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Correct control keeps motors rotating

The right motor control can mean the difference between profit and loss. It’s important to know IEC and NEMA standards when evaluating a system.

Almost nothing is accomplished in manufacturing without starting a motor. Whether powering a pump or conveyor, fan or drill, motors are the worker bees of automated manufacturing. Choosing the wrong motor control can lead to burned up motors, lost production time, poor quality, ruined batches and many other profit-draining situations.

The National Electrical Manufacturers Association (NEMA, www.nema.org) facilitates standards development in the United States for electrical equipment, and has a standard for contactors—heavy-duty switches designed to start and stop large electrical loads required by motors. Any electrician in an American plant will be familiar with putting a NEMA size 1 motor starter in a circuit controlling a typical squirrel cage motor rated at 10 HP, 460 VAC, 60 Hz.

In Europe, the International Electrotechnical Commission (IEC, www.iec.ch) fills the standards development role. European manufacturers and control designers have evolved a different emphasis from that of NEMA. European engineers’ goals involve reducing energy consumption and size of equipment.

Not surprisingly, the IEC standard for motor controllers differs from that of NEMA. Now that globalization of machinery is a fact of life, NEMA has issued “NEMA Standards Publication ICS 2.4-2003, NEMA and IEC Devices for Motor Service—A Guide for Understanding the Differences.” The goal of this publication is to educate American manufacturing personnel so that motor control will be correctly applied and all those nasty scenarios listed above will be avoided.

Size or application

A NEMA contactor is designed to meet the size rating specified in NEMA Standards. A philosophy of the NEMA Standards is to provide electrical interchangeability among manufacturers for a given NEMA size. Because the installer often orders a controller by the motor horsepower and voltage rating, and may not know the application or duty cycle planned for the motor and its controller, the NEMA contactor is designed by convention with sufficient reserve capacity to assure performance over a broad band of applications without the need for an assessment of life requirements.

IEC Standards do not define standard sizes. An IEC rating indicates that a manufacturer or laboratory has evaluated the contactor to meet the requirements of a number of defined applications, or utilization categories.

Utilization Category is an IEC term used to describe a specific type of application (see table). Convention has led to the assignment of several ratings to a given contactor for different utilization categories and voltages. The electrical system designer chooses which of several devices are preferred for the application, based on its ability to meet, or exceed, the required horsepower, voltage ratings and other factors, including performance. In general, any time the duty cycle includes significant jogging or plugging, a larger size IEC contactor is selected than would be needed for pure AC-3 applications. Manufacturers of IEC devices publish electrical contact life curves for a family of contactors to assist in selecting which contactor from a manufacturer’s product line is appropriate for a given application. These curves are not intended to predict the actual life of a specific member of the product line in a particular application, but are intended to provide an indication of the relative life of the products.

An engineer with knowledge of the application can choose the appropriate contactor by defining the correct utilization category, then calculating the anticipated contact life required by estimating the number of times the contactor will be switched in a given period. By appropriately sizing the contactor, it may be possible to reduce contactor size, and therefore cost.

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