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.e. 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.
If we see the figure above, we observe that there is a net shift of zero crossing of net flux in the air gap by an angle Ɵ in the direction of rotation for Generator and opposite to the direction of rotation for Motor. 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.
Interpoles are narrow poles placed at the GNA and fitted to the Yoke and also known as Commutating Poles or Compoles. 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.
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14 thoughts on “Purpose of Interpole in DC Machine”
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?
Cheers, Howard in NZ.
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. Let it be N1 and armature current be I so
N1I = x
Thus N1 i.e. interpole winding can be calculated.
pls ask if still you have any doubt. Thank you!
Thank you very much for your nice blog sir
If the idea behind interpoles is to completely nullify the armature flux, then how does the motor spin at all? Isn’t the principle of DC machine operation the interacting of two magnetic fields to produce motion? 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. It doesn’t remove the whole magnetic flux. So operation of DC machine is not affected but their performance enhances by use of Interpoles.
Does Interpole flux effect the flux under polar region
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?
Which is the good practi practice to change direction of DC motor.? Inter changing armature or feild connections?
sir, i have a dought.which effect is reduced by interpoles demagnetising effect or cross-arm
Armature cross flux is neutralized by the Interpoles in DC Machine.
What happens if the interpoles are connected in short circuit? Does this affect the power, voltage or something else?
You mean, both the interpoles short circuited?
If a self excited DC shunt motor is made to run as a DC shunt generator in the same direction as of direction of rotation of a DC motor, will it work? Will the polarity of main field poles and interpoles reverses automatically? Can we run it as generator in any direction?