Machinedesign 2213 Siemens Energy Automation 0 0

Will drives disappear?

June 1, 2000
Forget about matching the drive to the motor. Forget about most of the wiring and potential reflected wave effects. Integrated drive and motor packages eliminate these problems. However, every manufacturer of these emerging packages offers different features and functions. Here's a look at what's available

One day, drive selection may no longer be a part of your motor decision. You won't have to think about cable lengths and potential transient spikes. You won't have to think about motor-drive matching, or about proper wiring or installation. Instead, you'll just plug a packaged motor-drive unit to a power source, connect a communication cable, set up a few parameters, and start your process.

That day is almost here. Due to developments in signal and power electronics, integrated motor drive packages are beginning to appear. Most are in the fractional to 5 hp range, with a few available in 10 hp. In the next year to two years, you'll see 20 hp versions. "And you can expect to see even larger versions," says John McFarland, president, Baldor Electric Corp. "Probably up to about 40 hp, which will cover a large number of applications."

The one feature all of these packages have in common is that they promise savings in installation time and wiring costs. Beyond that, each package offers different features and functions.

From simple to servo

Typically referred to as smart, combined, or integrated, most of these packages combine an inverter with an ac induction motor. You can also find a few brushless dc versions. Until recently, there was only one integrated servo motor and drive that combined the motor, amplifier, inverter, encoder, and controller into one compact package. Even washdown versions of these packages are available.

In some packages, the integration is nothing more than a formerly stand-alone inverter that's been bolted onto the top or side of a motor with minimal consideration of potential thermal problems. With other packages, designers began fresh, creating inverters specifically for integration with motors. These designs use special signal and power electronics to minimize or eliminate noise, vibration, and heat problems. Several of these integrated units barely alter the motor form factor.

The best applications are those requiring simple velocity or torque control, such as those that use fans, pumps, or conveyors. "Other suitable applications include single or two-speed processes that could benefit from inverter technology," says Rudy Hauser, product manager, Siemens Energy & Automation Inc.

"You wouldn't use one of these inverter-motors for complex motion like that found in extruder applications or web processing," says Jeff Petro, product manager at Reliance Electric.

The integrated servo motors and drives, however, will handle applications that require some positioning because they include encoders and controllers in the packages. Thus, they can handle applications that use robots, pick-and-place equipment, or fixtures.

Technical advances pave the way

The introduction pace has picked up because of powerful, low-cost semiconductors; smaller power electronics; and better motion control algorithms. Engineers have also found solutions to the electrical noise and heat problems that occur when you crowd power electronics into a small space, or when you put power and signal electronics together.

"One of the benefits of these integrated packages," says Robert Bigler, vice president of engineering at Animatics Corp., "is that the design engineer has a totally controlled environment when wiring the drive to the motor." Even the source power can be controlled in an integrated package. Thus, the designer can solve noise problems by using special shielding on the wires to prevent cross signals or by placing the wires on the printed circuit board to ensure minimal noise and eliminate feedback loop problems. A few of these packages follow CE requirements for noise control.

“Because these packages have done the wiring between the motor and the drive, the OEM or end user doesn't have to worry about phase wiring," says Duran Miller, vp, Electronic Marketing, TB Wood's Inc. "Or transmission line effects like transient waves that come from long cables."

Installation is quicker and easier too. Such prewiring eliminates the need to match the motor to the inverter. In servo packages, because the wiring is inside the integrated inverter, it also eliminates problems from constant cable flexing.

Taking the heat off

All of these packages eliminate the need for, and the costs of, a drive enclosure. Because of this though, design engineers needed new solutions to ensure sufficient cooling of the motor and the inverter electronics.

Several designs mount the capacitors away from the motor and keep them cool by fans or continuous blowers and heat sinks. In some cases, the fan, which is powered by the inverter, also helps cool the motor. Should the inverter fail, it may be possible to continue to operate the motor but you'll have to provide some means of cooling it or de-rate it until the inverter can be fixed or replaced.

Because cooling efficiency varies, you'll find some motordrive packages with a wider range of speeds than others.

Though a few integrated designs use standard stand-alone inverters, most use specially designed inverters. These drives incorporate application-specific integrated circuits, DSPs, and memory chips. To protect them from heat, engineers position them on the board and stack the boards in such a way as to ensure efficient air flow.

Capacitors are sensitive to heat. One method of increasing their heat resistance is to replace large capacitors with smaller ones that are less temperature sensitive. Depending on the design, such a technique may also reduce the voltage dc link capacitance, which can result in higher ripple. This is controllable through application-specific integrated circuits.

Other thermal control solutions include low-loss IGBTs and special heat-sink designs. In general, most manufacturers recommend that these units not be used in applications where the ambient temperature is above 40 C. A few will operate to 50 C.

Vibration is generally not an issue with these specially designed inverters. Typically the chips and components are seated in sockets that protect them from vibrations, in some cases as high as 5 g.

Lastly, the position of the drive on the motor affects more than just efficient thermal dissipation, however. It changes a unit's diameter or length, which may influence your selection.

Service calls

Early versions of these packages required replacement of the whole unit when either the drive or the motor failed. Some existing versions still require this. Several manufacturers' designs, however, make replacement of any part easy and fast, usually doable in less than a half hour. Just unscrew a few bolts, unhook a few wires, remove the failed part and replace with a new one. You can replace the regulator board, the fan, fan shroud, and adaptor plates as well as the motor or the whole drive. If uptime is crucial, keep a few of these parts on hand as spares.

The overall reliability of these packages is generally better than stand-alone components primarily because there are fewer parts in them.

Other features

Most of these packages use adjustable- speed, V/Hz motors. A few packages provide just constant speed. Speed ratios vary from 4:1 to 30:1. A few manufacturers brought up the issue of the quality of the motor. Some claim competitor's speed ratios are limited because the motor is of lesser quality or because it wasn't designed for integration.

Some packages offer a range of acceleration- deceleration rates, from which you select the one that best matches your application. And there are constant-torque and variabletorque packages.

The servo packages tend to offer RS-485 communication to a controller. For the general-purpose devices, a few manufacturers plan to offer DeviceNet communications in the next introductions.

The programmable parameters vary considerably. Some offer a limited number of functions, others are fully programmable.

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