Machinedesign 2410 Combined Motor Drives899 0 0

Making the right match

Aug. 1, 2000
Integrated motor-drives eliminate the trouble of matching the motor to the drive, but they don't suit all applications. Here are a few pointers on making the best choice.

When the old motor on the exhaust fan finally dies, of course you'll replace it. And you'll do the same when the motor on your winder goes. In both cases, you may also want to replace the drives because of installation hassles and the complexities of matching drives to motors.

In that case, you may consider a combined motor-drive. These devices have been available for a few years and their benefits are fairly well known.

For an exhaust fan, a combined motor-drive turns out to be a good alternative to separate units. For a winder, however, it's a less than ideal choice because of limitations in speed and torque control. The point: Before purchasing a combined package, it's important to understand which applications make sense, and which should be left to traditional fixed speed controllers or individual drives and motors.

Finding the fit

If an application requires simple speed control, such as that needed by the exhaust fan mentioned above, a combined motor-drive would be an appropriate choice. Winders and centrifuges, on the other hand, require more advanced control of both torque and speed, and hence are often inappropriate for combined packages.

After determining torque and speed issues, then consider whether the application is in volatile or potentially hazardous environments. Most integrated drive-motor packages are not rated as explosionproof. In applications such as oil refineries, steel mills, and some food manufacturing that can't permit sparks or suspended particles, these drive packages may be inappropriate.

Horsepower range will also be a factor. Most existing drive-motor packages are available from fractional to about 5 hp, with a few reaching almost 20 hp.

Physical considerations

After settling on the basic application type, the next step is to evaluate the needs for motor-control complexity, panel space, communications, and plant-floor location.

Motor control complexity. Integrated drive-motors handle simple, open-loop requirements. In many cases where you use volts-per-hertz drives, integrated drive-motors can be an alternative.

They are usually easy to set-up, operate without a feedback device, and are low in cost making them a good choice for low-horsepower motor integration. Most units deal only with total motor current. They do not separate flux current from torque current, so torque cannot be regulated.

More complex applications that operate at low speeds yet must maintain high torque (like extruders, winders or web handling), as well as applications that demand high starting or accelerating torque (centrifuges) need a greater level of control that is usually available through sensorless vector drive technology.

Panel space. Saving panel space is one of the main benefits of integrated drive-motor packages. Often, they are a good choice for conveyor applications.

Instead of mounting the drive in the panel and running cables to the motor on the factory floor, integrated packages can be set up directly on the floor. Because the drive is mounted either at one end of the motor or on top of it, there is no increase in motor footprint and no depletion of panel space.

Networking. Some manufacturers offer connectivity to industrial networks in their integrated systems, and most offer basic codes for troubleshooting.

Networking takes an integrated package beyond simple troubleshooting. It enables a drive to communicate to a central automation system. Through this link, you can monitor for such factors as current speed, temperature, and lastknown fault - data that aid both preventative maintenance and troubleshooting.

Operator interface devices let you customize screens to monitor whether a drive-motor accepted messages from a controller, which can help to resolve or confirm process issues. In the process, you can more efficiently monitor the entire process, saving time and gaining greater control over it.

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Costs. When comparing costs between an integrated drive-motor and individual components, you'll find that initial costs are lower for the separate pieces. However, when you factor the costs of installation, including wiring, material, and labor, the integrated packages tend to cost about 15 to 40% less than separate drives and motors.

Plant floor location. And finally, the physical location of the integrated drive-motor can be important. Consider the distance between the drive and the panel, or if the equipment needs to be NEMA rated for wash downs, to meet FDA requirements.

Specialty machinery, such as industrial mixers and exhaust fume hood blowers, can benefit from integrated drive-motors because they eliminate reflected wave effects associated with most pulse-width modulation (PWM) inverters. Reflected wave phenomenon relates to the distance between the motor and the drive. The short lead lengths of integrated drive-motors, which are less than a few inches, significantly reduce exposure to these electrical effects. Long lead lengths between drives and motors may cause high voltages at the motor terminals that can seriously damage motor windings and lead to premature system failure.

Another location consideration is the amount of heat the drive-motor must endure. Most drive-motor packages use capacitors mounted away from motors, blowers, and heat sinks to disperse heat. Still, most manufacturers recommend that these packages not be used in applications where the ambient temperature is higher than 1048 F.

Some drive-motor designs include a self-powered, constantspeed blower fan that cools power electronics. This lets the motor run at any safe speed while the fan constantly blows cool air over the heatproducing components.

Together rather than apart

Integrated motor-drives eliminate many of the common inconveniences associated with drives and motors. Much of the installation work has already been done for you. There's no external wiring between the two components. So all you have to do is mount the integrated package in place and attach power leads.

The internal wiring is significantly shorter, virtually eliminating phase wiring or transmission line effects such as reflected waves. Plus, the design alleviates potential compatibility challenges because there's no need to match the drive to the motor. Most of these packages come factory wired, tested, and programmed with parameters such as slip compensation.

Some manufacturers integrate rather than combine the drive and motor components, which can improve performance. For example, a design may integrate a drive's power supply and IGBTs into the motor, placing them directly in the airflow of a constant speed blower that runs on its own power source. This will help decrease the chance of shutdowns due to overheating.

Finally, some applications can take advantage of the variable speed control offered by integrated packages without sacrificing ease of installation and reliability. For example, light industrial exhaust fans are usually made for two or three-speed operation based on push button control. With an integrated system, the controls are still available, usually mounted with the motor. There's no need for a separate control box located away from the fan or vent and the use of a single speed motor.

Stan Ho is product manager, Allen-Bradley Standard Drives, Rockwell Automation, Milwaukee.

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