In a DC machine, the carbon brushes are always placed at the magnetic neutral axis. In no load condition, the magnetic neutral axis coincides with the geometrical neutral axis. Now, when the machine is loaded, the armature flux is directed along the inter polar axis (the axis in between the magnetic poles)and is triangular in wave shape. This results an armature current flux directed along the brush axis and causes cross magnetization of the main field. This cross magnetization effect results in the concentration of flux at the trailing pole tip in generator action and at the leading pole tip in motor action.
The armature reaction is the effect of the armature flux on the main flux. In case of a DC motor the resultant flux is strengthened at the leading pole and weakened at the trailing pole tips.
What is Leading and Trailing Pole tip?
The tip of the pole from where the armature conductors come into influence is called leading tip and the other tip opposite in direction to it will be the trailing tip. For example, in the above figure if the motor rotates clockwise, then for North Pole, the lower tip is leading tip and for South Pole upper tip is leading tip. If the motion is reversed (in case of generator), the tips is interchanged. Due to cross magnetization, the magnetic neutral axis on load, shifts along the direction of rotation in DC generator and opposite to the direction of rotation in DC motor. If the brushes remain at their previous positions, then back emf in case of motor or generated e.m.f in case of generator would reduce and commutation would be accompanied by heavy sparking. This is because commutation occurs at the coils located on the brushes only, and the coil undergoing commutation comes under the influence of the alternate pole (changes its location from north to south pole or vice versa). Hence, the direction of current changes from +i to –i or vice versa in a small span of time. This induces a very high magnitude of reactance voltage (L × di/dt) in the coil which emerges out in the form of heat energy along with sparking, thus damaging the brushes and commutator segment. To reduce the adverse effects mentioned above and to improve the machine’s performance, following methods are used:
A natural solution to the problem appears to shift the brushes along the direction of rotation in generator action and against the direction of rotation in motor action, this would result into a reduction in air gap flux. This will reduce the induced voltage in generator and would increase the speed in motor. The demagnetizing mmf (magneto motive force) thus produced is given by:
Ia = armature current,
Z = total number of conductors,
P = total number of poles,
β = angular shift of carbon brushes (in electrical Degrees).
Brush shift has serious limitations, so the brushes have to be shifted to a new position every time the load changes or the direction of rotation changes or the mode of operation changes. In view of this, brush shift is limited only to very small machines. Here also, the brushes are fixed at a position corresponding to its normal load and the mode of operation. Due to these limitations, this method is generally not preferred.
The limitation of brush shift has led to the use of inter poles in almost all the medium and large sized DC machines. Inter poles are long but narrow poles placed in the inter polar axis. They have the polarity of succeeding pole (coming next in sequence of rotation) in generator action and proceeding (which has passed behind in rotation sequence) pole in motor action. The inter pole is designed to neutralize the armature reaction mmf in the inter polar axis. Since inter poles are connected in series with armature, the change in direction of current in armature changes direction of inter pole.
This is because the direction of armature reaction mmf is in the inter polar axis. It also provides commutation voltage for the coil undergoing commutation such that the commutation voltage completely neutralizes the reactance voltage (L × di/dt). Thus, no sparking takes place.
Inter polar windings are always kept in series with armature, so inter polar winding carries the armature current; therefore works satisfactorily irrespective of load, the direction of rotation or the mode of operation. Inter poles are made narrower to ensure that they influence only the coil undergoing commutation and its effect does not spread to the other coils. The base of the inter poles is made wider to avoid saturation and to improve response.
Commutation problem is not the only problem in DC machines. At heavy loads, the cross magnetizing armature reaction may cause very high flux density in the trailing pole tip in generator action and leading pole tip in the motor action.
Consequently, the coil under this tip may develop induced voltage high enough to cause a flash over between the associated adjacent commutator segments particularly, because this coil is physically close to the commutation zone (at the brushes) where the air temperature might be already high due to commutation process.
This flash over may spread to the neighboring commutator segments, leading ultimately to a complete fire over the commutator surface from brush to brush. Also, when the machine is subjected to rapidly fluctuating loads, then the voltage L× di/dt, that appears across the adjacent commutator segments may reach a value high enough to cause flash over between the adjacent commutator segments. This would start from the center of pole as the coil below it possesses the maximum inductance. This may again cause a similar fire as described above. This problem is more acute while the load is decreasing in generating action and increasing in motor action as then, the induced e.m.f and voltage L× di/dt will support each other. The above problems are solved by use of compensating winding.
Compensating winding consists of conductors embedded in the pole face that run parallel to the shaft and carry an armature current in a direction opposite to the direction of current in the armature conductors under that pole arc. With complete compensation the main field is restored. This also reduces armature circuit’s inductor and improves system response.Compensating winding functions satisfactorily irrespective of the load, direction of rotation and mode of operation. Obviously it is help in commutation as the inter polar winding gets relieved from its duty to compensate for the armature mmf under the pole arc.
Compensating windings major drawbacks:
- In large machines subject to heavy overloads or plugging
- In small motors subject to sudden reversal and high acceleration.
- The cross magnetizing armature reaction effect is mainly caused by armature conductors which are located under the pole arc. At high loads, this effect of armature reaction may cause excessive flux density in the trailing pole tip (in generator) and leading pole tip (in motor). Due to saturation in the pole shoe, the increase in flux density may be less than the reduction in the flux density in remaining section of the pole shoe. This would ultimately result into a net reduction in flux per pole. This phenomenon is thus known as the demagnetizing effect of cross magnetizing armature reaction, which is further compensated by the use of compensating windings.
- Inter polar winding and compensating windings are connected in series with the armature winding but on the opposite sides with respect to armature.
- The primary duty of inter polar winding is to improve the commutation process, and that of the compensating winding is to compensate for the increase or decrease in the net air gap flux i.e., to maintain its constant value.