ELECTRIC MOTOR CONTROLS
Once the proper motor is selected, understanding the many various control devices available and their uses and limitations becomes an important part related to reliable operation and protection of the motor and the personnel using the motor. Motor Control Topics There are four major motor control topics or categories to consider. Each of these has several subcategories and sometimes the subcategories overlap to some extent. Certain pieces of motor control equipment can accomplish multiple functions from each of the topics or categories. C The four categories include: 1) Starting the Motor Disconnecting Means Across the Line Starting Reduced Voltage Starting 2) Motor Protection Overcurrent Protection Overload Protection Other Protection (voltage, phase, etc) Environment 3) Stopping the Motor Coasting Electrical Braking Mechanical Braking 4) Motor Operational Control Speed Control Reversing Jogging Sequence Control • An understanding of each of these areas is necessary to effectively apply motor control principles and equipment to effectively operate and protect a motor.
All motors must have a control device to start and stop the motor called a “motor controller”. Motor Controller A motor controller is the actual device that energizes and de-energizes the circuit to the motor so that it can start and stop. • Motor controllers may include some or all of the following motor control functions: S S starting, stopping, over-current protection, overload protection, reversing, speed changing, jogging, plugging, sequence control, and pilot light indication. Controllers range from simple to complex and can provide control for one motor, groups of motors, or auxiliary equipment such as brakes, clutches, solenoids, heaters, or other signals.
Motor Starter The starting mechanism that energizes the circuit to an induction motor is called the “starter” and must supply the motor with sufficient current to provide adequate starting torque under worst case line voltage and load conditions when the motor is energized. • There are several different types of equipment suitable for use as “motor starters” but only two types of starting methods for induction motors: 1. 2. Across the Line Starting Reduced Voltage Starting
Across the Line Starting of Motors Across the Line starting connects the motor windings/terminals directly to the circuit voltage “across the line” for a “full voltage start”. • • This is the simplest method of starting a motor. (And usually the least expensive). Motors connected across the line are capable of drawing full in-rush current and developing maximum starting torque to accelerate the load to speed in the shortest possible time. All NEMA induction motors up to 200 horsepower, and many larger ones, can withstand full voltage starts. (The electric distribution system or processing operation may not though, even if the motor will).
Across the Line Starters 2
There are two different types of common “across the line” starters including 1. Manual Motor Starters 2. Magnetic Motor Starters
Manual Motor Starters
A manual motor starter is package consisting of a horsepower rated switch with one set of contacts for each phase and corresponding thermal overload devices to provide motor overload protection. • • • The main advantage of a manual motor starter is lower cost than a magnetic motor starter with equivalent motor protection but less motor control capability. Manual motor starters are often used for smaller motors - typically fractional horsepower motors but the National Electrical Code allows their use up to 10 Horsepower. Since the switch contacts remain closed if power is removed from the circuit without operating the switch, the motor restarts when power is reapplied which can be a safety concern. They do not allow the use of remote control or auxiliary control equipment like a magnetic starter does.
Figure 26. Manual Starter
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