Modern high frequency switching power devices, such as VFD’s employing Bipolar Junction Transistors (BJT) and even faster switching Insulated Gate Bipolar Transistors (IGBT), have produced unintended consequences, generally described as Electromagnetic Interference (EMI).

Fluting is characterized by the appearance of either pits or transverse grooves in the bearing race, which with mechanical wear results in bearing failure. Fluting is caused by Electrical Discharge Machining (EDM). The source of EDM is high frequency switching found in VFD’s.

The measured Rotor Shaft to Ground Voltage is an indicator of the potential for Bearing Current. Mechanical damage from fluting depends on the passage of electrical current thru the bearing. When voltage exceeds a critical value, the electric field intensity of the lubricant (grease) is surpassed and an EDM occurs, potentially raising the bearing current density to excessively high values; thus, the bearing’s mechanical life is reduced and fail in a short time.

Current density is defined as the EDM current magnitude divided by the bearing contact area.

All rotating machines develop bearing currents, whether dc or ac, large or small horsepower. Most induction motors are designed to have a maximum Vrg < 1 Vrms (less than 1 volt) with 60Hz ac line operation. Three mechanisms can cause the development of excessive Vrg – electromagnetic induction, electrostatic coupling from internal sources, and external sources.

Voltage source inverters serve as a source of Vrg through the capacitive coupling of the common mode voltage from stator to rotor. When the motor is running the bearing “floats”. With the bearing riding in the lubricant and forming a capacitor, the common mode source (VFD) charges the shaft/rotor to a voltage in excess of the lubricant’s electric field intensity (insulating capabilities), approximately 15 volts.

The Vrg must exceed a threshold of 3 to 30 volts before the potential of EDM (discharge) exists. If an EDM occurs, the current density suddenly increases, pitting the bearing and increasing mechanical wear.

Theory of EMI 

Due to high speed switching of modern power electronics, an ac Pulse Width Modulated (PWM) drive’s electrical signature is a source of EMI emissions. The drive enclosure, typically metal, is utilized to attenuate radiated noise, while conducted noise is transmitted via power lines connecting the drive to the ac source and load.

The two forms of conducted noise are differential (line to line) and common mode (zero sequence). The common mode voltage – the dominant excitation source – excites stray or parasitic coupling capacitances and contributes to Vrg and consequential bearing current discharge.

Proposed Solutions to Rotor Shaft Voltage Buildup

With the recognition that PWM inverter drives can reduce bearing life, numerous solutions have been proposed.

Conductive grease

One solution incorporates a conductive agent with the lubricant. A conductive grease is formed by suspending metallic particles in the grease. Laboratory test data on a 4 ball wear tester indicates the wear scar – mechanical surface damage – increases by approximately 60 % when the conductive agent is added. This indicates the conductive agent in the grease accelerates mechanical wear and would shorten the life of a bearing.

Shaft grounding kit

Another solution attaches a mechanical apparatus to the rotor shaft and “bleeds” the voltage with a brush. It is currently used in numerous applications. This approach requires a low resistance contact between the brush and rotor, a condition that field experience indicates is difficult because of brush wear and the build up of an oxide layer.

Ceramic type bearings

Another approach is a insulating layer is applied to the rotor shaft or ceramic bearing, forming an additional capacitance. This additional capacitance will redistribute Vrg between the two series capacitors. Thus reducing the voltage across the bearing relative to the standard machine construction. Thermal questions arise because the rotor heat normally transmitted by the bearing now must traverse the insulating layer before reaching the motor frame.

L-C filter

Other passive techniques use a potential transformer or coupling L-C filter. In both of these approaches common mode signals are formed and are either coupled to the line voltages with opposite phase, reducing the common mode voltage, or return common mode current to the inverter through an additional bridge rectifier.

ESIM motor

Finally, the Electrostatic Shielded Induction Motor (ESIM), has an internally mounted Faraday shield to diminish the coupling between the stator and rotor. With a shield, the electromagnetic torque is unaffected and the machine’s torque capability is not impaired. The shield’s effect on electrostatic coupling, however, is dramatic and nearly complete. The shield functions in a manner similar to shields used to reduce conducted EMI : A shield is typically inserted between a radiating source (Vsource) and the area to be protected (Vrg) along the conductive medium (Csr). The efficacy of the shield is determined by its attenuation of Vsource. The expectant attenuation ratio is the proportion of the area shielded to the totalarea between the source and receiver.

Inserting a Faraday shield into the air gap of a motor must be accomplished without short circuiting the stator laminations or bridging the air gap; shorting the stator laminations would induce circulating eddy currents and cause localized heating of the stator stack, bridging the air gap impedes the mechanical rotation and magnetic induction or torque producing component. The Faraday shield is applied onto an insulating layer in the air gap of the machine, and connected to ground.

Variable speed, constant torque, fixed speed and reduced current inrush are some terms with which you may or may not be familiar. These fall into the general term “motor control.” An explanation of these terms, and a step-by-step motor guide is explained to help you to determine your needs.

1. Fixed Speed Control – Starter.

If an application requires the motor to operate at a constant speed with little or no stopping and starting at full voltage, you will most likely require an electromechanical motor starter. Starters offer full-torque at start-up, but no speed control. With each new generation of starter the frame size becomes smaller but the work load remains the same. This allows you to save panel space, thus reducing the size of the enclosure and saving money.

2. Reduced Current In-Rush – Soft-Start/Stop.

If your application requires intermittent starting and stopping of the motor, you may need a solid-state reduced current inrush motor starter. These starters will reduce maintenance concerns related to the life of the contacts on an electromechanical motor starter. A solid-state starter has lower long term cost.

A reduced current motor starter functions by decreasing the starting current to the motor during start-up, allowing the motor to ramp up to full speed and off again. This puts less demand on the power distribution system. The drawback to these devices is when you reduce current, you reduce torque as well.

3. Variable Frequency Drives.

When the application requires variable speed throughout the process, the solution is a variable-speed drive (VFD). The primary function of any VFD is to control the speed, torque, acceleration, and direction of the motor that is being controlled. Unlike constant speed systems, VFDs offer an infinite number of speeds within the motor’s range.

VFDs can offer energy savings — running motors at variable speeds can optimize a process in order to save energy, and rebates from utilities may be available.

A VFD may also increase productivity and boost production yields by helping to automate production processes. Long term savings will be realized by reducing maintenance and equipment replacement costs. To best determine if a VFD fits the application, ask yourself: Do I need a wide range of speed? Will I need precise control of the motor speed and/or torque? If the answer is yes to one or both of these, then consider a VFD.

AC vs. DC Variable Speed Drives

Historically, applications that required precise motor control over a wide range of speeds have used DC variable speed drives. DC drives offer full torque at zero speed — this is due to independent controls over speed and torque. However, DC motors are more expensive and require a much higher degree of maintenance than AC motors. The newest technology is an AC drive that offers torque control, an important feature when energy efficiency and motor reliability is demanded. An AC drive initially may be more expensive than an equivalent DC unit, however, an AC motor will provide a faster return on investment.

AC Drive Advantages

Uses conventional, low cost, 3-phase AC induction motors for most applications

Preferred for tight motor mount applications

Smaller, lighter and more commonly available than DC motors

Suited for speeds over 2,500 r.p.m. since there are no brushes

Used when multiple motors in a system must operate simultaneously at a common frequency and/or speed

DC Drive Advantages

Less complex with single power conversion from AC to DC

Normally less expensive for most horsepower ratings

Long tradition of use as adjustable speed machines

Cooling blowers and inlet air flanges provide cooling air for a wide speed range at constant torque

Capable of providing starting and accelerating torques in excess of 400% of rated

Less noise. Some AC drives produce audible motor noise which is undesirable in some applications.

Constant Torque vs. Variable Torque

When torque requirements are independent of speed, your solution is a constant torque drive. A variable torque drive is best suited for applications such as fans, pumps and blowers. Under these applications, you will need full torque at top speed and diminishing torque as speed decreases.

Another question I get is:

Should I order a VFD equipped with a bypass?

I answer people by asking them a question, can you use a bypass with your system? Most people don’t think about this, but ask yourself before purchasing a bypass option. Simply put… if you need a backup system like a bypass … make sure it will work with your system. Example: if your application is a fan, can your system run at full speed or will the fan simply trip out the duct high static pressure sensor or worse cause damage to the ductwork.

One idea I recommend is purchase standard VFD’s and a spare VFD sized for your largest motor just in case. This way, if your VFD stops working, simply remove it and put in your spare.

If you can use a across the line bypass you have a few options, but both will get the job done with a flick of the switch.

Three Contactor Bypass Configuration

A three contactor bypass system offers the ability to isolate the inverter in order to service the unit while in the bypass mode. The service contactor can be closed to allow repairs and testing prior to switching out of the bypass mode. The bypass contactor and isolation contactor are mechanically & electrically interlocked so the inverter will not be damaged or back feed from the AC line voltage.

Two Contactor Bypass Configuration

The two contactor system allows the motor to be by-passes for emergency use. However, the drive cannot be isolated for service purposes. This configuration is used where system cost is a consideration. The down side is the system will need to be powered off while servicing the VFD.

Three phase line reactors offer an economical solution to a variety of application problems in variable-speed drive installations. Reactors solve problems on either the input or the output of the drive if the reactor is compensated to handle the effects of harmonics. To see where these reactors fit in today’s technology, we have to go back 25 years to the introduction of low-voltage industrial drives. Often, the installation required a voltage step-up or step-down and line isolation was almost universally recommended. The isolation transformer provided both line isolation and voltage transformation. It became standard practice to include a drive isolation transformer with nearly every drive installation.

The reactor acts as a current-limiting device and filters the waveform and attenuates electrical noise and harmonics associated with the inverter/drive output.

As the industry progressed, drive voltage ratings increased. Some even developed with dual voltage ratings. Multiple power systems appeared in industrial plants and dual voltage motors became more popular. With these and other improvements, it became less necessary to alter the line voltage supplying the drive. Then, the drive industry developed internal isolation and ground fault protection systems, thus the need for external isolation all but disappeared. The result was a significant cost reduction for a drive system.

Line reactors protect VFD’s, extending motor life, reducing power line distortion, attenuating harmonics and eliminating nuisance tripping.

Soon, users of these new, economical and efficient drives began experiencing nuisance problems not previously encountered with the older, isolation transformer protected systems. With the isolation transformer gone, the quality of the power delivered to the drive became more evident. The drives were very sensitive to line fluctuations and other nuisance problems not noticed before. A solution had to be found because the isolation transformer was too expensive to be put back into the circuit.

The line reactor was developed as a low-cost solution to the problem. The reactor acts as a current-limiting device and filters the waveform and attenuates electrical noise and harmonics associated with the inverter/drive output. In this respect, the line reactor even surpasses the isolation transformer at a fraction of the transformer’s cost. Line reactor costs are typically just 1/5 the cost of a comparable isolation transformer.

Among the harmonic compensated line reactors benefits are:

Virtual elimination of nuisance tripping of drives due to utility power factor correction capacitor switching

Attenuation of line harmonics

Extended switching component life (transistors, SCRS)

Extended motor life Reduced motor operating temperature (20 to 40 degrees C)

Reduced audible motor noise (3 to 5 db)

Minimized power disturbances

Filtered electrical noise (pulsed distortion and line notching)

Waveform improvement

Harmonic Compensation

As the name implies, line reactors are typically used on the line side of  a Variable Frequency Drive.

Harmonic compensated line reactors are specially designed to handle the waveform’s harmonic content. This compensates for the effect of higher total rms current as well as higher frequencies present in the waveform and may be used effectively on either the line or load side of any VFD.

Reactors are used on the load side of an VFD as a current-limiting device to provide protection for the drive under motor short circuit conditions. Here, the line reactor slows the rate of rise of the short circuit current and limits the current to a safe value. By slowing the rate of current rise the reactor allows ample time for the drive’s own protective circuits to react to the short circuit and trip out safely. Also, the reactor absorbs surges created by the motor load that might otherwise cause nuisance tripping of the drive. Machine jams, load swings and other application changes to the drive load cause motor load surges.

Looking at the load side reactor from the motor view, the ability of the reactor to filter the waveform produced by the VFD improves motor performance and the total system performance. Due to higher frequency pulses generated by the drive to produce the waveform, motors typically run hotter than normal, resulting in lower efficiency and shorter life. Unprotected motors must often be oversized to compensate for the higher frequencies and harmonic currents that are present in the drive output waveform. Waveform filtering by the reactor reduces the load side harmonic content, reduces thermal current affecting the motor and filters pulsed distortion. The reactor attempts to recreate a perfect sine wave, thus improving motor efficiency. This extends motor bearing life, increases horsepower output by 25-30%, and can reduce audible noise by as much as 3-5 decibels. Tests have shown that motor temperatures can be reduced as much as 20 to 40º C using a harmonic-compensated reactor.

On the line side of the VFD system, reactors also serve bidirectional functions. When the local utility switches power factor correction capacitors onto the electrical power grid, it creates voltage spikes. The proper impedance reactor in the input circuit virtually eliminates nuisance tripping of drives due to these voltage spikes. Also, the reactor can protect from line sags because it performs a line stabilizing function. Initially, this may seem unusual because the reactor adds impedance to the circuit, which causes a voltage drop. An important, overlooked factor is that the reactor has significant inductance so it opposes any rapid change in current. Most voltage sags are the result of excessive loading or current surges. Thus, by stabilizing the current waveform, the reactor can indirectly solve both overvoltage and undervoltage tripping problems.

Reactor Impedance

Line reactors are rated in percent impedance to retain some conformity with the ratings of conventional drive isolation transformers. We can determine the impedance rating of a conventional isolation transformer with the following procedure:

Short circuit the secondary winding.

Increase the primary voltage while monitoring secondary current.

Measure the primary voltage that causes rated secondary current to flow.

Compare this value with the rated primary voltage to obtain a ratio equal to the transformer impedance rating.

Reactor impedance must be measured differently because the reactor is a series, current-dependent device as opposed to the transformer that is a parallel, voltage-dependent device. To determine percent impedance of a single-phase reactor, measure its voltage drop with rated current flowing through it. Compare this voltage with the line voltage for percent impedance. You can connect two phases in series with single-phase voltage applied. Measure the total voltage drop across both coils and compare it with the system voltage for the impedance rating. For example, if the voltage drop across the reactor is 12V for a 480V line, the percent impedance is 12/480 X 100, or 2.5%.

Test a three-phase reactor with all three phases energized at rated current. With all phases energized, measure the voltage across any one phase and divide it by the system voltage. Multiply this value by 1.73 (square root of 3) and again by 100 for percent impedance. As an example, if the reactor drop is 8.3V with a 480V line, the percent impedance is 8.3/480 X 1.73 X 100, or 2.99%. If you energize only one phase of a three-phase reactor and compare the voltage drop with the system voltage for the impedance calculation, the calculated value indicates only 70-75% of actual value.

Line reactors are rated in percent impedance to retain some conformity with the ratings of conventional drive isolation transformers.

It is difficult for the user to test a reactor for conformance to a specification when reactors are rated only in percent impedance. For a more accurate test verification, it is helpful to find the reactor’s actual inductance. This can be done in a manner similar to the impedance calculation as follows:

First, energize all three phases of the reactor at rated current. The measured voltage equals the current times the inductive reactance (XL, which is 2(pi) times the frequency times the inductance). For a 60Hz system, the inductance equals the voltage divided by the current times 6.28 (2(pi)) times 60.

Using a meter or bridge system to test line reactors usually produces false readings for two reasons. First, this is only for single-phase testing so it indicates a value that is 25-30% less than actual. Second, meter or bridge tests are at such a low current level that the reactor’s core and gap remain unenergized.

Reactors are installed in HVAC equipment, machine tools, elevators, printing presses, UPS equipment, computer mainframes, harmonic filters, robotics equipment, ski lifts, wind generators, electric cars, trams, and many other types of equipment using drives or inverters.