Pages i - ix , to , to , to , to , to , to , to , to , to , to , Index. The interpole coil in a motor is connected to carry the armature current the same as in a generator. As the load varies, the interpole flux varies, and commutation is automatically corrected as the load changes. It is not necessary to shift the brushes when there is an increase or decrease in load.
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For understanding the role of Interpoles, we need to understand the effect of armature reaction in the DC Machine. The effect of armature mmf on the main field flux is to distort the main field flux and to reduce the net main field flux.
The figure below, shows the effect of armature mmf on the main field flux. It is quite clear from the above figure that the flux at the location of Carbon Brush i. A, B and A are not zero and therefore an EMF will be induced in the coils undergoing commutation and will lead to the sparking.
As we know that for better commutation, the coils short circuited by the brushes should have zero EMF induced in them. As the zero crossing of field flux is shifted due to armature reaction, the coils undergoing the commutation will have a net EMF induced in them. This induced EMF in the short circuited coil will delay the reversal of current in the short circuited coils and will result into poor commutation and sparking at the carbon brushes.
So the cheap and easy solution shall be to shift the Carbon Brush at Zero Crossing of the air gap flux. But this method of shifting the Carbon brush has a big disadvantage.
What is that? The Poles used in our smart solution is called the Interpole. For generator, the polarity of Interpoles must be same as that of main Pole ahead of it in the direction of rotation.
For Motor, the polarity of Interpole must be same as that of Main Pole behind it. So I expect that you understand the purpose of Interpoles as you only designed it.
But there is one more interesting role of Interpole. Interpole do not only nullify the effect of armature reaction but in addition, produces some extra mmf in the interpolar zone. This extra mmf in the interpolar zone induces rotational EMF in the short circuited coil undergoing commutation in such a direction to oppose the reactance voltage in the coil. Thus the resultant the resultant voltage in the short circuited coil becomes zero and the commutation is spark less.
Your comment and feedback is important to me. Thank you! Hi Aditya, thanks for your blog. If a dc motor with interpoles is driven as a generator over run the interpole polarity reverses so sparking is still reduced? How is the interpole winding calculated? Thank you Howard Simpson! Yes, the sparking will still reduce when the machine is used as Generator.
The winding calculation of interpole is quite easy. I hope you understood the concept of Interpoles in DC Machines. Since it is used to overcome the flux present at Geometric Neutral Axis GNA say x weber, therefore the winding of interpoles should be such as to make this flux x to zero.
If the idea behind interpoles is to completely nullify the armature flux, then how does the motor spin at all? If that is the case then how would the machine work if one of those magnetic fields were completely cancelled out ideally.
Use of Interpoles nullifies the armature flux at the location of carbon brush only. So operation of DC machine is not affected but their performance enhances by use of Interpoles. Which is the good practi practice to change direction of DC motor.?
Inter changing armature or feild connections? What happens if the interpoles are connected in short circuit? Does this affect the power, voltage or something else? Notify me when new comments are added. The question arises how to resolve this issue? As the Armature Reaction depends on the current flowing through the armature winding which in turn depends on the load current. So we need to find a smart way. Again, looking back to the figure, if it could be possible to make the resultant or net air gap flux zero at GNA, then there would not have been any detrimental effect of armature reaction on commutation.
Yes, this will work fine. So we can conclude our solution as,. Cheers, Howard in NZ. Thank you very much for your nice blog sir Reply. One question, If the idea behind interpoles is to completely nullify the armature flux, then how does the motor spin at all? Does Interpole flux effect the flux under polar region Reply. Ideally it should not affect the flux under pole. It is meant to work in interpolar region. How to check polarity of armature and interpole winding in a motor?
Armature cross flux is neutralized by the Interpoles in DC Machine.
Purpose of Interpole in DC Machine
Interpoles Author: E. To avoid the commutation difficulty actually encountered in old-style generators, narrow poles called interpoles or commutating poles are used, as indicated in Fig. These interpoles, being midway between main poles, are just opposite the armature poles shown in Fig. 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. 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. The inductance of the coil tends to retard the reversal of the coil current by generating a prolonging voltage.
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