Here You Go.

There is more than one way to change the speed of your electric AC motor, and some of the most commonly used methods are eddy current drives and variable frequency drives (VFDs).

The Need for Adjusting Speed

Not all electric motor applications need to run at motor's standard speed. An application may need more torque (which requires a reduction in speed) or more additional speed (which in turns means a reduction in torque). A motors' speed really should match what the application needs, and when they do not, there are significant losses in efficiency. When you consider some experts' estimate that 65% of industrial power consumption comes from electric motors, you realize how much of an impact this can have on your overall plant efficiency and energy consumption.

However, if a motor is not a perfect match to the loading needs of the application, the speed and torque can be adjusted through the use of system devices (valves, dampers, etc.). That approach, however, is not ideal and still leads to wasted power and inefficiency -- which is something you normally try to avoid. An alternate, more effective solution is the use of eddy current drives or variable frequency drives (VFDs).

Maintaining a plant can be costly as failures can and do occur. We plan for these situations in our budget meetings and look at historical spend data to set aside an appropriate likely amount to cover these costs. It’s smart planning. 

Finding ways to spend wisely without sacrificing quality or reliability can be a challenge but there are definitely ways it can be done. One thing to consider is a retrofit option when replacing or upgrading VFD’s (Variable Frequency Drives). In most cases a new drive can be installed using the existing wiring and enclosure.

So you are thinking about adding a variable frequency drive (VFD) to an electric motor in your plant. Great! Variable Frequency Drives are great controls that allow you to adjust the speed of the machine at the push of a button or program it into your automation platform.

But what could this do to your motor? There are a variety of factors to consider before adding a drive to an electric motor, such as:

Speed Range

Motors are designed to operate at a certain speed. If they are designed to operate on A VFD at certain speed ratios, those are listed (Dependent on torque type) on the nameplate. These motors have been tested by the manufacturer and are nameplated to be acceptable for use in those situations. This does not mean a motor that is not namplated for these ratios will not work, you just may want to consider if it designed to operate at a different speed or not.

Shaft currents can have a destructive effect on electric motor bearings. This article, with information from our friends at AEGIS Shaft Grounding Rings, is intended to inform professionals about these voltages and their effects on bearings in electric motors.

Because of the high-speed switching frequencies in PWM inverters, variable frequency drives induce shaft currents in AC motors. The switching frequencies of insulated-gate bipolar transistors (IGBT) used in these drives produce voltages on the motor shaft during normal operation through parasitic capacitance between the stator and rotor. These voltages, which can register 10-40 volts peak, are easily measured by touching an oscilloscope probe to the shaft while the motor is running. (Reference: NEMA MG1 Section 31.4.4.3)

Our friends at ABB put together the below video to demonstrate what a Variable Frequency Drive (VFD) or Variable Speed Drive (VSD) is and how they can help you have better control over your plant as well as increased efficiency.