Variable Frequency Drive Sizing

Have anyone done much vfd sizing for motors? I know you have to take torque and and horsepower into consideration alot. It appears most of the vfds I have seen tend to have larger conductors feeding it than one would calculate for a motor load without a vfd. Maybe its because the vfd is more oversized, or they used 150% NEC 2008 430.6(C). from the motor tables unlike only 125% sizing for conductors to the motor. If you want to buy VFD, you can go to our offical website: Fasttobuy.com. We provide We carry various VFD manufacturers from Mitsubishi to Delta VFD to provide you with a brand you know and trust. We supply both single-phase and three-phase drives in various voltages to help meet the specifications of your current system.

Any one have an instance where they had to size the vfd? Was the feed to the vfd allowed to be smaller than it would be for the motors itself? How did you size it? 

When a single VFD is utilized to manage numerous motors, VFD sizing and choice turn out to be much more complicated unless all the motors are began simultaneously. With numerous motors connected to 1 VFD, adding the horsepower of every to acquire a total load and choosing the VFD accordingly might not be adequate based on operating circumstances.

1 of much more motors cannot be began up whilst 1 or much more motors are currently operating unless the chosen drive is sufficiently oversized. To illustrate this point, think about the following instance with 3 460 VAC motors connected to 1 VFD.

Two from the motors are rated at five HP having a complete load amp (FLA) rating of six.two amps. The third motor is rated at ten HP with an FLA of 14 amps. If all motors are accelerated, decelerated and run in unison-the sum from the connected motor FLA enables use of a 20 HP drive. But if it had been essential to accelerate and run the five HP motors and after that begin the ten HP, the sum from the FLA would need to be recalculated.

The FLA for every from the five HP motors could be utilized within the calculations, however the locked rotor amps (LRA) for the ten HP would need to be taken into account. The LRA will be the quantity of present drawn by a motor at startup.

Simply because the ten HP motor wouldn’t be accelerated from zero frequency and voltage to its operating situation, it would appear in the drive as a fixed voltage/frequency line starter and would need its complete LRA rating to rapidly accelerate towards the drive’s output frequency.

Therefore, the amp draw around the drive when the ten HP drive is coming on line could be the FLA of every from the five HP motors, plus the LRA of ten HP motor that is 86.five amps. The total amp figure to become utilized for VFD sizing is therefore six.two FLA + six.two FLA + 86.five LRA = 98.9 Amps. This amp load would need upsizing to a 75HP VFD having a minimum continuous output rating of 99 Amps, a 50% improve in VFD size.

Final Recommendation

With modern power electronics and advanced microprocessor technology, Delta's AC Motor Drives are able to efficiently control motor speed, improve machine automation and save energy. Taking advantage of our strong position in power electronics technology, Delta's VFD Series of AC motor Drives has evolved rapidly. Each Drive series is designed to meet specific application needs. Our AC Drives accurately control speed and torque, smoothly handle an increased load, and provide numerous custom control and configuration operating modes. Our AC Motor Drive product line provides a full range of motor control technologies and is used throughout a wide range of industries, to enhance and improve machine automation. 

This guide is intended to become of use for common application sizing and isn't intended to become a complete guide. You will find applications and loads that might need unique sizing and consideration. When you're sizing or specifying a VFD for any application it pays to become conservative and leave some buffer space within the FLA and overload ratings. This really is particularly accurate in case your load is difficult to begin or sees heavy loading throughout operation. When you have any concerns about your application or in sizing a brand new VFD contact and speak to 1 of our application specialists before buying.