Motor controller

In addition to the controlling computer the system contains only a small control unit - PSP-4 (weight approx. 5 kg.) which among other system components includes a motor control system integrated closely with the PS-4 ultrasonic system. For communication between the PSP-4 control unit and the robot as well as robot power supply is used a single cable less than 10 mm. in diameter. 

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. 

For most plants, 01001 110 distribution systems start at the power company s service point on the plant s property. The choice of an electrical system will depend on many fartors relating to the particular projecl . A wide range of items, from switchgear, transformers, and motor control centers to cabling, earthing, and lighting, are required. Normally, competitive quotations would be obtained to take an estimate beyond the study stage. 

Air-Flow Control Process operating reqmrements and weather conditions are considered in determining the method of controlling air flow. The most common methods include simple on-off control, on-off step control (in the case of multiple-driver units), two-speed-motor control, variable-speed drivers, controllable fan pitch, manually or automatically adjustable louvers, and air recirculation. 

Frequently switchgear is used for motor control, particularly for large high-voltage motors. Switchgear (Fig, 29-6) must be used for motors... 

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. 

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. 

A rotor fails during start-up, possibly due to a lower supply voltage than desired or a smaller accelerating torque than required or reasons leading to similar conditions. In such cases the rotor tails first, due to higher rotor currents and a prolonged acceleration time or a locked rotor. At this instant, unless the motor control gear trips, the stator may also fail due to excessive heat. Instances can be cited where even... 

This field is very large and a detailed study of the subject is beyond the scope of this handbook. We will limit our discussions to the area of this subject that relates to the control of a.c. motors and attempt to identify the different solid-state devices that have been developed and their application in the control of a.c. motors. ()nly the more common circuits and configurations are discussed. The brief discussion of the subject provided here, however, should help the reader to understand this subject in general terms and to use this knowledge in the field of a.c. motor controls to achieve from a soft start to a very precise speed control and, more importantly, to conserve the energy of the machine which would be wasted otherwise. For more details of. statie controllers see the Further reading (Sr. nos. 2, 4., 5, 8 and 12) at the end of the chapter. To... 

Power MOSFF.Ts and IGBTs can handle much higher switching frequencies, compared o a thyristor. In an a.c. motor control, fast switching is mandatory and therefore transistors are preferred. 

Figure 6.47 Slip-ring motor control showing slip recovery system...

MCC - motor control centre. For details refer to Chapter 1.4. 

Figure 12.46 A simple power diagram for a 6.6 kV motor control with an isolator, contactor and a motor protection relay...

Lythall, R.T., AC Motor Control (on earth fault protection and thermistor protection). 

These may receive power from one or more sources of supplies and distribute them to different load centres, which may be a motor control centre (MCC) or a distribution board (DB), as illustrated in Figures 13.2... 

Figure 13.2 Details of a draw-out motor control centre (MCC) (Courtesy ECS)...

When there is only one process line and one MCC alone is adequate to control the entire process, it is possible to combine the PCC and the MCC into one unit to save on space and cost. The assembly may now be called a PMCC (power-cum-motor control centre). 

Lythall, R.T., Group motor control boards. Electrical Review 30 June (1961). 

Recommended practices for seismic gualitlcation of class IE equipment for nuclear power generating stations (NPGS) For qualifying class IE motor control centres for nuclear power generating stations (NPGS)... 

Time constant for open circuit voltage (when motor control will use delayed transfer to alternate sources on voltage loss). This value must include the effect of any capacitors applied on the load side of the motor controller. 

Switch on and off and control the speed of pump and fan motors. Control valves and dampers. 

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