Motor control starters

A third group includes silver—nickel, silver—cadmium oxide, and silver—graphite combinations. These materials are characterized by low contact resistance, some resistance to arc erosion, and excellent non sticking characteristics. They can be considered intermediate in overall properties between silver alloys and silver or copper—refractory compositions. Silver—cadmium oxide compositions, the most popular of this class, have wide appHcation in aircraft relays, motor controllers, and line starters and controls. 

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. 

Direct-Current Motor Control Control for dc motors runs the gamut from simple manual line starters to elaborate regulating systems. Only the starting problems are considered here since variable-speed drives and regulating systems are discussed elsewhere. 

The engineer providing the initial design of major facilities is faced with the decision of providing a motor control center building or indi id-ual (usually rack-mounted) motor starters and corresponding branch circuit protection devices. For installations using only several motors it fre-... 

For facilities that include large numbers of motors and other electrical equipment, it normally is both more economical and more convenient to furnish a building to enclose the required motor starters and distribution panels. This building is normally referred to as a motor control center (MCC). In addition to typically allowing less expensive non-explosion-proof equipment, these buildings are frequently environmentally controlled (air conditioned, and possibly heated in colder climates) to reduce equipment corrosion and enhance reliability. Maintenance is more easily performed indoors than if the equipment were installed outside and maintenance personnel were subject to extreme cold, rain, snow, or other adverse weather conditions. 

Four main units are used in the IMCS, which are the motor control unit (MCU), the feeder control unit (FCU), the circuit breaker control unit (CBCU) and the central control unit (CCU). A MCU is a microprocessor (micro-computer) based module which has integrated control, monitoring, protection functions, and a communication interface for the motor starter. An FCU is very similar to a MCU and interfaces communication for the plain feeder contactor or circuit breaker. A CBCU is also similar to a MCU but is used for incomers, interconnectors and busbar section circuit breakers. A CCU provides the facility to communicate simultaneously with MCUs, FCUs, CBCU, a distributed control system (DCS), system control and data acquisition (SCADA) and other digital information systems. Other discrete devices such as special protective relays can also be addressed by the CCU provided the software and porting systems are compatible. 

Some motor control centres are designed to allow the motor starters to reclose upon the restoration of the main busbar voltage following a supply disturbance. This is especially necessary for emergency and essential loads, e.g. cooling water pumps and lube oil pumps for engine-driven generators. If the... 

Electrical units. Transformer (main incoming), metering panel and switch yard, electrical motors, motor control center, cables, starters with remote actuators, lighting systems, etc. 

Motor control centre, safe laying of all electrical cables and wiring, positions of starters and push buttons (these shall be at suitable places)... 

In general, redundant motor starters are not used. Redundancy is applied in the form of contacts in the motor control circuit. Auxiliary contacts may be fed back to the SIS to verify starter status (position). 

A process technician typically traces power to the unit from a motor control center (MCC). The primary components of an electrical system are the MCC, motors, transformers, breakers, fuses, switch gears, starters, and switches. Specific safety rules apply to the operation of electrical systems. The primary safety system is the isolation of hazardous energy through lock-out/tag-out measures. Process technicians are required to have training in this area. Figure 7-22 shows the basic symbols and flow path associated with an electrical drawing. Electrical lines are typically run in cable trays to switches, motors, ammeters, substations, and control rooms. 

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... 

Manual starters are typically used on small motors in applications requiring infrequent starting. In general, comments applying to magnetic controls also pertain to manual motor controls. 

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. 

Solid-state starters usually incorporate additional features and functions providing additional motor control. These may include such things as overvoltage, overcurrent, phase-loss, phase-reversal, and overtemperature protection. 

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. 

Varying factors such as temperature, humidity, and atmospheric contamination may adversely affect the performance of motor controls. Misapplication of a control may also lead to serious trouble and is often regarded as the major cause of motor control problems. Visual inspection every 6 months or so, and less-frequent electrical checks with the proper instruments, will help to ensure that production will not be interrupted because of a starter failure that could have been prevented. 

Solid-state red need-voltage starters and controllers typically comprise only two main sections, power and logic. Faults to the motor control will occur in one of these two places. When a problem occurs in a motor control system, it is important to consider all aspects including the motor control, control wiring, sensing switches, programmable control eqnipment, etc. 

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. 

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. 

Motor control gear can often be located in a safe area remote from the motors in a flammable zone. For many processes where there is centralised control it is often an advantage to locate the starters in motor control centres, which can usually be situated in a safe area. 

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. 

The main motor controller is a separate controller, and depends upon the type of installation and motor. The motor could be AC, DC or inverter type. Rarely, it could be a hydraulic motor. The starter could be DOL (direct-online), particularly if a fluid coupling is fitted, it could be a soft-start inverter system, or a DC system. With an Inverter system the back-drive, also an inverter type, could be connected through the DC bus to allow power regeneration. The starter Itself could be actuated by a separate master system. Undoubtedly there will be interlocks with the starter, to cause it to de-energise with certain scenarios. 

Electrical. The plant electrical system is sometimes more important than the steam system. The electrical system consists of the utihty company s entry substation, any ia-plant generating equipment, primary distribution feeders, secondary substations and transformers, final distribution cables, and various items of switch-gear, protective relays, motor starters, motors, lighting control panels, and capacitors to adjust power factor. 

Conveyor installations maybe permanent or a combination of permanent and portable. The latter kind is often mounted on a bulk-deliveiy vehicle, which permits fast unloading into the customers silo by the carrier without effort or equipment from the customer. Controls range from simple motor starters and hand-connected hoses to sophisticated, microprocessor-elec tropueumatic control systems. 

Package-Handling Systems The control of packagehandling systems may depend on simple motor starters, on interlocked relays with photocell control, or on computers. Sohd-state controls are finding much application in the last two systems. 

---