Armature current

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. 

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. 

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

Fig. 4.14 DC servo-motor under armature control, e it) = Armature excitation voltage e it) = Backemf /a(t) = Armature current = Armature resistance = Armature inductance 6f = Constant field voltage if = Constant field current Tm = Torgue developed by motor 6 t) = Shaft angular displacement u] t) = Shaft angular velocity = dd/dt.

Anker, m. anchor (Elec.) armature, rotor, -spannung, /. (Elec.) armature voltage, -spule, /. (Elec.) armature coil, -strom, m. (Elec.) armature current. -wicklung, /. armature winding. 

Shunt connection (excitation independent of armature current),... 

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. 

ARMATURE CURRENT - The current flowing from the armature of a generator, to the armature of a motor. Not Including the current taken by the shunt field. 

ARMATURE DEMAGNETIZATION - The reduction in the effective magnetic lines of force, produced by the armature current. 

Motor torque is proportional to the armature current in shunt motors and to the square of the current in series motors. The conduction loss of the motor and servo-amplifier are both proportional to the current squared. Optimal efficiency is achieved by minimizing the form factor (Irms/lmean)- This can be done by increasing the switching frequency to reduce the ampHtude of the ripple. Benefits of increased efficiency are increased brush fife, gear life, and lower probabihty of field permanent magnet demagnetization. 

Duty cycle calculations where motor current is derived directly from the motor torque requirements are accurate for separately excited DC motors and synchronous motors, where torque current and armature current are equal. A double drum mine hoist operating at depth will typically have a duty cycle as represented in Figure 1. This duty cycle shows a transition from motoring to regeneration in the full speed zone of the shaft. 

Another popular rectifier circuit is the full-controlled three-phase full wave rectifier. This circuit is more expensive because six thyristors are used. However, the form factor is much better, about 1.01, and the ripple current is 360 Hz. The higher frequency makes it easier to filter the ripple current. The half-controlled three-phase bridge rectifier circuit may require armature current smoothing reactors to reduce the ripple current. Another problem associated with the non-uniform DC input to the motor is the commutation. The motor must commutate under a relatively high degree of leakage reactance. 

A potential safety hazard is the fact that with armature current feedback, the armature current is connected to the operator control and potentiometers maybe operated at high potentials (500 V). This problem can be eliminated using isolation transformers or DC to DC chopper circuits. 

Accelerating relay A relay nsed to aid in motor starting or accelerating from one speed to another. It may function by cnrrent-hmit (armature-current) acceleration connter-emf (armature-voltage) acceleration or definite-time acceleration. 

Short-circuit ratio In synchronous electric machines, the ratio of the field current for rated open-circuit armature voltage and rated frequency to the field current for rated armature current on a sustained symmetrical short circuit at rated frequency. 

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