What Is A Variable Frequency Drive

What is a variable frequency drive? Variable frequencydrives are also known as variable speed drives, VFD’s, adjustable speed drives, and inverters.

Variable frequency drives are solid state motor control systems designed to control the speed of an AC (alternating current) electric motor. Variable frequency drives operate as load controls within AC electric motor applications; and variable frequency drives can reduce energy costsup to 50% by speed reduction on electric motorswhere the full speed (RPM) of the electric motor is not required.Variable frequency drives are used in AC servo systems, air compressors, conveyor systems, lathes, mills, plastic extrusion, slitter lines, food processing, waste water treatment systems, submersible pumps, HVAC fans and blowers, and many more electric motor applications.

Many manufacturers apply AC variable frequency drives to rotating equipment because variable frequency drives reduce amperage spikes upon start up of large electric motors.Choosing the right AC variable frequency drive for an application will benefit rotating equipment by providing less wear on the electric motors where applied.Adjusting the acceleration and deceleration time of electric motors can extend the lifespan of an electric motor. AC variable frequency drives provide the ability to control the frequency of starting and stopping of an AC electric motor.This ability allowsan AC electric motor to only operate when needed for the equipment it’s rotating, and electric motors have a longer lifespan if they are not continuously operating when they don’t need to be.

Approximately one third of the world’s electrical energy is supplied by electric motors in fixed-speed centrifugal pump, fan, and air compressor applications. These fixed-speed applications hardly ever require the full load speed (RPM) of the electric motor in which they’re operating. By installing AC variable frequency drives to these applications, electric motor speeds are reduced, and power costs can be reduced by 50% or more. Technology has allowed cost and physical size reduction of AC variable frequency drives, and has improved performance through advances in semiconductor switching devices, simulation, control techniques, and control hardware and software.

Power Savings: What Is A Variable Frequency Drive

The majority of AC variable frequency drives in the market today contain electronic circuitry that converts 60 Hertz Line power into direct current. The variable frequency drive converts this line power into a pulsed output voltage that duplicates varying alternating current to a desired frequency (speed).A properly applied AC variable frequency drive when paired with an AC electric motor will significantly reduce operating costs for manufacturers. This is particularly true for variable torque loads such as fans, blowers, and pumps.Blowers are often used with dampers to control air flow; these dampers may operate either manually or automatically. When dampers are closed, 50% of the electric motor current will drop to approximately 60% of full load nameplate current. By utilizing an AC variable frequency drive in blower applications, the current draw of the motor will be reduced 30% for every 10% drop in speed. The same electric motor operating froman AC variable frequency drive at 50% speed, will draw approximately 20% of the full load current.

Example Application:

A 10 horsepower AC electric motor, rated 90% efficient, operating across the line without an AC variable frequency drive, with the dampers operating between 50 70%, for 2000 hours per year will require 11,996 KWH. If the KWH charge is $.08 per KWH, the cost to run this motor will be: $1,248.00 annually.The same 10 horsepower electric motor operating from an AC variable frequency drive, between 50 70% speed for 2000 hours per year will require 4,676 KWH. Operating cost at the same KWH rate will be: $432.00 per year. This represents a savings of $816.00 per year and should be enough to pay for the AC variable frequency drives investment and installation costs, within the first 12 months of operation.If any electric motor application operates more hours than in the above example, and/orthe KWH charge is higher, the savings will quickly compound.The energy saved on a utility bill from using a variable frequencydrive is often significant enough to pay for the variable speed frequencywithin a couple of months from installation date.Increasing and/or decreasing the start up time on an AC current electric motor via a variable frequencydrive can add years to the motor’s overall lifespan. Using a variable frequencydrive can also improve efficiency on production demands. Variable frequencydrives provide the ability to control the frequency of starting and stopping an AC electric motor.This ability provides a means by which an AC electric motor is only operating when needed. AC electric motors have a longer lifespan if they are not continuously operating when they do not need to be.

Types Of Drives: What Is A Variable Frequency Drive

Volts Per Hertz drives are the most common type of variable frequency drive and areknown as a V/Hz drives, or volts by hertz drives. V/Hz variable frequency drives are used inapplications such as fans, pumps, air compressors, and other related applications wherehigh starting torque is not required. V/Hz variable frequencydrive applications typically do not require full torque when the AC motor is operating at less than the base speed (RPM) of the electric motor. V/Hz variable frequencydrives are the most inexpensive type of variable frequencydrive. V/Hz variable frequencydrives do not provide full motor torque at low RPM.

Open-Loop vector drives are also known as “sensorless vector” variable frequencydrives. Open loop vector drives adapted the name “sensorless vector” because they do not use an external encoder for speed feedback to the motor.Open loop vector drives are used in applications where high starting torque and full torque at low speed (RPM) is required. Open-Loop vector drives operating a motor a zero RPM should not be used on crane or hoist applications. Most open-loop vector drives are used on CNC machines, mixers, mills, lathes, and other applications where high starting torque or full torque at low RPM is needed.Open loop vector drives are usually more expensive than a V/Hz variable speed drives.

Closed-Loop vector drives are used in applications where precise speed control (0.01%) is needed, or in applications where extensive programming is needed. Closed-Loop vector drives use an encoder on the motor to provide constant shaft position indication to the drive’s microprocessor. The encoder feedback allows the drive microprocessor to constantly control torque no matter how many RPM the motor is operating at. Closed-Loop vector drives are used to provide the motor to operate at full torque even at zero RPM. Closed-Loop vector drives are commonly used on hoist and crane applications because crane and hoist motors must produce full torque prior to it’s brake being released, or the load will drop and it will not be able to stop.

To learn more about variable frequency drives or for repair and replacement quotes, contact Precision Electric, Inc.

Yaskawa VFD Drives

Yaskawa VFD drives are designed to minimize harmful demand for input harmonic distortion. Yaskawa VFD drives up to25 HP are available with optional 3% or 5% equivalent line impedance DC Bus Reactors.At 30 HP and above, Yaskawa VFD drives include 3% impedance as standard. Some Yaskawa VFD drives include a dual diode bridgerectifier which can be configured to accept 12-pulse(delta-delta and delta-wye secondary) transformerinput. Yaskawa VFD drives can also be factory configured with an integral18-pulse transformer. These packages are typicallyused to meet the most strict harmonic requirementsat both drive input terminals and the point of commoncoupling.

Yaskawa VFD drives are factory-programmed and ready torun. An LCD interface enhances ease of use with itsmultiple language support and parameter copy feature thatallows duplication of settings between drives. In addition,a portable USB Copy Unit (1000 series only) providesa very convenient method of desktop configurationtransportable to the factory floor.All Yaskawa VFD drives have a split cover for easy access to thepower and control terminals. The power terminal compartmenteasily accommodates bend radius for cable connections. Controlwires are connected to a detachable terminal board for ease ofinstallation and maintenance.

Detachable cooling fans are easy to replace and on/off fan controlcan extend operating life. Accumulated operation time and coolingfan run time are recorded and can be displayed for preventivemaintenance programs.Start-up and configuration are simplified by the intuitiveprogramming menu and start-up procedure.DriveWizard Industrial is a PC-based support tool for drivecommissioning and maintenance. It provides a way to operatethe drive, change parameters, upload and download parameters,monitor and graph parameters, provide status and troubleshootingdata, and utilize a host of additional features with built-in helpmenus.

Yaskawa VFD Drives Product Overview

Yaskawa VFD DrivesThe Yaskawa A1000 is a multi-purpose VFD drive deliveringup to 1000 horsepower, and provides Yaskawas highest levelof vector performance including precise controlof both induction and permanent magnet motors.The Yaskawa A1000 VFD drive is a full featured drive, providing outstanding quality,performance, flexibility, and environmental friendliness through 1000HP.Enjoy network communications, feedback, and expandable I/O to controlanything from simple fans and pumps to complex machines. For newinstallations or retrofits, the Yaskawa A1000 VFD drive provides a single robust solution,regardless of the application.

The Yaskawa G7 VFD drive is the ultimate performancesolution with increased speed and torque response to provide servo-likeperformance from an induction motor. The Yaskawa G7 VFD drive has theworlds first 480V 3-level inverter architecture that eliminates orminimizes the installation problems associated with IGBT switching andprotects the entire motor-drive system. The Yaskawa G7 VFD drive performance makes it the ideal drive for high performancespeed, torque, or position control applications. Several control modesare provided in the G7. In open loop vector mode, the latest flux observeralgorithms extend speed range and provide maximum starting torque.In closed loop vector mode, 0.01% speed regulation and 1000:1 controlrange can be achieved. Zero-servo capability provides position controlat zero speed.

The Yaskawa P1000 VFD drive is the next generation in Industrial Fan and Pump control,designed for those applications that are variable torque. Simplicity,intuitiveness, and user friendliness were the key factors in the Yaskawa P1000 VFD drive design. The Yaskawa P1000 VFD drive includes a powerful set of pre programmedfan/pump application macros allowing for quick and easycommissioning. With its advanced motor control algorithm alongwith the next generation IGBT technology, motor noise and drivepackaging is reduced. The P1000 supports a wide range of network andcontrol options providing for the most cost effective solution.

The Yaskawa J1000 VFD drive is a general purpose AC drive; its PWM designprovides low motor noise and high starting torque, with a heavy dutycurrent overload rating of 150% for 60 seconds and a normal dutycurrent overload rating of 120% for 60 seconds. Volts per hertz control makesthe Yaskawa J1000 VFD drive suitable for most general applications. The J1000 is feature packed,low cost and compact. The digital operator includes a 5-digitLED status display. TheYaskawaJ1000 VFD drive has five multi-function digital inputs,one multi-function analog input, one multi-function digital output,and one multi-function analog output. An optional RS-422/485Modbus RTU serial communication port is available. An ideal choicewhenever low cost and small size are required.

The Yaskawa V1000 VFD drive is a high performance line of AC micro drives withlow motor noise and high starting torque. It provides two controlmethods; Volts per hertz,and open loop current vector control for precise speedregulation and higher torque at lower speeds. The Yaskawa V1000 VFD driveis intendedfor either heavy duty applications or normal duty applications. The Yaskawa V1000 VFD drive is the perfect choice wherever highperformance in a small size is required.

To learn more about Yaskawa VFD drives, please visit the Yaskawa Website or contact Precision Electric.






dc variable speed drives, abb 500 horsepower control cabinet

DC Variable Speed Drives

The first electric motors were designed and built for operation from direct current power.The following should be considered when choosing DC variable speed drives for production:

  • Wide speed range
  • Good speed regulation
  • Compact size and light weight
  • Ease of control
  • Low maintenance
  • Low cost

dc variable speed drive, 500 horsepower ABB variable speed drive

In order to determine if DC variable speed drives have the capability to provide the above characteristics, the DC drive has to be analyzed as two elements that make up the package. These two elements are the electric motor and the control.General purpose DC motors used on nearly all packaged drives have a very simple performance characteristic; the motor shaft turns at a speed directly proportional to the voltage applied to the armature.The Armature amperage of a DC motor is almost directly proportional to output torque regardless of speed. This characteristic indicates that a small fixed amount of current is required to turn the motor even when there is no output torque. This is due to the friction of the bearings, electrical losses in the motor materials and load imposed by the air in the motor winding.

DC motor speed is primarily determined by the applied armature voltage and DC motor torque is controlled by armature current.Understanding these concepts of DC motors provides the key to understanding the performance of DC variable speed drives. DC variable speed drives use a control module. This control module rectifies alternating current power and converts it to direct current for DC motor control. The control module also controls the DC output voltage and amperage in response to various control and feedback signals. The feedback signals then regulate the DC motor performance via speed, and torque.

The regulating function of DC variable speed drives provide an electronic circuit that monitors a number of inputs and sums these signals to produce an error signal. This error signal is processed and transformed into precisely timed pulses that are applied to the gates of the Silicon Controlled Rectifier(SCR) in the power bridge, thereby regulating the power output to the DC motor.In order to control DC motor speed accurately it is necessary to provide DC variable speed drives with a feedback signal related to motor speed. The standard method of doing this in a simple control is by monitoring the armature voltage and feeding it back into the regulator for comparison with the input set point signal.When armature voltage becomes high, relative to the set point, established by the speed potentiometer setting, an error is detected and the output voltage from the power bridge is reduced to lower the motor’s speed back to the “set point”. Similarly when the armature voltage drops an error of opposite polarity is sensed and the control output voltage is automatically increased in an attempt to re-establish the desired speed. The “Armature Voltage Feedback System” which is standard in most packaged drives is generally called a “Voltage Regulated Drive”.

A second and more accurate method of obtaining the motor speed feedback information is called “Tachometer Feedback”. In this case the speed feedback signal is obtained from a motor mounted tachometer. The output of this tachometer is directly related to the speed of the motor. Using Tachometer Feedback gives a DC variable speed drive improved regulation characteristics. When “tach feedback” is used the drive is referred to as a “Speed Regulated Drive”. Most DC drives are capable of being modified to accept tachometer signals for operation in the tachometer feedback mode.In some newer high performance “digital drives” the feedback can come from a motor mounted encoder that feeds back voltage pulses at a rate related to motor speed. These (counts) are processed digitally being compared to the “set point” and error signals are produced to regulate the armature voltage and speed.

In addition to the normal external adjustment such as the speed pot, there are anumber of common internal adjustments that are used on simple small analog type DC variable speed drives.The following is a description of the functions that these individual adjustments serve and theirtypical use.

Adjustments Of DC Variable Speed Drives & Motors

DC Variable Speed DrivesIn most cases when DC variable speed drives are initially installed, the speed potentiometer can be turneddown to its lowest point and the output voltage from the drive will go to zero, then the motor will stop.There are many situations where this is not desirable, because there are some machinesthat need to be kept running at a minimum speed and accelerated up to operating speed asnecessary. There is also a possibility that an operator may use the speed potentiometer tostop the motor to work on the machine. This can be a dangerous situation since the motor hasonly been brought to a stop by zeroing the input signal voltage. A more desirable situation iswhen the motor is stopped by opening the circuit to the motor or power to the control usingthe on/off switch. By adjusting the minimum speed up to some point where the motorcontinues to run even with the speed potentiometer set to its lowest point, the operator mustshut the control off to stop the motor. This adds safety into the system. The typicalminimum speed adjustment is from 0 to 30% of motor base speed.

The maximum speed adjustment sets the maximum speed attainable either by raising the inputsignal to its maximum point or turning the potentiometer to the maximum point. On a typicalDC motor the rated speed of the motor might 1750 RPM but the control might be capable ofrunning it up to 1850 or 1900 RPM. In some cases it’s desirable to limit the motor (and machinespeed) to something less than would be available at this maximum setting. The maximumadjustment allows this to be done. By turning the internal potentiometer to a lower point themaximum output voltage from the control is limited. This limits the maximum speed availablefrom the motor.

Digital DC variable speed drives are capable of constantly monitoring the DC motor current. The current drawn by the armature of theDC motor is related to the torque that is required by the load. Since this monitoring and controlis available an adjustment is provided in the drive that limits the output current to amaximum value.This function can be used to set a threshold point that will cause the motor to stall ratherthan putting out an excessive amount of torque. This capability gives the motor/drive combination the ability to prevent damage that might otherwise occur if higher values oftorque were available. This is handy on machines that might become jammed or otherwisestalled. It can also be used where the drive is operating a device such as the center winderwhere the important thing becomes torque rather than the speed. In this case the current limitis set and the speed goes up or down to hold the tension 0f the material being wound. Thecurrent limit is normally factory set at 150% of the motor’s rated current. This allows the motorto produce enough torque to start and accelerate the load and yet will not let the current(and torque) exceed 150% of its rated value when running. The range of adjustment istypically from 0 to 200% of the motor rated current.

IR compensation is a method used to adjust for the droop in a motor’s speed due to armature resistance. As mentioned previously, IR compensation is positive feedback that causes thecontrol output voltage to rise slightly with increasing output current. This will help stabilize the motor’s speedfrom a no load to full load condition. If the motor happens to be driving a load where the torque isconstant or nearly so, then this adjustment is usually unnecessary. However, if the motor is driving a loadwith a widely fluctuating torque requirement, and speed regulation is critical, then IR compensationcan be adjusted to stabilize the speed from the light load to full load condition. One caution isthat when IR compensation is adjusted too high it results in an increasing speed characteristic. This meansthat as the load is applied the motor is actually going to be forced to run faster. When thishappens it increases the voltage and current to the motor which in turn increases the motor speed further. If thisadjustment is set too high an unstable “hunting” or oscillating condition occurs that is undesirable.

The Acceleration Time adjustment performs the function that is indicated by its name. It willextend or shorten the amount of time for the motor to go from zero speed up to the set speed. It alsoregulates the time it takes to change speeds from one setting (say 50%) to another setting(perhaps 100%). So this setting has the ability to moderate the acceleration rate on the DC variable frequency drive.If an acceleration time that’s too rapid is called for, “accelerationtime” will be overridden by the current limit. Acceleration will only occur at a rate that is allowed by theamount of current the control passes through to the motor. On most small DC variable speed drives, the acceleration time is not linear. What this means is that a change of 50 RPM may occur more rapidly when the motor is at low speed than it does when the motor is approaching the set pointspeed. This is important to know but usually not critical on simple applications where these drives are used.This is an adjustment that allows loads to be slowed over an extended period of time. Forexample, if power is removed from the motor and the load stops in 3 seconds, then the deceleration timeadjustment would allow an increased time and “power down” the load over a period of 4, 5, 6 ormore seconds.

To learn more about DC variable speed drives or for DC variable speed drive repair and replacement quotes, contact Precision Electric, Inc.



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types of vfd drives

Types Of VFD Drives

There are 3 general types of VFD drives:

Each of these types of VFD drives can be divided into different variations. Every type of VFD drive system runs an electric motor and a speed control unit. VFD drive technology in today’s manufacturing processes mainly consist of solid state electronic components in a single control system. Older VFD drive systems use mechanical moving parts that in short periods of time, result in equipment failure due to worn parts from constant motion.

Eddy Current Systems – Types Of VFD Drives

types of vfd drivesThese types of VFD drives use an eddy current electric motor system. An eddy current type of VFD drive consists of a fixed speed motor and an eddy current clutch. The clutch contains a fixed speed rotor and a variable speed rotor separated by a small air gap. A direct current in a field coil produces a magnetic field that determines thetorquetransmitted from the input to the output rotor. The controller provides closed loop speed regulation by varying the clutch current, allowing the clutch to transmit enough torque to operate at the desired speed. Speed feedback is provided by an integral AC tachometer.

Eddy current VFD drives are less efficient than all other types of variable frequency drives.Nearly all eddy current motor drive systems are obsolete today. Eddy Current motor drives were designed and manufactured decades ago and there are very few manufacturing facilities across the globe that still use them in production. When an Eddy Current motor drive system fails, it’s usually too expensive to repair and impossible to replace. When an Eddy Current motor drive reaches the end of its life cycle and can no longer be repaired, manufacturers retrofit the Eddy Current Drive Systems with an AC Induction Motor and an AC VFD Drive system.

DC Systems – Types Of VFD Drives

types of vfd drivesThese types of VFD drives use DC (direct current) electric motors. DC (direct current) motors have a rotating armature winding (winding in which a voltage is induced) but non-rotating armature magnetic field, and a static field winding (winding that produces the main magnetic flux) or permanent magnet. Different connections of the field and armature winding provide different inherent speed/torque regulation characteristics. The speed of a DC motor can be controlled by changing the voltage applied to the armature or by changing the field current. The introduction of variable resistance in the armature circuit or field circuit allowed speed control. Modern DC motors are often controlled bypower electronicssystems called DC VFD drives.

There are different types of DC VFD drives, but the most common are known as DC Motor Speed Control Systems.The speed of a DC motor is directly proportional to armature voltage and inversely proportional to motor flux; either armature voltage or field current can be used to control the motor speed. DC Motors have become expensive and today most DC motor speed control systems are retrofitted with an AC induction motor and AC VFD drive. AC VFD drives are less expensive than DC Drive systems, are more available, and are more energy efficient than DC VFD Drive systems.

AC Systems – Types Of VFD Drives

types of vfd drivesThese types of VFD drives use AC (alternating current) electric motors. AC VFD drives are also known as Variable Frequency Drives, VSDs (variable speed drives), AFDs (adjustable speed drives), inverters, and micro drives. AC variable frequency drives are used in many applications such as AC Servo Systems, Air Compressors, Conveyor Systems, Lathes, Mills, Food Processing production lines, Waste Water treatment systems, Submersible Pumps, HVAC fans and blowers, and many more applications.Approximately one third of the world’s electrical energy is supplied by electric motors in fixed-speed centrifugal pump, fan, and air compressor applications. This proves thatsignificant energy efficiency improvement opportunities are available if older DC motor speed control systems and eddy current drive systems are retrofitted with AC variable frequency drive systems.Most industrial manufacturers replace their eddy current and DC variable frequency drive systems with AC variable frequency drive systems.

Technology has reduced the physical size and cost of AC variable frequency drives, and has improved performance through advances in semiconductor switching devices, simulation, control techniques, and control hardware and software.Manufacturers capitalize on AC VFD drive technology to save money on power consumption, increase quality control, decrease production downtime, and improve overall efficiency on production lines.


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