Phase ConvertersBy Rick Christopherson

Note:This article is being completely rewritten with new information, more explanation, new graphics, and a source for the parts to make a converter. Check back later for updates.

Sometimes a woodworker will find himself with a 3-phase tool, but his shop only supplies single phase, 240 volt power. 3-phase motors are common for industrial grade tools, as their efficiency is higher than their single phase counterparts. In order to operate a 3-phase motor on single phase power, we need to make an artificial three phase system. This is accomplished by using a phase converter.

The phase converter will artificially generate the third leg of a three phase system from the two poles of a single phase system. This is not a perfect transformation, but it does get the job done. There are two types of phase converters: thestatic phase converter, and therotary phase converter. The rotary phase converter is built using a static phase converter, plus an idler motor.Balanced Static Phase ConverterThe static phase converter is the basic building block for any phase converter, and so it makes sense to start with that. With the static phase converter, the motor will only receive about 80% of its normal operating power. Additionally, you need a separate static phase converter for each three phase motor. These are the primary drawbacks to the static phase converter. However, the static phase converter is very simple to build. This loss of power is balanced by the lower cost of the static phase converter. Thesignificantcost savings is why static converters are so popular.

The drawing above shows a balanced static phase converter with a starting circuit. The starting circuit provides enough current to the motor to get it started, but then must be disconnected to prevent too much current from flowing through the motor after it starts. There are more rudimentary phase converters, but this design provides better performance with lower running currents.

Getting StartedThe first step is to select the capacitor sizes that will be needed. This is generally just trial and error, as the capacitor sizes will depend on the motor. I went to an electronic surplus store and located several capacitors for a couple of dollars each. These capacitors should be rated for at least 250 volts AC. For my 1 Hp lathe, I ended up using a 5 uF (microfarad) cap between L1&L3, and a 12.5 uF cap between L2&L3, and about 80 uF for the starting capacitor. These numbers should be a good starting point, in that their ratio should remain similar for larger motors. For my 5 Hp rotary converter, these numbers are 25 uF for L1&L3 and 50 uF for L2&L3, which is approximately 5 times larger than those for the 1 Hp converter.

At the surplus store, I located capacitors ranging from 1, 3, 5, 10, 12.5, 15, and 20 microfarads in the 250 volt range. What you will want to do, is get several capacitors of different sizes so you can fine tune the converter. Keep in mind that you can make a capacitor smaller or larger by combining it with other capacitors. Two capacitors connected in parallel will add to each other, and two capacitors in series will "get smaller".

Explaining how we add capacitors together is a little complex, so I am hoping that most of this can be understood by example. From the lefthand circuit in drawing above, two 10 uF caps in parallel will total 20 uF. We justaddtheir values together. In the next circuit, we have two capacitors in series. Two 10 uF caps in series will total 5 uF. For series connections, we add theirreciprocalvalues, and then take the reciprocal of that. For combinationseries/parallelcircuits, we look at the circuit as two separate problems. In the next sample, we have two caps in series giving us 5 uF as before, which is then in parallel with another cap, so the total is 15 uF. In the last sample, we have two caps in parallel giving us 20 uF, which is then in series with another, and the result is 6.6 uF. Using these various combinations, we can get nearly any size capacitor we want. I used 10 uF capacitors for this example, but they don't need to be the same size. Take the third sample as an example: this would be the same as a parallel combination of a 5 uF and a 10 uF.

I have to admit, that when I bought capacitors for the lathe, I was way off the first time. I picked up a bunch of 20 uF caps, but these were too large. Because I bought enough of these to build converters for two lathes, I had enough of them to make some complex combinations during the initial sizing. After I found out the final sizes, I went back to the store and picked up smaller capacitors for the final assembly. If you think about it, even making a mistake in your initial purchase is still cheaper than buying a store bought phase converter.

Initial SetupTo start out with, I would set up the converter using the numbers provided above. For a 1 Hp converter, I would use 5 & 12 uF caps between the lines, and whatever capacitors are left over are used for the starting circuit. If the 5 and 12 aren't available, use whatever is close for now. For the initial capacitor sizing, the following rule should be helpful:

CL1-L3= 4 to 5 times the motor horsepowerCL2-L3= 10 to 15 times the motor horsepowerCstarter= 40 to 100 microfarads just for testing

For testing purposes, I used a standard light switch to control the starting capacitors. For some of the configurations, I did not have enough capacitors left over to provide a sufficient starter. For these situations, I used a pull-cord (like starting a lawnmower) to get the motor started.

After you have the initial setup, it is time to turn the motor on. Close the switch for the starting circuit, and press the start switch. If the motor takes more than 2 seconds to get up to full speed, shut it down immediately. You can either add another capacitor to the starting circuit, or try using a pull string. If your motor turns backwards, then just reverse the wires on your motor. (At the motor, take any two of the three wires and swap them.) Once the motor is up and running, it is time to fine tune the capacitors.

Tuning the CapacitorsTo fine tune your capacitors, you need to check the voltages between each phase of the motor. The 3 phases used by the motor are A-B-C, or also refered to L1-L2-L3. To check the phase voltage, place each probe from a voltmeter on the respective phase. Phase A-B (Phase A to B) will be 240 volts, which is the line voltage of your house. One of the phases will be low, and the other will be high. You will need to adjust the capacitors until these voltages get close to being balanced out. If you find one of these voltages extremely high, turn the motor offimmediately.There are some configurations where one phase's voltage can exceed 350 volts. This will put high currents through the motor, and that is not good. Forgetting to turn your start circuit off is one example of this.

To determine the best configuration of a phase converter, I have found it best to create a data table and write down the various values. I first vary one capacitors size larger and smaller from the initial configuration. Then I vary the other capacitor. You should be aware that when you connect a new capacitor to one which was just powered, there will probably be some sparks as one capacitor charges up the other. This is disconcerting, but it is normal. Make sure you turn off the power before rearranging the capacitors.

The data table below is from my lathe. This isn't the order I collected the data points in. Instead, I sorted the data in this table based on the capacitor size between (A) and (C), so that the information is more presentable. Unfortunately, I had tried a couple of larger combinations before I started writing down the results, so these are missing. All I can remember about these first trials was that I started with 20 uF and 40 uF, but the voltages were so far out of line, that I quickly realized I needed to use much smaller values.

A-C Caps(uF)B-C Caps(uF)A-C voltsB-C volts

3.810230212

3.820260234

413.3240220

416249226

4.710233212

4.713.3242218

4.720263233

4.740300260

510234212

512.5243218

513.3242218

516250224

520263233

6.610236208

6.613.3245215

6.616253221

6.620266230

To see this data better, I have created a graph which shows the relationship between the two voltages. Graphing the data isn't necessary, nor did I use this, but it may help you see how different configurations effect the outcome.

From the data table and chart, there are a couple of configurations which work fairly well. Notice that at no time do all three phases reach 240 volts. This would be a perfect conversion. The best I could attain was 240 volts from (A) to (C) with 220 volts from (B) to (C). While I could get more power out of my motor when the voltage from (B) to (C) was closer to 240 volts, this would make the voltage from (A) to (C) much higher than 240 volts, which results in too much current flowing through that set of windings.

These are the conditions I looked at when selecting the capacitor sizes:

1. I wanted the phase voltages to be as close to being equal as possible.

2. I did not want any voltage to exceed 240 volts.

3. While still adhering to items 1 and 2, I wanted to use the least number of capacitors.

With respect to item #3, even though the motor performance was best when using 4 uF and 13.3 uF, this required 8 capacitors to achieve this combination. Since the performance when using 5 uF and 12.5 uF capacitors wasn't much different, I chose this combination. This decision was made after the testing, when I returned to the store and located 5 uF and 12.5 uF capacitors. (I want this phase converter to be small enough to fit in a junction box bolted to the inside of one of the legs.)

Configuring the Starting CircuitThe purpose of the starting circuit is to get the motor up to speed as fast as possible. The longer it takes to get the motor up to speed, the longer high currents will be present in the motor's windings. For the most part, the larger the starting capacitor, the faster it starts. My desire is to have the motor completely up to speed within one second with the least number of capacitors. Even though using a monstrous sized capacitor would bring the motor up to speed very fast, this would also run the risk of causing damage too. To select the right size, I just keep adding more capacitors to the circuit until the motor starts quickly.

What I find to be more intriguing is the potential for the starting circuit to be automatic. My rotary phase converter has been in service for about a year now, and there have been times when I forgot to shut the starter back off. There are three methods for activating the starter circuit: a manual on/off switch, a momentary contact switch (push and hold button), and two types of fully automatic switches.

Manual on/off SwitchThe simplest method for engaging the starting circuit is the use of a standard on/off switch. This requires the operator to turn the switch on before starting the motor, and turning it off after the motor is up to speed. The drawback to this, is that it requires the operator to activate and deactivate the switch. Failure to do either of these will result in excessively high currents in the motor.

Momentary PushbuttonThe momentary pushbutton is better, since you can't forget to turn your start circuit off when the motor is up to speed. With this, the start circuit is only engaged for as long as you hold the switch in. The drawback, is that the switch can be released too soon, before the motor gets up to speed. This would result in high currents flowing through the motor unless the start switch was again pressed. And if the switch was not released on time, currents would again be high after the motor gets up to speed.

Off-Delay TimerWith most switches and relays, when you tell them to turn off, they turn off. With an off-delay timer, when you tell it to turn off, it hesitates for a little bit before it turns off. This is ideal for a starting circuit, where we only need it to be active for a second or so. With an off-delay timer, we would use the momentary push button to activate the relay, and releasing the button would activate the timer.

The benefit to this setup is that you can't forget to turn the starter off, yet it will always remain active long enough to get the motor started. The drawback to this setup is almost trivial. If the timer is programmed too long, then the starter will remain active slightly after the motor is up to speed. Unless you had a method for sensing the actual speed (RPM's) of the motor, this is the most foolproof method for operating the starting circuit.

Current or Voltage Sensing CircuitNew Update:Since the time this article was written, I have completed the self-starting aspect of my lathe's converter. The discussion below is a duplicate from the writeup on that project.How this works, is when I press the normal start button on the lathe, the starting capacitors are already engaged. As the motor comes up to speed, a relay senses the increase in the third phase voltage. As this relay becomes active, it opens a switch, which disconnects the starting capacitors. As a side effect, if I grab the hand wheel and slow the motor down, the starting circuit re-engages to bring it back up to speed. This is what made me decide to make the unit self-starting. Before I completed the self-starting aspect of the converter, I loaded the motor just to see how much power I was getting. I slowed the motor too much, and it would not come back up to speed. I didn't want this to happen during normal use. The diagram above is the same as before, except I replaced the normal switch with a relay, and added a diode and variable resistor. A very significant benefit to this circuit is that it automatically compensates when the motor is slow to start.

After I managed to build the self-starting converter, I made a significant observation. When the lathe was set for low speed, the motor came up to speed quickly, as it's load was low. When the lathe was set at a higher speed, inertia made the lathe start slower, and it took longer to come up to full speed. This circuit compensated for the longer start-time, and remained engaged for a longer period of time. Had I used the "off-delay timer", this would not have been the case.

Initial Concept:While I was fine tuning the phase converter's capacitors, I noticed that the voltage from line 2 to the generated line 3 started out at 16 volts before I started the motor. As the motor came up to speed, this voltage gradually increased until it reached its final value of 220 volts. Since the voltage seemed to be related to the speed of the motor, I figured that if I could harness this variation, I could control when the starting capacitors were removed from the circuit. To do this, I needed to come up with a "voltage controlled switch".

Relay: The relay uses the normally closed contacts, which means that when there is no power to its coil, the contacts are touching. The relay's coil is rated for 120 volts at 60 Hz (AC power). Since I am feeding it with 240 volts (actually it is 220 volts in this case), I needed to reduce the voltage. Although I never bothered taking actual measurements, this relay would become active when the coil voltage reached about 70 volts. All I had to do was make sure that the coil's voltage reached 70 volts when the motor was near full speed.

Variable Resistor:The variable resistor acts as a fine tuning control. Some of the circuit's voltage is expended across this resistor. By adjusting the amount of resistance in the dimmer, I control how much voltage the relay gets. I had already burned up a couple of normal variable resistors because they were not rated to handle this kind of current. It suddenly hit me that a dimmer switch for home lighting was cheap, and rated for this kind of power consumption. Since I couldn't find a common variable resistor with large power capabilities, I went to a home center and picked up a common wall dimmer.

As I adjusted the dimmer, the starter control would either turn off too soon, or not at all. By making these adjustments, I controlled how long the starter remained active with respect to the motor's speed.

Diode: I don't know why I needed this, but it was a necessary part nonetheless. My relay was rated to operate on AC voltage. A diode blocks part of the voltage which results in a "sloppy" DC voltage. Without this diode, the system was self-defeating. As soon as the start button was pushed, the starting capacitors resulted in a high enough voltage to activate the relay. The relay would then disengage the capacitors, but they were the cause of the higher voltage. As soon as the capacitors were disengaged, the voltage was too low to activate the relay, and the relay would re-engage the capacitors. The bottom line was that the relay would flicker on and off, but the motor would not start. I fully expected this to happen when I used a DC rated relay, but the AC rated relay shouldn't have done this. I can only surmise that it is because I cut the voltage in half by using the diode.

The Rotary Phase ConverterWith the discussion above on the static phase converter, there isn't much to explain about the rotary converter. The rotary converter is nothing more than a second motor in the circuit which is acting as a generator. With the static converter, the tool's motor performed this function, but at the cost of some loss of power. With the rotary converter, the idler motor is under no physical load, but it cleans up the signal a little. If you examine the drawing below compared to the first drawing, the only difference is that we added the idler motor.

The output of the rotary phase converter is closer to being a true 3-phase source than the static converter. This provides more power to the tool motor, and also brings it up to speed faster. The rotary converter is best served when you have a motor which is started and stopped frequently, and you need the full power of the motor. Furthermore, a single rotary converter can drive several different 3-phase tools.

Setting up the rotary converter is the same as the static converter described above. The only decision to be made is the size of the idler motor. The idler motor needs to be larger than the largest tool which will be operated.

Since the static converter will provide a motor with 80% of it's normal operating power, and the rotary phase converter uses a static phase converter as a starter, your idler motor should be 125% of your tool(s) motor size. That is, if your tool is a 5 horsepower motor, your idler should be between 6 and 7 horsepower. It is always better to err on the high side, so I would use a 7 horsepower idler motor. If the converter will operate more than one tool, make sure the current rating of the idler motor is 125% of the sum of the tool motors.

Single to 3-PHASE Power Conversion

This is a compilation of data received by my request sent to MOON-NET.-----

From: K3PGP - John To: MOON-NET@VM.STLAWU.EDU Subject: 3 phase power from single phase source ?Date: Tuesday, March 23, 1999 5:50 PM

I have seen reference to people using a three phase motor and a capacitorbank to generate three phase 208 vac from a single phase 220 vac line. Inone particular case I saw a 15 HP motor being used to supply 208 vac at 25amps per leg to a transmitter power supply. The source was single phase 220vac. Unfortunately I am unable to obtain any further details.

Does anyone know how the motor is hooked up to do this? How do I determinewhat size motor I need and the hookup and value of the capacitors?

Unfortunately I have an EME project pending that requires three phase 208vac power at approx. 25 amps per leg. All that is available at the site issingle phase 220 vac. Any help would be appreciated.

Thanks...

John - K3PGPhttp://www.k3pgp.org-==-------Responses were received from:

Ken W6GHV, Jim N9JIM ex-WB9AJZ, Mike Murphy KA8ABR, Tom W2DRZ, Russ K2TXB, Kent D. O'Dell KA2KQM, Olivier CT1FWC / F6HGQ, Stan WA1ECF, Mike WD0CTA, Tom KB2BAH, Cliff K7RR, Dave N7DB, and Ted VE3BQN.Below is the summary of those responses. Although much of this applies to running motors the same system can be applied to running any 3-phase equipment including transmitters from a single phase source.

If I have missed anyone or have failed to give credit please let me know!

The answer to my question is Y-E-S and the basic idea was best summarized by Russ K2TXB and is posted below.

NOTE: When connected like this the motor will NOT start. It will only hum. You need to wrap a rope around the shaft and manually start it spinning, just like a lawn mower engine. Another option is capacitor start which is described in the following article.

For those of you that want more details the following compilation article is provided. I will update this article as soon as my 15 HP motor gets here and I have a chance to run some actual tests with it driving the 3-phase transmitter power supply.

Many quality used industrial machines are available at attractive prices that have 3 phase electric motors. Most residential homes do not have access to 3 phase electric power at a reasonable price. If the home shop builder decides to use these machines they must either replace the 3 phase motors with single phase motors or find a way to use the single phase power at their house to run them. This article explains how to build a rotary phase converter that will convert your single phase 220 VAC electric power to 3 phase 220 VAC to power your industrial machines.Safety should be your first concern and any electrical wiring should follow your local electrical code. That being said, some typical wire sizes, overload, and short circuit protection methods will be described to get you started. Also, the metal frame of the motors and your machines should be grounded. This safety ground normally does not conduct any electricity. It is present in case a current carrying conductor accidentally touches the metal frame. This provides a low resistance path for the electricity to flow instead of going through your body to earth ground.

There are two basic types of phase converters on the market which will allow 3 phase motors to run using single phase input to the converter. These types are referred to as static and rotary. The static converter is basically only a start circuit that once the motor starts, disengages and lets the motor run on single phase power. The disadvantage of this method is that the motor winding currents will be very unbalanced and the motor will not be able to run above about two-thirds its rated horsepower. The rotary converter provides current in all 3 phases and although not perfect, will allow a motor to provide all or nearly all its rated horsepower. If the motor has a service factor of 1.15 to 1.25 then you should be able to use full rated horsepower. The service factor can be found on the motor nameplate and is usually abbreviated S.F. The reasons that the electric power is not perfect are very technical and can include small amounts of voltage and current imbalance as well as the phase angles between phases not being perfect. The voltage and current balancing is straight forward if you have access to a voltmeter or preferably a clamp-on type ammeter. But even if you don't have these meters, using the approximate values of run capacitors specified in this article the currents should be close and you will be able to get nearly full horsepower from your 3 phase motors.

The terminology used to described the phase converter parts needs clarification. The rotary part of the rotary phase converter is a standard 3 phase electric motor called the idler motor. It is called this because typically it has no mechanical load connected to its shaft. Since applying single phase power to a 3 phase motor will not start it rotating, a means to start the idler motor turning near rated speed is necessary. This can be done in several ways. A pull rope can be used, a small single phase electric motor can be used, or a start capacitor can be used. If the mechanical means are used, power to the idler is not applied until after the motor is spinning and the rope or power to the single phase motor is removed. To balance the voltages and currents in the 3 phase output a pair of run capacitors can be used. A disconnect switch is required by most local electrical codes for each piece of equipment. If a plug and receptacle is used to connect power to the equipment, this meets the disconnect requirement. Overload protection is required for each motor. This can be built-in to the motor or provided separately. Check the motor nameplate, if it does not say built-in overload protection, then it must be supplied separately. Typically, a thermal overload relay and a magnetic contactor are used for controlling the motor. The magnetic contactor is a heavy duty relay for turning motors on and off. It is designed to handle the high starting currents of motors. There are also mechanical (manual) contactors available with thermal overload protection as part of the switch. For the purpose of this article the two wires carrying the single phase 220 VAC power will be called lines 1 and 2. These are connected to terminals 1 and 2 of the idler motor, respectively. The wire coming from the third terminal of the idler motor will be called line 3.

To build a rotary phase converter follow the general schematic shown below:

Figure 1The single phase 220 VAC input is brought in on lines 1 and 2, labeled L1 and L2 in figure 1. Time delay cartridge fuses are used for short circuit protection. 1R-1 and 1R-2 are the main contacts for the magnetic contactor (power relay.) The coil for this relay is denoted 1R. The run capacitors are wired between lines 1-3 and lines 2-3. The overloads are part of a thermal overload relay with a normally closed contact labeled OL-1. This contact will open if any overload is tripped. Opening this contact disables the flow of current through the 120 VAC control circuit deenergizing the coil 1R. The idler motor terminals are labeled T1, T2, and T3. The start circuit uses relay 2R and its contact 2R-1 to connect the start capacitor across lines 1 and 3 while the start push button is held in. In the control wiring, the auxiliary contact of relay 1, labeled 1R- X, maintains power to the coil 1R after the start push button is released. The 3 phase output power is connected after the main contacts (1R-1 and 1R-2) so that power from lines 1 and 2 are not connected to the output unless the phase converter is running.

A simpler alternative, which eliminates the separate start circuit and also eliminates the set of run capacitors between lines 2-3 is called a self starting phase converter. This design is discussed later in this article.

Choose the wire size based on the current that will flow in the wire. Table 1 can be used for guidance and is based on 3 phase, 220 VAC motors and 125% of motor nameplate current. Use only copper wire with a minimum size of #14. It is acceptable to use larger wire than listed in table 1.

Table 1Minimum suggested wire sizes.MotorHpMotorCurrentWireSize

1/22.0#14

3/42.8#14

1.03.6#14

2.06.8#14

3.09.6#14

5.015.2#12

7.522.0#10

If a run of wire longer than 50 feet is used such as from the circuit breaker panel to the phase converter, choose the wire size to keep the voltage drop in the wire less than 3 percent. Remember to add the currents of all devices that will draw power from this feed wire. Table 2 can be used for guidance and is based on copper wire.

Table 2Minimum suggested wire size for low voltage drop. Amps vs feet.Currentin Amps60Ft90Ft120Ft150Ft180Ft210Ft

5#14#14#14#14#14#14

6#14#14#14#14#14#12

7#14#14#14#14#12#12

8#14#14#14#12#12#12

9#14#14#12#12#10#10

10#14#14#12#12#10#10

12#14#12#12#10#10#10

14#12#12#10#10#10#8

16#12#12#10#10#10#8

18#10#10#10#8#8#8

20#10#10#10#8#8#8

25#10#10#8#8#6#6

30#8#8#8#6#6#6

Selecting the idler motor is the first step. It should be a 3 phase motor rated to operate at the line voltage and frequency that is available, normally 220 VAC, 60 Hertz. The phase converters tested here were wye (star) wound. Some motors are delta wound. Many motors have more than 3 leads so that it can be wired for more than one voltage. Dual voltage wound motors typically have 9 leads as shown below.

Figure 2Check the motor nameplate, if for voltage it lists 220/440 then it can be wired one way for 220 volts and another way for 440 volts. If you are not sure, disconnect all wires and measure the resistance between wires and compare to figure 2. The same motor would have the amperage listed as 15/7.5 meaning it will draw 15 amps when connected for 220 VAC and 7.5 amps when connected for 440 VAC. The speed rating is not important; from 1100 to 3600 RPM are all fine. The higher speed might produce slightly better phase angles, but the lower speed is generally easier to start. Ball bearing motors are recommended rather than motors with sleeve bearings. If the motor has oil cups it is a sleeve type bearing, if it has grease fittings or no fittings at all it is a ball bearing type. Spin the motor to be sure the bearings are good. Also, when buying a used motor connect an ohmmeter between each lead and the frame to verify that no short circuits are present. That is a sign that the insulation inside the motor is defective. For guidance, the cost of a used 3 phase motor of 2 horsepower or less should be about $20; for larger motors use about $10 per horsepower. The horsepower rating of the idler motor should be the same or higher than the largest 3 phase motor that you will use. If you have equipment that starts with the motor loaded, such as an air compressor, then 1.5 times the motor horsepower would be recommended.

The start capacitor should be rated for at least 250 VAC. The inexpensive electrolytic type can be used. If the idler motor is 1 horsepower or less the more expensive oil filled type used for run capacitors can also be used because the small size is not too expensive. The self starting phase converter uses the same set of oil filled capacitors for both starting and as run capacitors. The electrolytic type will lose capacitance over the years and therefore should be purchased new. It can be identified by the round, black, plastic case. The microfarad rating should be chosen by the horsepower rating of the idler motor. Since the idler motor is started without a mechanical load, the size is not critical and for guidance anything between 50 and 100 microfarads per horsepower will work. The larger rating will bring the motor up to speed faster and draw more current while starting. A 220- 250 VAC, 270-324 microfarad start capacitor sells new for about $15.

The run capacitors are optional. The converter will work fine without them, however you may only be able to get about 80% power from your 3 phase motors due to low current in the third line. The run capacitors are commonly rated for 330 or 370 VAC. The oil filled type must be used. These are rated for continuous AC duty while the electrolytic type are not and could explode. The oil filled type will not loose capacitance over the years and therefore can be purchased used or surplus. A new 50 microfarad run capacitor might cost $50 while used or surplus only $7. It can be identified by the metal case and oval shape (sometimes rectangular or even round.) The purpose of the run capacitors is to balance the voltage and current in the 3 phase lines. One set is connected between lines 1 and 3. The other is connected between lines 2 and 3. A set may be needed because if more than about 50 microfarads are needed, two or more separate capacitors must be connected in parallel to obtain the desired value. The best way to size these is by trial and error using a clamp-on type ammeter on the 3 phase lines while the 3 phase motor is running. For perfect balance each set may be a different value. For guidance or if perfect balancing of the currents is not needed, the microfarad rating can be estimated by the horsepower rating of the idler motor. Using equal capacitance of 12 to 16 microfarads per horsepower should result in a satisfactory balance.

The effect of the run capacitors on voltage and current in the 3 phase lines is shown infigure 3andfigure 4. Infigure 3, a 3/4 horsepower idler motor needed about 18 microfarads between both lines 1-3 and lines 2-3. Infigure 4, a 5 horsepower idler motor needed about 70 microfarads between the phases. This idler was best balanced with 80 microfarads between lines 1-3 and 60 microfarads between lines 2-3, although 70 microfarads between each was only slightly worse.

During the current balancing tests the 3 phase motor was only turning the spindle on the lathe, no metal was being cut. This was to obtain a repeatable, albeit small, load. Table 3 shows the current balance using various run capacitors.

The self starting phase converter uses capacitance between only one phase (1-3) instead of using 2 sets as recommended here. The result of trying this with the same 5 horsepower phase converter is shown in figure 5. The balance of voltages and currents improved compared to no run capacitors, but not as well as putting capacitance between both lines 1-3 and lines 2-3. In either case, as a side benefit, the single phase current draw which includes both the phase converter and the load motor power consumption will also be reduced dramatically as shown in figure 6. When no 3-phase motors were operating and only the idler was running, the single phase current without run capacitors was 14.8 amperes and with the run capacitors it was only 4.4 amperes as shown by the triangles in figure 6. This 70 percent reduction in current is impressive, but due to the change in power factor the actual power consumption only changed from 379 watts to 295 watts or 22 percent.

Table 31/2 HP lathe motor turning spindle only. Single Phase Line

Amps Volts pf Watts Three Phase Lines

------ Amps ------ Capacitance

Line1 Line2 Line3 pf Watts 1-3 2-3

17.22 246.2 0.16 685 2.37 2.42 0.43 0.45 289 0 0

15.85 246.7 0.16 627 2.27 2.33 0.59 0.43 279 10 10

10.13 246.6 0.22 545 1.91 2.09 1.29 0.39 279 50 50

8.67 246.2 0.26 557 1.83 2.06 1.52 0.37 279 60 60

7.15 245.6 0.29 512 1.68 2.00 1.72 0.32 240 70 70

7.13 245.6 0.29 504 1.81 1.88 1.76 0.32 249 80 60

To assure that the size of run capacitors would not be far off while cutting metal, a couple data points were taken at a spindle speed of 130 RPM and a feed rate of 0.004 inches/revolution while turning down the diameter of a piece of mild steel. The original diameter was 1.850 inches. The first cut of 0.030 reduced the diameter twice that to 1.790. The second cut of 0.060 started from the 1.790 diameter and reduced it to 1.670. Table 4 lists the results which show a balance similar to when the same capacitance was used and the spindle was not cutting metal.

Table 460 microfarads between lines 1-3 and lines 2-3. Single Phase Line

Amps Volts pf Watts 3 Phase Line

----- Amps ------

Line 1 Line 2 Line 3 pf Watts

8.67 246.2 0.26 557 1.83 2.06 1.52 0.37 279 Spindle only

8.71 247.1 0.26 565 1.83 2.08 1.53 0.40 303 0.030 inch cut

8.85 247.1 0.30 648 1.90 2.18 1.58 0.50 387 0.060 inch cutThere are two relays shown in the schematic infigure1. The number 1 relay is the main power relay and should have a motor horsepower rating suitable for the idler motor size. These are often referred to as magnetic contactors. It has two main poles to switch the 220 VAC single phase lines and an auxiliary set of contacts used to latch the coil of the relay energized when the main contacts are closed. The idler is shut off by pressing the stop button which opens the circuit to the coil causing the contactor to open. The number 2 relay is used to connect the start capacitor to the circuit. A relay is used so that the high starting currents do not go through the push button. A motor rated relay can be used or if a current rated relay is used select it to carry at least 2 times the nameplate current. The actual current depends on the size of the start capacitor and can be estimated using the following equation.

i = 2 (3.14) (frequency) (voltage) (capacitance)/10^6

i = 2 (3.14) ( 60 ) ( 220 ) ( 300 )/10^6 = 24.9 amps

Electrical codes require a disconnect for each piece of equipment. The disconnect switch (or plug) separates all current carrying conductors from the line voltage. For 220 VAC single phase systems this is 2 wires (a 2 pole switch), for 3 phase systems this is 3 wires (a 3 pole switch.) Since the phase converter is supplied with single phase power it can use a 2 pole disconnect or 2 of the 3 poles of a 3 pole switch. Each piece of equipment using the 3 phase power should also have its own 3 pole service disconnect. Many of these have fuses as part of the switch and are referred to as fused disconnects. For motor applications this is helpful since the motor overloads do not sufficiently protect from short circuits like fuses do. The use of time delay, cartridge fuses are common with motor circuits. Some local codes allow the use of the branch circuit disconnect or circuit breaker as the service disconnect for the equipment if it is within sight of the equipment. The disconnect of the phase converter can often meet this requirement in home shops.

The idler motor is started first and typically left running while the 3 phase motors in the shop are turned on and off as needed. More than one motor at a time can be operated and each running motor will act as a phase converter for the others so the total horsepower running can be 2 to 3 times the idler motor horsepower. If a manual switch is used instead of a magnetic contactor, then the push button to engage the start capacitor must be held in before the manual switch is turned on. When the idler motor starts (about 1 second or less) then the push button for the start capacitor is released.

Commercial vendors of static converters allow using the static converter to start an idler motor so that several motors can be run at the same time. However, some of these commercial units use voltage or current sensing relays to engage the start capacitor. If a motor near the size of the idler (which the static converter is sized for) is started, the start-up current can drop the line voltage for a fraction of a second and result in the start capacitor engaging. This can overload the static converter since other motors are running. The design recommended here does not have this limitation since the start capacitor is only engaged when the operator pushes the start button.

Self Starting Phase ConverterA self starting phase converter is simpler and less expensive than the converter shown infigure 1.A self starting schematic is shown below.

Figure 7However, the current and voltage balance in the 3-phase output varies more with load so that some unbalance is present at loads other than the one for which capacitance was selected.

For many shops the small amount of unbalance is acceptable and most commercial rotary phase converters are the self starting type. Inside one commercial 2 horsepower rotary phase converter was two 30 microfarad capacitors in parallel, this is effectively 60 microfarads. Since only two wires went between the capacitor bank and the motor, these must be connected across only one phase. In a 3 HP converter of a different manufacturer, three 40 microfarad capacitors were used (120 microfarads total.)

For the simplest converter, without a separate start circuit, using 25-30 microfarads per idler horsepower between one of the input lines and the third (generated) line will provide an acceptable phase converter. Too little capacitance and the idler either will not start, or it will start very slowly. Since the time delay fuses typically used for motor short circuit protection will allow some amount of over current for starting for about 5 seconds, it is recommended that enough capacitance be used to start the idler faster than that. Excess capacitance will cause the 3-phase voltages to exceed the input line voltage, especially when the idler is not loaded. Tables 5 and 6 show the voltages with various capacitance for a 5 HP and a 3 HP phase converter, respectively. The lathe used to put a load on the converter for the tests in tables 5 and 6 has a 1/2 HP motor; the drill press used has a 3/4 HP motor. As more 3-phase load was applied, the voltages across lines 1-3 and 2-3 were reduced as shown in the tables. Also shown in tables 5 and 6 are the times the idler needed to start. Comparefigure 4andfigure 5and decide if the improvement in output balancing is worth the extra effort of a separate start circuit which is required if equal capacitance is connected across both lines 1-3 and 2-3.

Table 55 HP self starting idler. Start Time 3-Phase Voltages

Seconds L1-L2 L1-L3 L2-L3120 microfarads: 2.6 247.1 262.8 238.7 No load

246.9 255.4 231.0 Lathe

247.1 251.0 227.2 Lathe & Drill press

130 microfarads: 1.6 246.9 264.8 243.7 No load

246.6 258.6 234.8 Lathe

246.2 253.7 229.8 Lathe & Drill press

150 microfarads: 1.0 247.9 270.3 253.6 No load

246.6 263.2 244.0 Lathe

247.8 259.2 238.8 Lathe & Drill pressTable 63 HP self starting idler. Start Time 3-Phase Voltages

Seconds L1-L2 L1-L3 L2-L3

50 microfarads: 0.8 245.6 249.4 225.0 No load

245.6 239.0 220.0 Lathe

70 microfarads: 0.8 245.5 260.4 238.7 No load

100 microfarads: 0.6 246.1 277.7 256.1 No load

245.9 262.5 245.6 Lathe

245.6 255.9 236.6 Lathe & Drill press

120 microfarads: 0.6 245.5 288.0 265.7 No load

245.7 270.3 254.9 Lathe

245.3 261.5 245.9 Lathe & Drill press