Machinedesign 2520 Sirius 3r Protective Devices 0 0

Directions in motor protection

June 1, 2000
Through better sensing technology and algorithms, the latest motor protection devices guard motors from an array of common problems. And they will do even more over the next few years

Protection devices, whether contactors, overload relays, or motor starters, are moving beyond simple overcurrent surveillance. Many now safeguard motors against specific causes of overheating. A few are becoming proactive, compensating for problems before they force a system shutdown.

Plus, like so many other pieces of plant equipment, communication has become a vital part of their increasing capabilities. Combined with the intelligence afforded by microprocessors, protection devices can send and receive commands, working with other controllers and ultimately extending motor life.

More protection

Most of the latest motor protection devices detect phase loss and they do it by looking at the phase going out. Usually, they check to see if there's a current increase in the two remaining phases of a three-phase motor.

Other methods of phase detection offer more data sooner. "Some of the latest protection devices detect subtle phase imbalances that occur when a phase begins to go out," says Scott Lasko, product manager, Rockwell Automation, Milwaukee. Wis. "They monitor imbalance and trip before the imbalance causes thermal damage. This can also give customers the option of inhibiting the trip a little while longer before shutting down the process."

A few motor protection devices go a step further, taking proactive measures. One even rebalances voltage loss or surge in a system. If line voltage is off by as little as 4%, it can raise motor temperature by 25%.

There are several circumstances that contribute to voltage imbalance. Losses often occur naturally on a line. In other cases, various loads may be improperly balanced throughout a plant. Also, too many devices entering or exiting the power line in close succession or simultaneously will cause surges or drops.

Protective devices are now available that sense such conditions and set about rebalancing the voltage. One such device uses a microprocessor to signal thyristors, which in effect, phase back the high-voltage legs and boost the lower leg to rebalance voltage.

In addition to proactive voltage management, you may soon see vibration monitoring added to motor protectors. Of course there are ways to detect vibration now, separate from protection devices. The least expensive is to figure out the most vibration a system can handle and install a sensor that senses only for that condition. A more practical way is through a sensor mounted to the side of a motor or the side of a load on a gearbox. Alternatively, engineers can mount a sensor next to the motor shaft.

A better way is to look at the electronic signal of the current and voltage. Several researchers are doing just that by connecting sensors and wiring to the rotor bars, spinning the rotor, and recording the waveform distortions. The present problem is deducing what the distortions mean. So work continues on analysis and algorithm development.

As developers find ways to better sense and control existing motor operating conditions as well as discover others they can manage, they are also shifting from single-function to multiple- function protection devices. "Today's overload relay is not just a phase loss monitor," continues Lasko. "It's also a jam relay and a ground-fault relay, even a current monitor all built into one product."

The next step will be to combine the multi-functioning protection devices with other components, such as power supplies, needed to operate a motor-drive system. Many components can be embedded onto a printed circuit board. In addition to having a range of protective functions on a standard product, such a design will also use less space.

Early detection

All of these developments are leading to pre-event diagnostics. "Several of these advances, such as some of the sensor technologies and algorithms, are ones that people wouldn't normally associate with motor protection," says Lee Smith, product line manager for enclosed control, Cutler- Hammer, Milwaukee. "But the capabilities are really starting to come out even though the industry is still a year or two away before some of them are truly economical."

The cost of current sensing, for example, has been dropping over the last few years. The result is that several developers are looking into Hall effect sensors as replacements for current transformers. Because of automotive requirements and volume, these sensors are now smaller, more accurate, and less costly. Furthermore, they offer several advantages over current transformers, such as a wider frequency sensing range. In some cases, they will even sense direct current.

Concurrently with falling sensor costs, developers have been working on creating algorithms that more closely model motor behavior. "The think tanks are close to developing a body of knowledge that can lead to better algorithms," adds Smith. In some cases, the models are going beyond just detecting deviations.

"Algorithms in development are looking down stream in the process with current, waveform, and signature analysis," Smith continues. "Eventually, we'll have enough data about the waveform that it will tell controllers, and therefore engineers, what's going on with the process. Researchers aren't yet able to tell you whether bearing number 3 will go down in two weeks, but they're on the point of it because the algorithms are getting better, microprocessors are more powerful, and engineers are gaining a better understanding of what a bad waveform looks like."

The algorithms are powerful enough today to give hints to some of the load conditions affecting bearings. In a pump, for example, algorithms and sensors can help determine if there's cavitation or water hammer occurring.

Get the message out

Once these devices gather all the data, they can't keep it a secret. So you'll find that most protection devices have the ability to transmit data over the common industrial networks. Even bi-metal overload relays are being connected to networks.

Most protectors incorporate application specific integrated circuits with the appropriate protocols. Others communicate through external adapters.

"Motor control centers in particular are making use of communications capabilities," says John Nethery of Siemens Automation, Alpharetta, Ga. The ability to connect many starters provides more access to plant operations.

There is a caution with connecting analog sensors to a network, though. Big chunks of analog data will really slow some network transmission rates. The industry will have to turn to other network choices for analog data that offer high transmission rates of at least several hundred megabits per second. One possibility might be FireWire. Stay tuned.

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