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Showing posts with label phase converter supplier in the metro manila. Show all posts
Showing posts with label phase converter supplier in the metro manila. Show all posts

Thursday, September 26, 2013

BUILDING A PHASE CONVERTER...what you will learn...for your philippine electrical concerns...needs...installation...orders... safeelectrical2013@gmail.com

BUILDING A PHASE CONVERTER

Includes Self Starting

By Jim Hanrahan



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safeelectrical2013@gmail.com



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 figure 1. 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. 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.

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.

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 in figure 1. 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 the converter shown in figure 1. 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.
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. 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

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Before You Buy a 3-Phase Converter...for your philippine electrical concerns...needs...installation...orders...safeelectrical2013@gmail.com

Before You Buy a 3-Phase Converter


A word of caution to 3-phase converter purchasers.
There are many phase converters on the market and, provided they are properly sized, all will work well. Unfortunatelythere are no industry standards that apply to phase converters, so to guarantee you are getting a price on phase converters of identical capacity, you must know the right questions to ask. You'll find tips on this page to help you avoid paying more for less. But the best tip of all? Buy from a company that will ask you these questions in the first place to ensure you get a 3-phase converter that will do the job--Kay Industries.
Here are some things to look for:
  • Never compare phase converters solely on their nameplate HP rating.
  • Always ask for the NEMA frame size of the phase converter you are being quoted. Compare prices only on phase converters of equal frame size, not equal HP.
  • Never believe anyone who tells you that you can get the same output from a smaller frame size.
  • Always ask for the weight of the phase converter if you can't find out the frame size. Weight is a great proxy for phase converter capacity. A 150 lb. phase converter absolutely cannot do the same job a 300 lb. phase converter, regardless of its nameplate rating. That's just common sense.
  • Never buy a phase converter that runs at 3,600 RPM unless it comes with earplugs
  • Always insist upon automatic controls if your load cycles on and off repeatedly. It will save energy and pay for itself.
We strongly recommend to all 3-phase converter purchasers that they ask these questions of their prospective supplierbefore placing an order and reject any phase converter supplier that cannot or will not respond.
  • What is the NEMA frame size of the phase converter?
  • How much does the phase converter weigh?
  • At what RPM does the phase converter operate?
  • Is the phase converter available with automatic controls?
In addition, you should ask:
  • What is the noise level of the phase converter in decibels?
  • What happens if this phase converter will not do the job?
  • Can I get my money back?
  • Can I get credit toward a replacement?
  • What is your policy on return freight?
Buy from a company you can trust
A properly sized and selected phase converter will perform its function for years, service free and without compromising the performance of the load equipment. But the critical factor is proper sizing..

 SAFE ELECTRICAL SUPPLY AND SERVICES will always ask detailed questions about your application to insure that you get the correct converter for the job.

for your philippine electrical concerns...needs...installation...orders...safeelectrical2013@gmail.com

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DIGITAL PHASE CONVERTER ..what you will learn.... for your philippine electrical concerns...needs...installation...orders...safeelectrical2013@gmail.com

Digital phase converter[edit]

A digital phase converter creates a three phase power supply from a single phase supply. A digital signal processor (DSP) is used to controlpower electronic devices to generate a third voltage, which along with the single-voltage from the supply creates a balanced three-phase power supply.
AC power from the utility is converted to DC, then back to AC. The power switching devices used in this process are insulated gate bipolar transistors (IGBT).
In one type of digital phase converter the input rectifier consists of IGBTs in series with inductors. The IGBTs are controlled by software in the DSP to draw current from the single-phase line in a sinusoidal fashion, charging capacitors on a constant voltage DC bus. Because the incoming current is sinusoidal, there are no significant harmonics generated back onto the line as there are with the rectifiers found in mostVFDs. The controlled rectifier input allows power factor correction.
The output inverter consists of IGBTs that draw on the power of the DC bus to create an AC voltage. A voltage created by power switching devices like IGBTs is not sinusoidal. It is a pulse-width modulated (PWM) waveform very high in harmonic distortion. This PWM voltage is then passed through an inductor/capacitor filter system that produces a sine wave voltage with less than 3% total harmonic distortion (standards for computer grade power allow up to 5% THD). By contrast, VFDs generate a PWM voltage that limits their versatility and makes them unsuitable for many applications. Software in the DSP continually monitors and adjusts this generated voltage to produce a balanced three-phase output at all times. It also provides protective functions by shutting down in case of utility over-voltage and under-voltage or a fault. With the ability to adjust to changing conditions and maintain voltage balance, a digital phase converter can safely and efficiently operate virtually any type of three-phase equipment or any number of multiple loads.
The solid state design results in a relatively small package with no moving parts except for small cooling fans. The converters operate at 95%–98% efficiency.[citation needed] When the converter is energized with no load, it consumes very little power.[citation needed]




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ROTARY PHASE CONVERTER: what you will learn.... for your philippine electrical concerns...needs...installation...orders...safeelectrical2013@gmail.com

Rotary phase converter


A simple homemade phase converter
A rotary phase converter, abbreviated RPC, is an electrical machine that produces three-phase electric power from single-phase electric power. This allows three phase loads to run using generator or utility-supplied single-phase electric power.

The original "Kandó" design of phase converter[edit]

The first locomotive with a phase converter (only for demonstration purposes)
The Kandó phase converter (1933)
The "Kandó" locomotive the first locomotive using a phase converter system
At the beginning of the 20th century there were two main principles of electric railway traction current systems:
  1. DC system
  2. 16 2/3 Hz three phase system
All of them needed a separated supply system or converters to the standard 50Hz electric network. Kálmán Kandó recognized that the electric traction system must be supplied by single phase 50Hz power from the standard electric network, and it must be converted in the locomotive to 3 phase power for traction motors. He created an electric machine called phase converter. It was a single phase synchronous motor and a three phase synchronous generator with common stator and rotor. It had two independent windings. The winding placed closer to the rotor is a three phase (or variable phase) synchronous generator, the other is a winding of a single phasesynchronous motor. The winding of the motor takes the power from the overhead line and the generator provides the power for the three (or more) phase traction motors.

Types[edit]

A rotary phase converter (RPC) may be built as a motor-generator set. These completely isolate the load from the single-phase supply and produce balanced three-phase output. However, due to weight, cost, and efficiency concerns, most RPCs are not built this way.
Instead, they are built out of a three-phase induction motor or generator, called an idler, on which two of the terminals (the idler inputs) are powered from the single phase line. Therotating flux in the motor produces a voltage on the third terminal. A voltage is induced in the third terminal that is shifted by 120° from the voltage between the first two terminals. In a three-winding motor, two of the windings are acting as a motor, and the third winding is acting as a generator.

Power quality[edit]

A common measure of the quality of the power produced by an RPC or any phase converter is the voltage balance, which may be measured while the RPC is driving a balanced load such as a three-phase motor. Other quality measures include the harmonic content of the power produced and the power factor of the RPC motor combination as seen by the utility. Selection of the best phase converter for any application depends on the sensitivity of the load to these factors. Three-phase induction motors are very sensitive to voltage imbalances.
The quality of three-phase power generated by such a phase converter depends upon a number of factors including:
  • Power capacity of the phase converter (idler horsepower rating).
  • Power level demands of the equipment being supplied. For instance, "hard starting" loads such as heavily loaded machinery or well pumps may have higher requirements than other loads rated at the same horsepower.
  • Power quality demands of the equipment being supplied (CNC equipment may have more stringent power quality requirements than a welding machine)
  • Use of techniques to balance the voltage between the three legs.

Quality improvement[edit]

RPC manufacturers use a variety of techniques to deal with these problems. Some of the techniques include,
  • The insertion of capacitors between the terminals to balance the power at a particular load.
  • The use of idlers with higher power ratings than the loads.
  • The construction of special idler motors with more windings on the third terminal to boost the voltage and compensate for the sag caused by the load.
  • The use of electronics to switch in capacitors, during start up or otherwise, based on the load.
  • The use of filters.

How RPCs work[edit]

The main principles of RPC operation are as follows: Three-phase induction motors have three terminals called "legs", usually numbered (arbitrarily) L1, L2, and L3. A three-phase induction motor can be run at two-thirds of its rated horsepower on single-phase power applied to any pair of legs, once spun up by some means. A three-phase induction motor that is spinning under single-phase power applied to legs L1 and L2, generates an electric potential (and can deliver power through) leg L3, although without some form of current injection, special windings in the idler, or other means the voltage will sag when a load is applied. Power factor correction is a very important consideration when building or choosing an RPC. This is desirable because an RPC that has power factor correction will consume less current from the single-phase service supplying power to the phase converter and its loads. Balanced voltage between the three legs of power is important for operational life of the equipment receiving that power. Unbalanced three-phase power can damage the equipment that it is meant to operate.

Use of RPCs[edit]

General[edit]

RPCs may be used anywhere three-phase devices need to be used, but only single phase power is available. Three-phase motors cannot start on single phase power without a device to generate three-phase power. Since prices (and quality) of used three-phase motors are usually more favorable than those of their single-phase counterparts, demand exists for phase converters. This is also true because single-phase electric motors generally are not available in sizes over 15 hp (11 kW) due to their complexity, starting requirements and relative high expense and low efficiency compared to three-phase motors. In fact, single-phase motors larger than 5 hp (3.7 kW), though available, are rarely seen in use for these reasons. RPCs are sold by various vendors, but often also are home made.

Electric railways[edit]

Rotary phase converters are used in the opposite direction (three-phase to single-phase) for electric railways. In Europe, electricity is normally generated as three-phase AC at 50 Hertz. Five European countries, Germany, Austria, Switzerland, Norway and Sweden have standardised on single-phase AC at 15 kV 16⅔ Hz for railway electrification. Phase converters are, therefore, used to change both the phase and the frequency.

Alternatives to rotary converters[edit]

Alternatives exist to rotary phase converters for operation of three-phase equipment on a single-phase power supply.
  • Static phase converters are actually not a converter at all. They start the motor spinning and then "single-phase" the motor. They also require de-rating of the motor being run, typically to 60% or less of its three-phase rating. A 10 hp (7.5 kW) motor can be run only at 6 hp (4.5 kW). The power balance and thus motor efficiency is extremely poor. Failure to run a motor at reduced load will result in overheating and quite often destruction of the motor. Many machine manufacturers and dealers specifically state that the use of a static converter will render any warranty null and void.
  • Three phase inverters and related variable frequency drives, may, under certain circumstances, produce large amounts of harmonic distortion in their output, and the efficiency of the inverter-motor combination can be poor. When poor efficiency in a VFD powered motor has been investigated, in every case documented to date, the root cause for the loss in efficiency was either improper installation or a failure to adhere to best practices. Almost universally, a VFD increases the efficiency of a system and reduces operating costs.
  • Inverter drives (VFDs)can damage some motors not rated for use with an inverter but this is not a concern when motor voltages are less than 440 VAC. Motor manufacturers may void warranties on non-inverter rated motors if they are run at a variable speed or with an inverter, but most three phase motors over 10 hp (7.5 kW) sold today are rated for use with an inverter drive. This damage can be prevented by the use of load and line reactors or in Europe a DU/DT filter. This reactor or filter reduces the voltage stack up that can occur on older high impedance motors such as wound rotor motors. The filter prevents the windings of the motor form insulation breakdown caused by voltage stack up.
  • VFDs that have a capacity greater than 3 hp can be costly thus making the RPC an attractive option.