool”. This is where it gets cool.

Basically you have installed a Micro-Computer Control on your electric motor. You can control the ramp-up speed so you do not have the BIG-BANG on startup that dims your lights in the garage or makes your machine jump and ramp-down speed control “this is call dynamic braking” which is a great safety option for saws where your blade does not continue to spin for a few minutes after it is turned off. Again I talk about low-voltage controls such as emergency stop, reverse and forward control. Programmable multi-speed setting and you can have secondary control stations. Another great feature is you can control the RPM of the motor either using a preset frequency (Hz) or using a POT (potentiometer) to control the RPM. Now most motors have a minimal RPM range of 5:1, but some motors have a full inverter range, so I suggest you consult your motor manufacturer to be safe.

Single-phase motors cannot be used with VFD’s nor can you use them for variable Hz applications. I am not talking about single-phase multi-rpm electric motors. That is a different subject because they are designed to work with 50 or 60Hz constant power supply not a variable frequency application. So let’s not confuse them. Single phase motors get a jolt from a capacitor to get them moving and run on dumb luck the rest of time, where 3 phase motors run on all three points of the delta triangle giving you smooth torque and efficiency. This is why industry does not use single-phase power.

Now to sum things up: VFD’s are great to use as a power conversion from single-phase power to three-phase power for the use of electric motors. The added conveniences such as full rated power of your machine, compact and light weight, wide range of motor controls and efficiency just makes this a wise choice in most cases.

(VFD’s) To be continued.

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