Armature Reaction in DC Machine

In a DC machine, carbon brushes are placed at the magnetic neutral axis, which matches the geometrical neutral axis when there’s no load. When the machine is loaded, the armature flux moves along the inter-polar axis, forming a triangular wave shape. This flux, directed along the brush axis, causes cross magnetization of the main field, concentrating flux at the trailing pole tip in generators and the leading pole tip in motors.
Armature reaction is defined as the effect of the armature flux on the main flux. In a DC motor, this reaction strengthens the flux at the leading pole and weakens it at the trailing pole tips.

What is Leading and Trailing Pole tip?

The leading tip is where armature conductors first interact with the pole, while the trailing tip is opposite. For instance, if a motor rotates clockwise, the North Pole’s lower tip is the leading tip, and the South Pole’s upper tip is the leading tip. When the direction reverses (like in a generator), the tips swap. Cross magnetization shifts the magnetic neutral axis with the load—along the rotation direction in DC generator and opposite in DC motor. If brushes stay in their previous positions, the back emf in motors or generated emf in generators drops, causing heavy sparking during commutation. This happens because the coil under commutation switches its pole influence, rapidly changing current direction and inducing high reactance voltage, resulting in heat and sparking. To mitigate these issues and enhance performance, several methods are used:

Brush Shift

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:
Where,
I_a = 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.

Inter Pole

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.

Compensating Winding

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.

NOTE:

1. 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.
2. Inter polar winding and compensating windings are connected in series with the armature winding but on the opposite sides with respect to armature.
3. 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.