Armature

The 20 wt % Mo Remahoy was used primarily in a single product, as the bias magnet in an armature-type telephone receiver which was produced in more than 10 units aimuahy. Because hot forging is necessary in its manufacture, Remahoy receiver magnets have been replaced by the cold-formable Cr—Co—Fe magnets. 

Solenoid Valves The electric solenoid valve has tw o output states. Wlien sufficient electric current is supplied to the coil, an internal armature moves against a spring to an extreme position. This motion causes an attached pneumatic or hvdraiilic valve to operate. Wlien current is removed, the spring returns the armature and the attached solenoid valve to the deenergized position. An intermediate pilot stage is sometimes used when additional force is required to operate the main solenoid valve. Generallv, solenoid valves are used to pressurize or vent the actuator casing for on/off control-valve application and safetv shutdown applications. 

Design In stirred miUs, a central paddle wheel or disced armature stirs the media at speeds from 100 to 1500 r/min. The media oscillate in one or more planes and commonly rotate veiy slowly. 

In the Attritor (Union Process Inc.) a single vertical armature rotates several long radial arms. These are avaUatle in batch, continuous, and circiilation types. Morehouse-Cowles media miUs comprise a... 

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. 

Direct-current motor fields are on the stator. The rotor is the armature. The magnetic field does not rotate like the field in ac machines. Current in the armature reacts with the stator field to produce torque. 

E = applied voltage, V V = counterelectromotive force (generated voltage), V R = armature resistance, H I = armature current, A k = constant dependent on motor design n = speed, r/min ( ) = magnetic-field flux... 

The major differences between ac and dc starters are necessitated by the commutation limitation of dc motors, which is the ability of the individual commutator segments to interrupt their share of armature current as each segment moves away from the brushes. Normally 250 to 275 percent of rated current can be commutated safely. Since motor-starting current is limited only by armature resistance, line starting can be used only for veiy small [approximately 1492-W (2-hp)] dc motors. Otheiwise, the commutator would flash over and destroy the motor. External resistance to limit the current must be used in starting to prevent this. 

Manual rheostats can be used in series with the motor armature for the current-limiting func tion. If the rheostat has ample thermal capacity, it can also Be usedto vaiy speed. If this system is used, interlocks should be included to prevent closing of the contactor unless maximum resistance is in the circuit. 

The above methods provide speed variation in steps, as in squirrel cage motors or in two machines or more, as in frequency converters, and cannot be u.sed for a process line, which requires frequent precise speed controls. Until a few years ago there was no other option with all such applications and they had to be fitted with d.c. motors only. D.C. motors possess the remarkable ability of precise speed control through their separate armature and field controls. In d.c. motors the speed... 

Armature voltage control (Field strengthening region)... 

With years of research and development in the field of static drives, it is now possible to identify and separate the.se two parameters (f, and /, ) and vary them individually, as in a d.c. machine, to achieve extremely accurate speed control, even slightly better than in d.c. machines. In d.c. machines the armature current and the field strength arc also varied independently. A.C. machines can now be used to provide very precise speed control, as accurate as 0.001% of the set speed, with closed-loop feedback controls. This technique of speed control is termed I ield-oriented control (FOC) and is discussed below. 

For field-oriented controls, a mathematical model of the machine is developed in terms of rotating field to represent its operating parameters such as /V 4, 7, and 0 and all parameters that can inlluence the performance of the machine. The actual operating quantities arc then computed in terms of rotating field and corrected to the required level through open- or closed-loop control schemes to achieve very precise speed control. To make the model similar to that lor a d.c. machine, equation (6.2) is further resolved into two components, one direct axis and the other quadrature axis, as di.sciis.sed later. Now it is possible to monitor and vary these components individually, as with a d.c. machine. With this phasor control we can now achieve a high dynamic performance and accuracy of speed control in an a.c. machine, similar to a separately excited d.c. machine. A d.c. machine provides extremely accurate speed control due to the independent controls of its field and armature currents. 

Remain constant for speed variations within the required speed N, as the field current is kept fixed and only the armature voltage is varied. For speed variations beyond N. however, when the armature voltage is kept constant and the field current is varied, the magnetizing losses afso vary... 

Armature current (j) = Torque curve (2) = Output curve... 

The armature of the machine will normally have a residual voltage of around 8 V (for LT machines) across the terminals when running at the synchronous speed. If not, as when the generator is operated after a long shutdown, a d.c. voltage of 12 V can be applied through a battery for a few seconds to obtain the required residual voltage. 

Rotating armature These have a rotating armature and a static field excitation system. The output from the armature is taken through the sliprings. 

This device controls the generator and maintains a steady-stale armature voltage automatically within the predefined limits. It also serves to control the reactive kVAr loading during a parallel operation or when the machine is being used as a synchronous condenser for reactive power compensation through a quadrature droop control (QDC) as noted below. 

At low p.f.s the generator operates at a low level of excitations (armature reaction demagnetizing). During a fault, therefore, when the p.f. of the circuit falls it will also cause a fall in the excitation level and in turn in the terminal voltage. A low voltage, however, would reduce the severity of the fault. 

---