How to Match VFD To Fan Motor

Updated: Jun 3

How to Match Variable Frequency Drive (VFD) to Existing Fan Motor, and How To Match Proper Fan Motor for Variable Frequency Drive?


Whichever way you approach this, we can agree these don’t go like PB&J. There are two answers to these questions: a short, simple one and a long one. We’ll cover both in this article.


You would expect to find instructions or a guide to motor/VFD compatibility on the motor nameplate or motor specification sheet - be ready to be disappointed. Motor and VFD manufacturers are not making it simple for users - and we’ll tell you why.


To make your life easier when you are installing an Ecogate system, the Installation’s Best Practice is included in the Ecogate Power MASTER VFD User & Installation Guide. We also offer Ecogate Engineering; our technician visits your facility and as a part of the complete system engineering we will evaluate the existing fan motor and recommend a matching Power MASTER VFD. The engineering costs can be credited to an Ecogate system purchase. We also offer new system design and consultation services.



The Simple Answer


The simple answer can be used under these conditions:


  1. The cable length between the VFD output and the fan motor is “short”. For Ecogate’s Power MASTER (with ABB ACS880), the short cable is 600ft or shorter (this length is based on 25 years of Power MASTER VFD and fan motor installation experience, which is shorter compared to ABB’s general recommendation). We always recommend installing the Power MASTER VFD as close as possible to the fan motor (for technical reasons, but more importantly it is also cost-effective as the cable to the large fan motors is expensive).

  2. All wiring and grounding is done per Ecogate’s Best Practice recommendation Summary: Use the VFD rated cable or RHW, XHHW-2, or THHN cables in the metallic conduit for both the VFD input and output and use best practice low-impedance grounding; you can read more about it here and in the Ecogate Power MASTER User and Installation guide.

  3. You are using an “inverter duty” motor or a “modern” motor (less than 30 years old) with Insulation Class F or better Note: The insulation class as listed on the motor nameplate has nothing to do with voltage insulation rating for the VFDs; it is the thermal rating of the motor wiring. The insulation Class F is recommended minimum, or “thumbs up” for use with the VFD if above conditions are met.

  4. You have only one fan motor per one VFD connected (it is possible to connect more fan motors to one VFD, but additional rules apply),

  5. You are using a low switching frequency (the default Power MASTER settings are 4.5kHz, in parameter 97.01).


If the above conditions are met, matching your motor & VFD should be simple:


  1. The voltage rating of the VFD must match the motor/facility’s power supply (i.e. in the USA 230 V class or 460V class, in Canada 230V, 460V, or 575V).

  2. The nominal current rating in A of the VFD must be the same or higher than the motor’s current rating.

  3. The nominal power rating of the VFD must be the same or higher than the motor rating.

Notes:

  1. When you are using the fan motor with the VFD, ignore the motor’s Service Factor (motor service factor 1.15 means that the motor can be used for a short period of time with 15% overloading). I.e. if the motor is used with the VFD, the motor service factor is always 1.00. Why? If the motor is driven by the VFD, the VFD creates additional heating for the motor. The 1.15 service factor motor can handle either an overload of 15%, or run on an inverter, but not both.

  2. There are multiple ratings for the VFD: Nominal, Maximal, Light-Duty Overload (defined as current allowing 110% overload for 1 minute every 10 minutes at 40 °C), and Heavy-Duty Overload. Which one should you use? If you are using a standardly designed Ecogate system, use the Nominal current and Nominal power rating of the VFD.

  3. ABB publishes power ratings in kW while most US motors are rated in HP. To convert from horsepower (HP) to kilowatts (kW), multiply horsepower by 0.746.


If you feel that this is not a simple answer, read the long answer…



Long Answer


VFDs are not using sinusoidal output voltage/current, instead, they’re using full voltage outputs to simulate the effect of a sinusoidal voltage/current waveform.


The non-sinusoidal variable frequency output of the VFD has several effects, including increased motor losses, increased dielectric stresses on motor windings, electromagnetic noise, and shaft currents. If the VFD and motor are not correctly specified, these effects can combine to damage a motor’s insulation and severely shorten its operating life.


The high switching frequency of the IGBTs (semiconductor transistors used for the VFD outputs) leads to fast changes in voltage in short periods of time (how fast the voltage changes in time is measured using the dV/dt value in volts per microsecond). Steep fronted waves with large dV/dt (fast rise times) lead to voltage overshoots. Additionally, if the motor impedance is greater than the impedance of the conductor cable between the motor and the drive, the voltage waveform will reflect at the motor terminals.


This is similar to the Modbus line’s issues, where we add a terminating resistor at the end of the Modbus line to eliminate waveform reflection. This reflection creates a standing wave (similarly to how light reflects off a mirror). Longer motor cables result in a higher amplitude of standing waves.


To see the effect of high voltage at motor terminals based on the length of cable between the VFD and the motor, you can see a simulation by Andy Schroedermeier using this link.


Here is an example of the voltage at the motor terminal with a short cable between the VFD output and motor (the blue curve is line to line voltage at motor terminals versus time; the orange is voltage if you add dV/dt filter to the output of the VFD):



The peak voltage is about 750V (the voltage peak can be reduced by using dV/dt filter, see the orange curve).



If the cable is five times longer, the voltage peak will increase to over 1,100 V:



Go to Andy's simulation page; you can change the length of the cable using the slider to see the effect of the cable length on motor voltage.


In the USA, NEMA (National Electrical Manufacturers Association) publishes standards for electrical products (in the rest of the world IEC standards are used). The NEMA Motor Standard MG1 Part 31.4.4.2 (alternatively IEC 60034-25) specifies allowable rise times and peak voltages for inverter-rated motors that are directly connected to silicon IGBT-based VFDs.


NEMA MG 1-2011, Part 31, specifies that insulation systems for low voltage (≤ 600 V) VFD-duty motors should be designed to withstand an upper limit of 3.1 times the motor’s rated line-to-line voltage. This is equivalent to an upper limit of 1,426 peak Volts at the motor terminals for a motor rated at 460 V (in reality utility voltage is typically 480V, therefore 3.1 * 480V = 1,488 V). The rise times of the VFD waveform must equal or exceed 0.1 microseconds (fast-rising waveform combined with high peak voltage is dangerous to the motor wire insulation).


These are the values that the motor must withstand to work reliably with the VFD.

Unfortunately, motor manufacturers typically don’t publish compatibility information with the NEMA MG1 Part 31.4.4.2 on the motor nameplate, or in the motor specification sheet. On the other hand, most motor manufacturers only use motor winding wire that is spike-resistant enough to meet the peak voltage requirements of NEMA MG-1 Part 31.4.4.2.


Notes:

  1. Systems that are rated for 230V generally do not have a problem with reflected waves because the motor insulation systems are generally rated for 460V systems (as motors are typically designed for use at 230V or 460V). Reflected waves on 230V systems would have peak voltages no greater than 1000 volts (the requirement is 230V * 3.1 = 713V).

  2. If you can, use the VFD with the lowest carrier frequency (this will decrease the waveform’s dV/dt rise time).

  3. Avoid running multiple motors from the same VFD (it can be used if the cables to the motors are short, and each motor must have its own overload protection; the motors can not be daisy-chained, and must be connected by separate cables to the VFD output).

  4. If the cable will be long and the rise time will be short, dV/dt output filters can be installed between the drive and motor. Keep in mind that filters are expensive and they will increase total losses, therefore try to use short cables between the VFD and fan to avoid needing to use the output filters.


To make your life easier when you are installing an Ecogate system, the Installation’s Best Practice is included in the Ecogate Power MASTER VFD User & Installation Guide. We also offer Ecogate Engineering; our technician visits your facility and as a part of the complete system engineering we will evaluate the existing fan motor and recommend a matching Power MASTER VFD. The engineering costs can be credited to an Ecogate system purchase. We also offer new system design and consultation services.




Author, Ales Litomisky



1. Cable codes explanation: T = Thermoplastic, X = Cross-linked Polyethylene, HH = High Heat Resistance; RHW (“R” stands for rubber or neoprene outer insulation, the “H” stands for a heat resistance of 75 degrees Celsius or “HH” for 90 degrees Celsius, and the “W” stands for the ability of the wire to withstand water damage; THHN (thermoplastic high-heat-resistant, nylon-coated wire), XHHW (cross-linked polyethylene,High Heat-resistant, Water-resistant).

2. Why 3.1 multiplier? Because reflected waveform can double the voltage, then it is multiplied by √2 =1.42 (conversion from RMS voltage to Peak voltage), and because the effect of high voltage peaks on wire insulation is not fully understood it is multiplied by “fudge” (safety) factor 1.1 (in summary 2*√2*1.1 = 3.11).



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