Interpoles

Interpoles One way to reduce the effects of armature reaction is to place small auxiliary poles called "interpoles" between the main field poles. The interpoles have a few turns of large wire and are connected in series with the armature. Interpoles are wound and placed so that each interpole has the same magnetic polarity as the main pole ahead of it, in the direction of rotation.

Interpoles …. The field generated by the interpoles produces the same effect as the compensating winding. This field, in effect, cancels the armature reaction for all values of load current by causing a shift in the neutral plane opposite to the shift caused by armature reaction. The amount of shift caused by the interpoles will equal the shift caused by armature reaction since both shifts are a result of armature current.

MOTOR REACTION IN A GENERATOR When a generator delivers current to a load, the armature current creates a magnetic force that opposes the rotation of the armature. This is called MOTOR REACTION. When the conductor is stationary, no voltage is generated and no current flows. Therefore, no force acts on the conductor. When the conductor is moved and the circuit is completed through an external load, current flows through the conductor. This sets up lines of flux around the conductor in a direction opposite to its rotation; i.e; Motoring reaction

VOLTAGE CONTROL Voltage control is either manual or automatic. In most cases the process involves changing the resistance of the field circuit. By changing the field circuit resistance, the field current is controlled. Controlling the field current permits control of the output voltage. The major difference between the various voltage control systems is merely the method by which the field circuit resistance and the current are controlled.

VOLTAGE REGULATION VOLTAGE REGULATION should not be confused with VOLTAGE CONTROL. Voltage regulation is an internal action occurring within the generator whenever the load changes, while Voltage control is an imposed action, usually through an external adjustment, for the purpose of increasing or decreasing terminal voltage.

Field Rheostats

AMPLIDYNES AMPLIDYNES are dc generators that are designed to act as high-gain amplifiers. By short- circuiting the brushes in a normal dc generator and adding another set of brushes perpendicular to the original ones, an amplidyne is formed. Its power output may be up to 10,000 times larger than the power input to its control windings.

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Starting of DC Motors At  the  moment  a  DC  motor  is  started  the  armature  is  stationary  and  there  is  no  counter  EMF being generated.  The only component to limit starting current is the armature resistance, which, in most DC motors is a very low value (approximately one ohm or less), as shown in Equation

Starting DC Motor In order to reduce this very high starting current, an external resistance must be placed in series with  the armature  during the  starting period.   To  show why  this is  essential,  let us  consider a 10-hp motor with an armature resistance of 0.4 ohms.  If the motor were supplied by a 260 VDC source, the resulting current would be as shown in Equation given: Ia=(260-0)/0.4 =650 Amperes

Starting DC Motor This large current is approximately twelve times  greater  than  actual  full-load current for  this motor. The high current would, in all probability,  cause  severe  damage  to  the   brushes, commutator,  or  windings.   Starting   resistors  are  usually  incorporated  into  the   motor  design  to limit starting current to 125 to 200 percent of full load current .