FORD ENGINEER TURNS TO ALGOR FEA FOR MULTIPLE ALTERNATOR DESIGNS

Ford Motor Company Product Design Engineer Arthur Gajewski is shown here with the rotor pole piece and a complete rotor assembly. Note the pole piece FEA model on the computer screen.

Most people are familiar with the automotive alternator. The alternator provides the power necessary to operate the electrical components of an automobile, such as the lights and radio. It also recharges the battery. While at first glance an alternator may appear to be a relatively simple device, it is, in actuality, a critical component subject to tough design criteria.

Varied Requirements

Arthur Gajewski, Product Design Engineer with the Ford Motor Company's Electrical and Fuel Handling Division, knows the complexity of automotive alternators. Since the requirements of various car and truck models vary widely, Mr. Gajewski is often asked to provide an alternator design that meets strict requirements for size, weight and power output.

The model shown below was recently used in the design of an alternator's rotor pole piece. Each rotor has a pair of pole pieces with a coil of wire in the cavity between the walls of the opposing poles. The low carbon steel pole pieces provide a path for the magnetic flux created by the coil. As the rotor spins, its flux field induces an alternating voltage in the stationary armature called the stator. The alternating current is then rectified to direct current for use in the automobile's electrical system.

The model created for both the centrifugal deflection and modal analyses contains 103 elements and 228 nodes.

A visualization of the deflection analysis results.

Modal analysis results.

Two Critical Analyses

According to Mr. Gajewski, there are two important analyses performed on new alternator pole piece designs. "As a standard design practice," he says, "centrifugally induced deflections and stresses are determined for each new pole piece. Additionally, the first few natural frequency modes are analyzed to ensure that they will not contribute to the noise the machine can generate in its operational speed range.

"The centrifugal deflections are important because there is an airgap of 0.40 mm between the rotating poles and the stationary armature. Alternators are usually designed to survive short duration, high speed bursts of from 22,000 to 25,000 RPM. This is typical of the high engine speeds experienced when operating in low gear ratios such as in passing or other rapid acceleration situations. If deflections are equal to or greater than the airgap," he continues, "the machine could malfunction. And, if centrifugal stresses are too high, the pole pieces could permanently deform."

According to Mr. Gajewski, it is important to carefully analyze natural frequencies because, "In operation, the pole piece fingers undergo alternating magnetic forcing functions. The frequency of these forcing functions varies directly with rotational speed. If the frequency of the forcing function is close to the natural frequency of the pole piece, the potential for resonance exists."

Usually, this resonance generates an acoustical disturbance. Therefore, modal analysis allows prediction of the natural frequencies and the pole piece can be configured so that the first few modes occur at frequencies above those corresponding to speeds within the normal operating range of the alternator."

The Model

To perform the necessary analyses, Mr. Gajewski constructed a single Algor FEA model consisting of 228 nodes and 103 elements. The design was based on a conceptual layout which originated in AutoCad and was imported into Superdraw II. Because of symmetry, he was able to restrict the model to one-sixth of the actual part. The model was constructed and analyzed on an IBM PS/2 Model 70, 386 running at 16 Mhz.

The Results

"Centrifugal analysis, at 22,000 RPM reveals a displacement at the finger tip of 0.21 mm or one-half the width of the airgap," says Mr. Gajewski. "Algor predicts that the stresses are not significant enough at this speed to cause the pole piece to fail."

As for the modal analysis, Mr. Gajewski says, "The first mode resonances coincided with rotational speeds that are at the upper end of the operating speed range. I achieved excellent correlation between laboratory testing and Algor predictions."

On the subject of Algor FEA software, Mr. Gajewski says, "The centrifugal loading capability is fantastic! Other FEA software packages do not offer centrifugal loading analysis. The meshing menu in Superdraw II permits easy meshing of very complex shapes and the animation feature allows me to visualize the mode shapes following modal analysis."

Mr. Gajewski also uses Algor software to perform thermal analysis. He is currently studying temperature distributions in a variety of components under consideration for use in future alternator designs.