Recently patented invention reduces speed ripple of dc motors caused by commutator bars and other motor characteristics. May give new life to old machines
Adjustable-speed dc drives generally maintain close speed regulation. However, the commutator bars in dc motors cause small but sometimes significant speed fluctuations, especially while operating at lower motor speeds. As a commutator bar passes into and outof contact with a brush, the dc motor speed fluctuates. According to Robert M. Fridhandler, a senior research analyst with a large pulp and paper manufacturing company, this ripple is typically 0.2% to 0.8% of the motor’s set speed. Although insignificant in most applications, this seemingly small amount of speed fluctuation can create severe problems in precision processes, such as photographic film coating lines, precision grinding operations, and finished metal rolling. In some cases, users add flywheels in the system to smooth the speed.
Some industry sources credit these fluctuations with expediting ac drive advances.
However, there may be another method for eliminating these tell-tail signs of prime-mover speed fluctuations. Mr. Fridhandler, of Blauvelt, N.Y., was recently assigned patent 5,167,002 for an idea that enables a dc drive to reduce these fluctuations by 40% to 60%. Although not a complete elimination, it is a significant improvement. In some applications, this improvement is expected to be sufficient enough to eliminate the requirement for adding system inertia. Even more importantly, the ripple reduction may enable existing dc drives to improve product quality without extensive retrofit programs.
This invention, which is informally called a speed ripple controller (SRC), can be installed on existing applications as well as incorporated in new installations. Mr. Fridhandler anticipates that the biggest need for his invention is for retrofit and modernization applications.
How it works
A conventional dc adjustable-speed drive rectifies ac plant power and controls it to produce adjustable-voltage dc. This, in turn, determines the speed of a dc motor. For accurate speed control, a tachometer connected to the motor shaft sends a voltage signal back to the controller (drive). The controller then checks for errors between the commanded speed and the actual speed as sensed by the tachometer, and makes any needed correction in the voltage applied to the motor.
However, commutator bars, pole-face variations, and other motor construction details cause speed ripples. Usually, such ripples are at higher frequencies than the motor control circuit can handle. So the controller is unable to smooth the ripple.
Enter the SRC. In summary, it establishes the magnitude and frequency of the ripple, then generates antiwaves at just the right time to cancel part of the ripple.
To do this, the SRC first establishes each time a specific point on the armature is at a specific position, say 12 o’clock. This sensing can be provided by a magnetic pickup, encoder, or an optical sensor and reflective tape on the motor shaft, Figure 1. This position information is fed to a fast Fourier transform analyzer (FFT) and to an arbitrary function generator (AFG).
Voltage from the dc tachometer goes to the drive control circuit as a speed feedback signal and to the FFT, which determines phase, amplitude, and frequency of speed fluctuations that are synchronous to the rotating system.
The AFG generates the proper frequency, amplitude, and phase relationship of an anti wave so it can minimize the speed variations of the dc motor. It is imperative for the AFG to maintain the proper phase relationship of the antiwave in relation to the motor speed variation, else the speed ripple controller will be ineffective or will make matters worse instead of better.
In general, Mr. Fridhandler’s invention reduces, for example, the ripple by 40% to 60%, while operating at 75% of motor base speed, Figure 2. The value, “AC ripple at motor shaft speed,” is frequently unknown or overlooked. This relatively low-frequency component often accounts for 40% of the total ac ripple.
Moreover, the speed improvement by the SRC varies with motor speed. As expected, undamped ripple increases as speed decreases, Figure 3. Therefore, at slower speeds, the speed ripple controller makes more improvement than at higher speeds. Mr. Fridhandler believes that many engineers are seeking this lowspeed improvement.
Working with Mr. Fridhandler, a few companies are conducting feasibility studies on manufacturing and marketing the SRC. However, there has been no commitment on either side as of early July.
When asked about the cost of such a controller, Mr. Fridhandler commented that the first unit would be rather costly, but in quantities, the cost of manufacture, including investment in the design of a dedicated, self-tuning chip, could be a few hundred dollars. n
Invention by Robert M. Fridhandler, Blauvelt, N.Y.