Phase ConvertersBy Rick Christopherson
Note:This article is being completely rewritten with new
information, more explanation, new graphics, and a source for the
parts to make a converter. Check back later for updates.
Sometimes a woodworker will find himself with a 3-phase tool,
but his shop only supplies single phase, 240 volt power. 3-phase
motors are common for industrial grade tools, as their efficiency
is higher than their single phase counterparts. In order to operate
a 3-phase motor on single phase power, we need to make an
artificial three phase system. This is accomplished by using a
phase converter.
The phase converter will artificially generate the third leg of
a three phase system from the two poles of a single phase system.
This is not a perfect transformation, but it does get the job done.
There are two types of phase converters: thestatic phase converter,
and therotary phase converter. The rotary phase converter is built
using a static phase converter, plus an idler motor.Balanced Static
Phase ConverterThe static phase converter is the basic building
block for any phase converter, and so it makes sense to start with
that. With the static phase converter, the motor will only receive
about 80% of its normal operating power. Additionally, you need a
separate static phase converter for each three phase motor. These
are the primary drawbacks to the static phase converter. However,
the static phase converter is very simple to build. This loss of
power is balanced by the lower cost of the static phase converter.
Thesignificantcost savings is why static converters are so
popular.
The drawing above shows a balanced static phase converter with a
starting circuit. The starting circuit provides enough current to
the motor to get it started, but then must be disconnected to
prevent too much current from flowing through the motor after it
starts. There are more rudimentary phase converters, but this
design provides better performance with lower running currents.
Getting StartedThe first step is to select the capacitor sizes
that will be needed. This is generally just trial and error, as the
capacitor sizes will depend on the motor. I went to an electronic
surplus store and located several capacitors for a couple of
dollars each. These capacitors should be rated for at least 250
volts AC. For my 1 Hp lathe, I ended up using a 5 uF (microfarad)
cap between L1&L3, and a 12.5 uF cap between L2&L3, and
about 80 uF for the starting capacitor. These numbers should be a
good starting point, in that their ratio should remain similar for
larger motors. For my 5 Hp rotary converter, these numbers are 25
uF for L1&L3 and 50 uF for L2&L3, which is approximately 5
times larger than those for the 1 Hp converter.
At the surplus store, I located capacitors ranging from 1, 3, 5,
10, 12.5, 15, and 20 microfarads in the 250 volt range. What you
will want to do, is get several capacitors of different sizes so
you can fine tune the converter. Keep in mind that you can make a
capacitor smaller or larger by combining it with other capacitors.
Two capacitors connected in parallel will add to each other, and
two capacitors in series will "get smaller".
Explaining how we add capacitors together is a little complex,
so I am hoping that most of this can be understood by example. From
the lefthand circuit in drawing above, two 10 uF caps in parallel
will total 20 uF. We justaddtheir values together. In the next
circuit, we have two capacitors in series. Two 10 uF caps in series
will total 5 uF. For series connections, we add
theirreciprocalvalues, and then take the reciprocal of that. For
combinationseries/parallelcircuits, we look at the circuit as two
separate problems. In the next sample, we have two caps in series
giving us 5 uF as before, which is then in parallel with another
cap, so the total is 15 uF. In the last sample, we have two caps in
parallel giving us 20 uF, which is then in series with another, and
the result is 6.6 uF. Using these various combinations, we can get
nearly any size capacitor we want. I used 10 uF capacitors for this
example, but they don't need to be the same size. Take the third
sample as an example: this would be the same as a parallel
combination of a 5 uF and a 10 uF.
I have to admit, that when I bought capacitors for the lathe, I
was way off the first time. I picked up a bunch of 20 uF caps, but
these were too large. Because I bought enough of these to build
converters for two lathes, I had enough of them to make some
complex combinations during the initial sizing. After I found out
the final sizes, I went back to the store and picked up smaller
capacitors for the final assembly. If you think about it, even
making a mistake in your initial purchase is still cheaper than
buying a store bought phase converter.
Initial SetupTo start out with, I would set up the converter
using the numbers provided above. For a 1 Hp converter, I would use
5 & 12 uF caps between the lines, and whatever capacitors are
left over are used for the starting circuit. If the 5 and 12 aren't
available, use whatever is close for now. For the initial capacitor
sizing, the following rule should be helpful:
CL1-L3= 4 to 5 times the motor horsepowerCL2-L3= 10 to 15 times
the motor horsepowerCstarter= 40 to 100 microfarads just for
testing
For testing purposes, I used a standard light switch to control
the starting capacitors. For some of the configurations, I did not
have enough capacitors left over to provide a sufficient starter.
For these situations, I used a pull-cord (like starting a
lawnmower) to get the motor started.
After you have the initial setup, it is time to turn the motor
on. Close the switch for the starting circuit, and press the start
switch. If the motor takes more than 2 seconds to get up to full
speed, shut it down immediately. You can either add another
capacitor to the starting circuit, or try using a pull string. If
your motor turns backwards, then just reverse the wires on your
motor. (At the motor, take any two of the three wires and swap
them.) Once the motor is up and running, it is time to fine tune
the capacitors.
Tuning the CapacitorsTo fine tune your capacitors, you need to
check the voltages between each phase of the motor. The 3 phases
used by the motor are A-B-C, or also refered to L1-L2-L3. To check
the phase voltage, place each probe from a voltmeter on the
respective phase. Phase A-B (Phase A to B) will be 240 volts, which
is the line voltage of your house. One of the phases will be low,
and the other will be high. You will need to adjust the capacitors
until these voltages get close to being balanced out. If you find
one of these voltages extremely high, turn the motor
offimmediately.There are some configurations where one phase's
voltage can exceed 350 volts. This will put high currents through
the motor, and that is not good. Forgetting to turn your start
circuit off is one example of this.
To determine the best configuration of a phase converter, I have
found it best to create a data table and write down the various
values. I first vary one capacitors size larger and smaller from
the initial configuration. Then I vary the other capacitor. You
should be aware that when you connect a new capacitor to one which
was just powered, there will probably be some sparks as one
capacitor charges up the other. This is disconcerting, but it is
normal. Make sure you turn off the power before rearranging the
capacitors.
The data table below is from my lathe. This isn't the order I
collected the data points in. Instead, I sorted the data in this
table based on the capacitor size between (A) and (C), so that the
information is more presentable. Unfortunately, I had tried a
couple of larger combinations before I started writing down the
results, so these are missing. All I can remember about these first
trials was that I started with 20 uF and 40 uF, but the voltages
were so far out of line, that I quickly realized I needed to use
much smaller values.
A-C Caps(uF)B-C Caps(uF)A-C voltsB-C volts
3.810230212
3.820260234
413.3240220
416249226
4.710233212
4.713.3242218
4.720263233
4.740300260
510234212
512.5243218
513.3242218
516250224
520263233
6.610236208
6.613.3245215
6.616253221
6.620266230
To see this data better, I have created a graph which shows the
relationship between the two voltages. Graphing the data isn't
necessary, nor did I use this, but it may help you see how
different configurations effect the outcome.
From the data table and chart, there are a couple of
configurations which work fairly well. Notice that at no time do
all three phases reach 240 volts. This would be a perfect
conversion. The best I could attain was 240 volts from (A) to (C)
with 220 volts from (B) to (C). While I could get more power out of
my motor when the voltage from (B) to (C) was closer to 240 volts,
this would make the voltage from (A) to (C) much higher than 240
volts, which results in too much current flowing through that set
of windings.
These are the conditions I looked at when selecting the
capacitor sizes:
1. I wanted the phase voltages to be as close to being equal as
possible.
2. I did not want any voltage to exceed 240 volts.
3. While still adhering to items 1 and 2, I wanted to use the
least number of capacitors.
With respect to item #3, even though the motor performance was
best when using 4 uF and 13.3 uF, this required 8 capacitors to
achieve this combination. Since the performance when using 5 uF and
12.5 uF capacitors wasn't much different, I chose this combination.
This decision was made after the testing, when I returned to the
store and located 5 uF and 12.5 uF capacitors. (I want this phase
converter to be small enough to fit in a junction box bolted to the
inside of one of the legs.)
Configuring the Starting CircuitThe purpose of the starting
circuit is to get the motor up to speed as fast as possible. The
longer it takes to get the motor up to speed, the longer high
currents will be present in the motor's windings. For the most
part, the larger the starting capacitor, the faster it starts. My
desire is to have the motor completely up to speed within one
second with the least number of capacitors. Even though using a
monstrous sized capacitor would bring the motor up to speed very
fast, this would also run the risk of causing damage too. To select
the right size, I just keep adding more capacitors to the circuit
until the motor starts quickly.
What I find to be more intriguing is the potential for the
starting circuit to be automatic. My rotary phase converter has
been in service for about a year now, and there have been times
when I forgot to shut the starter back off. There are three methods
for activating the starter circuit: a manual on/off switch, a
momentary contact switch (push and hold button), and two types of
fully automatic switches.
Manual on/off SwitchThe simplest method for engaging the
starting circuit is the use of a standard on/off switch. This
requires the operator to turn the switch on before starting the
motor, and turning it off after the motor is up to speed. The
drawback to this, is that it requires the operator to activate and
deactivate the switch. Failure to do either of these will result in
excessively high currents in the motor.
Momentary PushbuttonThe momentary pushbutton is better, since
you can't forget to turn your start circuit off when the motor is
up to speed. With this, the start circuit is only engaged for as
long as you hold the switch in. The drawback, is that the switch
can be released too soon, before the motor gets up to speed. This
would result in high currents flowing through the motor unless the
start switch was again pressed. And if the switch was not released
on time, currents would again be high after the motor gets up to
speed.
Off-Delay TimerWith most switches and relays, when you tell them
to turn off, they turn off. With an off-delay timer, when you tell
it to turn off, it hesitates for a little bit before it turns off.
This is ideal for a starting circuit, where we only need it to be
active for a second or so. With an off-delay timer, we would use
the momentary push button to activate the relay, and releasing the
button would activate the timer.
The benefit to this setup is that you can't forget to turn the
starter off, yet it will always remain active long enough to get
the motor started. The drawback to this setup is almost trivial. If
the timer is programmed too long, then the starter will remain
active slightly after the motor is up to speed. Unless you had a
method for sensing the actual speed (RPM's) of the motor, this is
the most foolproof method for operating the starting circuit.
Current or Voltage Sensing CircuitNew Update:Since the time this
article was written, I have completed the self-starting aspect of
my lathe's converter. The discussion below is a duplicate from the
writeup on that project.How this works, is when I press the normal
start button on the lathe, the starting capacitors are already
engaged. As the motor comes up to speed, a relay senses the
increase in the third phase voltage. As this relay becomes active,
it opens a switch, which disconnects the starting capacitors. As a
side effect, if I grab the hand wheel and slow the motor down, the
starting circuit re-engages to bring it back up to speed. This is
what made me decide to make the unit self-starting. Before I
completed the self-starting aspect of the converter, I loaded the
motor just to see how much power I was getting. I slowed the motor
too much, and it would not come back up to speed. I didn't want
this to happen during normal use. The diagram above is the same as
before, except I replaced the normal switch with a relay, and added
a diode and variable resistor. A very significant benefit to this
circuit is that it automatically compensates when the motor is slow
to start.
After I managed to build the self-starting converter, I made a
significant observation. When the lathe was set for low speed, the
motor came up to speed quickly, as it's load was low. When the
lathe was set at a higher speed, inertia made the lathe start
slower, and it took longer to come up to full speed. This circuit
compensated for the longer start-time, and remained engaged for a
longer period of time. Had I used the "off-delay timer", this would
not have been the case.
Initial Concept:While I was fine tuning the phase converter's
capacitors, I noticed that the voltage from line 2 to the generated
line 3 started out at 16 volts before I started the motor. As the
motor came up to speed, this voltage gradually increased until it
reached its final value of 220 volts. Since the voltage seemed to
be related to the speed of the motor, I figured that if I could
harness this variation, I could control when the starting
capacitors were removed from the circuit. To do this, I needed to
come up with a "voltage controlled switch".
Relay: The relay uses the normally closed contacts, which means
that when there is no power to its coil, the contacts are touching.
The relay's coil is rated for 120 volts at 60 Hz (AC power). Since
I am feeding it with 240 volts (actually it is 220 volts in this
case), I needed to reduce the voltage. Although I never bothered
taking actual measurements, this relay would become active when the
coil voltage reached about 70 volts. All I had to do was make sure
that the coil's voltage reached 70 volts when the motor was near
full speed.
Variable Resistor:The variable resistor acts as a fine tuning
control. Some of the circuit's voltage is expended across this
resistor. By adjusting the amount of resistance in the dimmer, I
control how much voltage the relay gets. I had already burned up a
couple of normal variable resistors because they were not rated to
handle this kind of current. It suddenly hit me that a dimmer
switch for home lighting was cheap, and rated for this kind of
power consumption. Since I couldn't find a common variable resistor
with large power capabilities, I went to a home center and picked
up a common wall dimmer.
As I adjusted the dimmer, the starter control would either turn
off too soon, or not at all. By making these adjustments, I
controlled how long the starter remained active with respect to the
motor's speed.
Diode: I don't know why I needed this, but it was a necessary
part nonetheless. My relay was rated to operate on AC voltage. A
diode blocks part of the voltage which results in a "sloppy" DC
voltage. Without this diode, the system was self-defeating. As soon
as the start button was pushed, the starting capacitors resulted in
a high enough voltage to activate the relay. The relay would then
disengage the capacitors, but they were the cause of the higher
voltage. As soon as the capacitors were disengaged, the voltage was
too low to activate the relay, and the relay would re-engage the
capacitors. The bottom line was that the relay would flicker on and
off, but the motor would not start. I fully expected this to happen
when I used a DC rated relay, but the AC rated relay shouldn't have
done this. I can only surmise that it is because I cut the voltage
in half by using the diode.
The Rotary Phase ConverterWith the discussion above on the
static phase converter, there isn't much to explain about the
rotary converter. The rotary converter is nothing more than a
second motor in the circuit which is acting as a generator. With
the static converter, the tool's motor performed this function, but
at the cost of some loss of power. With the rotary converter, the
idler motor is under no physical load, but it cleans up the signal
a little. If you examine the drawing below compared to the first
drawing, the only difference is that we added the idler motor.
The output of the rotary phase converter is closer to being a
true 3-phase source than the static converter. This provides more
power to the tool motor, and also brings it up to speed faster. The
rotary converter is best served when you have a motor which is
started and stopped frequently, and you need the full power of the
motor. Furthermore, a single rotary converter can drive several
different 3-phase tools.
Setting up the rotary converter is the same as the static
converter described above. The only decision to be made is the size
of the idler motor. The idler motor needs to be larger than the
largest tool which will be operated.
Since the static converter will provide a motor with 80% of it's
normal operating power, and the rotary phase converter uses a
static phase converter as a starter, your idler motor should be
125% of your tool(s) motor size. That is, if your tool is a 5
horsepower motor, your idler should be between 6 and 7 horsepower.
It is always better to err on the high side, so I would use a 7
horsepower idler motor. If the converter will operate more than one
tool, make sure the current rating of the idler motor is 125% of
the sum of the tool motors.
Single to 3-PHASE Power Conversion
This is a compilation of data received by my request sent to
MOON-NET.-----
From: K3PGP - John To: [email protected] Subject: 3 phase
power from single phase source ?Date: Tuesday, March 23, 1999 5:50
PM
I have seen reference to people using a three phase motor and a
capacitorbank to generate three phase 208 vac from a single phase
220 vac line. Inone particular case I saw a 15 HP motor being used
to supply 208 vac at 25amps per leg to a transmitter power supply.
The source was single phase 220vac. Unfortunately I am unable to
obtain any further details.
Does anyone know how the motor is hooked up to do this? How do I
determinewhat size motor I need and the hookup and value of the
capacitors?
Unfortunately I have an EME project pending that requires three
phase 208vac power at approx. 25 amps per leg. All that is
available at the site issingle phase 220 vac. Any help would be
appreciated.
Thanks...
John - K3PGPhttp://www.k3pgp.org-==-------Responses were
received from:
Ken W6GHV, Jim N9JIM ex-WB9AJZ, Mike Murphy KA8ABR, Tom W2DRZ,
Russ K2TXB, Kent D. O'Dell KA2KQM, Olivier CT1FWC / F6HGQ, Stan
WA1ECF, Mike WD0CTA, Tom KB2BAH, Cliff K7RR, Dave N7DB, and Ted
VE3BQN.Below is the summary of those responses. Although much of
this applies to running motors the same system can be applied to
running any 3-phase equipment including transmitters from a single
phase source.
If I have missed anyone or have failed to give credit please let
me know!
The answer to my question is Y-E-S and the basic idea was best
summarized by Russ K2TXB and is posted below.
NOTE: When connected like this the motor will NOT start. It will
only hum. You need to wrap a rope around the shaft and manually
start it spinning, just like a lawn mower engine. Another option is
capacitor start which is described in the following article.
For those of you that want more details the following
compilation article is provided. I will update this article as soon
as my 15 HP motor gets here and I have a chance to run some actual
tests with it driving the 3-phase transmitter power supply.
Many quality used industrial machines are available at
attractive prices that have 3 phase electric motors. Most
residential homes do not have access to 3 phase electric power at a
reasonable price. If the home shop builder decides to use these
machines they must either replace the 3 phase motors with single
phase motors or find a way to use the single phase power at their
house to run them. This article explains how to build a rotary
phase converter that will convert your single phase 220 VAC
electric power to 3 phase 220 VAC to power your industrial
machines.Safety should be your first concern and any electrical
wiring should follow your local electrical code. That being said,
some typical wire sizes, overload, and short circuit protection
methods will be described to get you started. Also, the metal frame
of the motors and your machines should be grounded. This safety
ground normally does not conduct any electricity. It is present in
case a current carrying conductor accidentally touches the metal
frame. This provides a low resistance path for the electricity to
flow instead of going through your body to earth ground.
There are two basic types of phase converters on the market
which will allow 3 phase motors to run using single phase input to
the converter. These types are referred to as static and rotary.
The static converter is basically only a start circuit that once
the motor starts, disengages and lets the motor run on single phase
power. The disadvantage of this method is that the motor winding
currents will be very unbalanced and the motor will not be able to
run above about two-thirds its rated horsepower. The rotary
converter provides current in all 3 phases and although not
perfect, will allow a motor to provide all or nearly all its rated
horsepower. If the motor has a service factor of 1.15 to 1.25 then
you should be able to use full rated horsepower. The service factor
can be found on the motor nameplate and is usually abbreviated S.F.
The reasons that the electric power is not perfect are very
technical and can include small amounts of voltage and current
imbalance as well as the phase angles between phases not being
perfect. The voltage and current balancing is straight forward if
you have access to a voltmeter or preferably a clamp-on type
ammeter. But even if you don't have these meters, using the
approximate values of run capacitors specified in this article the
currents should be close and you will be able to get nearly full
horsepower from your 3 phase motors.
The terminology used to described the phase converter parts
needs clarification. The rotary part of the rotary phase converter
is a standard 3 phase electric motor called the idler motor. It is
called this because typically it has no mechanical load connected
to its shaft. Since applying single phase power to a 3 phase motor
will not start it rotating, a means to start the idler motor
turning near rated speed is necessary. This can be done in several
ways. A pull rope can be used, a small single phase electric motor
can be used, or a start capacitor can be used. If the mechanical
means are used, power to the idler is not applied until after the
motor is spinning and the rope or power to the single phase motor
is removed. To balance the voltages and currents in the 3 phase
output a pair of run capacitors can be used. A disconnect switch is
required by most local electrical codes for each piece of
equipment. If a plug and receptacle is used to connect power to the
equipment, this meets the disconnect requirement. Overload
protection is required for each motor. This can be built-in to the
motor or provided separately. Check the motor nameplate, if it does
not say built-in overload protection, then it must be supplied
separately. Typically, a thermal overload relay and a magnetic
contactor are used for controlling the motor. The magnetic
contactor is a heavy duty relay for turning motors on and off. It
is designed to handle the high starting currents of motors. There
are also mechanical (manual) contactors available with thermal
overload protection as part of the switch. For the purpose of this
article the two wires carrying the single phase 220 VAC power will
be called lines 1 and 2. These are connected to terminals 1 and 2
of the idler motor, respectively. The wire coming from the third
terminal of the idler motor will be called line 3.
To build a rotary phase converter follow the general schematic
shown below:
Figure 1The single phase 220 VAC input is brought in on lines 1
and 2, labeled L1 and L2 in figure 1. Time delay cartridge fuses
are used for short circuit protection. 1R-1 and 1R-2 are the main
contacts for the magnetic contactor (power relay.) The coil for
this relay is denoted 1R. The run capacitors are wired between
lines 1-3 and lines 2-3. The overloads are part of a thermal
overload relay with a normally closed contact labeled OL-1. This
contact will open if any overload is tripped. Opening this contact
disables the flow of current through the 120 VAC control circuit
deenergizing the coil 1R. The idler motor terminals are labeled T1,
T2, and T3. The start circuit uses relay 2R and its contact 2R-1 to
connect the start capacitor across lines 1 and 3 while the start
push button is held in. In the control wiring, the auxiliary
contact of relay 1, labeled 1R- X, maintains power to the coil 1R
after the start push button is released. The 3 phase output power
is connected after the main contacts (1R-1 and 1R-2) so that power
from lines 1 and 2 are not connected to the output unless the phase
converter is running.
A simpler alternative, which eliminates the separate start
circuit and also eliminates the set of run capacitors between lines
2-3 is called a self starting phase converter. This design is
discussed later in this article.
Choose the wire size based on the current that will flow in the
wire. Table 1 can be used for guidance and is based on 3 phase, 220
VAC motors and 125% of motor nameplate current. Use only copper
wire with a minimum size of #14. It is acceptable to use larger
wire than listed in table 1.
Table 1Minimum suggested wire
sizes.MotorHpMotorCurrentWireSize
1/22.0#14
3/42.8#14
1.03.6#14
2.06.8#14
3.09.6#14
5.015.2#12
7.522.0#10
If a run of wire longer than 50 feet is used such as from the
circuit breaker panel to the phase converter, choose the wire size
to keep the voltage drop in the wire less than 3 percent. Remember
to add the currents of all devices that will draw power from this
feed wire. Table 2 can be used for guidance and is based on copper
wire.
Table 2Minimum suggested wire size for low voltage drop. Amps vs
feet.Currentin Amps60Ft90Ft120Ft150Ft180Ft210Ft
5#14#14#14#14#14#14
6#14#14#14#14#14#12
7#14#14#14#14#12#12
8#14#14#14#12#12#12
9#14#14#12#12#10#10
10#14#14#12#12#10#10
12#14#12#12#10#10#10
14#12#12#10#10#10#8
16#12#12#10#10#10#8
18#10#10#10#8#8#8
20#10#10#10#8#8#8
25#10#10#8#8#6#6
30#8#8#8#6#6#6
Selecting the idler motor is the first step. It should be a 3
phase motor rated to operate at the line voltage and frequency that
is available, normally 220 VAC, 60 Hertz. The phase converters
tested here were wye (star) wound. Some motors are delta wound.
Many motors have more than 3 leads so that it can be wired for more
than one voltage. Dual voltage wound motors typically have 9 leads
as shown below.
Figure 2Check the motor nameplate, if for voltage it lists
220/440 then it can be wired one way for 220 volts and another way
for 440 volts. If you are not sure, disconnect all wires and
measure the resistance between wires and compare to figure 2. The
same motor would have the amperage listed as 15/7.5 meaning it will
draw 15 amps when connected for 220 VAC and 7.5 amps when connected
for 440 VAC. The speed rating is not important; from 1100 to 3600
RPM are all fine. The higher speed might produce slightly better
phase angles, but the lower speed is generally easier to start.
Ball bearing motors are recommended rather than motors with sleeve
bearings. If the motor has oil cups it is a sleeve type bearing, if
it has grease fittings or no fittings at all it is a ball bearing
type. Spin the motor to be sure the bearings are good. Also, when
buying a used motor connect an ohmmeter between each lead and the
frame to verify that no short circuits are present. That is a sign
that the insulation inside the motor is defective. For guidance,
the cost of a used 3 phase motor of 2 horsepower or less should be
about $20; for larger motors use about $10 per horsepower. The
horsepower rating of the idler motor should be the same or higher
than the largest 3 phase motor that you will use. If you have
equipment that starts with the motor loaded, such as an air
compressor, then 1.5 times the motor horsepower would be
recommended.
The start capacitor should be rated for at least 250 VAC. The
inexpensive electrolytic type can be used. If the idler motor is 1
horsepower or less the more expensive oil filled type used for run
capacitors can also be used because the small size is not too
expensive. The self starting phase converter uses the same set of
oil filled capacitors for both starting and as run capacitors. The
electrolytic type will lose capacitance over the years and
therefore should be purchased new. It can be identified by the
round, black, plastic case. The microfarad rating should be chosen
by the horsepower rating of the idler motor. Since the idler motor
is started without a mechanical load, the size is not critical and
for guidance anything between 50 and 100 microfarads per horsepower
will work. The larger rating will bring the motor up to speed
faster and draw more current while starting. A 220- 250 VAC,
270-324 microfarad start capacitor sells new for about $15.
The run capacitors are optional. The converter will work fine
without them, however you may only be able to get about 80% power
from your 3 phase motors due to low current in the third line. The
run capacitors are commonly rated for 330 or 370 VAC. The oil
filled type must be used. These are rated for continuous AC duty
while the electrolytic type are not and could explode. The oil
filled type will not loose capacitance over the years and therefore
can be purchased used or surplus. A new 50 microfarad run capacitor
might cost $50 while used or surplus only $7. It can be identified
by the metal case and oval shape (sometimes rectangular or even
round.) The purpose of the run capacitors is to balance the voltage
and current in the 3 phase lines. One set is connected between
lines 1 and 3. The other is connected between lines 2 and 3. A set
may be needed because if more than about 50 microfarads are needed,
two or more separate capacitors must be connected in parallel to
obtain the desired value. The best way to size these is by trial
and error using a clamp-on type ammeter on the 3 phase lines while
the 3 phase motor is running. For perfect balance each set may be a
different value. For guidance or if perfect balancing of the
currents is not needed, the microfarad rating can be estimated by
the horsepower rating of the idler motor. Using equal capacitance
of 12 to 16 microfarads per horsepower should result in a
satisfactory balance.
The effect of the run capacitors on voltage and current in the 3
phase lines is shown infigure 3andfigure 4. Infigure 3, a 3/4
horsepower idler motor needed about 18 microfarads between both
lines 1-3 and lines 2-3. Infigure 4, a 5 horsepower idler motor
needed about 70 microfarads between the phases. This idler was best
balanced with 80 microfarads between lines 1-3 and 60 microfarads
between lines 2-3, although 70 microfarads between each was only
slightly worse.
During the current balancing tests the 3 phase motor was only
turning the spindle on the lathe, no metal was being cut. This was
to obtain a repeatable, albeit small, load. Table 3 shows the
current balance using various run capacitors.
The self starting phase converter uses capacitance between only
one phase (1-3) instead of using 2 sets as recommended here. The
result of trying this with the same 5 horsepower phase converter is
shown in figure 5. The balance of voltages and currents improved
compared to no run capacitors, but not as well as putting
capacitance between both lines 1-3 and lines 2-3. In either case,
as a side benefit, the single phase current draw which includes
both the phase converter and the load motor power consumption will
also be reduced dramatically as shown in figure 6. When no 3-phase
motors were operating and only the idler was running, the single
phase current without run capacitors was 14.8 amperes and with the
run capacitors it was only 4.4 amperes as shown by the triangles in
figure 6. This 70 percent reduction in current is impressive, but
due to the change in power factor the actual power consumption only
changed from 379 watts to 295 watts or 22 percent.
Table 31/2 HP lathe motor turning spindle only. Single Phase
Line
Amps Volts pf Watts Three Phase Lines
------ Amps ------ Capacitance
Line1 Line2 Line3 pf Watts 1-3 2-3
17.22 246.2 0.16 685 2.37 2.42 0.43 0.45 289 0 0
15.85 246.7 0.16 627 2.27 2.33 0.59 0.43 279 10 10
10.13 246.6 0.22 545 1.91 2.09 1.29 0.39 279 50 50
8.67 246.2 0.26 557 1.83 2.06 1.52 0.37 279 60 60
7.15 245.6 0.29 512 1.68 2.00 1.72 0.32 240 70 70
7.13 245.6 0.29 504 1.81 1.88 1.76 0.32 249 80 60
To assure that the size of run capacitors would not be far off
while cutting metal, a couple data points were taken at a spindle
speed of 130 RPM and a feed rate of 0.004 inches/revolution while
turning down the diameter of a piece of mild steel. The original
diameter was 1.850 inches. The first cut of 0.030 reduced the
diameter twice that to 1.790. The second cut of 0.060 started from
the 1.790 diameter and reduced it to 1.670. Table 4 lists the
results which show a balance similar to when the same capacitance
was used and the spindle was not cutting metal.
Table 460 microfarads between lines 1-3 and lines 2-3. Single
Phase Line
Amps Volts pf Watts 3 Phase Line
----- Amps ------
Line 1 Line 2 Line 3 pf Watts
8.67 246.2 0.26 557 1.83 2.06 1.52 0.37 279 Spindle only
8.71 247.1 0.26 565 1.83 2.08 1.53 0.40 303 0.030 inch cut
8.85 247.1 0.30 648 1.90 2.18 1.58 0.50 387 0.060 inch cutThere
are two relays shown in the schematic infigure1. The number 1 relay
is the main power relay and should have a motor horsepower rating
suitable for the idler motor size. These are often referred to as
magnetic contactors. It has two main poles to switch the 220 VAC
single phase lines and an auxiliary set of contacts used to latch
the coil of the relay energized when the main contacts are closed.
The idler is shut off by pressing the stop button which opens the
circuit to the coil causing the contactor to open. The number 2
relay is used to connect the start capacitor to the circuit. A
relay is used so that the high starting currents do not go through
the push button. A motor rated relay can be used or if a current
rated relay is used select it to carry at least 2 times the
nameplate current. The actual current depends on the size of the
start capacitor and can be estimated using the following
equation.
i = 2 (3.14) (frequency) (voltage) (capacitance)/10^6
i = 2 (3.14) ( 60 ) ( 220 ) ( 300 )/10^6 = 24.9 amps
Electrical codes require a disconnect for each piece of
equipment. The disconnect switch (or plug) separates all current
carrying conductors from the line voltage. For 220 VAC single phase
systems this is 2 wires (a 2 pole switch), for 3 phase systems this
is 3 wires (a 3 pole switch.) Since the phase converter is supplied
with single phase power it can use a 2 pole disconnect or 2 of the
3 poles of a 3 pole switch. Each piece of equipment using the 3
phase power should also have its own 3 pole service disconnect.
Many of these have fuses as part of the switch and are referred to
as fused disconnects. For motor applications this is helpful since
the motor overloads do not sufficiently protect from short circuits
like fuses do. The use of time delay, cartridge fuses are common
with motor circuits. Some local codes allow the use of the branch
circuit disconnect or circuit breaker as the service disconnect for
the equipment if it is within sight of the equipment. The
disconnect of the phase converter can often meet this requirement
in home shops.
The idler motor is started first and typically left running
while the 3 phase motors in the shop are turned on and off as
needed. More than one motor at a time can be operated and each
running motor will act as a phase converter for the others so the
total horsepower running can be 2 to 3 times the idler motor
horsepower. If a manual switch is used instead of a magnetic
contactor, then the push button to engage the start capacitor must
be held in before the manual switch is turned on. When the idler
motor starts (about 1 second or less) then the push button for the
start capacitor is released.
Commercial vendors of static converters allow using the static
converter to start an idler motor so that several motors can be run
at the same time. However, some of these commercial units use
voltage or current sensing relays to engage the start capacitor. If
a motor near the size of the idler (which the static converter is
sized for) is started, the start-up current can drop the line
voltage for a fraction of a second and result in the start
capacitor engaging. This can overload the static converter since
other motors are running. The design recommended here does not have
this limitation since the start capacitor is only engaged when the
operator pushes the start button.
Self Starting Phase ConverterA self starting phase converter is
simpler and less expensive than the converter shown infigure 1.A
self starting schematic is shown below.
Figure 7However, the current and voltage balance in the 3-phase
output varies more with load so that some unbalance is present at
loads other than the one for which capacitance was selected.
For many shops the small amount of unbalance is acceptable and
most commercial rotary phase converters are the self starting type.
Inside one commercial 2 horsepower rotary phase converter was two
30 microfarad capacitors in parallel, this is effectively 60
microfarads. Since only two wires went between the capacitor bank
and the motor, these must be connected across only one phase. In a
3 HP converter of a different manufacturer, three 40 microfarad
capacitors were used (120 microfarads total.)
For the simplest converter, without a separate start circuit,
using 25-30 microfarads per idler horsepower between one of the
input lines and the third (generated) line will provide an
acceptable phase converter. Too little capacitance and the idler
either will not start, or it will start very slowly. Since the time
delay fuses typically used for motor short circuit protection will
allow some amount of over current for starting for about 5 seconds,
it is recommended that enough capacitance be used to start the
idler faster than that. Excess capacitance will cause the 3-phase
voltages to exceed the input line voltage, especially when the
idler is not loaded. Tables 5 and 6 show the voltages with various
capacitance for a 5 HP and a 3 HP phase converter, respectively.
The lathe used to put a load on the converter for the tests in
tables 5 and 6 has a 1/2 HP motor; the drill press used has a 3/4
HP motor. As more 3-phase load was applied, the voltages across
lines 1-3 and 2-3 were reduced as shown in the tables. Also shown
in tables 5 and 6 are the times the idler needed to start.
Comparefigure 4andfigure 5and decide if the improvement in output
balancing is worth the extra effort of a separate start circuit
which is required if equal capacitance is connected across both
lines 1-3 and 2-3.
Table 55 HP self starting idler. Start Time 3-Phase Voltages
Seconds L1-L2 L1-L3 L2-L3120 microfarads: 2.6 247.1 262.8 238.7
No load
246.9 255.4 231.0 Lathe
247.1 251.0 227.2 Lathe & Drill press
130 microfarads: 1.6 246.9 264.8 243.7 No load
246.6 258.6 234.8 Lathe
246.2 253.7 229.8 Lathe & Drill press
150 microfarads: 1.0 247.9 270.3 253.6 No load
246.6 263.2 244.0 Lathe
247.8 259.2 238.8 Lathe & Drill pressTable 63 HP self
starting idler. Start Time 3-Phase Voltages
Seconds L1-L2 L1-L3 L2-L3
50 microfarads: 0.8 245.6 249.4 225.0 No load
245.6 239.0 220.0 Lathe
70 microfarads: 0.8 245.5 260.4 238.7 No load
100 microfarads: 0.6 246.1 277.7 256.1 No load
245.9 262.5 245.6 Lathe
245.6 255.9 236.6 Lathe & Drill press
120 microfarads: 0.6 245.5 288.0 265.7 No load
245.7 270.3 254.9 Lathe
245.3 261.5 245.9 Lathe & Drill press