The NEC defines a controller in a few different ways. In Art. 100, a controller is described as “a device or group of devices that serves to govern, in some predetermined manner, the electric power delivered to the apparatus to which it is connected.” Getting a little more specific, a definition in Art. 430.2 states, “A controller is any switch or device that is normally used to start and stop a motor by making and breaking the motor circuit current.” For this article, we'll concentrate on controllers, specifically the magnetic motor starter variety of controllers.
The magnetic motor starter is an electromagnetically operated set of contacts that starts and stops the connected motor load. A control circuit with momentary contact devices connected to the coil of the magnetic motor starter performs this start and stop function. A 3-pole full-voltage magnetic motor starter is made up of the following components: a set of stationary contacts, a set of movable contacts, pressure springs, a solenoid coil, a stationary electromagnet, a set of magnetic shading coils, and the moving armature.
It's also important to remember that a magnetic motor starter is a contactor that has the addition of an overload relay assembly that provides running overload protection to the motor. Selection of the thermal overload relay is done using the manufacturer's table included with the magnetic motor starter. In addition, ensure you know the full load current (FLC) of the motor, the service factor (SF) of the motor, and the ambient temperature in which the equipment is being operated. Thermal units are based on an ambient temperature of 40°C (104°F).
Typical magnetic motor starters that are commonly available include: full voltage (across-the-line), reduced voltage, and reversing. As the name implies, a full-voltage or across-the-line magnetic motor starter (Fig. 1) applies full voltage to the motor. This means the magnetic motor starter is designed to properly handle the levels of inrush current that will develop as the motor is started. Designed to limit the effects of inrush current during motor startup, reduced voltage starters are available in electro-mechanical and electronic formats. See “Standard Motor Control Circuit Primer” in the June issue of EC&M on page 18 for a more thorough discussion of the types of reduced voltage starters.
Reversing starters are designed to reverse shaft rotation of a 3-phase motor. This is accomplished by interchanging any two-line conductors that supply the motor load. The reversing magnetic motor starter (Fig. 2) features a forward and a reverse starter as part of the assembly. Electrical and mechanical interlocks are provided to ensure only the forward or the reverse starter can be engaged at any given time — but not at the same time.
NEMA magnetic motor starters are available in various voltage and horsepower ratings with designations ranging from size 00 through size 9. These NEMA sizes classify a magnetic motor starter by voltage and maximum horsepower. Coil voltages are typically available in 24V, 120V, 208V, 240V, 277V, 480V, and 600V varieties. The magnetic motor starter is also offered in different types of enclosures, depending on the environment in which the equipment will operate. Typical protective enclosures are: NEMA 1 (general-purpose), NEMA 4 (watertight), NEMA 12 (dust-tight), and NEMA 7 (hazardous location).
IEC-style magnetic motor starters are usually available in a modular format with a power base and a control unit. Three-phase power bases are available in 208V, 230V, 460V, and 575V varieties with corresponding maximum horsepower ratings. The control unit functions as the adjustable overload relay assembly, which is different from the fixed style of thermal overload unit found in a NEMA-type magnetic motor starter. IEC devices are usually smaller in size and less expensive than comparable NEMA-type devices. IEC magnetic motor starters are often supplied as part of OEM (original equipment manufacturer) equipment.
If we compare the NEMA magnetic motor starter to the IEC magnetic motor starter, the following differences would be noticed:
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An IEC device is physically smaller than a comparable NEMA device.
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An IEC device is usually less expensive than a comparable NEMA device.
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An IEC device has a life cycle of approximately one million operations while a comparable NEMA device has a life cycle of almost four times that number.
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An IEC device has an adjustable overload relay assembly while a comparable NEMA device has a fixed and removable overload relay assembly.
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An IEC device should normally be protected with fast-acting, current-limiting fuses while a NEMA device can be protected with conventional time delay fuses.
The end-user should carefully consider all of these requirements before making the decision to install either a NEMA magnetic motor starter or an IEC magnetic motor starter in a specific application. The National Electrical Manufacturers Association (NEMA) and International Electro-Technical Commission (IEC), two standardization bodies that classify electrical equipment, are also good resources for additional information.
Vidal is president of Joseph J. Vidal & Sons, Inc., Throop, Pa.
Author's Note: I would like to dedicate this article to my dad, Joe, who passed away unexpectedly on June 10, 2007. My dad was in the electrical construction industry for more than 50 years and worked up until two days before his passing. He introduced me to this business at a very young age, encouraging me to further my education as an engineer. I will truly miss his guidance and inspiration.
