Sustainable shaft grounding with variable frequency drives

October 7th, 2014, Published in Articles: Vector

 

Currents induced on motor shafts by variable speed drives (VFDs) can wreak havoc with motor bearings, shortening motor life dramatically and causing costly repairs. A reliable method of shaft grounding is essential to mitigate these currents and realise the full potential of VFDs.

In the field of flow control, the potential for increased efficiency with VFDs is especially dramatic. Many centrifugal fans and pumps run continuously, but often at reduced loads. Because the energy consumption of such devices correlates to their flow rate cubed, the motors that drive them will use less power if controlled by VFDs. In fact, if a fan’s speed is reduced by half, the horsepower needed to run it drops by a factor of eight.

In this light, throttling mechanisms that restrict the work of motors seem old-fashioned and wasteful. In constant-torque applications where the main objective is more accurate process control (reciprocating compressors, conveyors, mixers, etc.), a VFD can be programmed to prevent the motor from exceeding a specific torque limit.

Built-in bearing protection

Hard to predict, VFD-induced shaft currents cause cumulative damage to a motor’s bearings, even in many motors marketed as “inverter-ready.” Because the problem is best addressed in the design stage of a system, the best solution arguably would be a motor with built-in bearing protection, available at a reasonable cost. Minimal, voluntary standards issued by the US National Electrical Manufacturers Association (NEMA) for IGBT-inverter-controlled motors rated for 600 V or less state that such motors should be designed to withstand repeated surges of 1600 V (or 3,1 times the motor’s rated voltage) and rise times of 0,1 microsecond. However, since no-one enforces these standards and testing is problematic, motor manufacturers are free to make whatever claims they like, and most models boasting extra protection from VFD currents have beefed-up insulation for the windings, but not for the bearings.

Shaft grounding is cost-effective

Fortunately, the problem can usually be mitigated by retrofitting previously installed motors. Whether a VFD-controlled motor is being used to run an air-conditioning fan in a “green” building or to run a conveyor on an energy-efficient assembly line, shaft grounding is a cost-effective way to achieve sustainability.

Causes of motor failure

These are some of the many ways inverters (VFDs) can cause motor failure. When a VFD-controlled motor fails, warranty claims against motor and VFD manufacturers may not pan out. Because systems using VFDs are so varied and the potential causes of failure so numerous, even when the VFD and motor are properly rated and perfectly matched to each other and neither is inherently defective, the liability question usually amounts to a circle of pointing fingers.

Repetitive electromagnetic interference (EMI)

Typically, the most vulnerable parts of motors controlled by VFDs are the windings and the bearings. The cause of their damage is repetitive electromagnetic interference (EMI, although it often goes by other names) arising from the non-sinusoidal current produced by a VFD’s power-switching circuitry. Through wiring, it has been called high-frequency line noise, harmonic content, eddy currents, parasitic capacitance, capacitive coupling, magnetic dissymmetry, electrostatic build-up, high-voltage ringing, reflective voltage, overshoot, steep voltage wavefronts, and common mode voltage.

Through radiated waves, it is known as radio frequency interference (RFI). These unwanted currents can cause degradation of insulation, bearings, coil varnish, etc., which is cumulative and can eventually lead to motor failure. More specifically, causes of such failure include but are not limited to high peak voltages, fast voltage rise times, the corona effect, and induced shaft currents.

High switching frequencies

High peak voltages arising from the high switching frequencies of modern VFDs are a major concern, especially if a single VFD is used to control multiple motors or if the line connecting a VFD with a motor is more than 15 m long. As a rule, the longer the cable, the lower its impedance. If the load impedance is higher than the line impedance, current is reflected back toward the VFD, creating voltage spikes at the motor terminal that can be twice as high as the DC bus voltage.

Cumulative bearing damage

Often overlooked until it is too late to save the motor is the cumulative bearing damage caused by VFD-induced shaft currents. Hard to predict but easier to prevent, these currents are best addressed in the design stages of a system. Without some form of mitigation, shaft currents discharge to ground through bearings, causing unwanted electrical discharge machining (EDM) which erodes the bearing race walls and leads to excessive bearing noise, premature bearing failure, and subsequent motor failure.

Closer look at bearing damage

Fig. 1: Voltage peaks on the shaft of a motor can damage bearings.

Fig. 1: Voltage peaks on the shaft of a motor can damage bearings.

Short of dismantling the motor, there are two main ways to check for bearing damage – measuring vibration and measuring voltage. Neither method is foolproof. By the time vibration tests confirm bearing damage, it is usually far advanced. Likewise, the main benefit of voltage tests may be the relief they provide if the results indicate no shaft voltage. If a baseline voltage measurement is taken right after a VFD has been installed, subsequent monitoring may provide early warning of harmful current loops.

Shaft currents can be measured by touching an oscilloscope probe to the shaft while the motor is running (see Fig. 1). These voltages repeatedly build up on the rotor to a certain threshold, then discharge in short bursts along the path of least resistance, which all too often runs through the motor’s bearings.

Extremely fast voltage rise times (dV/dT)

Serious, cumulative electrical bearing damage can be attributed to the extremely fast voltage rise times (dV/dt) associated with the insulated gate bipolar transistors (IGBTs) found in today’s typical pulse-width-modulated VFD. The discharge rate tends to increase with the carrier frequency. Discharges through bearings can be so frequent that, before long, the entire bearing race wall becomes riddled with fusion craters known as frosting. Since many of today’s motors have sealed bearings to keep out dirt and other contaminants, electrical damage has become the most common cause of bearing failure in VFD-controlled AC motors.

Fig. 2: Pitting of a bearing race wall at regular intervals leads to fluting.

Fig. 2: Pitting of a bearing race wall at regular intervals leads to fluting.

In the phenomenon known as fluting (see Fig. 2), the operational frequency of the VFD causes concentrated pitting at regular intervals along the race wall, forming washboard-like ridges. Fluting can cause excessive noise and vibration which forewarn of imminent bearing failure.

Searching for a solution

Motor failures caused by VFD-induced shaft currents can result in significant unplanned downtime. In addition, these failures affect the performance and mean time between failure (MTBF) of the original equipment systems in which the motors are used. In some production applications, even a momentary stoppage due to motor failure can cost more than $250 000, excluding the cost of repairing/replacing the motor.

Clearly, there is a need for a device which mitigates bearing damage from VFD-induced shaft currents.

Recommends bearing insulation

Section IV, Part 31.4.4.3 of NEMA Standard MG1 (motors and generators) recommends bearing insulation at one end of the motor if the NEMA-motor-frame size is 500 or larger and the peak shaft voltage is greater than 300 mV. In these larger motors, bearing damage may be due in part to magnetic dissymmetries that result in circulating end-to-end shaft currents.

Installing shaft grounding brushes

For smaller motors, the same standard recommends insulating both bearings with high-impedance insulation or installing shaft grounding brushes to divert damaging currents around the bearings. For these motors, a VFD can generate high-frequency common mode voltage, which shifts the 3-phase-winding neutral potentials significantly from ground. Because the damaging voltage oscillates at high frequency and is capacitively coupled to the rotor, the current path to ground can run through either one bearing or both.

Another path to ground

The NEMA standard is quick to point out, however, that bearing insulation will not prevent damage to other connected equipment. When the path to the bearings is blocked, the damaging current seeks another path to ground. That other path can go through a pump, gearbox, tachometer, encoder or break motor, which can consequently wind up with bearing damage of its own.

Bearing protection ring

Fig. 3: Bearing protection ring.

Fig. 3: Bearing protection ring.

The ideal solution would be a low-cost, maintenance-free device which redirects shaft currents safely along a very low-impedance path from shaft to ground, protecting connected equipment as well as bearings. This device could be installed by the motor manufacturer or retrofitted in the field in virtually any VFD application. One product which meets all these criteria is the bearing protection ring (see Fig. 3), a relatively recent invention which overcomes the dirt-collection, corrosion, and wear problems of conventional grounding brushes. A few manufacturers have introduced motors with bearing protection rings already installed but, at the time of writing, such motors are the exception, not the rule.

Conclusion

Regardless of the application, the success of an automated control system depends upon its design. If in-house engineers lack the special expertise required, they should enlist the services of a qualified systems integrator who understands the engineering specifications, operating conditions, and performance curves for the whole system.

Potential problems can be anticipated and resolved with informed decisions at every stage, from specification to commissioning of the system.

Especially important is the selection of VFDs and motors for such systems. They should be rated for compatibility, not only with each other but also with other components of the system. A savvy specifier will choose a motor which is truly equipped for use with today’s fast-switching VFDs – one with adequate protection against bearing damage as well as winding damage.

The importance of grounding to protect motor bearings has been underestimated for too long. An economical, long-term method of shaft grounding is a must to minimise harmful currents and realise the full “green” potential of VFDs. Until all OEM motors marketed for use with VFDs are truly “inverter-ready,” retrofitting them with shaft grounding is the best approach.

Acknowledgement

This article was originally published in Industrial Electrix, January – March 2014, and is reproduced here with permission.

Contact Electro Static Technology, info@est-aegis.com

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