| VARIABLE FREQUENCY DRIVES for Phase Conversion and Motor Control Jack Hardman, Great Falls, Virginia |
SUMMARY
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INTRODUCTION
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Because
3 phase commercial power is not available at my home, I am
forced to use single phase 240 VAC to run my pipe organ
blower. I started 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 by the closure of a disconnect switch
and then a conventional magnetic starter.
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. A
small 3 hp rotary phase converter is shown here. A rotary
phase converter generally looks like a shaftless electric motor
with an extra metal box attached to contain the phase shift capacitors
needed to get its three phase motor to start and run on single phase
since power is being 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 or more.  While 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 users of larger motors have three phase
commercial power available, and all VFDs will run on 3 phase. 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". 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).
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 "DC bus". 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 Width Modulation.
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. If
the 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, exagerate 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 phase shifting capacitor bank. 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, the blower 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.
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, 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 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 blower to slow down quickly instead of 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. Many drives can also be programmed remotely from a computer using software and cables available for the purpose from the drive manufacturer. 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 drive itself 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 shutdown. 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 blower when the magnet power supply is turned on making keying voltage available to 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 process. 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 tradeoffs 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 installation. 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 computer 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.
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 for granted. 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 the rotary phase converter I first tried. 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.
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