Resistance armature

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. 

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.

Manual or magnetic with field and armature resistance. 

Where Rd and Rg are the resistances present in their respective axis, usually both are equal to R the armature resistance. 

Most practical generators have an armature resistance Ra that is much less in value than the synchronous reactances Xj and Xq. Consequently the equations in sub-section 3.5.2 can be further simplified without incurring a noticeable error. They become,... 

For most LV and all HV generators it is often acceptable to ignore the armature resistance as far as calculating the magnitude of first-cycle fault currents is concerned. It is usual to assume that the... 

X-to-R ratio of generators is high, e.g. between 20 (for LV generators) and 100 (for HV generators). However, the value of armature resistance is of most importance when considering the downstream circuit-breaker fault clearance capabilities. This aspect is described in sub-sections 7.2.7 and 7.2.11. The calculation of current magnitudes may be carried out in several ways depending upon the amount and accuracy of the data available. 

The above expression is adequate for data that are typically available for the industry. The armature resistance Ra is only present in the time constant Ta. (Krause offers a more complete solution in which the omission of Ra is minimised. The effect is then to modify the time constant Ta... 

ARMATURE RESISTANCE - The resistance of the wire used in the windings of the armature, measured between the rings or brushes, or from positive to negative terminals. 

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. 

Constant speed direct-current motors are usually started by decreasing the amount of an external resistance in the armature circuit. The starting current may exceed full-load current by about 50 per cent. From 15 to 30 sec. should be allowed to bring a motor up to speed. This may be insured by automatic operation. The rheostat may incorporate low-voltage and overload releases, and should contain the motor-field circuit closing mechanism. Ordinary rheostats (starters) cost from 3 to 6 per cent as much as motors, the smaller sizes costing relatively more. 

It is a particular characteristic in the solution of differential equations involving resistances and inductances that a DC component accompanies the symmetrical AC component. The magnitude of the DC component can equal that of the peak AC component since both are determined by X J. The decay of the DC component can be reasonably slow and is determined by which is a function of X J and the armature winding resistance Ra With machines that have significant values of X J and particularly low values of R, the value of Ta can become relatively high. When Ta is high in relation to Tj and rj it is possible that the initial AC decay is faster than the DC decay. When this happens the AC instantaneous current does not reach zero until several cycles have passed. This puts an extra strain on the circuit breaker and can cause problems at the point when it starts to open to clear the fault current. 

ARMATURE REGULATING RESISTORS - Are resistors, designed to regulate the speed or torque of a loaded motor, by placing a resistance in the armature or power circuit. 

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