for Phase Conversion and Motor Control
Hardman, Great Falls, Virginia
Frequency Drives (VFD) can convert single phase to three phase power
when needed, even for many larger integral horsepower motors.
- VFDs can only drive three phase
single phase motors.
- They can soft start a high inertia
load to limit or completely eliminate the large inrush current normally
associated with motor starting.
- VFDs take the place of a magnetic
at the same time offering superior motor protection and precise control.
- While their initial cost may seem
high, their installed cost compares very favorably with traditional
motor starting techniques; but VFDs also offer so many really
significant advantages over conventional magnetic starters at the same
- VFDs are more efficient electrically
other phase conversion
devices. Less power is wasted to heat.
- Motors do not need to be derated
when driven by a VFD. Because VFDs deliver balanced three phase
current, motors can be operated at their full rated horsepower for
their specified duty cycle.
- VFDs allow easy adjustment of
motor speed to match changing load requirements.
can be programmed to change motor speed dynamically in response to
a system related variable, such as a drop in static wind pressure.
- VFDs can potentially damage old
weak motor insulation. The use of a Load Reactor can minimize
the risk; but this is usually unnecessary. Rewinding with inverter duty
rated wire intended for use with VFDs will eliminate the risk.
- VFDs can be programmed to shorten
or lengthen a motor's normal deceleration time when the motor is
powered off. Energy recovered from braking may have to be
dissipated in an external resistor.
case study demonstrates how to use the remarkable capabilities of a
Variable Frequency Drive to solve many common motor control problems.
Because 3 phase commercial power is not available at my home, I am
forced to use single phase 240 VAC to run my 25 hp pipe organ blower. I
out using a rotary transformer type of phase converter to provide the
three phase power required for the motor. It was big, noisy, and slow
to start, and caused a pretty severe brownout emphatically noticed on
every light in the house. Each of the several battery backup power
supplies in the house would issue its warning beep and briefly switch
over to battery operation until the phase converter got up to full
speed. Then another brownout took place when the Spencer blower motor
was started. Each of these starts was an "across-the-line"
start initiated first by the closure of a disconnect switch and then
a conventional magnetic starter.
A small 3
hp rotary phase converter is shown here. A rotary phase converter is a
shaftless 3-phase electric motor with an extra metal box
attached to contains the phase shift capacitors needed to get its three
phase motor to start and run on single phase. Single phase power
is applied to only one of the motor's three field windings.
Once up to
speed, the load is then switched to all three phase windings of the
motor. The two unpowered windings derive their power in the same way
that a transformer's primary winding can induce a current into its
secondary winding, hence the name rotary transformer.
Rotary phase converters were commonly used before practical solid state
alternatives were developed. In spite of their popularity, I learned
from my unhappy experience that a rotary phase converter was not to be
the solution to my phase conversion problem; so I started thinking
about alternatives. I knew that Variable Frequency Drives could make
the required single to three phase conversion; but after studying many
drive specifications found on the Internet, I was disappointed to learn
that drives powered from single phase are only offered for motors up to
2 or 3 hp. But what if your motor is larger than that? Well,
since the initial writing of this page in 2003, a few manufacturers
have finally introduced new models that are factory specified to
operate from 120 or 240 volts AC single phase, and are rated for 10 hp
VFDs designed to run on single phase electric power have been readily
available to operate motors up to 3 hp, there have been very few
offered to operate larger integral horsepower motors. There is very
little call for such a thing since most larger motors are used in a
business which already has three phase commercial power available, and
all VFDs will run on 3 phase as further described below.
It is usually possible to use single phase power with larger motors,
however, if the drive is carefully selected for the size and type of
load. Specifically this has to do with rectifier sizing, and the type
of load being driven. The 40 hp Hitachi VFD pictured here is less than
10" x 15-1/2" x 7".
Many sales people will say that if you just double the size of the
drive, it will work on a single phase power source. For example, to run
a 7-1/2 hp motor, use a 15 hp VFD. For a 10 hp motor, use a 20 hp
drive, etc. This is a rather arbitrary formula that is probably safe
for many applications; but at least for larger drives, correctly
programmed, it could be expensive overkill. It’s better to try to
calculate the size drive needed. Most of the engineers I have spoken
with used the following logic to determine the size needed. Read the
full load amperes (FLA) on the nameplate
of the motor to be driven. Remember that this current rating
is for 3 phase power. To calculate the equivalent number of amperes
needed from the single phase power line, multiply the 3 phase FLA times
the square root of 3 (1.73).
Single Phase Full Load
Amperes = (3 phase FLA) X 1.73
Select a drive with a rated INPUT
current capacity at least as large as the calculated single phase
amperes. Don’t cut it too fine. Leave a little safety margin. Some
sales engineers suggest a 10% fudge factor. And don’t forget to program
your drive to limit output current to your motor’s FLA amperes! This
will prevent the drive from exceeding its input diode current ratings.
This procedure is good for comparing drive prices from different
manufacturers; but when you are ready to order a VFD, be sure your
sales engineer knows you are intending to run the drive from single
phase, and get him to confirm and approve your selection.
Some types of loads, like reciprocating piston pumps and compressors,
for example, require a Constant Torque VFD designed for or at least
derated for the constant torque application. Fortunately, an organ
blower is a variable torque fan load, and the less expensive Variable
Torque drive can be used. In this context, the torque needed to rotate
a fan varies with the speed of rotation. Most VFDs can be programmed to
operate in either mode, with reduced current being available when
driving a Constant Torque load.
An Encoderless Vector Drive or Flux Vector Drive is not required for
simple fan loads. A Variable Frequency Drive is sometimes referred to
as an AC Drive, Adjustable Speed Drive (ASD), and occasionally simply
as an "Inverter". The Input Section of a VFD takes alternating current
and rectifies it to Direct Current (DC). The voltage ripple remaining
after rectification is minimized by using capacitors which act as
short-term reservoirs to help stabilize the DC voltage available on the
The heart of a VFD is really a power inverter - a device that converts
DC power to AC power. The inverter section of a VFD typically uses a
microprocessor to manage 3 bipolar pairs of semiconductor switches to
synthesize three pseudo sine wave output voltages which are zero, 120,
and 240 degrees apart in their phase relationships. The semiconductor
switch pairs feed from the DC bus, and deliver pulses of Direct Current
to each of the three phase load terminals of the drive in sequence. The
microprocessor controls the pulse durations and polarity to synthesize
a pseudo sine wave of variable frequency. This is commonly called Pulse
Block diagram of a VFD showing major components
The need for careful selection of the drive is
really the need to match
to the intended load the size and current ratings of the silicon diode
rectifiers that make up the front end of a VFD. It is much easier to
supply stable DC bus voltage when there are three phase-independent
full wave bridge rectifiers operating at zero, 120 degrees and 240
degrees apart, than to have to draw all power from just one single
alternating current source. It’s really very much like
comparing a single cylinder internal combustion engine to a three
cylinder engine of equal power and torque. The three cylinder engine
simply runs a whole lot smoother than a single cylinder engine.
front end of the VFD (the diode rectifier section) is large enough to
supply the required current demanded by the anticipated load, then the
drive should work successfully when powered from a single phase source.
If each one of the 3 rectifier packages is by itself large enough to
support the intended load, then a single phase power source can be
used, and the 2 unused diode packages can be left unterminated. In
practice, I connect one side of the single phase line to L1, and the
other side of the single phase line to both L2 and L3 to
allow these two rectifier packs to share the load.
Most drives are optimized for use on 3 phase power. Drives intended for
single phase come with oversized filter capacitors. Larger capacitors
are needed to better smooth the rectified DC since only one of the
three phase diode bridges is under power. Also, larger capacitors do a
better job dealing with the increased ripple amplitude and decreased
ripple frequency applied to the DC bus as a result of running on single
phase. The wider voltage swings in the capacitor's charge/discharge
cycle, when running on single phase, exaggerate the heating effect on
the capacitor bank. This heating effect is minimized when the drive is
powered from 3 phase current.
So if only single phase power is available, then one very important
benefit gained from using a VFD is that you don’t need a separate phase
converter with its attendant cost, installation, noise, electrical
inefficiencies, and eventual maintenance. And all the phase converters
I am aware of have current balance problems where leg currents vary
with load. Motor loads don’t like this, even if they are able to grin
and bear it for a while. VFDs drive each 3 phase leg in exactly the
same way; so when all 3 motor windings have the same impedance, as they
normally do, then leg currents will remain equal even as the blower
load and speed vary. This is why a motor must be derated when powered
by a rotary phase converter, or worse yet by a simple phase shifting
capacitor bank, which are often misleadingly referred to as "solid
Another important benefit of using a VFD is the ability to soft start
the load. Instead of a sudden demanding across-the-line start of the
phase converter, followed by another across-the-line start of the organ
blower motor, a VFD-powered motor simply starts turning and very
gracefully accelerates without complaint to the programmed operating
speed over a programmed period of time. By spreading the motor
acceleration over a longer period of time, you can completely eliminate
all the inrush current demanded by a motor to get it up to running
speed from a dead stop. For a squirrel cage induction motor, this
inrush current is typically on the order of 5-6 times full load current
(FLA). For a larger motor, this sudden demand for current often causes
a very noticeable brownout to the lighting and other electrical loads
in operation when the motor is started, as mentioned above. The cold
start ramp time to attain normal running speed is easily programmed on
a VFD. I have mine set to 30 seconds so as to completely eliminate any
discernible voltage drop. Dimming lights in the house no longer signal
the start of the organ blower.
Organ blowers are normally selected to provide a little headroom in
supplying the organ’s required wind pressure and volume. To provide for
this reserve capacity, the blower runs faster than necessary most of
the time. A worst case example is when no pipes are playing at all.
Similarly, when the organist is playing a light registration, the
blower is still running full bore ready to handle the worst case stop
combination when it comes along.
Obviously, one useful feature of a variable frequency drive is the
ability to vary the speed of the blower motor. This makes it easy to
match the running speed of the motor to the actual load. For a pipe
organ, it means being able to speed up the blower when a little more
wind or pressure is required. Conversely, if less wind would adequately
satisfy the needs of the organ, then you can very easily slow things
down to better match the motor speed to the real load. This capability
has special value to organ people. For example, during the organ tuning
process when only 1 or 2 pipes are playing at once, the blower can be
slowed down to allow its "static" pressure to approach the normal
regulated wind pressure of the pipes being tuned. Not only does this
save electric power, but also it reduces the frictional heat gain from
churning the very small consumption of wind required to tune pipes.
VFDs usually offer several optional programming methods which can be
utilized to accomplish this speed adjustment in very accurate discreet
programmable steps, or dynamically as described below.
Another way to utilize the control features of a VFD is to use remote
pressure sensing to control the running speed of the blower. This can
be done using a simple wind pressure switch wired to the VFD to
arbitrarily respecify the running speed of the motor, with a
programmable time constant to establish the sensitivity or rate of
response of the drive to the changing control signal. The revised or
alternate running speed is programmed in advance.
A more sophisticated approach would be to cause motor speed to change
dynamically by using an analog pressure transducer to control the
drive’s output frequency (motor speed). Once again, as wind consumption
increases, as more and more pipes are called into play, the drop in
residual wind pressure feeds back to the VFD to cause motor
acceleration until the desired wind pressure has been restored. And
when the musical arrangement thins out with fewer pipes playing,
reducing demand for wind volume, the motor will slow down to conserve
energy and reduce noise and frictional heating of the wind. So another
benefit of using a VFD is that it can be programmed to automatically
compensate for varying load conditions.
Block diagram of PID Control feedback operation available on some VFDs
The blower's wind pressure is sensed by an analog pressure sensor. Two
types of sensor are commonly available. One has a variable voltage
output, and the other has a variable current output. Cost is usually
under $100. Program the drive to match the type of sensor you use. A
Set point is programmed which represents the motor speed needed to
produce the desired static wind pressure under no-load conditions (no
pipes playing). Another parameter is programmed to specify how much
influence the pressure sensor is to exert on the calculated motor speed
needed to maintain the currently sensed wind pressure close to the
desired or Set point wind pressure. The drive constantly senses,
recalculates, and adjusts the motor speed (frequency) to maintain the
desired wind pressure regardless of the wind load imposed on the blower
at the moment. In this case the motor speed will increase as more pipes
are played. And conversely, at the end of the tune, the blower will
slow down again when wind consumption returns to its normal idle value.
This is all interesting theory, but practical issues such as regulator
efficiency and motor speed recovery time can limit their application. A
VFD is not a practical substitute for properly designed organ wind
pressure regulators; but it can help stabilize the static pressure in
the main wind trunk ahead of the various regulators feeding wind
chests. The accuracy of the pressure regulators must be considered,
especially when the differential between static and regulated wind
pressures is minimal. A relatively large change in static pressure can
sometimes have a surprising negative effect on the regulated wind
pressure, possibly resulting in undesired changes in tuning pitches.
Tuning with a marginal reserve static pressure can lead to pitch
changes when the normal static pressure is restored. This is due to the
imperfect nature of pressure regulators. Use some discretion and good
judgment when playing with your VFD's capabilities. As with most
things, a lack of understanding can lead to unexpected results
Another opportunity to utilize a VFD's variable speed capability occurs
when the organist finishes playing a tune and hits the General Cancel
before spinning around to receive applause and speak to his audience.
The organ relay can signal the drive to slow down to a walk to minimize
noise, power consumption, and frictional heating of the churning air in
the blower. When the organist starts to register for his next piece,
the blower quickly speeds up to its programmed normal running speed.
Recovery time depends on the horsepower reserve of the motor and drive,
the level of current limiting programmed into the drive, and the speed
of the idling motor. Trial and error will allow you to program
realistic speed differentials into the drive. Ramp time is separately
programmable for this situation on some drives.
To help maintain an instrument in tune, a VFD can be made to turn the
blower motor very slowly when the organ is not
being played. This moves ambient air and moisture through the
organ very slowly in an effort to stabilize the temperature and
humidity of the windchests and pipes. The electrical cost to do this is
minimal since it takes very little power to turn the blower very
slowly. This can be activated manually, of course, but there are
typically several different ways to program a drive to accomplish this
automatically as a result of "turning off" the organ.
While it may not be important in the context of organ blowers, VFDs
typically have the ability to employ controlled braking to the motor
load. Several optional braking schemes are usually available. When
necessary, recovered energy is converted to heat in a resistor. So if
you’d like your motor-driven load (organ blower in this case study) to
slow down quickly instead of just coasting to a
stop, it’s easy to do if you are using a VFD. Just a little
more programming of the drive is needed to enable this feature.
Incidentally, most VFDs are programmed using a keypad and alphanumeric
display built into the drive case. The keypad on some drives can be
remotely located by using a long extension cable between the drive and
the remote keypad. Many drives can also be programmed remotely from a
computer using software and cables available for the purpose from the
A VFD does not need an expensive magnetic starter. A fused disconnect
is all that’s required. The drive is normally left under power all the
time, and consumes very little standby current until commanded to start
the motor. One of the functions of a magnetic starter is to protect the
motor from overload. Heaters in the starter respond to motor current
and open the control circuit when an unusual or protracted overload
situation occurs. VFDs offer superior motor protection in their
programmable current limiting capabilities. Current limitation also
protects the VFD from self destruction by exceeding its design limits.
Dramatic electrical problems like a short circuit in the output cause a
self protecting shutdown to take place. Error codes are presented on
the programming display to identify the cause for a protective
To start a VFD, a switch closure or voltage appearance is all that’s
needed. For example, a VFD can be triggered to start the organ blower
the magnet power supply is switched on to provide keying voltage for
the organ. Just run a wire from the keying voltage power supply to the
appropriate VFD control input.
Incidentally, most VFDs offer a "Frequency Arrival" signal to indicate
that normal operating frequency has been attained. This signal can be
used to turn on a pilot light, or start another electrically controlled
Yes there are some nice features and benefits that come with the use of
VFDs for organ blowers. But of course, there are some problems and
trade-offs too. To start with, VFDs may seem to cost a little more than
magnetic starters. For me, I think the inherent special features and
flexibility of a VFD easily justify its cost. I accept that some may
disagree. But when you consider installed cost, a VFD is bound to cost
a lot less than 2 magnetic starters, a phase converter, and all the
additional time, labor, and materials needed to complete the
VFDs operate in the digital domain. This is at once a blessing and a
curse. The continuous spiking of high voltage DC square waves to the
motor windings can encourage premature insulation failure. Unless they
have been rewound, old motors will still have old insulation. Most new
motors are now designed and specified to be "inverter duty rated". That
means that they have superior insulation designed to be highly tolerant
of the constant abuse of the pulsing high voltage DC used to generate
the 3 pseudo sine wave output lines. I say "pseudo" because the output
wave shape from a VFD is only an approximation of a pure sine wave. And
when located at a distance from the drive, the motor can see what
begins to look like a hairy square wave. We are used to thinking of a
sine wave as having a nice smooth harmonic free shape. A VFD’s output
is synthesized digitally, and the fast rise time of the DC pulses means
that lots of harmonics are always present to some extent.
The drives that I have studied use pulse width modulation to change the
average current delivered to the load during a certain period of time.
In the process of trying to imitate the shape of a sine wave, the
microprocessor controlled semiconductor switches deliver bursts of DC
from the bus to their respective output terminals. At the
beginning of the sine wave, the pulses are very narrow, and as the
prototype waveform rises in amplitude, the pulses are made wider by
maintaining the semiconductor switches ON (closed) longer. This causes
an increase in the average current available to the load during that
part of the alternating current cycle. The pulses get narrower again as
the waveform approaches the zero crossing point. The cycle then repeats
but with reverse polarity DC for the second half of the sine wave.
Before using an old motor with a VFD, it is sometimes recommended that
a 5% impedance Load Reactor be inserted between the drive and the
motor. This will reduce the number and intensity of harmonics that are
sent to the motor, and minimize the effects of high voltage spiking on
the old insulation. For a more critical application ask your local
motor shop to rewind your motor using modern better insulated wire
designed specifically for inverter operation. Because the new wire can
operate safely at higher temperatures, the chances are that the rewound
motor can also safely deliver more horsepower as a bonus.
VFD powered motors typically produce a unique whistling sound when
running. The whistling sound is the result of the digital pulsing of
the power to the motor. The chances are that this audible noise will
not be a problem since the blower itself is inherently noisy. It can be
minimized by reprogramming the digital pulse rates to a higher carrier
frequency, or by inserting a 5% impedance Load Reactor between the
drive and the motor.
Oscillograms showing improved waveform by
adding a 5% load reactor
If there is a considerable distance between the drive and the motor, a
load reactor is desirable to prevent motor overheating from wasteful
harmonic content in the generated output power. Harmonic content is
real power, too high in frequency to contribute to motor torque, but
still there to heat the motor windings. Load reactors function as low
pass filters to significantly reduce the harmonic content of the
VFD-generated 3 phase power delivered to the motor. When the motor is
reasonably close to the drive, a load reactor is probably not needed,
and can be added later if found necessary.
The filter capacitors in a VFD have a limited life and must be replaced
occasionally. For industrial motors running hard 12 hours a day, the
recommended time interval is usually 5 years or so depending on the
actual run time of the load. Replacement capacitors don't
cost that much, but you should be aware of the limited life of these
capacitors, especially when the rectifier section of the drive is
working especially hard as it always does when powered from single
phase current. In typical home or church organ service, they should
last much much longer.
A switch is a pretty simple thing. Anybody can flip it on or off. But a
VFD takes a bit more understanding, thought, and care during
installation and programming to make it work best in any particular
application. From my own experience, some tweaking was required to make
everything work the way I wanted. The larger the drive and higher the
currents, the more important all this becomes. Once the drive has been
installed and programmed, then it's operation becomes routine and taken
I am no expert on VFDs, but through reading and many conversations with
VFD design engineers, in addition to my own practical experience with
several VFDs, I have managed to learn quite a lot about these
remarkably clever devices. I have tried to present the VFD in a fair
light describing both their benefits and limitations. I tried to
explain why I prefer a VFD to other phase conversion technologies. Even
when three phase power is already available, a VFD still offers many
advantages over a conventional magnetic starter, as described above.
I’d be glad to compare notes with anyone who would like to consider
using a Variable Frequency Drive.
Pictures and Suggested
50" diameter 25 hp 3-stage Spencer organ blower
Variable Frequency Drive and Load Reactor
Single phase power in on the left
and three phase out on the right
Selection, installation and programming
information for Hitachi brand
Variable Frequency Drives. Check Standard Specs to see which models
will deliver the required horsepower.
Selection, installation, programming
information for AC Tech brand
Variable Frequency Drives.
Sub-Micro Series SCM & SCF are recommended for fan loads
Information on how to select and use Line
and Load Reactors from MTE