Many electricians applyVariable Frequency Drives (VFDs)toAC Motors that are not inverter rated. Manyare not even aware that inverter rated motors exist.

Inverter rated motors are not required for inverter applications, however, when aVFD is applied to an inverter rated motor, the inverter rated motor has less chance of premature winding failure. Inverter rated motors have unique windings designed for voltage spikes up to approximately 1500 Volts.

Variable Frequency Drives (VFDs) create artificial sine waves which allow an AC Motor to function; normal AC Motor windings are designed for voltage up to around 600 volts. When these spikes exceed the maximum, the winding insulation starts to break down and eventually will burn up. Inverter rated motors are optimized with higher rated insulated (pulse-shielded) copper windings.

These type of windings can handle voltage spikes up to 1500 volts and higher beforethe insulation will begin to break down. Inverter rated motors also have tighter bearing air gaps than normalAC Motors. Thesetighter air gaps are designed to prevent voltage buildup in the proximity of bearing housings which can cause bearing failure.

The greater the distance the motor is from the drive the more profound the effects of inverter generated voltage spikes. When the wire from an inverter to the motor is 50 ft or longer, an Inverter rated motor should always be applied. Always chose an inverter rated motor on new applications.

Operating a Non-Inverter rated motor with an inverter will void the warranty. When the warranty claim is evaluatedby a qualifiedservice shop and/or manufacturer, they will determine the cause of failure to be Operating of a Non-Inverter rated motor with an Inverter.

Consult with a professional prior to making a decision on what type of motor to use in yourVFD application. Many customers tend to use an existingAC motor for the VFD application if the motor is already available, and replace itwith an inverter-rated motor when the original motor fails. There is some risk in this practice should the failed motor damage your new inverter.

The general lifespan of anAC Motor is dependant on several factors such as:

• Environment
• Routine Maintenance
• Using the correct type of motor andVFD for application.

Many electricians today apply Variable Frequency Drives (VFDs) for numerous reasons on AC Motor applications. One advantage of using aVFD is to extend the lifespan of anAC Motor.VFDs allow soft-start which in turn creates less wear and tear on a motor, however, if the motor being applied is non inverter rated, some of these benefits may not apply.

This does not mean you should only use aVFD on inverter rated motor only, this just means you have overall greater benefits on inverter motors than basicAC motors.

Often times those using a Variable Frequency Drive (VFD) may find themselvesneedingto connect a higher horsepowerVFD to asingle phase input power source.Since most higher horsepowerVFDsonly acceptthree phaseinput as a power source, they are left with little options or alternatives.Don’t fret, there is a solution.

If you are using a Variable Frequency Drive (VFD)rated forthree phase input and the only power source you have available to you issingle phase input, then you canderate theVariable Frequency Drive (VFD)to accept thesingle phase input power source. You can almost always use a VFD rated forthree phase input with asingle phase input power source. Of course, if it is available, try touse asingle phase input rated VFD if your power source is single phase.

When only athree phase input VFD is available,it is acceptableandcommon practicetoderate theVFDto work with asingle phase input power source.Variable Frequency Drive (VFD)availability and installation procedures may vary from one manufacturer to another.

Before youderate yourVFD, it is most important to ensure theVFD you are using is properly suited for your application. The following are some basic guidlines to help you in determining whether or not your Variable Frequency Drive (VFD) is suitable for your application:

1. Gather motor nameplate data including horsepower (HP), current (Amps), motor voltage, input line voltage andpower sourcephase.
2. Determine which type ofVFD your application will require. The type will fall under the category of eitherVolts per Hertz (V/Hz), closed-loop vector, or open-loop vector (Sensorless Vector).

The internal components of thethree phase input Variable Frequency Drive (VFD)is rated for the appropriate current expected whenthree phase input power is applied. When usingsingle phase inputinstead, the line side current from the single phaseis always higher.To “derate”is the process ofensuring that these component are rated for the higher current that will flow from thesingle phase input instead of thethree phase input.

You canderate aVFD by:

1. Determining the Horsepower of the Motor theVFD will be connected too, then
2. ChoosingVFD with a Horsepowerhigher than the Horsepower of the motor to compensate for the additionalinput currentfrom the single phase power source.

The simplest formula usedfor these types of applications is:

VFD Input Current > Motor Current Rating * 1.73

TheVFD input current must be equal to or greater thanthe Motor Current Rating * 1.73.

When installing mostthree phase input Variable Frequency Drives(VFDs)on an application wheresingle phase input power is used, you will almost always connect the input line leads to L1 and L2 of theVFD. L3will be left open with nothing connected. Consult with theVFD manufacturer or knowledgeable integrator to be sure.

Example ApplicationtoDerate aThree Phase Input Variable Frequency Drive(VFD) to work with aSingle Phase Input power source:

An application has a 230VACsingle phase input power sourceand needs to connect it to aconveyorthat has a Variable Frequency Drive(VFD) connected to a 10Horsepower 230 VAC 3 phase induction motor.Let us assumeit has been determined that this application willoperatewell with a simple Volts per Hertz (V/Hz)VFD.The issue is, since there are noVFD manufacturersthat offer a 10Horsepower (HP)single phase input Variable Frequency Drive (VFD),we will need toderate aVFD with athree phase input forsingle phase input. Most manufactuers ofVFDs only offer products up to 3 Horsepower (HP) forsingle phase input,three phase output; some products such as AC Tech SCF series do offer standardsingle phase input,three phase output products available up to 5 Horsepower (HP) range.

The10 Horsepower (HP)AC motornameplate reveals that the motor is rated for approximately 27 ampsat 230 VAC.We must use theequation above:

• VFD Input Current > Motor Current Rating * 1.73
• VFD Input Current > 27 Amps * 1.73
• VFD Input Current > 46.71

Now it has been determined this application will need a 230 VAC 3 phaseVolts per Hertz (V/Hz)Variable Frequency Drive (VFD) with an input current rated at orabove 47.0 amps.

Our company’sVFD of choice is theAC Technology/LenzeSMVector (SMV) product. Although thisVFD is open-loop vector capable andthis application only requires a standard Volts per Hertz (V/Hz)VFD, the AC Tech SMVector (SMV)VFD is agreat alternative to any manufacturer ofVolts per Hertz (V/Hz)products because the SMV is often the same price or cheaperas any other Volts per Hertz (V/Hz) product andcan operate in eitherVolts per Hertz (V/Hz) mode oropen-loopvector. TheseVFDs are also available in both Nema 1 and Nema 4x enclosures.

Looking into the product catalog, we find an SMV model fitting the requirementsavailable rated at15Horsepower (HP) that has a230 VACthree phase input rated for 48 input amps.

An ACVariable Frequency Drive(VFD) is commonly referred to as an Inverter.This is because of the way aVFDworks. The following details the inner workings of aVFD:

1. Alternating Current (AC) power is applied to the input of theVFDand feeds a bridge rectifier.
2. The rectifier converts the Alternating Current (AC) voltage into Direct Current (DC) voltage.
3. The Direct Current (DC) voltage then feeds the Direct Current (DC) buss capacitors on theVFDwhere it is stored for use by a transistor or Insulated-Gate Bipolar Transistor (IGBT).
4. Direct Current (DC) from the capacitors feed the input of the transistor(s).
5. The transistor(s) then continuously turns on and off at the appropriate frequency to build a new sine wave for use by the motor connected to the output of theVFD.

The process above is often referred to as inversion because it changes from one form to another then back again.

The voltage frequency, as distributed in the USA, is 60 cycles per second and the unit of measurement is Hertz (Hz).The output frequency and voltage of an ACVariable Frequency Drive(VFD) is variable and controlled by the speed at which the output transistor is continuously turned on and off.

The variable speedis controlled digitally in modernVFDs and changed by the operator through programming, an operator interface, or by changing an analog input to theVFDthat is programmed as speed reference input.