Variable Frequency Drive for Asynchronous Motors

Losses: Xc, Xl, hysteresis, core heating, mechanical resistance, and other dynamics introduced by the application.

A VFD allows regulation of speed and torque in a properly designed motor so that maximum efficiency of the motor can be attained throughout the motor RPM range used for the application.

GA

I wish more people understood that.

Less work = less power. This is the big efficiency saving.

The little bit of reduction that may be possible in magnetization energy can be lost in the drive itself - so no real net gain.

There are fixed (no load) losses that occur because you are establishing the magnetic field around the stator and rotor, and variable (load) losses proportional to the mechanical work being performed by the motor caused by the additional current passing through the stator windings and the rotor bars.

It is not always necessary to use a VFD. Before you run out and buy one, you should examine everything about the motor, what it is driving, and how it interacts with the overall process that it is placed in. If your motor only draws 55% of its full load current, then it is only loaded to (approximately) 50% of its rated output.

From a simplistic viewpoint, it is twice as large as it needs to be; from a detailed viewpoint, you may find that it is oversized because it needs to start a high inertia load in a short period of time, maybe repeatedly, and the starting torque requirements of the load exceed that of a motor sized for the running torque.

If it is driving a variable load, then you need to examine how the speed to the shaft of the load is varied. If the load speed is a function of the process, then you need to examine how the speed change occurs. If it is through a variable speed mechanism, then the motor always runs at full speed and incurs additional mechanical losses in speed change.

The point is you must understand evrything about how, why, and when your motor/process consumes energy, long before you run off and buy the "latest and greatest" energy saving device.

It occurs to me that the application may require varying torque values during normal operation such as initial starting or restarting under loaded conditions which most likely is the reason for an apparently oversized motor having been installed.

If you will identify how much and when maximum torque is needed throughout the cycle of operation it may be possible to purchase a motor with a specific torque curve which would allow a much smaller HP to be used thereby realizing some savings.

If you are able to do this the cost will be far less than purchasing a VFD, and a VFD rated motor, then suffering the cost of installation.

Depending on your location most motor OEM's provide on-site technical support to help you determine the best direction to go.

I would suggest you make the call to a reputable motor OEM and find out what the best options are before spending money on a "fix" that will not work.

With adequate filters it works OK on standard motors. But the cost may exceed a new motor, especially in small frame sizes.

These days most industrial motors in our part of the world have a vfd rating. Not cost effective to run two product lines.