Consulting-Specifying Engineer - November 2007 - (Page S5)

on-peakperformance Applications Primary considerations Three main considerations for using a BBS in an engine starting system include the following: 1) Diodes must be large enough to carry 100% of the worst-case starter cranking current. 2) Larger batteries must be used to compensate for power loss in the BBS. 3) Care must be taken not to connect a single enginedriven alternator to both batteries, even through diodes. If an alternator is absolutely necessary, connect it to only one battery. It is also important to select a BBS with sufficiently high diode-ampere rating. If a BBS diode is not large enough to carry 100% of the worst-case starter cranking current, the diode could fail catastrophically. For example, from the graph in Figure 2 below, the starter rolling current for a 69-liter diesel with SAE 30 oil is about 1,600 amps at 25ºF. The breakaway current for this engine can be up to 2,800 amps. The BBS for this engine, oil viscosity, and minimum temperature must be rated for both 2,800 amps breakaway current and 1,600 amps rolling current for the full starting cycle. Note that the actual current available from a BBS assembly is always lower than the rated current of the component diode. This is because the diode must be derated for safe operation in a BBS due to waste heat generated during cranking duty. Besides more cranking current margin, there is another benefit of using larger diodes: lower voltage drop while carrying full cranking current. Larger diodes reduce power loss, which means that the battery has to be upsized less than it would with a higher loss diode. Diodes must be firmly attached to a mass of thermally conductive metal (the heatsinks) to ensure the diode junction temperatures remain well below the diode manufacturer’s specified maximum under worst-case conditions. Even though normal cranking cycles may be short in duration, less than 45 seconds typical, our company, for example, designs to the worstcase specified cranking duration of 90 seconds (two consecutive cranking periods of 45 seconds each) as defined in NFPA 110 “Standard for Emergency and Standby Power Systems,” section 5.6.4.4. Undersized diodes run hotter in normal operation—risking catastrophic failure—and due to excessive voltage drop, may not deliver sufficient voltage to the starter to start the engine. >>A properly engineered BBS consists of two high-current diodes, specialized heat sinks, voltage surge protection, and high-current terminals assembled and mounted in a metal housing. Example of Total Start Current Demand - 2 MWe Genset 3,000 2,500 69 liter diesel, SAE 30 oil, mfgr data 69 liter diesel, SAE 10W-30 oil, mfgr data 85 liter diesel, mfgr worst case data 2,000 Two starter motors for 69L or 85L diesel: Worst-case break-away current draw 1,500 1,000 Amps 500 0F 10 F 18 F 25 F 32 F 39 F 46 F 54 F 61 F 68 F 75 F 81 F Figure 2 - The graph shows that starter rolling current for a 69-liter diesel with SAE 30 oil is about 1,600 amps at 25ºF. The breakaway current for this engine can be up to 2,800 amps. november2007 5

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - November 2007

Consulting-Specifying Engineer Contents Editor's Viewpoint Letters In The News M/E Roundtable Emergency Power for Healthcare 2007 Products of the Year Industrial-strength Lighting Hazardous HVAC Codes and Standards Management Report Jobs/Classifieds Specifier's Notebook On-Peak Performance Contents Best Battery Selector in Engine Starting Applications NEC 708: Practical Impact on Backup Power Systems Protecting Power: Specifying Outdoor Generator Enclosures Investing in Backup Power Systems Coes, Consultants, Manufacturers, and Standby Power Systems

Consulting-Specifying Engineer - November 2007

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.