Since the introduction of variable speed electronic controls for DC and AC motors, users have noted a troubling problem; voltage potential is sometimes induced on the shaft of the motor.  The resulting current can discharge through gearbox bearings, through the motor bearings, or through the encoder or tachometer attached to the motor.  This discharge causes the premature failure of motor bearings and/or encoder bearings. This paper discusses the phenomenon of encoder and motor damage due to motor shaft currents through encoders, and proposes several remedies and guidelines .

Problem

Both AC and DC motors controlled by electronic speed controls can experience “shaft currents”, where voltage is induced on the motor shaft, and the resulting current attempts to discharge to ground, or circulate through the motor through any conductive, contacting surface.  Several excellent papers have been published on the topic, including Rockwell’s 3/11/02 “Inverter-Driven Induction Motors Shaft and Bearing Current Solutions” (1), “Don’t Lose Your Bearings” (2) by A. Muetze and A. Binder, IEEE 1077-2618/06, and “Mitigating Stray Currents in AC Drives Installations”(3) by Adalberto José Rossa.  These shaft currents can discharge through arcing contact between the bearings and bearing races, causing the characteristic fluting damage to the races.  This paper does not examine the causes of shaft currents, only the resulting impact on motor/encoder combinations. 

The discharge of motor shaft currents can damage encoder bearings, and improper encoder installation and design can cause shaft current discharges that damage motor bearings.  Solving these problems is essential to avoiding premature motor bearing failure and/or encoder bearing failure.

Background

As detailed in the shaft grounding papers listed (1,2,3), any current path to ground, or return path between the shaft and motor frame can be highly undesirable.  This is especially true when the current path is combined with the typical vector duty motor configurations shipped today, with a non-isolated bearing on the drive end and one insulated/isolated bearing on the non-drive end. (Figure 1)  If the encoder creates a path to ground and/or the motor frame, current may circulate through the non-isolated drive-end motor bearing, and it can discharge through the encoder bearings. (Figure 2)

Industrial encoders were originally designed as stand-alone devices that were coupled to the motor or load (Solid Shaft/Coupled).  As magnetic technology was introduced, and miniaturization and hardening of optical designs progressed, encoder manufacturers began to offer encoders for direct motor mount.  These designs fell into two broad categories:  Modular, and Hollow Shaft.  Modular encoders are typically mounted to a machined C-face on the motor, and the rotor is mounted to the shaft.  Hollow Shaft encoders are mounted to and supported by the motor shaft and secured by an antirotation arm or tether.  Solid shaft styles are also still in wide use.

Solid Shaft Encoders:
Standard solid shaft encoders (also called coupled) are not electrically isolated: The metal encoder shaft is linked to the motor shaft with a conductive coupling; current is carried through the encoder bearings to the frame, and the encoder is solidly bolted to a surface that is typically connected to the motor frame, such as a flange or foot.  Electrical outputs are often connected via metallic grounded conduit, and some encoder designs do not isolate circuit ground from the encoder housing.  This means shaft currents can quickly end the life of motor (or encoder) bearings. (Figure 3)

Because the encoder can mount in the typical grounding brush mounting location on an older motor, some solid shaft encoders offer shaft grounding kit options.  Because they are mounted on the non-drive end of the motor shaft, they have the potential to create a circulating current path through ground (Figure 4), which can damage the uninsulated motor bearing as previously outlined.

Modular Encoders:
Modular encoders (also called pancake or C-face mount) are inherently isolated electrically by their design and do prevent shaft current discharge.  Although the encoder stator/housing is grounded to the motor frame, the rotor is electrically isolated from the sensor mounted in the stator by the air gap between the sensor and the rotor.  Usually this gap is at least 0.010” [0.25mm], sufficient to electrically isolate the motor shaft’s relatively low voltage potential from the housing.

However, modular encoders may offer shaft grounding brush options, which create a path to ground, potentially endangering the uninsulated drive motor bearing in a motor with a single isolated bearing on the non-drive end.

Hollow Shaft Encoders:
Hollow shaft encoders (also called tethered) are not inherently electrically isolated; the typical metal hollow shaft directly grips the motor shaft; current can be carried through the encoder bearings to the frame, and the antirotation arm forms the return path to the motor housing.  As in solid shaft designs, some manufacturers connected or capacitor-coupled circuit ground to the housing. Moreover, some industrial encoder connectors feature metallic conduit adapters, offering a third path to ground through the encoder bearings.

Encoder manufacturers recognized the problem of antirotation arms early on, and added insulating washers to the antirotation arm mounting kit.  However, many installers were (and still are) unclear on the purpose of these insulators and arm, and hoping for a more a sturdy mounting, they directly bolted the antirotation arm to the encoder, and shaft currents were free to flow once more.  Encoder manufacturers responded with insulating nylon sleeves or coatings in their shaft assemblies to eliminate the need to isolate the arm with plastic washers.  Users then discovered that the shaft sleeve plastics flow easily under clamping pressure, and do not have the same coefficient of expansion as the steel motor shaft.  With the temperature cycling characteristics of vector duty motors, hollow shaft encoders with nylon sleeve designs have been slipping off motor shafts in ever-increasing numbers.

Hollow shaft encoder bearings may also be destroyed by motor shaft current discharge just like motor bearings(3).  Most encoder bearings are less than 1/10 the size of motor bearings.  This makes encoder bearings much more vulnerable to damage from motor shaft currents.  Fluting and the corresponding bearing destruction may occur within days or months.  Many users have reported large-scale hollow shaft encoder failures when shaft currents were unchecked. 

Adding to the woes created by hollow shaft encoders, users began to demand shaft grounding kits. In some cases, the users hoped that the grounding brush would eliminate both the motor bearing damage and the hollow shaft encoder bearing failures they had experienced.  While shaft grounding brushes on the non-drive end can, in some cases, protect the encoder bearings, they do create a path for destruction of an un-insulated motor drive-end bearing in a motor with a single non-drive end isolated bearing.  Many customers now prohibit the use of shaft grounding kits on modern motors.

Application Guidelines:

The first guideline is a simple one: Apply shaft grounding wisely:

• Don’t add shaft grounding brushes to the same end of a motor as the insulated or isolated bearing when the other motor bearing is un-insulated.  The resulting circulating current can cause premature motor drive-end bearing failure.
• If the motor does not have any insulated or isolated bearings or the motor has two insulated bearings, shaft grounding kits may be applied at the encoder or drive end with no ill effects.

Second, follow encoder manufacturers’ guidelines for installation:

• If the hollow shaft encoder antirotation arm is provided with insulating washers, use them.  For solid shaft encoders, select an insulated coupling that isolates the encoder shaft from the motor shaft.

Finally, select an encoder that by its construction methods prevents shaft current discharge through the encoder without special installation concerns:

• The encoder should prevent discharge to ground through encoder signal connections and conduit adapters, as well as prevent circulating currents through the encoder frame and bearings.
• Consider encoders that are inherently isolated, and therefore prevent motor and encoder damage from shaft currents through the encoder, like those from Avtron Industrial Automation, Inc., author of this technical article.

References

(1) “Inverter-Driven Induction Motors Shaft and Bearing Current Solutions”, Rockwell Automation Industry White Paper, 3/11/02
(2) “Don’t Lose Your Bearings”, A. Muetze and A. Binder IEEE 1077-2618/06
(3) “Mitigating Stray Currents in AC Drives Installations” (Parts 1,2 & 3), Adalberto José Rossa, Drivesmag.com: 10 November 2011-09 January 2012

Timing the announcement with the AHR show this week in Chicago, Danfoss has introduced the VLT® HVAC Basic Drive - a small, full-featured variable speed drive that promises reliable, low-cost HVAC performance for basic fan and pump operations.

“In some fan and pump operations advanced drive features are unnecessary and, because they are superfluous, simply add to overall costs.  The VLT HVAC Basic Drive is an ideal solution that strikes the optimum balance between price and variable speed drive performance in these straightforward HVAC installations,” says Ed Smith, a company representative.

VLT HVAC Basic Drives minimize wear on HVAC equipment and maximize system up-time, while reducing HVAC system operating costs up to 15%. 

The company says that the VLT HVAC Basic Drive is the most compact drive in its class and with its specifications to reduce panel space requirements. Numerous built-in features reduce, and in some applications may even eliminate, the need for additional external equipment such as gateways, PI controllers and PLCs.  An Automatic Energy Optimizer function reduces energy consumption by up to 15%, while “sleep mode” functionality can help further reduce operating costs and extend drive life.  Bypass frequencies minimize operating noise, vibration and resonance issues.

VLT HVAC Basic Drives also feature a “start up wizard” that makes drive set-up fast and simple, and easy tool access further aids fast and effective commissioning and operation.  A robust single-piece enclosure provides reliable, maintenance-free operation in ambient temperatures up to 50 degrees C, with no external cooling required.  A unique cooling concept provides problem-free performance, even in harsh environments, without forced air flowing over the electronics.

Hitachi America, Ltd. has announced two new 100V Class models in their WJ200 series, rated at ½ hp and 1 hp, 100-120 VAC, single-phase input, and 200-240 VAC, 3-phase output.

The WJ200 is a sensorless vector (SLV) drive that is capable of 200% or greater torque across the speed range and has improved speed regulation (the company reports dramatic improvements – fluctuation half that of previous models.)

The WJ200 is feature rich:

Integrated PLC-like functionality: With its powerful Easy Sequence (EzSQ) logic controller function, programs up to 1024 lines can be created on a PC using a structured language editing tool. These programs can then be transferred to the WJ200 to control its operation and allow sophisticated control schemes to be created without the use of external controllers.

One drive for two motor realms: The capability to drive both induction motors and permanent magnet (PM) motors.

EzCOM peer-to-peer communications: One drive is designated as the “administrator,” and controls the network. Other drives on the network can be master or slave, with masters able to write data to any designated slave(s). Master/slave roles are rotated under the control of the administrator automatically. Up to 8 masters can reside on the network, and up to 32 drives (up to 247 drives, if external signal repeaters are used). The administrator can be master or slave also.

The WJ200 also is RoHS compliant. Option boards will be released in the near future to allow communication via Ethernet/IP, DeviceNet, CompoNet, ProfiBus, CANopen, and others.

An electrician explains the basic operation of an ABB VFD.

Siemens boasts about its automation offer and how it increases productivity and energy efficiency in this promotional video.

Siemens explains how to upgrade old machines with energy efficient motors and drives in this promotional video.

Drives are coupled with motor systems for one of two basic reasons: 1.) affinity laws promise greater energy efficiency when full speed operation isn’t required, or 2.) something about the process demands variable operation and control. Sometimes the needs are combined and the drive, or drives, become very important elements in a machine, building, water or factory system. This article will focus on energy efficiency.

Since the drive came into view as a green technology -- a tool to lower consumption and costs associated with energy -- the economics of drives have improved. For example, since 2001, the base costs of a drive, purchased, installed and operating, are flat or slightly down, while the electricity price paid by businesses is up.

As long as these two numbers move in the directions that they have been moving, more businesses will become drive owners, whether the people in them know what a drive is or not. It would seem to be a matter of necessity.

However, as competition increases, project budgets tighten, and economic trends remain volatile, small differences in energy performance matter. More importantly, service advantages among drive suppliers can be the difference between a selecting a smart, green system to start with, or regressing to full throttle operation in the interest of stretching limited construction funds. Drives are not ubiquitous, yet. In the U.S., 84% of motors are not yet drive controlled and in the E.U., 76% of motors are not drive controlled.

To try to understand why, we asked energy consultants to explain the challenges that they face specifying drives. The top two reasons: effort to predict energy ROI, and complex or custom application work.

Interestingly, those same energy consultants tell us that they struggle to prove energy payback concretely. While it is common practice among consultants to rely on drive suppliers to offer pre-sale predictions of energy performance and payback, it is rare that an installed drive is proven, through audit or performance testing, to be delivering on its economic promise.

More than 50% of consultants say that they have no idea if their projects payback and another 20% say that the payback estimate is proxy for compliance (but it shouldn’t be.)

So we think that 2012 will be the year of the audit. Watch as suppliers visit job-sites, parametric data, calculators and kW meters in hand, to show why drives are, and will be, crucial to reducing energy consumption, costs, and to lowering carbon footprint. And watch as consultants begin to demand that such services are included by, even required of, accepted suppliers in winning contracts.

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*We interviewed more than 190 mechanical and HVAC consultants and users of DASH Energy ROI Software in North America for this data.

Highlights from their answers:

• "Drives are a must have in all HVAC projects. The real issue is that customers become familiar with one brand or product."
• "HVAC systems all have them. The issue is who, not what."
• "The market seems to believe in them now. When we're convinced of short payback we don't resist."
• "We know we need them. We listen to the market for brand preference."

Data to follow in subsequent posts.

ABB and Technicon explain the science of energy savings with AC drives.

Global AC drives manufacturer Vacon has delivered new variable-speed AC drives with a total power of 58 megawatts to Indian-based AnRak Aluminium Ltd. The AC drives are for a new aluminium refinery which is being built near the City of Visakhapatnam in India's southeastern state of Andhra Pradesh.

More than 300 AC drives ranging from 2.2 kW to 1.5 MW will be used to control the aluminium refining process. The factory is to be completed in March 2012 with a total production capacity of 1.5 million tons of refined aluminium per year.