How to Match VFD To Fan Motor

How to Match a Variable Frequency Drive (VFD) to an Existing Fan Motor and How to Choose the Right Fan Motor for a Variable Frequency Drive

No matter how you look at it, these two don't go together like peanut butter and jelly. There are two answers to these questions: a short, simple one and a long one. We'll cover both in this article.

You might expect to find instructions or a guide for motor/VFD compatibility on the motor nameplate or motor specification sheet, but prepare for disappointment. Motor and VFD manufacturers haven't made it simple for users, and we'll explain why.

To make installation easier when implementing an Ecogate system, we've included the Installation's Best Practices in the Ecogate Power Master VFD User & Installation Guide. Additionally, we offer Ecogate Engineering services, where our technician visits your facility, evaluates your existing fan motor, and recommends a matching Power Master VFD as part of the complete system engineering process. The engineering costs can be credited towards an Ecogate system purchase. We also provide new system design and consultation services.

The Short Answer

The simple answer is applicable 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), a short cable is 600ft or shorter. This length is based on 25 years of Power Master VFD and fan motor installation experience and is shorter than ABB's general recommendation. We always advise installing the Power Master VFD as close as possible to the fan motor for technical reasons and cost-effectiveness since cables for large fan motors are expensive.

2. All wiring and grounding are done per Ecogate's Best Practice recommendations. In summary: Use VFD-rated cables or RHW, XHHW-2, or THHN cables in metallic conduit for both the VFD input and output. Implement low-impedance grounding best practices, which you can read more about [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 that the insulation class listed on the motor nameplate is unrelated to voltage insulation ratings for VFDs; it refers to the thermal rating of the motor wiring. Insulation Class F is the recommended minimum or a "thumbs up" for VFD use if the above conditions are met.

4. You have only one fan motor per VFD connected (connecting more fan motors to one VFD is possible, 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 and VFD should be simple:

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

2. The nominal current rating (in amperes) of the VFD must be equal to or greater than the motor's current rating.

3. The nominal power rating of the VFD must be equal to or greater than the motor rating.

Notes:

1. When using a fan motor with a VFD, disregard the motor's Service Factor (a motor service factor of 1.15 means the motor can handle a 15% overload for a short period). If the motor is driven by a VFD, the motor service factor is always 1.00 because the VFD creates additional heating for the motor. Therefore, a 1.15 service factor motor can handle either a 15% overload or running on an inverter, but not both. 2. VFDs have multiple ratings: Nominal, Maximal, Light-Duty Overload (defined as a current allowing a 110% overload for 1 minute every 10 minutes at 40 °C), and Heavy-Duty Overload. Which one should you use? If you're using a standardly designed Ecogate system, use the Nominal current and Nominal power rating of the VFD.

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

If you feel this isn't a simple answer, read on for the long answer…

The Comprehensive Answer

VFDs don't use sinusoidal output voltage/current. Instead, they use 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 significantly shorten its operating life.

The high switching frequency of the IGBTs (semiconductor transistors used for the VFD outputs) results in rapid voltage changes in short periods of time. These 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 issues with the Modbus line, where we add a terminating resistor at the end of the Modbus line to eliminate waveform reflection. This reflection creates a standing wave, much like light reflecting off a mirror. Longer motor cables result in higher amplitude standing waves.

To visualize the effect of high voltage at motor terminals based on the length of the cable between the VFD and the motor, you can view a simulation by Andy Schroedermeier [here].

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

Voltage at the Motor Terminal

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:

Voltage at the Motor Terminal (5x Times Longer Cable)

In the USA, NEMA (National Electrical Manufacturers Association) publishes standards for electrical products. 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 equates 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.

These are the values that the motor must withstand to work reliably with the VFD. Unfortunately , motor manufacturers typically don't provide compatibility information with the NEMA MG1 Part 31.4.4.2 on the motor nameplate, or in the motor specification sheet. However, most motor manufacturers do use motor winding wire that is spike-resistant enough to meet the peak voltage requirements of NEMA MG-1 Part 31.4.4.2.

Please note the following:

1. Systems rated for 230V generally don't have issues with reflected waves because the motor insulation systems are typically rated for 460V systems. Reflected waves on 230V systems would have peak voltages no greater than 1000 volts (the requirement is 230V * 3.1 = 713V).

2. If possible, use the VFD with the lowest carrier frequency, as this will decrease the waveform's dV/dt rise time.

3. Avoid running multiple motors from the same VFD, unless the cables to the motors are short, and each motor has its own overload protection. The motors cannot be daisy-chained and must be connected by separate cables to the VFD output.

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

To simplify the installation of an Ecogate system, we include the Installation's Best Practice in the Ecogate Power Master VFD User & Installation Guide. We also offer Ecogate Engineering; our technician will visit your facility and as part of the complete system engineering, 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).