Read or Download Basics of Control Components(en)(120s) PDF
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Extra resources for Basics of Control Components(en)(120s)
Contactors cannot sense when the motor is being loaded beyond its rated conditions. They provide no overload protection. Most motor applications require overload protection. However, some smaller-rated motors have overload protection built into the motor (such as a household garbage disposal). Overload relays, similar to the one shown below, provide this protection. The operating principle, using heaters and bimetal strips, is similar to the overloads discussed previously. 43 Motor Starter 44 Contactors and overload relays are separate control devices.
When a motor is started with full voltage, starting current can be as high as 600% of full-load current on standard squirrel cage motors. It can be even higher on high efficiency motors. There are situations where this method of starting is not acceptable. On large motors the high starting current is reflected back into the power lines of the electric utility, causing lights to flicker and in more serious situations can cause computers to malfunction. S. require reduced-voltage starting on large-horsepower motors.
Underwriters Laboratory (UL), for example, specifies a maximum horsepower rating for which a contactor can be used. L. 508. All Siemens contactors are rated in accordance with at least one of the previous organizations test procedures. Some carry multiple ratings. For example, Furnas INNOVA starters meet or exceed NEMA and UL standards. Siemens Sirius starters meet or exceed NEMA, IEC, and UL standards. Furnas INNOVA PLUS Starters Furnas INNOVA PLUS starters are available in NEMA sizes 0 through 4.