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In recent years it has been widely documented the advantages in using a variable frequency drives on pumps, fans, compressors and other equipment. The advantages gained in both productivity and reducing energy consumption plus the life of equipment is significant. The AC Variable Frequency Drive market is huge, due to the great advances in highly reliable AC power electronics and high power microprocessors.

The advantages to Variable Speed Drives are:

• Energy is saved by replacing mechanical fluid flow controllers with integrated motor speed control. This translate into cost saving and a reduction in greenhouse gas emissions.
• The soft start of variable speed drives eliminates voltage dips and reduces starting shock on motor, coupling, gears and driven equipment reduces maintenance on equipment.
• Running at reduced speed reduces the wear on all drive train components and also reduces the need for high maintenances items such as dampers, throttling valves, etc.
• With the range of low control is generally higher in variable speed drives the likelihood of surges and vibration is reduced.
• When operating speeds in excess of 3000 rpm for 2-pole AC motor can be done without the use of gearing.

Basic Variable Frequency Drive theory:

Using a variable frequency drive for a specific application is no mystery when you understand the requirements of the load. The variable frequency drive must have ample current capability for the motor so the motor can produce the required torque for the load. A point to remember that machine torque is independent of motor speed and that load horsepower increases linearly with rpm.

Variable frequency drives  divided into the following individual load type:

• Constant Torque Loads: These loads represent 90% of all general industrial machines (other than pumps and fans).  Some of these load types are general machinery, hoists, conveyors, printing presses, positive displacement pumps, some mixers and extruders, reciprocating compressors, and rotary compressors.
• Constant Horsepower Loads: These loads are typically found in machine-tool industry and center driven winder applications. Some of these constant horsepower loads are winders, core-driven reels, wheel grinders, large driller machines, lathes, planers, boring machines, and core extruders.

In the past these loads were considered DC drive application only. Now with high performance flux vector variable frequency drives many of these applications can be handled by AC variable frequency drives.

Variable Torque loads: These loads are most found in variable flow applications, such as pumps and fans. Some example of applications include fans, centrifugal lowers, centrifugal pumps, propeller pumps, turbine pumps, agitators, and axial compressors. The variable frequency drive offers the greatest saving in these loads because horsepower varies as the cube of speed and torque varies as square of speed for these loads. A example of this is if a motor speed is reduced 20%, motor horsepower is reduced by a cubic relationship (.8 x .8 x .8) or by 51%.  Utilities often times offer rebates for customers investing in variable frequency drive.

Sizing Variable Frequency Drive for the load

The first think you must understand the requirements of the load. It helps also if you understand the difference between horsepower and torque. Most electrical people tend to think of loads in horsepower rating instead of torque ratings.

 Let's us first discuss what torque is. Torque is an applied force that tends to produce rotation and is measured in lb-ft or lb-in. All loads have a torque requirement that must be met by the motor. The main purpose of the motor is to develop enough torque to meet the requirements of it load. Torque can be thought of as the "ooumph". The motor has to have enough "ooumph" to get the load moving and keep it moving under all the conditions that may apply.

Horsepower is the time rate at which work is being done. One hp is the force required to lift 33,000 lbs 1ft in 1 min. If you want to get the work done in less time, just get more horsepower.

Know your load, for example, a 1hp motor operating at 1800 rpm will develop 2.92 lb-ft of torque. Every load has distinct torque requirements that vary with the load's operation. These torques must be supplied by the motor via the variable frequency drive. You should have a clear understanding of these torques.

• Break-away torque : Torque required to start a motor.
• Accelerating torque: Torque to bring load to operating speed within a given time.
• Running torque: torque required to keep the load moving at all speeds.
• Peak torque: occasional peak torque required by the load, such as a load being dropped on a conveyor.
• Holding torque: torque required by the motor when operating as a brake, such as down hill loads and high inertia machines.

Guidelines to help ensure a correct match of Variable Frequency Drive to Motor:

1. Identify the operating profile of the load to which the variable frequency drive is to be applied. Include any or all of the "torques" discussed above. Using a recording true rms ammeter to record the motor's current draw under all operating conditions.  Find the highest "peak" current readings under the worst conditions. Also, see if the motor has been working in an overloaded condition by checking the motor full-load amps (FLA). An overloaded motor operating at reduced speeds may not survive the increased temperatures as a result of the reduced cooling effects of the motor at these lower speeds.

2. Determine why the load operation needs to be changed. Very often variable frequency drives have been applied to applications where all that was required was a "soft start" reduced voltage controller. The need for the variable frequency drives should be based on the ability to change the load's speed as required. In those applications where only one speed change is required, a variable frequency drive may not be necessary or practical.

3. Size the variable frequency drive to the motor based on the maximum current requirements under peak torque demands. Do not size the variable frequency drive based on horsepower ratings. Many applications have failed because of this. Remember, the maximum demands placed on the motor by the load must also be met by the variable frequency drive.

4. Evaluate the possibility of required over sizing of the variable speed drive. Be aware that motor performance (break-away torque, for example) is based upon the capability of the variable frequency drive used and the amount of current it can produce. Depending on the type of load and duty cycle expected, over sizing of the variable frequency drives may be required.

Key Variable Frequency Drives specifications:

There are many specifications but the following are the most important.

 Continuous run current rating: This is the maximum rms current the variable frequency drive can safely handle under all operating conditions at a fixed ambient temperature (usually 40 degrees C). Motor ball load sine wave currents must be equal to or less than this rating.

 Overload current rating: This is an inverse time/current rating that is the maximum current the variable frequency drive can produce for a given time frame. Typical ratings are 110% to 150% overcurrent for 1 min, depending on the manufacturer. Higher current ratings can be obtained by oversizing the variable frequency drive. This rating is very important when sizing the variable frequency drive for the currents needed by the motor for break-away torque.

 Line voltage: As with any motor controller, an operating voltage must be specified. Variable frequency drives are designed to operate at some nominal voltage such as 240VAC or 480VAC, with an allowable voltage variation of plus or minus 10%. Most motor starters will operate beyond this 10% variation, but variable frequency drives will not and will go into a protective trip. A recorded voltage reading of line power deviations is highly recommended for each application.

Applications to watch out for:

1. Will the variable frequency drive operate more than one motor? The variable frequency drive must be sized based on the total peak currents of all motor loads under worst operating conditions.. The Variable Frequency Drive must be sized based on this maximum current requirement. Additionally, individual motor protection must be provided here for each motor.
2. Will the load be spinning or coasting when the variable frequency drive is started? This is very often the case with fan applications. If the load is already in motion, it will be out of sync with the variable frequency drive. With a variable frequency drive, when it is first started, it begins at a low frequency and voltage and gradually ramps up to a preset speed. The variable frequency drive will attempt to pull the motor down to the lower frequency, (it may require high current levels) usually causing an overcurrent trip. Variable frequency manufacturers offer drives with an option for synchronization with a spinning load. This variable frequency drive ramps at a different frequency.
3. Will the power supply source be switched while the variable frequency drive is running? This occurs in many buildings, such as hospitals, where loads are switched to standby generators in the event of a power outage. Some drives will ride through a brief power outage while others may not. If your application is of this type, it must be reviewed with the drive manufacturer for a final determination of drive capability.
4. Is the load hard to start? These are the motors that dim the lights in the building when you hit the start button. Remember, the variable frequency drive is limited in the amount of overcurrent it can produce for a given period of time. These applications may require oversizing of the variable frequency drive for higher current demands.
5. Are starting or stopping times critical? Some applications may require quick starting or emergency stopping of the load. In either case, high currents will be required of the drive. Again, oversizing of the variable frequency drive maybe required.
6. Are external motor disconnects required between the motor and the variable frequency drives? Service disconnects at motor loads are very often used for maintenance purposes. Normally, removing a load from a variable frequency drive while operating does not pose a problem for the variable frequency drive. Introducing a load to a variable frequency drive by closing a motor disconnect while the variable frequency drive is operational can be fatal to the variable frequency drive. When a motor is started at full voltage, as would happen in this case, high currents are generated, usually about six times the full load amps of the motor current. The variable frequency drives would see these high currents as being well beyond its capabilities and would go into a protective trip or fail altogether. A simple solution for this condition is to interlock the variable frequency drive run permissive circuit with the service disconnects via an auxiliary contact at the service disconnect. When the disconnect is closed, a permissive run signal restarts the variable frequency drive at low voltage and frequency.
7. Are there power factor correction capacitors being switched or existing on the intended motor loads? Switching of power factor capacitors usually generates power disturbances in the distribution system. Many variable frequency drives can and will be affected by this. Isolation transformers or line reactors may be required for these applications

Power factor correction at variable frequency drive-powered motor loads is not necessary as the variable frequency drive itself does this by using DC internally and then inverting it into an AC output to the motor. All variable frequency drive manufacturers warn against installing capacitors at the variable frequency drive output.

Our staff,  of specialist, at  Variable Frequency Drive Co. are always ready to answer any questions on any of your applications.

Contact our experienced variable frequency drive specialists at Variable Frequency Drive Co at 510-490-2187 to get your quote, place your order and arrange for shipping.  Our electrical specialists are standing by and ready to help

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