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From Motors and Drives

Protection and Ratings

Motor Protection
The typical method of starting a three-phase induction motor is by connecting the motor directly across the power line. Line starting a motor is done with a three-phase contactor. To adequately protect the motor from prolonged overload conditions, motor overloads are installed, typically in the same enclosure as the three-phase contactor. These overloads (OLs) operate as heater elements-heating to the point of opening the circuit, and mechanically disconnecting the circuit (Figure 3-31).

Overloads can be purchased with a specific time designed into the element. Classes 10, 20, and 30 are the usual ratings for industrial use. A class 10 overload indicates that the overload will allow 600% inrush current for 10 s before opening the circuit. Class 20 overloads would allow 600% inrush current for 20 s, and a class 30 would allow 30 s of operation. The current draw from a typical induction motor, as well as the torque produced can be seen in Figure 3-32.

Line starting an induction motor, as shown in Figure 3-32, would allow the motor to develop rated torque, as soon as the motor starter button is pressed. This is because across the line, the motor has the benefit...


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Products & Services
Motor Starters and Contactors
Motor starters and contactors are motor-starting devices. They combine a contactor with overload protection and are designed for starting AC or DC motors.
AC Servomotors
AC servomotors are responsive, high-acceleration motors typically constructed as permanent magnet synchronous motors.
DC Motors
DC motors are most commonly used in variable speed and torque applications. They include brushless and gear motors, as well as servomotors.
AC Motor Drives
AC motor drives interface controllers to AC motors. They match the control signals (voltage and power levels) as well as the signal type (analog or digital). They also provide power conversion, amplification, and the sequencing of waveform signals.

Topics of Interest

AC Motor Types Introduction AC motors can be divided into two major categories-asynchronous and synchronous. The induction motor is probably the most common type of asynchronous motor (meaning speed...

IEC Ratings At this point, it would be helpful to briefly review IEC motor ratings and then compare IEC with NEMA. The motor market today has become more global, with IEC rated motors on equipment...

Over Temperature Conditions Placing the motor into overload conditions is one cause of over-temperature. High ambient temperatures and dirty or clogged air filters on the machine or motor blowers also...

Control of Speed, Torque, and Horsepower Control of Speed The speed of a squirrel cage motor depends on the frequency and the number of poles for which the motor is wound. The higher the frequency,...

AC and DC motors are the two major types in use today that are related to the industrial and HVAC applications. These motors provide the speed, torque, and horsepower necessary to operate the...