Motor control short-circuit

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... 

Main motors need starter overload and short circuit protection. High rupture fuses (HRC) will protect the motor against short circuit conditions, and will interrupt the electrical supply in milliseconds of the fault occurring. It is essential that fuses of this type are always fitted. Conventional overload protection, thermal or magnetic, can offer no protection to a motor with an extended acceleration time. Thermistor overload protection is the only true protection for a motor under these conditions. A thermistor is embedded in each of the motor s three windings and connected in series. The resistance of these thermistors is designed to increase rapidly at a set temperature, depending upon the insulation class of the motor. The thermistors are connected to an electronic amplifier control unit in the starter enclosure, and will trip the starter contacts when required. The device will not reset until the motor has sufficiently cooled. 

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. 

This is a very useful nomogram to determine the performance of a motor with the help of only no-load and short-circuit test results. In slip-ring motors, it also helps to determine the external resistance required in the rotor circuit to control the speed of the motor and achieve the desired operating performance. Slip-ring motors are discussed in Chapter 5. The concept behind this nomogram is that the locus of the rotor and the stator currents is a circle. Consider the equivalent circuit of an induction motor as shown in Figure 1.15, where... 

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. 

Brush lifting and short-circuiting devices can be employed for motors required to run continuously for a long period to minimize wear and tear of slip-rings and brushes. However, when speed control is required or switching operations are frequent, continuous contact brush gear assembly must be employed. 

A motor control centre (MCC) after short-circuit... 

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. 

Thyristor bridges are used where the DC output voltage needs to be varied. For example for control purposes such as varying the speed of motors or for protective purposes such as limiting the maximum DC output current that can flow when an external short circuit occurs. 

Electrical cable schedule Voltage drop calculations Electrical area classification Electrical instrument interface drawings Underground electrical cable and cable trench arrangement Electrical equipment layout drawings Motor control center and power layout Load and short-circuit analysis VSDs configuration Plant lighting plan... 

Provide dust control equipments for crushing and grinding units since the dust can enter electrical circuits, junction boxes of motors, any openings in starters or relays, and cause short circuits leading to breakdowns. The frequent plant stoppages can disturb process settings and the product quality. 

Motor control circuit The circuit that carries the electric signals directing controller performance but does not carry the main power current. Control circuits tapped from the load side of motor branch circuits short-circuit protective devices are not considered to be branch circuits and are permitted to be protected by either supplementary or branch-circuit overcurrent protective devices. 

Solid-State Overload Relays. These overload relays are of two types, self-powered and externally powered.The self-powered relay (Fig. 5.137) uses currents induced by the current flowing to the motor for power, while the externally powered units contain an electronic power supply and require a separate source of control power to operate. The advantage of the setf-powered units is that they are sinyler, require no extra wiring, and are self-protected from short circuits. 

For motors with greater than fractional horsepower ratings, devices such as fuses, circuit breakers, and self-protected combination controllers must be installed ahead of the motor control apparatus to protect branch-circuit conductors, motor control apparatus, personnel, and the motor itself against fault conditions that may be the result of short circuits or grounds. 

When a fault occurs, the overcurrent device must safely open and interrupt the fault. Overcurrent devices such as fuses, circuit breakers, and self-protected combination controllers, which interrupt fault currents, must have an interrupting rating equal to or greater than the available short-circuit current at their line-side terminals. Control devices, such as motor starters and overload relays, must have a short-circuit current rating equal to or greater than the available short-circuit current. That is to say, they must be able to withstand the fault current for the time it takes the overcurrent device to interrupt the fault. In practice, the actual current which flows during a fault is less than the available short-circuit current, and it is this current which the control device must withstand. 

Overcurrent devices and motor controls must be provided with marking which indicates their interrupting or short-circuit current ratings, as appropriate. For available currents above 10,000 A, discrete components, such as motor starters, must also be provided with marking which indicates the proper protective device which must be provided ahead of the component. This enables the user to properly coordinate the protective device and motor controls with the available short-circuit current. 

A disconnect switch or a nonautomatic circuit interrupter can be used to provide an electrical circuit disconnect function. Manual motor controllers marked Suitable as Motor Disconnect may also be used. Short-circuit protection is provided for these devices when fuses are added to the disconnect switch or thermal and/or magnetic trip units are added to the nonautomatic circuit interrupter. Self-protected combination controllers may be used to provide both the disconnect function and the short-circuit protection for motor circuits. 

Self-Protected Combination Controllers. A self-protected combination controller operates on the same principal as the thermal-magnetic circuit breaker. In addition to providing the short-circuit protection and disconnecting function, the self-protected combination controller also provides the motor control function. 

The common enclosure design (Fig. 5.141) and the use of combination starters offer both economy and ease of installation in multiple motor control installations. In addition, motor control centers (MCCs) provide proper coordination between short-circuit protective devices and the controller. Since MCCs are engineered systems, the components are closely coordinated to work together, and the unit is rated for a particular value of short-circuit interrupting duty at the point of its installation. MCCs may contain a molded-case circuit breaker and starter, or a fused switch and a starter. 

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