Figure —Armature reaction. Now you have two fields — the main field, view A, and the field around the armature coil, view B. View C of figure shows . These interpoles, being midway between main poles, are just opposite the The interpole windings are connected in series with the armature. Stator of a DC machine. ▫ Stator is the stationary part. ▫ Main poles as field winding. ▫ Further windings. ▫ Interpole. ▫ Compensation eliminate unwanted effects.

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Gramme-ring wound armatures are seldom used for this reason. The field strength is independent of winidngs armature current. Automatic starters are not covered in detail in this module.


By reversing either field or armature connections. The armature poles are attracted to field poles of opposite polarity, causing the armature to rotate. They are the Gramme-ring and the drum-wound types.

To avoid the commutation difficulty actually encountered in old-style generators, narrow poles called interpoles or commutating inferpole are used, as indicated in Fig.


As the armature begins to turn, counter EMF increases; and, since the counter EMF opposes the applied voltage, the armature current is reduced. It may then be removed after counter EMF has been built up. Direction of armature current, and direction of magnetic flux in field. Unloaded operation of large machines is dangerous.

What current flows in the interpole windings? The interpole windings are connected in series with the armature windings, and therefore their correction ampere-turns increase or decrease in the same proportion as do the distorting ampere-turns of the armature. It interple have a load connected to avoid damage from excess speed.


In fact, most DC generators can be made to act as DC motors, and vice versa. This EMF opposes ihterpole applied voltage, and limits the flow of armature current. These interpoles, being midway interoole main poles, are just opposite the armature poles shown in Fig. It is not necessary to shift the brushes when there is an increase or decrease in load.

By winding the armature in a way that places the entire coil where it is exposed imterpole maximum flux. It is very difficult for a designer to calculate the exact number of turns to be placed on the interpoles, and it is sometimes necessary to connect an adjustable diverter around the interpole windings to permit shunting of a part of the total armature current around those coils.

In practice the interpoles are made stronger than just sufficient to neutralize the armature flux, in order to aid in the reversal of current in the coils being commutated.

Shunt-motor speed varies only slightly with changes in load, and wijdings starting torque is less than that of other types of DC motors. Its starting torque is greater than other types of DC motors. As the load varies, the interpole flux varies, and commutation is automatically corrected as the load changes.

What is the purpose of starting resistors? High torque turning force at low speed. If their polarities be made the same as those of the armature poles opposite to them, and if their strength be made equal to that of the armature poles, then the distortion of the main field at the commutating point, as in Fig.


Electrical Engineering is a free introductory textbook to the basics of electrical engineering. The device to be driven by the motor. The speed and torque characteristics are a combination of the desirable characteristics of both series and shunt motors.

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged. The proper division of surge current can be obtained by inserting, in series with the interpole shunt, a conductor wound around an iron core to form an inductance just equal to that of the interpole coils.

The inductance of the coil tends to retard the reversal of the coil current by generating a prolonging voltage.

In a generator the interpoles must be of the same polarity as the main poles which they precede in the direction of rotation. The following information provides a summary of the main subjects for review. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer.

Therefore, the interpole still has the proper polarity to provide automatic commutation. The field strength varies with changes in armature current.

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