Funny you should ask:
Forrest Addy
>Funny you should ask: is this enough?
The ultimate and most expensive solution for the three phase power from single phase problem is easily the electronic phase converter. This clever gadget takes line power and from it creates the third leg through PWM technology. It�s timing and waveform are accurate and the full nameplate ratings of the motor can be realized. Thi is a fixed frequency device not intended to control motors by itself. It's designed to provide full and complete three phase power from a single phase elecctrical source. I understand complex CNC Machine tools may be connected to the electronic phase converter without regard to line legs and generated legs. Hook it up and start to work.
Finally we come to the �variable frequency drive� (VFD). There�s a lot to learn when it comes to making shrewd decisions regarding VFD�s Few home shop workers have experience with heavy duty industrial electronics. The key to learning new things is to connect the basic concepts to a vocabulary. It�s no different than learning tennis or machine work than when learning enough about electricity to make a decision about VFD's and whether they will help you with powering your three phase motor. Be patient. The learning doesn't happen overnight. The understanding will come if you apply yourself.
The speed of an induction motor is controlled by the frequency of the AC power feeding it. When plugged connected to AC power induction motors run only at their designed speed.
Variable frequency drives are a box of solid state electronics that converts 60 Hz AC to variable frequency three phase. This allows you to operate a three phase (and three phase only) induction motor at any RPM you set. The motor runs at full torque to 60 HZ and drops off in proportion above 60 Hz. They are very simple for ordinary home shop machnists to work with if you can follow instructions when setting them up.
The VFD goes between the power and the motor. On the box are some control push buttons including the "start" and "stop" button. There is also a means to increase and decrease the speed of the motor - usually a knob like the volume control of an old radio. The initial connection amounts to hooking up five wires and a ground. Inside the box are terminals for connecting it to a more convenient arrangement of external push-button controls (the original control buttons furnished with your machine if you like) but you can also run it from the push buttons on the box.
A manual should come with your VFD should you make the plunge. Do not buy any VFD that does not come with a manual. If you're electrically challenged you won't decipher the manual overnight. You'll have to study it, taking in bit at a time like you'd eat an elephant.
As for cost, $200 (used price for a pre-owned 2 HP single phase rated unit) doesn't seem expensive to me for such a talented box. It will power (separately by means of a receptacle on the VFD and a plug for each three phase motor) other three phase motors should you mount them on your drill press, lathe, mill, etc. That smooth, quiet, three phase power and variable speed adds a lot of functionality to a machine shop, and the gadget has built in economizer circuitry that will lower you power bill a tad.
Be sure to get all the manuals that go with them. There's up to a hundred settings (parameters) you set to customize it to suit your motor and application including maximum current, acceleration and a deceleration, stall prevention, and lots of other features. You can run a 1/4 HP motor from a 10 HP drive if you wish but that's stretching it.
There are many drive brands Yaskawa, Hitachi, Mitsubishi, Baldor, GE, Teco to mention only a few. Don't rush off and buy one over the counter from a retail dealer. First determine your needs, then check around your local big city mechanical drives or electric motor suppliers. Ask for take-off or trade-ins. You may be able to get last year's drive for 1/2 price. Be sure to get a 230 Volt drive preferably one suited for single input although a 3 phase only drive will work on single phase if appropriately de-rated as mentioned earlier. If you screw up and get a 460 volt VFD there's no solution but a step up transformer.
Some drives will step up 115 volt to 230 volts internally up to 1 HP. This is a drive targeted at consumers with a bad home shop habit and no way to get 230 volts in their shops. They are pricier than 230 or 460 volt drives. If you�re looking at 115 Volt VFD�s and wondering how they will work on a 5 HP motor maybe it would be a good time to run in a 230 volt service to your shop.
Used later model VFD's from the internet auction sites may be quite acceptable. Beware of older technology units not because they function poorly but because they can be noisy and emit an aduible squeal that even I can hear. eBay usually has two pages of VFD's. Select "Business and Industry" as a category and key in "VFD", "inverter drive", or "AC drive" as search objects.
Here are a couple of on-line sources. Once you get comfortable with the concepts of VFD technology and feel like shopping, phone them and dicker. Check out http://www.dealerselectric.com/ . Go to "Browse our inventory" then click on "inverter drives." Another one is at http://vfds.com/vfdprice.htm#120 Volt Models. These people (and many others sell 120 volt input drives. They have a step-up circuit inside that lets them run 230 volt motors. These are more expensive. http://www.driveswarehouse.com
I have seven VFD's powering my machine tools from my 20HP metalworking hydraulic planer to my smallest drill press. I'll never go back to single phase power for major shop equipment.
The de-rating question of VFD's for the home shop user may require more than a simple 50% answer. It's a matter of how much: the math works out to a three phase rated VFD running on single phase adequately serves a motor up to 2/3 it's rated capacity. If the VFD is designed for single phase power then you can use it to full load capacity.
More advanced consideration for de-rating VFD�s include questions like. How much single phase 120 Hz ripple in the DC buss can the VFD can handle without the logic or the power section being adversely affected under full load at @ 60 Hz? How much extra current and filter capacity is there in the rectifier section?
As the motor speed for a full torque load is reduced, the demand from the DC buss decreases accordingly until about 41 Hz is reached. At that point full load motor current can be theoretically be drawn from the VFD's load terminals. The rectifier and DC buss still thinks it's working into a de-rated load because of the output transistor's PWM duty cycle at the lower voltage/Hz.
So it's full reted current to the load terminals up to about 41 Hz and a linear decrease thereafter to about 70 percent of full load current. This will limit the VFD's input diodes to no more then the three phase full loat current. As for ripple current that's a fuction of the installed filter capacity and has to be figured (or scoped) for your particular VFD.
A three phase rated VFD destined to feed a 1.15 service factor motor run to full name plate ratings from single phase power needs to be DOUBLE rated, that is a 5 HP motor requires a 10 nominal HP drive. The de-rating protects the input diodes which may be rated for three phase amps.
Most modern feature-rich drives allows the drive to be programmed at constant HP in this band of operation. There will be no harm to the motor or drive. The motor will merely act as if overloaded and slow down but at the current limit set when the drive was programmed.
Most of these concerns are moot because few - very few - people in home shops take full HP cuts for more than a minute or so.
There's never a simple answer to any question regarding AC.
By the way, connect nothing to the third line terminal of the VFD. No ground, not to the single phase line. Resist temptation and leave it unused and innocent ot wire. Someone mentioned connecting the neutral to something.
Connect the neutral to nothing but the return leg of any 115 circuits forming part of the electrical systen. Do NOT connect the branch circuit neutral to ground at any point in the electrical circuit except to the neutral/ground rail at the service entrance in compliance with local code and NEC. Connect the ground to the chassis of the VFD at the ground terminal provided.
Also be sure to continue the ground conductor through to the motor and other electrical loads following the VFD's installation book guidelines to avoid ground loops.
Concerning questions about running motors from VFD's at low RPMS
Often home shop machinists wish to take advantage of VFD technology for their single phase machine tools but a conversion from single phase to VFD control takes some thinking. For one thing a VFD will not run a single phase motor. Period. For another you cannot replace a step pulley or multi-speed geared spindle transmission and vary the spindle RPM�s with the VFD alone. Any attempt to do so results in frustration as the spindle speed is reduced at the VFD�s speed control knob. It�s a matter of mechanical advantage.
Some complain that �VFDs lose torque at low RPM�s.� That�s false. Torque is twisting effort. Power is a function of torque times RPM. An induction motor is a constant torque device. As you reduce the motor RPM with the VFD the motor's torque remains constant but the RPM's and consequently power drops proportionate to the VFD's setpoint frequency. A 1 HP motor designed to run at 60 Hz develops 1/2 HP at 30 Hz, 1/4 hp at 15 Hz, and so on. No motor torque is lost as the RPM's are reduced only the delivered power.
What is mistakenly called "torque loss" is actually loss of tangential force. When it's necessary to run larger cutters at lower RPM the torque requirement goes up. If you attempt to obtain lower RPM by dialing down the VFD bacause the moment at the cutting edges is larger you also reduce the available power at the cutter. If a full HP cut is desired the spindle speed has to be mechanically - not electronically - reduced to suit the cutter RPM.
For that reason you cannot dispense with a machine tool's multi-step mechanical reductions if you wish to take advantage of the motor's full HP. Use the VFD to fill in between the steps for max HP. For low power operations you can simply dial down the VFD if you wish to be lazy and avoid shifting belts or gears but only for lighter cuts in proportion to motor RPM.
Thrift store or garage sale? Don't buy it if pre-1995. Get a return agreement and have a local motor guy check it out. These drives are nearly bullet-proof but their NEMA 1 enclosures are built like a bird cage. If the internal electronics are permeated by conductive dust (like from an abrasive shop saw) the logic and switching circuits will suffer. Older VFD's are 6 step and squeal when they run and are bereft of important features. The newer PWM drives are quiet and more resistant to faults.
As for the economies of VFD Vs a phase converter, there are several. A spinning phase converter running idle draws idle current depending on about a dozen variables. Actual charges are strongly dependent on duty cycle and power factor. As long as the phase converter is running you pay for the KW/hrs the converter ticks up on the meter whether you are drawing power from it or not.
As for conversion efficiency, I'd guess that a machine tool drive motor run from a VFD consumes about 1/3 the power over a year that a comparable motor fed from an across the line starter.
All modern VFD�s have "economizer circuitry". If the motor is run at part load, the VFD drops the line voltage to some level where the motor runs at reduced voltage closer to full load amps and its greatest conversion efficiency. My lathe motor is 10 HP and is rated for 230 Volts 3 phase @ 34 Amps. When I'm coasting along taking a light cut, the motor draws about 23 Amps from the VFD at reduced voltage but the VFD draws only about 4 Amps from the line. No this is not smoke and mirrors but simply how real-world VFD's work with real world three phase motors.
Most industrial motors are over-rated in their application and therefore run inefficiently. About 1/4 of all electrical energy is consumed by motors 50 HP or less. We in the US wouldn�t have to build new power generating plants for years if every three phase motor was furnished with a VFD.
The subject of an older motor's behavior with VFD's includes many urban legend style stories based more on self-accreting hearsay than actual performance. Not to trivialize the point but motor heating and insulation failure are concerns frequently overstated by popular belief particularly on 230 Volt systems.
Every motor has a duty cycle rating which is the percentage of time it can be safely run to full ratings. Most induction motors have duty cycle ratings of 100% or more meaning there is no limitation to operation except ambient the air temperature can be no greater than the safe insulation class temperature rating minus the motor's rated temperature rise. Since motors have thermal mass it takes time for the motor's copper temperature to increase to dangerous levels even at overload current. Makes sense; otherwise a motor would burn out at start-up.
Most induction motors running power tools have a relatively low duty cycle, even during stock reduction roughing cuts. The time between cuts counts as idle time permitting the motor a breather when it can cool somewhat.
You can run only one motor at a time from a VFD and you really should run the motor from the control panel not switch the motor after the drive. You may desire to purchase one VFD and run several items of equipment from it by plugging them in separately as you need them. Just remember that you may wish to adjust parameters every time you change motor for the built-in motor protection features to work properly.
Inverter rated induction motors have extra insulation strength to resist transient over-voltages induced in the windings by the switching frequency. New motors have stator iron properties favoring high frequencies therefore high voltage transients and insulation breakdown can be more of a problem when run from VFD's. Older motors have older iron which incidentally better blots up high frequency switching transients so it causes less of a voltage spike problem than would seem. Thus older tech motors mitigate effects newer motors have to be designed for.
It's long been the rule that induction motor insulation (and any other electrical appliance or device for that matter) has to with stand double the connectable voltage plus 500 Volts. So long as the motor is dry, VFD switching transients will cause no trouble for the home shop machinist running hid equipment from 230 Volts..
