How To Run 3 Phase Motor On Single Phase Using Capacitor

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Single to 3-Phase Power Conversion

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

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From: K3PGP - John <k3pgp@ALLTEL.NET>
To: MOON-NET@VM.STLAWU.EDU <MOON-Internet@VM.STLAWU.EDU>
Subject field: 3 stage ability from single phase source ?
Date: Tuesday, March 23, 1999 5:50 PM

I accept seen reference to people using a three phase motor and a capacitor
depository financial institution to generate three phase 208 vac from a single phase 220 vac line. In
ane particular case I saw a 15 HP motor beingness used to supply 208 vac at 25
amps per leg to a transmitter power supply. The source was single phase 220
vac. Unfortunately I am unable to obtain any further details.

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

Unfortunately I have an EME project pending that requires 3 phase 208
vac power at approx. 25 amps per leg. All that is available at the site is
single phase 220 vac. Any aid would be appreciated.

Thanks...

John - K3PGP

http://www.k3pgp.org

-=<EOF>=-
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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 practical to running any 3-phase equipment including transmitters from a single phase source.

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

The answer to my question is Y-Due east-South and the basic idea was all-time summarized by Russ K2TXB and is posted below.

NOTE: When connected like this the motor volition NOT start. It volition simply hum. Y'all need to wrap a rope around the shaft and manually start it spinning, just like a lawn mower engine. Some other choice is capacitor get-go which is described in the post-obit article.

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

Many quality used industrial machines are available at attractive prices that have three phase electric motors. Nigh residential homes do not take access to 3 stage electric power at a reasonable price. If the home shop builder decides to apply these machines they must either replace the iii stage motors with single phase motors or find a mode to use the single phase power at their house to run them. This article explains how to build a rotary stage converter that volition catechumen your single stage 220 VAC electrical power to 3 phase 220 VAC to power your industrial machines.

Prophylactic should be your first concern and whatsoever electrical wiring should follow your local electrical code. That being said, some typical wire sizes, overload, and short circuit protection methods volition exist described to go yous started. Also, the metal frame of the motors and your machines should exist grounded. This safety ground normally does not conduct any electricity. Information technology is present in case a current carrying usher accidentally touches the metallic frame. This provides a low resistance path for the electricity to flow instead of going through your body to world footing.

There are ii 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 showtime 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 volition be very unbalanced and the motor will not be able to run above about 2-thirds its rated horsepower. The rotary converter provides current in all three phases and although not perfect, will allow a motor to provide all or almost all its rated horsepower. If the motor has a service factor of 1.15 to one.25 then you lot should be able to use total rated horsepower. The service factor can be plant on the motor nameplate and is usually abbreviated South.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 every bit the stage angles between phases not being perfect. The voltage and current balancing is straight forward if you have access to a voltmeter or preferably a clench-on type ammeter. Just fifty-fifty if you don't take these meters, using the estimate values of run capacitors specified in this article the currents should be close and you will be able to get about total horsepower from your 3 phase motors.

The terminology used to described the phase converter parts needs description. The rotary role 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 ability to a three phase motor will non start it rotating, a means to start the idler motor turning well-nigh rated speed is necessary. This can be done in several ways. A pull rope can be used, a small-scale 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 non applied until subsequently 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 stage output a pair of run capacitors tin be used. A disconnect switch is required past virtually local electrical codes for each slice of equipment. If a plug and receptacle is used to connect ability 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 non say built-in overload protection, and so 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. At that place are also mechanical (manual) contactors bachelor with thermal overload protection as part of the switch. For the purpose of this commodity the two wires carrying the single phase 220 VAC power volition be called lines one and 2. These are connected to terminals 1 and two of the idler motor, respectively. The wire coming from the tertiary terminal of the idler motor volition be called line 3.

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

Effigy 1

The single stage 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 excursion protection. 1R-1 and 1R-ii 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 function of a thermal overload relay with a usually closed contact labeled OL-one. This contact will open up if any overload is tripped. Opening this contact disables the flow of current through the 120 VAC command excursion deenergizing the coil 1R. The idler motor terminals are labeled T1, T2, and T3. The start circuit uses relay 2R and its contact 2R-one 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 one, labeled 1R- X, maintains power to the ringlet 1R after the first push button is released. The 3 stage output power is connected afterward the main contacts (1R-ane and 1R-2) and then that ability from lines 1 and ii are not connected to the output unless the phase converter is running.

A simpler alternative, which eliminates the separate kickoff excursion and also eliminates the set up of run capacitors between lines 2-3 is chosen a self starting phase converter. This design is discussed later in this commodity.

Choose the wire size based on the current that volition flow in the wire. Tabular array ane tin 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. Information technology is adequate to use larger wire than listed in table 1.

Table one
Minimum suggested wire sizes.

Motor Hp	Motor Current	Wire Size
ane/2	2.0	#xiv
3/4	2.8	#xiv
ane.0	3.vi	#14
2.0	6.eight	#xiv
3.0	9.half-dozen	#14
5.0	15.2	#12
vii.5	22.0	#x

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

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

Current in Amps	60 Ft	xc Ft	120 Ft	150 Ft	180 Ft	210 Ft
v	#xiv	#fourteen	#14	#14	#14	#14
half dozen	#xiv	#14	#14	#14	#14	#12
7	#14	#fourteen	#14	#14	#12	#12
eight	#xiv	#14	#14	#12	#12	#12
9	#14	#fourteen	#12	#12	#10	#10
10	#fourteen	#14	#12	#12	#10	#x
12	#14	#12	#12	#10	#10	#10
14	#12	#12	#10	#10	#ten	#8
16	#12	#12	#10	#ten	#x	#8
xviii	#10	#ten	#x	#viii	#8	#eight
20	#10	#10	#x	#8	#8	#8
25	#10	#10	#8	#8	#6	#six
30	#8	#8	#8	#half-dozen	#6	#6

Selecting the idler motor is the first step. It should be a 3 stage motor rated to operate at the line voltage and frequency that is available, commonly 220 VAC, sixty Hertz. The stage converters tested here were wye (star) wound. Some motors are delta wound. Many motors take more than 3 leads so that information technology can be wired for more than ane voltage. Dual voltage wound motors typically have ix leads as shown below.

Effigy ii

Check the motor nameplate, if for voltage information technology lists 220/440 so it tin be wired i way for 220 volts and another fashion for 440 volts. If you are non sure, disconnect all wires and measure the resistance between wires and compare to figure 2. The same motor would have the amperage listed every bit 15/7.v meaning it volition draw 15 amps when continued for 220 VAC and 7.5 amps when continued for 440 VAC. The speed rating is not of import; from 1100 to 3600 RPM are all fine. The higher speed might produce slightly ameliorate phase angles, merely the lower speed is more often than not easier to start. Ball begetting motors are recommended rather than motors with sleeve bearings. If the motor has oil cups it is a sleeve blazon begetting, if information technology has grease fittings or no fittings at all information technology 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 within the motor is defective. For guidance, the cost of a used 3 phase motor of 2 horsepower or less should be near $20; for larger motors use about $x per horsepower. The horsepower rating of the idler motor should exist the same or higher than the largest 3 stage motor that you will use. If you have equipment that starts with the motor loaded, such as an air compressor, and then ane.five times the motor horsepower would be recommended.

The start capacitor should be rated for at least 250 VAC. The inexpensive electrolytic type tin exist used. If the idler motor is ane horsepower or less the more than expensive oil filled type used for run capacitors tin as well be used because the small size is not too expensive. The self starting stage converter uses the aforementioned set of oil filled capacitors for both starting and as run capacitors. The electrolytic blazon will lose capacitance over the years and therefore should be purchased new. Information technology can be identified by the round, black, plastic example. The microfarad rating should be called by the horsepower rating of the idler motor. Since the idler motor is started without a mechanical load, the size is not disquisitional and for guidance anything between 50 and 100 microfarads per horsepower will piece of work. The larger rating will bring the motor up to speed faster and draw more electric current while starting. A 220- 250 VAC, 270-324 microfarad starting time 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 become most fourscore% power from your 3 phase motors due to low current in the 3rd line. The run capacitors are commonly rated for 330 or 370 VAC. The oil filled blazon must be used. These are rated for continuous AC duty while the electrolytic blazon are not and could explode. The oil filled type will non loose capacitance over the years and therefore tin exist purchased used or surplus. A new 50 microfarad run capacitor might cost $fifty while used or surplus just $seven. It can be identified by the metal instance and oval shape (sometimes rectangular or even circular.) The purpose of the run capacitors is to balance the voltage and current in the 3 phase lines. Ane set up is connected betwixt lines 1 and 3. The other is connected betwixt lines 2 and 3. A set may be needed considering if more nigh 50 microfarads are needed, two or more split capacitors must be connected in parallel to obtain the desired value. The best way to size these is past trial and error using a clamp-on type ammeter on the 3 phase lines while the 3 phase motor is running. For perfect residual 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 residual.

The effect of the run capacitors on voltage and current in the 3 phase lines is shown in figure iii and figure 4 . In effigy 3 , a 3/4 horsepower idler motor needed most eighteen microfarads between both lines 1-3 and lines 2-3. In figure 4 , a five horsepower idler motor needed nigh 70 microfarads between the phases. This idler was best balanced with 80 microfarads between lines one-3 and lx microfarads betwixt lines 2-3, although 70 microfarads between each was only slightly worse.

During the current balancing tests the 3 stage motor was only turning the spindle on the lathe, no metal was being cut. This was to obtain a repeatable, admitting small, load. Tabular array iii shows the current balance using diverse run capacitors.

The self starting phase converter uses capacitance betwixt merely one phase (1-3) instead of using 2 sets equally recommended here. The event of trying this with the aforementioned 5 horsepower stage converter is shown in effigy 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-three. In either case, as a side benefit, the single phase electric current draw which includes both the phase converter and the load motor power consumption will also be reduced dramatically as shown in figure half dozen. When no 3-phase motors were operating and only the idler was running, the single stage current without run capacitors was 14.8 amperes and with the run capacitors information technology was just 4.4 amperes equally shown by the triangles in figure half dozen. This 70 percent reduction in current is impressive, but due to the change in ability factor the actual ability consumption merely changed from 379 watts to 295 watts or 22 pct.

Tabular array 3
ane/ii HP lathe motor turning spindle but.

                                                                  Single Phase Line                                                     Amps   Volts   pf Watts    Three Phase Lines                                                          ------ Amps ------                Capacitance                             Line1  Line2  Line3    pf  Watts  1-three   2-3  17.22  246.two 0.16  685    2.37   2.42   0.43    0.45  289     0     0 fifteen.85  246.seven 0.xvi  627    two.27   2.33   0.59    0.43  279    x    10 ten.13  246.6 0.22  545    1.91   2.09   1.29    0.39  279    l    50  eight.67  246.two 0.26  557    1.83   ii.06   1.52    0.37  279    threescore    60  vii.15  245.6 0.29  512    1.68   2.00   i.72    0.32  240    70    70  7.13  245.6 0.29  504    1.81   1.88   1.76    0.32  249    eighty    60

To clinch that the size of run capacitors would not be far off while cutting metal, a couple information points were taken at a spindle speed of 130 RPM and a feed charge per unit of 0.004 inches/revolution while turning down the diameter of a piece of balmy steel. The original bore was ane.850 inches. The first cut of 0.030 reduced the diameter twice that to 1.790. The 2d cut of 0.060 started from the 1.790 diameter and reduced it to 1.670. Table 4 lists the results which show a residuum similar to when the same capacitance was used and the spindle was not cut metallic.

Table iv
60 microfarads between lines 1-3 and lines 2-3.

                                          Single Stage Line                                                          Amps  Volts  pf  Watts

                                          3 Stage Line                                   ----- Amps ------                                Line 1  Line 2  Line 3   pf  Watts  8.67  246.two 0.26  557    1.83    2.06    1.52   0.37  279  Spindle only   eight.71  247.one 0.26  565    1.83    2.08    1.53   0.forty  303  0.030 inch cutting   8.85  247.i 0.xxx  648    1.90    2.18    1.58   0.fifty  387  0.060 inch cut

In that location are two relays shown in the schematic in figure1 . The number i relay is the primary power relay and should have a motor horsepower rating suitable for the idler motor size. These are often referred to as magnetic contactors. Information technology has two main poles to switch the 220 VAC single phase lines and an auxiliary ready of contacts used to latch the scroll of the relay energized when the main contacts are closed. The idler is shut off by pressing the cease push 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 then that the high starting currents do non go through the push button. A motor rated relay tin can be used or if a current rated relay is used select it to carry at least 2 times the nameplate current. The actual electric current depends on the size of the kickoff capacitor and can be estimated using the following equation.

                                                  i = 2 (iii.xiv) (frequency) (voltage) (capacitance)/10^6              i = 2 (three.14) (   sixty    ) (  220  ) (    300    )/x^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 stage systems this is 3 wires (a three pole switch.) Since the stage converter is supplied with single phase power it can utilise a 2 pole disconnect or ii 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 take fuses as part of the switch and are referred to every bit fused disconnects. For motor applications this is helpful since the motor overloads do non sufficiently protect from short circuits similar fuses practise. The apply of time delay, cartridge fuses are common with motor circuits. Some local codes allow the use of the branch circuit disconnect or circuit billow as the service disconnect for the equipment if it is inside sight of the equipment. The disconnect of the phase converter can often meet this requirement in home shops.

The idler motor is started commencement 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 fourth dimension can exist operated and each running motor will human action as a phase converter for the others then the total horsepower running can be 2 to iii times the idler motor horsepower. If a manual switch is used instead of a magnetic contactor, and so the button to engage the first capacitor must exist held in before the manual switch is turned on. When the idler motor starts (most 1 second or less) then the push push button for the start capacitor is released.

Commercial vendors of static converters allow using the static converter to start an idler motor then that several motors can be run at the same time. However, some of these commercial units use voltage or current sensing relays to appoint the start capacitor. If a motor most 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 showtime capacitor engaging. This tin can overload the static converter since other motors are running. The design recommended hither does not accept this limitation since the starting time capacitor is just engaged when the operator pushes the start push .

Self Starting Phase Converter

A self starting phase converter is simpler and less expensive than the converter shown in effigy ane. A cocky starting schematic is shown below.

Figure 7

Yet, the current and voltage balance in the 3-stage output varies more with load so that some unbalance is present at loads other than the ane for which capacitance was selected.

For many shops the small amount of unbalance is acceptable and near commercial rotary stage converters are the self starting type. Inside one commercial 2 horsepower rotary phase converter was two 30 microfarad capacitors in parallel, this is finer sixty microfarads. Since just two wires went between the capacitor banking company 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 full.)

For the simplest converter, without a separate beginning excursion, using 25-30 microfarads per idler horsepower betwixt 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 nigh 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 non loaded. Tables 5 and 6 show the voltages with various capacitance for a 5 HP and a 3 HP stage converter, respectively. The lathe used to put a load on the converter for the tests in tables 5 and half-dozen has a 1/2 HP motor; the drill press used has a 3/iv HP motor. Equally more 3-stage load was applied, the voltages across lines i-3 and 2-3 were reduced every bit shown in the tables. Also shown in tables 5 and six are the times the idler needed to showtime. Compare figure four and effigy 5 and make up one's mind if the improvement in output balancing is worth the actress effort of a separate offset circuit which is required if equal capacitance is connected across both lines i-three and 2-3.

Table five
v HP self starting idler.

                          Start Time   3-Phase Voltages           Seconds  L1-L2  L1-L3  L2-L3

            120 microfarads:   2.6     247.1  262.8  238.7 No load                          246.9  255.4  231.0 Lathe                                        247.ane  251.0  227.ii Lathe & Drill printing  130 microfarads:   1.6     246.9  264.viii  243.7 No load                          246.6  258.6  234.8 Lathe                                        246.two  253.7  229.8 Lathe & Drill press  150 microfarads:   1.0     247.9  270.iii  253.vi No load                          246.6  263.2  244.0 Lathe                                         247.8  259.ii  238.8 Lathe & Drill press

Table 6
3 HP self starting idler.

                          Start Time   3-Phase Voltages           Seconds  L1-L2  L1-L3  L2-L3   fifty microfarads:   0.eight     245.6  249.4  225.0 No load                          245.6  239.0  220.0 Lathe   lxx microfarads:   0.viii     245.5  260.four  238.7 No load                             100 microfarads:   0.6     246.1  277.seven  256.1 No load                          245.ix  262.5  245.6 Lathe                                        245.6  255.9  236.six Lathe & Drill printing  120 microfarads:   0.vi     245.five  288.0  265.7 No load                          245.seven  270.three  254.9 Lathe                                        245.three  261.five  245.9 Lathe & Drill press

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