New motor-mounted A-S drive controller

July 1, 2000
For industrial and commercial applications, new technologies will enable mounting drive controller directly on a motor

Many have been asking, “Why not mount an adjustable-speed drive controller directly on a motor, preferably on a standard NEMA-frame ac motor?” Although motors with built-on controllers have been made, they are generally limited to dc subfractional and fractional-horsepower motors and appliance motors. For integral ratings, the controller has been too big. But, that is changing.

Technological breakthroughs will enable integral horsepower ac motors to include sophisticated controls. To make this happen, a team of engineers at Baldor Electric are partnering with a vendor team to develop an integrated module that contains both the control and power circuits for an ac adjustablespeed drive. Moreover, this module will be small enough to fit on ac motors rated 1- hp and larger, Figure 1. Company officials report that new units — designated “Baldor SmartMotor” — will have many of the basic control features that are included in the rest of their adjustable-speed drives. Later this year, Baldor expects to offer Smart- Motors with ratings through 10 hp.


Achieving such a goal requires making a large investment in research and development. Therefore, one must ask, “Why is the company deploying such resources to a ‘maybe’ product?” Roland S. Boreham, Jr., Baldor chairman, feels strongly that the market wants and needs such a device, that the basic technologies now exist, and that the company has the technology partnership to make it work. Over the long haul, Mr. Boreham expects to get a good return on the investment. Vice President of Research and Engineering, James Kimzey, delineated that the new drives will:

• Save space by eliminating the need for a separate controller (which is presently often as big as the motor).
• Reduce installed costs because power wiring between the motor and controller is eliminated.
• Eliminate motor problems caused by “line ringing.” Mounting the controller on the motor eliminates long power lines between the two units, which can produce high-voltage transients that can cause motor failure.
• Wipe out the finger pointing between the motor and controller manufacturers when a problem comes up.
• Enable using adjustablespeed drives in applications that previously could not justify the expenses of a drive.

Typical applications

At first glance, one is tempted to limit the probable applications to those with relatively simple requirements — fans, pumps, blowers, mixers, packaging machines, plus conveyors and other material handling equipment.

Although these are the most common applications, Mr. Kimzey points out that other applications include any machine where motors are distributed throughout a unit. In such cases, the motors can be connected to a common power system (or bus). Then the lowervoltage, lowpower control wires are run to the control point. This arrangement is less costly than running individual power wires to each motor from remotely located drive controllers.

Jerry Peerbolte, vice president of marketing, said that the sales are expected to be 50% stock units sold through distributors and 50% modified motors and customized designs sold direct to OEMs.

The module

The key to this product is developing a module that has the required “smarts” plus the power control capability in a package small enough to easily mount on a standard ac motor, Figure 2.

The “smarts” include most of the functions that the company offers in its other adjustable-speed drives, both ac and dc. These include: Start, stop, forward, reverse, speed control, preset speeds, adjustable acceleration and deceleration rates and curves (S-curve or other), readouts of speed and motor current in rpm and amperes or in engineering units, displays in English rather than in codes, motor and controller protection, selectable modulating frequency from 1.25 to 18 kHz, stall protection, remote control capability from handheld keypad or computer, and many others.

In addition to offering similar functions, SmartMotors will use the same codes and method of operation with the same type of input as other Baldor controls. Thus, a user needs to learn only one system.

Within the module, a mathematical model of the motor will serve two functions and open avenues for future developments. First, the model enables accurately calculating the motor temperature. If it exceeds specified values, the controller will shut the system down. Second, the model improves motor operational stability, especially for larger motors.

Heat removal

In addition to developing a unit small enough to fit on the motor it is controlling, the whole program also hinges on designing a heat removal system that will keep the module within specified thermal limits. To do this, the engineering team is designing a multifinned assembly that mounts on the motor so the air forced by the external motor fan goes through the module cooling fins as well as over the motor shell. Although the two outside fins, Figure 3, contact the motor, the rest of the fins are above the motor outer surface.

To reduce the heat produced by the motor, the SmartMotor will utilize Baldor Super E, high-efficiency ac induction motor designs. These motors operate at lower temperatures than standard motors.

As the development continues, heat runs will determine application conditions requiring a separately powered fan for cooling both motor and control.

Voice your opinion!

To join the conversation, and become an exclusive member of Machine Design, create an account today!