Static control, motors

Another concept is brushless excitation, in which an ac generator (exciter) is direc tfy coupled to or mounted on the motor shaft. The ac exciter has a stator field and an ac rotor armature which is directly connected to a static controllable rectifier on the motor rotor (or a shaft-mounted drum). Static control elements (to sense synchronizing speed, phase angle, etc.) are also rotor-mounted, as is the field discharge resistor. Changing the exciter field adjusts the motor field current without the necessity of brushes or slip rings. Brushless excitation is suitable for use in hazardous atmospheres, where conventional brush-type motors must have protective brush and slip-ring enclosures. 

Static controls and braking of motors 6/163 T(, = braking torque in mkg... 

With the availability of V/f drives and other advanced technologies through static controls, as discussed in Sections 6.2 to 6.4, the use of standard squirrel cage motors for such applications is a preferred choice. 

A DC motor is feedback-controlled by a current sub-control loop and a primary speed control loop. In order to close the control loop, the actual current value is fed back to the current control loop and a speed signal to the speed control loop. While current is measured in the power converter, a shaft encoder on the motor is required for speed signal feedback. Either a tacho-generator or a digital encoder is used as a speed transmitter. If speed measurement accuracy is not very important, the speed feedback can be measured via armature voltage. In this case, this measurement can also be done within the power converter. Static control accuracy reaches... 

Static controller A controller in which the major portion of all of the basic functions are performed through the control of electric or magnetic phenomena in solids such as transistors, etc. Synchronous motor coutroller A controller consisting of a three-pole starter for the ac stator circuit, a contactor for the dc field circuit, an automatic synchronizing device to control the dc field contactor, and a cage-winding protective relay to open the ac circuit without synchronizing, in order to start a synchronous motor, accelerate it to synchronous speed, and synchronize it to supply frequency. 

A static controller may be used to control the operating speeds of wound-rotor motors. In one method, a controlled saturable reactor is placed in the rotor circuit with the accelerating resistance. For fixed operating speeds, reactor saturation (which controls the motor speed) can be varied using a control resistor. Static controllers can also be used to reverse the direction of motor rotation by placing saturable reactors in the motor primary circuit rather than reversing contactors. Controlled reactor saturation directs the reversal of the motor rotation. 

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