Figure 1, a Leroy Somer Alternator rated at 20 kVA (16 kW), popularly used by Caterpillar and other major AC Generator producers, next to the Polar model 8220 “DC” alternator, rated at 16 to 22 kW (depending on RPM).
It is easy to argue numbers but diﬀicult to make sense of all of it. This is particularly true if the presenter is arguing “apples with oranges”.
The purpose of this paper is to analytically examine the diﬀerences in Polars’ technology versus Caterpillar Cummins and Generac technologies.
For those with some mechanical and electrical backgrounds, common sense will prove the point. The Caterpillar and Cummins White Papers argue in favor of AC generators, but the examples they used in the comparison with DC generator technologies left Polars’ technology out of the picture.
There are many types of AC and DC alternators; the Caterpillar and Cummins White Papers implied that all DC generators use the same technology and have the same eﬀiciencies. This is not the case.
Decades ago, Polar coined the term DC alternator for a brush-less permanent magnet alternator with low voltage and high frequency output (400 to 800 Hz) optimized for conversion into DC current. Polars’ optimization did not require switch mode power supply as illustrated by Caterpillar and Cummins. The high frequency AC current passes through a simple diode bridge circuit then connects to and charges the battery directly, requiring no other electronics.
In Telecom, the term rectifier refers to a battery charger/power supply. In other engineering fields, a rectifier refers to a diode bridge. This is where some confusion arises, the Polar DC generator uses a simple diode bridge and few other DC generator systems use a switch mode circuit to regulate voltage and current.
The best fuel eﬀiciency data is from field trials, not laboratory testing. Typically, laboratory testing fails at simulating real world use of generators. There are too many variables to account for in simulating the real world and at the same time laboratory engineers tend to simplify testing parameters. Also it is unreasonable to compare two manufacturers’ equipment performance by using product data sheets. For an accurate comparison, both tests should use the same test technicians, fuels, weather conditions, loads, test equipment, and operate in actual field conditions.
For eﬀicient electric motors and alternators, most engineers target their design eﬀiciencies between 90% to 96%. Designers of electromagnetic machinery and power supplies very rarely include the parasitic losses into their performance.