Motor control overload

Overload Protection Overload relays lor protecting motor insulation against excessive temperature are located either in the motor control or in the motor itsell. The most common method is to use thermal overciirrent relays in the starter. These relays have heating characteristics similar to those ol the motor which they are intended to protect. Either motor current or a current proportional to motorline current passes through the relays so that relay heating is comparable to motor heating. 

High-voltage contactor-type motor controls depend on power fuses for short-circuit protection. The fuses are coordinated with the overload relays to protect the motor circuit over the full range of fault conditions from overload conditions to solid maximum-current short circmts. 

Material and utilities requirements for new products—will they overload existing system Can staggering of their running time be done to distribute the load more uniformly on motor control centres, electrical cables, hoists, DG sets, belt conveyors. Reactors, and cooling systems—if there are some common ingredients for old and new products ... 

SOLID-STATE VARIABLE-SPEED AC MOTOR CONTROLS CONTROL OF DC MOTORS DC Motor Starting and Braking Speed Control DC Drive Characteristics PROTECTION OF MOTOR CONTROLLERS Electrical Enclosure Types and Specifications Motor Overload (Running) Protection Other Types of Motor Protection Short-Circuit Protection Protection of Solid-State Motor Controls COMBINATION STARTERS Interrupting Rating... 

Current-responsive protector Devices that inclnde time-lag fuses, magnetic relays, and thermal relays (normally located in the motor or between the motor and controller) that provide a degree of protection to the motor, motor control apparatns, and branch-circuit conductors from overloads or failures to start. 

Solid-State Motor Controller. A solid-state motor controller is a device consisting of an electronic control assembly and one or more solid-state power assemblies. It is used with an electromechanical motor starter. A controller can be used for single-speed, multispeed, and reversing apphcations when the proper electromechanical device is also nsed. A controller typically has no means of motor overload protection or abihty to completely remove power from the motor. 

For a single-speed nonreversing apphcation, a motor controller is wired in series between a motor starter and motor. The motor starter provides the power isolation and overload protection while the controller switches the motor current on and off. 

Solid-State Motor Starter. A sohd-state motor Starter is a device consisting of an electronic control assembly, one or more sohd-state power assemblies, and motor overload protection. The main difference between a controUer and starter is that the starter can be used without any other motor control device. 

Motor controllers—commonly called starters—are usually magnetically ojterated devices with thermal overload protection built in through the application of melting ahoy Unks. A preventive maintenance program is essential for motor starters if they are to continue to function properly. Remote controls with operating hghts indicate only that the controls functioned. Therefore, it is necessary to make periodic inspections of these devices. 

Panelboards serving lighting and appliance circuits which must be switched on at fixed times and off at other times can well be equipped with magnetic contactors. These devices operate in a manner similar to motor controllers except they contain no overload devices. They permit remote switching of panelboards without using the breakers as a switching device. 

The starting of an induction motor does not relate to simple switching alone. It also involves its switchgears to control its starting inrush current, starting torque, or both, and its overload and short-circuit protection. 

When a pump has a motor drive, the process engineer must verify that the motor will not overload from extreme process changes. The horsepower for a centrifugal pump increases with flow. If the control valve in the discharge line fully opens or an operator opens the control valve bypass, the pump will tend to run out on its curve, giving more flow and requiring more... 

For centrifugal and axial compressors, some form of override control is recommended for constant speed motor drivers to sense motor over load and override the process control until the cause of overload has... 

In addition to the basic control loops, all processes have instrumentation that (1) sounds alarms to alert the operator to any abnormal or unsafe condition, and (2) shuts down the process if unsafe conditions are detected or equipment fails. For example, if a compressor motor overloads and the electrical control system on the motor shuts down the motor, the rest of the process will usually have to be shut down immediately. This type of instrumentation is called an interlock. It either shuts a control valve completely or drives the control valve wide open. Other examples of conditions that can interlock a process down include failure of a feed or reflux pump, detection of high pressure or temperature in a vessel, and indication of high or low liquid level in a tank or column base. Interlocks are usually achieved by pressure, mechanical, or electrical switches. They can be included in the computer software in a computer control system, but they are usually hard-wired for reliability and redundancy. 

Protective overrides can be added to optimized load-following controls, so that the system is protected against excessively low pressures on the suction side of the compressor or from overloading the compressor s motor drive. 

In addition to following the load, protective overrides prevent the development of excessively low suction pressures (PIC-02), which could result in drawing oil into the compressor and from overloading the drive motor and thereby tripping the circuit breaker (KWIC). In order to prevent reset windup when the controller output is blocked from affecting the SIC set point, external feedback (EFB) is provided for all three controllers. 

Select standard size motor. A motor that is loaded to 85 percent by a 79.1-hp impeller will require a minimum size of (79.1 hp)/0.85 = 93.1 hp, which means a 100-hp (75-kW) motor. This motor and impeller assembly is correctly sized for conditions with the design gas flow. However, because of the gassed power factor, that is, P/P0 = 0.38, should the gas supply be lost for any reason, the impeller power would increase to 78/0.38 = 205 hp and seriously overload the motor. To avoid this problem, some method (typically electrical control) prevents motor operation without the gas supply. When the gas supply is off, the control either stops the agitator motor or, in the case of a two-speed motor, goes to a lower speed. 

The control system is designed to sense certain component and system failures. Pressure switches are located to sense overpressure, excessive suction, low chlorine pressure, and low eductor vacuum. A flow switch senses low flow. Motor starters sense motor overloading. Evaporator low-temperature switch senses low water temperature. 

Motors. Certain fundamental considerations must be taken into account in selecting the correct motor and control for a given job. The first factor to consider is the size or horsepower rating of the motor required to handle the job with standard drives, most machinery builders have established the exact brake horsepower demanded by their particular machine, so that a motor of the nearest higher standard horsepower rating is recommended. On drives of an intermittent nature, where peak loads or momentary overloads and a frequent number of starts, stops, and/or reversals occur, the motor rating is based not only on torque capabilities... 

The very flexible nature of the thyristor controller allows the motor to have accurate control plus excellent overload protection. Most thyristor controllers are furnished with maximum current limits for motor armature current and for short-circuit current protection. During conditions of rapid acceleration or heavy load the armature current will rapidly become high and so the maximum current limiter will automatically hold the armature current until the duty is reduced. Thyristor controllers also make it possible to gain accurate control of the torque or load at zero speed. This is very desirable when handling anchors and the drill string. 

Jobs involving the operation of complex machinery (e.g., aircraft, construction, and factory control rooms) are primary applications of the perceptual/motor approach. Likewise, many product-inspection and equipment-monitoring jobs can tax attention and concentration capabilities of workers. But jobs in many other occupations may also impose excessive attention and concentration requirements. For example, some managerial, administrative, professional, and sales jobs can be excessively demanding on the information-processing capabilities of workers, thus causing errors and stress. And nearly all jobs have periods of overload. Perceptual/motor design principles can often be applied to reduce these demands of jobs. 

Hoists for long hft with motor driven trolley, cab or floor controlled, variable speed, mechanical or electric load braking, double reeving, single or twin load hooks, overload relays, AC current. FOB cost = 40000 for lifting capacity = 3 Mg with n = 0.82 for the range 2-20. For DC current X 1.05. L-tM = 1.5. L/M = 0.22. 

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