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Building a Phase Converter

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 in below.

3 phase power converter

The 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 1.
Minimum suggested wire sizes.

                  Motor       Motor      Wire
                   HP        Current     Size
                  ----       -------  --------

                   1/2         2.0       #14
                   3/4         2.8       #14
                   1.0         3.6       #14
                   2.0         6.8       #14
                   3.0         9.6       #14
                   5.0        15.2       #12
                   7.5        22.0       #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 2.
Minimum suggested wire size for low voltage drop.

               Current   Length of wire in feet:
                Amps     60    90    120   150   180   210

                  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 in figure 2.

Typical 3-Phase Motor Wiring Diagrams
Figure 2


Check 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.

ideler motor
Figure 3
Figure 4

The effect of the run capacitors on voltage and current in the 3 phase lines is shown in figure 3 and figure 4. In figure 3, a 3/4 horsepower idler motor needed about 18 microfarads between both lines 1-3 and lines 2-3. In figure 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.


Figure 5
Figure 6

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 3.
1/2 HP lathe motor turning spindle only.

  Single Phase Line        Three Phase Lines
Amps   Volts   pf Watts    ----- 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 4.
60 microfarads between lines 1-3 and lines 2-3.

  Single Phase Line      Three Phase Lines
Amps  Volts  pf  Watts   ----- 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 cut

There are two relays shown in the schematic below.

3 phase power converter

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 Converter

A self starting phase converter is simpler and less expensive than a converter A self starting schematic is shown in figure 7. However, 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.

self starting phase converter
Figure 7

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. Go back and compare figure 4 and figure 5 and 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 5.
5 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
                           247.1  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 press

Table 6.
3 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

Author – Jim Hanrahan

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