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:
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).
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.
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.
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),
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:
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).
The nominal current rating in A of the VFD must be the same or higher than the motor’s current rating.
The nominal power rating of the VFD must be the same or higher than the motor rating.
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.
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.
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.