Phase Converters By 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. conv erter. Check back later for updates. Sometimes a woodworker will find himself with a 3-phase tool, but his shop only supplies single phase, !" volt power. 3-phase motors are common for industrial grade tools, as their efficiency is higher than their single phase counterparts. #n 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 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 $ob done. There are two types of phase converters% the static phase converter , and the rotary phase converter . The rotary phase converter is built using a static phase converter, plus an idler motor. Balanced Static Phase Converter The static phase converter is the basic building block for any phase converter, and so it makes sense to start with that. &ith the static phase converter, the motor will only receive about '"( of its normal operating power. )dditionally )dditionally,, you need a separate static phase converter for each three phase motor. motor. These These are the primary drawbacks to the the static phase converter. *owever, the static phase converter is very simple to build. This loss of power is balanced by the lower cost of the static phase converter. The significant cost savings is why static converters are so popular.
The drawing above shows a balanced static phase p hase 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 running currents. Getting Started The first step is to select the capacitor si+es that will be needed. This is generally $ust trial and error, as the capacitor si+es will depend on the motor. # went to an electronic surplus store and located several capacitors for a couple of dollars each. These capacitors should be rated for at least " volts )C. or my *p lathe, # ended up using a u /microfarad0 cap between 1213, and a . u cap between 1213, and about '" u for the starting capacitor. These numbers should be a good starting point, in that their their ratio should remain similar similar for larger motors. or my *p rotary converter, these numbers are u for 1213 and " u for 1213, which is approximately times larger than those for the *p converter.
)t the surplus store, # located capacitors ranging from fr om , 3, , ", ., , and " microfarads in the " volt range. &hat you will want to do, is get several capacitors of different si+es so you can fine tune the converter. eep in mind that you can make a capacitor smaller or larger by combining it with other capacitors. Two Two capacitors connected in parallel will add to each other, and two capacitors in series will 4get smaller4.
5xplaining how we add capacitors together is a little complex, so # am hoping that most of this can be understood by example. rom the lefthand circuit in drawing above, two " u caps in parallel will total " u. &e $ust add their values together. #n the next circuit, we have two capacitors in series. Two Two " u caps c aps in series will total u. or series connections, we add their reciprocal values, and then take the reciprocal of that. or combination series/parallel circuits, we look at the circuit as two separate problems. #n the next sample, we have two caps in series giving us u as before, which is then in parallel with another cap, so the total is u. #n the last sample, we have two caps in parallel giving us " u, which is then in series with another, and the result is 6.6 u. 7sing 7sing these various combinations, we can get nearly any si+e capacitor we want. # used " u capacitors for this example, but they don8t
need to be the same si+e. Take the third sample as an example% this would be the same as a parallel combination of a u and a " u. # have to admit, that when # bought capacitors for the lathe, # was way off the first time. # picked up a bunch of " u caps, but these were too large. 9ecause # bought enough of these to build converters for two lathes, # had enough of them to make some complex combinations during the initial si+ing. )fter # found out the final si+es, # went back to the store and picked up smaller capacitors for the final assembly. #f you think about it, even making a mistake in your initial purchase is still cheaper than buying a store bought phase converter. Initial Setup To start out with, # would set up the converter using the numbers provided above. or a *p converter, # would use 2 u caps between the lines, and whatever capacitors are left over are used for the starting circuit. #f the and aren8t available, use whatever is close for now. or the initial capacitor si+ing, the following rule should be helpful%
C1-13 : ! to times the motor horsepower C1-13 : " to times the motor horsepower Cstarter : !" to "" microfarads $ust for testing or testing purposes, # used a standard light switch to control the starting capacitors. or some of the configurations, # did not have enough capacitors left over to provide a sufficient starter. or these situations, # used a pull-cord /like starting a lawnmower0 to get the motor started. )fter 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. #f the motor takes more than seconds to get up to full speed, shut it down immediately. ;ou can either add another capacitor to the starting circuit, or try using a pull string. #f your motor turns backwards, then $ust reverse the wires on your motor. /)t the motor, take any two of the three wires and swap them.0
configurations where one phase8s voltage can exceed 3" volts. This will put high currents through the motor, and that is not good. orgetting to turn your start circuit off is one example of this. To determine the best configuration of a phase converter, # have found it best to create a data table and write down the various values. # first vary one capacitors si+e larger and smaller from the initial configuration. Then # vary the other capacitor. ;ou should be aware that when you connect a new capacitor to one which was $ust powered, there will probably be some sparks as one capacitor charges up the other. This is disconcerting, but it is normal. >ake sure you turn off the power before rearranging the capacitors. The data table below is from my lathe. This isn8t the order # collected the data points in. #nstead, # sorted the data in this table based on the capacitor si+e between /)0 and /C0, so that the information is more presentable. 7nfortunately, # had tried a couple of larger combinations before # started writing down the results, so these are missing. )ll # can remember about these first trials was that # started with " u and !" u, but the voltages were so far out of line, that # ?uickly reali+ed # needed to use much smaller values.
)-C Caps /u0 9-C Caps/u0
)-C volts
9-C volts
3.'
"
3"
3.'
"
6"
3!
!
3.3
!"
"
!
6
!@
6
!.A
"
33
!.A
3.3
!
'
!.A
"
63
33
!.A
!"
3""
6"
"
3!
.
!3
'
3.3
!
'
6
"
!
"
63
33
6.6
"
36
"'
6.6
3.3
!
6.6
6
3
6.6
"
66
3"
To see this data better, # have created a graph which shows the relationship between the two voltages. Braphing the data isn8t necessary, nor did # use this, but it may help you see how different configurations effect the outcome.
rom the data table and chart, there are a couple of configurations which work fairly well. otice that at no time do all three phases reach !" volts. This would be a perfect conversion. The best # could attain was !" volts from /)0 to /C0 with " volts from /90 to /C0. &hile # could get more power out of my motor when the voltage from /90 to /C0 was closer to !" volts, this would make the voltage from /)0 to /C0 much higher than !" volts, which results in too much current flowing through that set of windings. These are the conditions # looked at when selecting the capacitor si+es% . # wanted the phase voltages to be as close to being e?ual as possible. . # did not want any voltage to exceed !" volts. 3. &hile still adhering to items and , # wanted to use the least number of capacitors. &ith respect to item D3, even though the motor performance was best when using ! u and 3.3 u, this re?uired ' capacitors to achieve this combination. Since the performance when using u and . u capacitors wasn8t much different, # chose this combination. This decision was made after the testing, when # returned to the
store and located u and . u capacitors. /# want this phase converter to be small enough to fit in a $unction box bolted to the inside of one of the legs.0 Configuring the Starting Circuit The 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 motor8s windings. or the most part, the larger the starting capacitor, the faster it starts. >y desire is to have the motor completely up to speed within one second with the least number of capacitors. 5ven though using a monstrous si+ed capacitor would bring the motor up to speed very fast, this would also run the risk of causing damage too. To select the right si+e, # $ust keep adding more capacitors to the circuit until the motor starts ?uickly.
&hat # find to be more intriguing is the potential for the starting circuit to be automatic. >y rotary phase converter has been in service for about a year now, and there have been times when # forgot to shut the starter back off. There are three methods for activating the starter circuit% a manual onEoff switch, a momentary contact switch /push and hold button0, and two types of fully automatic switches. Manual on/off Switch The simplest method for engaging the starting circuit is the use of a standard onEoff switch. This re?uires the operator to turn the switch on before starting the motor, and turning it off after the motor is up to speed. The drawback to this, is that it re?uires the operator to activate and deactivate the switch. ailure to do either of these will result in excessively high currents in the motor. Momentar Push!utton The momentary pushbutton is better, since you can8t forget to turn your start circuit off when the motor is up to speed. &ith this, the start circuit is only engaged for as long as you hold the switch in. The drawback, is that the switch can be released too soon, before the motor gets up to speed. This would result in high currents flowing through the motor unless the start switch was again pressed. )nd if the switch was not released on time, currents would again be high after the motor gets up to speed. "ff#$ela Timer &ith most switches and relays, when you tell them to turn off, they turn off. &ith 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. &ith 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 can8t 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. #f the timer is programmed too long, then the starter will remain active slightly after the motor is up to speed. 7nless you had a method for sensing the actual speed /F=>8s0 of the motor, this is the most foolproof method for operating the starting circuit. Current or %oltage Sensing Circuit New Update: Since the time this article was written, I have completed the self-starting aspect of my lathe's converter. The discssion !elow is a dplicate from the writep on that pro"ect.
*ow this works, is when # press the normal start button on the lathe, the starting capacitors are already engaged. )s the motor comes up to speed, a relay senses the increase in the third phase voltage. )s this relay becomes active, it opens a switch, which disconnects the starting capacitors. )s a side effect, if # 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. 9efore # completed the self-starting aspect of the converter, # loaded the motor $ust to see how much power # was getting. # slowed the motor too much, and it would not come back up to speed. # didn8t want this to happen during normal use. The diagram above is the same as before, except # replaced the normal switch with a relay, and added a diode and variable resistor. ) very significant benefit to this circuit is that it automatically compensates when the motor is slow to start. )fter # managed to build the self-starting converter, # made a significant observation. &hen the lathe was set for low speed, the motor came up to speed ?uickly, as it8s load was low. &hen 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. *ad # used the 4offdelay timer4, this would not have been the case. #nitial Concept% &hile # was fine tuning the phase converter8s capacitors, # noticed that the voltage from line to the generated line 3 started out at 6 volts before # started the motor. )s the motor came up to speed, this voltage gradually increased until it reached its final value of " volts. Since the voltage seemed to be related to the speed of the motor, # figured that if # could harness this variation, # could control when the starting capacitors were removed from the circuit. To do this, # needed to come up with a 4voltage controlled switch4. Felay% The relay uses the normally closed contacts, which means that when there is no power to its coil, the contacts are touching. The relay8s coil is rated for " volts at 6" *+ /)C power0. Since # am feeding it with !" volts /actually it is " volts in this case0, # needed to reduce the voltage. )lthough # never bothered taking actual measurements, this relay would become active when the coil voltage reached about A" volts. )ll # had to do was make sure that the coil8s voltage reached A" volts when the motor was near full speed. Gariable Fesistor% The variable resistor acts as a fine tuning control. Some of the circuit8s voltage is expended across this resistor. 9y ad$usting the amount of resistance in the dimmer, # control how much voltage the relay gets. # had already burned up a couple of normal variable resistors because they were not rated to handle this kind of current. #t suddenly hit me that a dimmer switch for home lighting was cheap, and rated for this kind of power consumption. Since # couldn8t find a common variable resistor with large power capabilities, # went to a home center and picked up a common wall dimmer. )s # ad$usted the dimmer, the starter control would either turn off too soon, or not at all. 9y making these ad$ustments, # controlled how long the starter remained active with respect to the motor8s speed. Hiode% # don8t know why # needed this, but it was a necessary part nonetheless. >y relay was rated to operate on )C voltage. ) diode blocks part of the voltage which results in a 4sloppy4 HC voltage. &ithout this diode, the system was self-defeating. )s 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. )s 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. # fully expected this to happen when # used a HC rated relay, but the
)C rated relay shouldn8t have done this. # can only surmise that it is because # cut the voltage in half by using the diode. The &otar Phase Converter &ith the discussion above on the static phase converter, there isn8t 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. &ith the static converter, the tool8s motor performed this function, but at the cost of some loss of power. &ith the rotary converter, the idler motor is under no physical load, but it cleans up the signal a little. #f 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 fre?uently, and you need the full power of the motor. urthermore, 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 si+e 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 '"( of it8s normal operating power, and the rotary phase converter uses a static phase converter as a starter, your idler motor should be ( of your tool/s0 motor si+e. That is, if your tool is a horsepower motor, your idler should be between 6 and A horsepower. #t is always better to err on the high side, so # would use a A horsepower idler motor. #f the converter will operate more than one tool, make sure the current rating of the idler motor is ( of the sum of the tool motors.
Single to 3-PHASE Power Conversion
This is a compilation of data received by my re?uest sent to ><<-5T. ----rom% 3=B= - Iohn Jk3pgpK)11T51.5TL To% ><<-5TKG>.ST1)&7.5H7 J><<-5TKG>.ST1)&7.5H7L Sub$ect% 3 phase power from single phase source M Hate% Tuesday, >arch 3, @@@ %" => # have seen reference to people using a three phase motor and a capacitor bank to generate three phase "' vac from a single phase " vac line. #n one particular case # saw a *= motor being used to supply "' vac at amps per leg to a transmitter power supply. The source was single phase " vac. 7nfortunately # am unable to obtain any further details. Hoes anyone know how the motor is hooked up to do thisM *ow do # determine what si+e motor # need and the hookup and value of the capacitorsM 7nfortunately # have an 5>5 pro$ect pending that re?uires three phase "' vac power at approx. amps per leg. )ll that is available at the site is single phase " vac. )ny help would be appreciated. Thanks...
Iohn - 3=B=
http://www'()pgp'org
-:J5<L:###### Fesponses were received from% en &6B*G, Iim @I#> ex-&9@)IN, >ike >urphy )')9F, Tom &HFN, Fuss TO9, ent H. <8Hell )P>,
ike &H"CT), Tom 99)*, Cliff AFF, Have AH9, and Ted G539P. 9elow is the summary of those responses. )lthough much of this applies to running motors the same system can be applied to running any 3-phase e?uipment including transmitters from a single phase source. #f # have missed anyone or have failed to give credit please let me knowQ
The answer to my ?uestion is ;-5-S and the basic idea was best summari+ed by Fuss TO9 and is posted below.
>any ?uality used industrial machines are available at attractive prices that have 3 phase electric motors. >ost residential homes do not have access to 3 phase electric power at a reasonable price. #f 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 " G)C electric power to 3 phase " G)C 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 si+es, overload, and short circuit protection methods will be described to get you started. )lso, the metal frame of the motors and your machines should be grounded. This safety ground normally does not conduct any electricity. #t 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. #f the motor has a service factor of . to . 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.. 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. 9ut even if you don8t 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. #t 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. ) pull rope can be used, a small single phase electric motor can be used, or a start capacitor can be used. #f 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. ) disconnect switch is re?uired by most local electrical codes for each piece of e?uipment. #f a plug and receptacle is used to connect power to the e?uipment, this meets the disconnect re?uirement.
Figure 1 The single phase " G)C input is brought in on lines and , labeled 1 and 1 in figure . Time delay cartridge fuses are used for short circuit protection. F- and F are the main contacts for the magnetic contactor /power relay.0 The coil for this relay is denoted F. The run capacitors are wired between lines -3 and lines -3. The overloads are part of a thermal overload relay with a normally closed contact labeled <1-. This contact will open if any overload is tripped.
Choose the wire si+e based on the current that will flow in the wire. Table can be used for guidance and is based on 3 phase, " G)C motors and ( of motor nameplate current. 7se only copper wire with a minimum si+e of D!. #t is acceptable to use larger wire than listed in table . Table 1 Minimum suggested wire si*es' Motor Motor ,ire +p Current Si*e -/. .' 0-1 )/1 .'2 0-1 -' )'3 0-1 .' 3'2 0-1 )' 4'3 0-1 5' -5'. 0-. 6'5 ..' 0-
#f a run of wire longer than " feet is used such as from the circuit breaker panel to the phase converter, choose the wire si+e to keep the voltage drop in the wire less than 3 percent. Femember to add the currents of all devices that will draw power from this feed wire. Table can be used for guidance and is based on copper wire. Table Minimum suggested wire si*e for low voltage drop' 7mps vs feet' Current in 7mps 5 3 6 2 4 - -. -1 -3 -2 . .5 )
3 4 -. -5 -2 .- 8t 8t 8t 8t 8t 8t 0-1 0-1 0-1 0-1 0-1 0-1 0-1 0-1 0-1 0-1 0-1 0-. 0-1 0-1 0-1 0-1 0-. 0-. 0-1 0-1 0-1 0-. 0-. 0-. 0-1 0-1 0-. 0-. 0- 0- 0-1 0-1 0-. 0-. 0- 0- 0-1 0-. 0-. 0- 0- 0- 0-. 0-. 0- 0- 0- 02 0-. 0-. 0- 0- 0- 02 0- 0- 0- 02 02 02 0- 0- 0- 02 02 02 0- 0- 02 02 03 03 02 02 02 03 03 03
Selecting the idler motor is the first step. #t should be a 3 phase motor rated to operate at the line voltage and fre?uency that is available, normally " G)C, 6" *ert+. The phase converters tested here were wye /star0 wound. Some motors are delta wound. >any motors have more than 3 leads so that it can be wired for more than one voltage. Hual voltage wound motors typically have @ leads as shown below.
Figure Check the motor nameplate, if for voltage it lists "E!!" then it can be wired one way for " volts and another way for !!" volts. #f you are not sure, disconnect all wires and measure the resistance between wires and compare to figure . The same motor would have the amperage listed as EA. meaning it will draw amps when connected for " G)C and A. amps when connected for !!" G)C. The speed rating is not importantR from "" to 36"" F=> are all fine. The higher speed might produce slightly better phase angles, but the lower speed is generally easier to start. 9all bearing motors are recommended rather than motors with sleeve bearings. #f 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. )lso, 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. or guidance, the cost of a used 3 phase motor of horsepower or less should be about "R for larger motors use about " 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. #f you have e?uipment that starts with the motor loaded, such as an air compressor, then . times the motor horsepower would be recommended. The start capacitor should be rated for at least " G)C. The inexpensive electrolytic type can be used. #f the idler motor is horsepower or less the more expensive oil filled type used for run capacitors can also be used because the small si+e 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. #t 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 si+e is not critical and for guidance anything between " and "" microfarads per horsepower will work. The larger rating will bring the motor up to speed faster and draw more current while starting. ) "- " G)C, A"-3! microfarad start capacitor sells new for about . The run capacitors are optional. The converter will work fine without them, however you may only be able to get about '"( power from your 3 phase motors due to low current in the third line. The run capacitors are commonly rated for 33" or 3A" G)C. The oil filled type must be used. These are rated for continuous )C 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. ) new " microfarad run capacitor might cost " while used or surplus only A. #t can be identified by the metal case and oval shape /sometimes rectangular or even round.0 The purpose of the run capacitors is to balance the voltage and current in the 3 phase lines.
The effect of the run capacitors on voltage and current in the 3 phase lines is shown in figure ) and figure 1. #n figure ), a 3E! horsepower idler motor needed about ' microfarads between both lines -3 and lines -3. #n figure 1, a horsepower idler motor needed about A" microfarads between the phases. This idler was best balanced with '" microfarads between lines -3 and 6" microfarads between lines -3, although A" microfarads between each was only slightly worse.
Huring 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 /-30 instead of using sets as recommended here. The result of trying this with the same horsepower phase converter is shown in figure . The balance of voltages and currents improved compared to no run capacitors, but not as well as putting capacitance between both lines -3 and lines -3. #n 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. &hen no 3-phase motors were operating and only the idler was running, the single phase current without run capacitors was !.' amperes and with the run capacitors it was only !.! amperes as shown by the triangles in figure 6. This A" percent reduction in current is
impressive, but due to the change in power factor the actual power consumption only changed from 3A@ watts to @ watts or percent. Table 3 E *= lathe motor turning spindle only.
Single Phase Line Amps Volts pf Watts
17.22 1#."# 1!.13 ".7 7.1# 7.13
24.2 24.7 24. 24.2 24#. 24#.
!.1 !.1 !.22 !.2 !.2$ !.2$
"# 27 #4# ##7 #12 #!4
Three Phase Lines ------ Amps -----Line1 Line2 Line3 2.37 2.27 1.$1 1."3 1." 1."1
2.42 2.33 2.!$ 2.! 2.!! 1.""
!.43 !.#$ 1.2$ 1.#2 1.72 1.7
pf
Watts
!.4# !.43 !.3$ !.37 !.32 !.32
2"$ 27$ 27$ 27$ 24! 24$
Capacitance 1-3 2-3 ! 1! #! ! 7! "!
! 1! #! ! 7! !
To assure that the si+e of run capacitors would not be far off while cutting metal, a couple data points were taken at a spindle speed of 3" F=> and a feed rate of ".""! inchesErevolution while turning down the diameter of a piece of mild steel. The original diameter was .'" inches. The first cut of "."3" reduced the diameter twice that to .A@". The second cut of "."6" started from the .A@" diameter and reduced it to .6A". Table ! lists the results which show a balance similar to when the same capacitance was used and the spindle was not cutting metal. Table ! 3 microfarads !etween lines -#) and lines .#)' Single Phase Line Amps Volts pf Watts 3 Phase Line ----- Amps -----Line 1 Line 2 Line 3 pf ".7 ".71 "."#
24.2 !.2 247.1 !.2 247.1 !.3!
##7 ## 4"
1."3 1."3 1.$!
2.! 2.!" 2.1"
1.#2 1.#3 1.#"
!.37 !.4! !.#!
Watts 27$ 3!3 3"7
Spin%le onl& !.!3! inch c't !.!! inch c't
There are two relays shown in the schematic in figure-. The number relay is the main power relay and should have a motor horsepower rating suitable for the idler motor si+e. These are often referred to as magnetic contactors. #t has two main poles to switch the " G)C single phase lines and an auxiliary set of contacts used to latch the coil of the relay energi+ed 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 relay is used to connect the start capacitor to the circuit. ) relay is used so that the high starting currents do not go through the push button. ) motor rated relay can be used or if a current rated relay is used select it to carry at least times the nameplate current. The actual current depends on the si+e of the start capacitor and can be estimated using the following e?uation.
i ( 2 )3.14* )fre+'enc&* ),oltage* )capacitance*1! i ( 2 )3.14* )
!
* )
22!
* )
3!!
*1! ( 24.$ amps
5lectrical codes re?uire a disconnect for each piece of e?uipment. The disconnect switch /or plug0 separates all current carrying conductors from the line voltage. or " G)C single phase systems this is wires /a pole switch0, for 3 phase systems this is 3 wires /a 3 pole switch.0 Since the phase converter is supplied with single phase power it can use a pole disconnect or of the 3 poles of a 3 pole switch. 5ach piece of e?uipment using the 3 phase power should also have its own 3 pole service disconnect. >any of these have fuses as part of the switch and are referred to as fused disconnects. or 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 e?uipment if it is within sight of the e?uipment. The disconnect of the phase converter can often meet this re?uirement 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. >ore 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 to 3 times the idler motor horsepower. #f 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. &hen the idler motor starts /about second or less0 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. *owever, some of these commercial units use voltage or current sensing relays to engage the start capacitor. #f a motor near the si+e of the idler /which the static converter is si+ed for0 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 .
Sel" Starting P#ase Converter ) self starting phase converter is simpler and less expensive than the converter shown in figure -' ) self starting schematic is shown below.
Figure $ *owever, 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. or many shops the small amount of unbalance is acceptable and most commercial rotary phase converters are the self starting type. #nside one commercial horsepower rotary phase converter was two 3" microfarad capacitors in parallel, this is effectively
6" microfarads. Since only two wires went between the capacitor bank and the motor, these must be connected across only one phase. #n a 3 *= converter of a different manufacturer, three !" microfarad capacitors were used /" microfarads total.0 or the simplest converter, without a separate start circuit, using -3" microfarads per idler horsepower between one of the input lines and the third /generated0 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 seconds, it is recommended that enough capacitance be used to start the idler faster than that. 5xcess capacitance will cause the 3-phase voltages to exceed the input line voltage, especially when the idler is not loaded. Tables and 6 show the voltages with various capacitance for a *= and a 3 *= phase converter, respectively. The lathe used to put a load on the converter for the tests in tables and 6 has a E *= motorR the drill press used has a 3E! *= motor. )s more 3-phase load was applied, the voltages across lines -3 and -3 were reduced as shown in the tables. )lso shown in tables and 6 are the times the idler needed to start. Compare figure 1 and figure 5 and decide if the improvement in output balancing is worth the extra effort of a separate start circuit which is re?uired if e?ual capacitance is connected across both lines -3 and -3. Table % *= self starting idler. Start Time 3-Phase Voltages Seconds L1-L2 L1-L3 L2-L3 120 microfarads: 2.6 247.1 262. 23.7 !o load 246." 2##.4 231.0 Lathe 247.1 2#1.0 227.2 Lathe $ %rill &ress 130 microfarads:
1.6
1#0 microfarads:
1.0
246." 246.6 246.2
264. 2#.6 2#3.7
243.7 !o load 234. Lathe 22". Lathe $ %rill &ress
247." 270.3 2#3.6 !o load 246.6 263.2 244.0 Lathe 247. 2#".2 23. Lathe $ %rill &ress
Table & 3 *= self starting idler. Start Time 3-Phase Voltages Seconds L1-L2 L1-L3 L2-L3 #0 microfarads:
0.
24#.6 24#.6
24".4 23".0
22#.0 !o load 220.0 Lathe
70 microfarads:
0.
24#.#
260.4
23.7 !o load
100 microfarads:
0.6
246.1 24#." 24#.6
277.7 262.# 2##."
2#6.1 !o load 24#.6 Lathe 236.6 Lathe $ %rill &ress
120 microfarads:
0.6
24#.# 24#.7 24#.3
2.0 270.3 261.#
26#.7 !o load 2#4." Lathe 24#." Lathe $ %rill &ress
