Compensating Winding

Compensating Winding

A compensating winding is used in large DC machines with weak magnetic field excitation and is associated with significant load fluctuation like rolling mills, turbo generators, etc.

The function of commutating winding is to neutralise the cross-magnetizing effect of armature reaction. It provides uniform flux distribution under the pole faces.

The compensating winding is also known as auxiliary winding embedded in slots located in the main pole faces illustrated in the figure below.

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In the absence of compensating winding, the flux shifts backward or forward with a change in load condition. This shifting of flux induces statically induced EMF in the armature coils.

The magnitude of induced EMF depends on the rapidity of change in load and the amount of difference. And the magnitude of this EMF is as high as to strike an arc between the consecutive commutator segments.

The arc may be converted into a flashover around the commutator and results in short-circuiting the entire armature.

The compensating winding is connected in series with armature in such a way that the current’s direction in compensating winding in any pole face will be opposite to the direction of current through the adjacent armature conductors.

The compensating winding is designed in such a way that the MMF produced by the compensating winding is equal to the MMF produced by the number of armature conductors that lie under a pole face.

While calculating the conductors per pole face in compensating winding, the current that passes through the compensating conductor is the armature current Ia. And the current that passes through the armature conductor is the armature current Ia divided by the number of parallel paths A.

The interpoles and compensating windings both are used to minimise the effect of armature reaction. For large and/or high-speed machines, armature reaction creates the most severe conditions.

A compensating winding increases the cost of a machine and doubles the armature copper loss. But by adding compensating windings, the machine can handle the most violent load fluctuation.

Hence, 0.5 kW, 1500 RPM or 30 kW, 150 RPM machines do not require interpole or compensating windings. 5 kW, 1500 RPM machines required only interpoles and 100 kW, 1500 RPM must require both interpole and compensating windings.

It is used in DC generators and motors of ward-Leonard sets, especially in steel mills.

Number of Compensating Winding

It must be noted that the compensating windings must provide sufficient MMF to counterbalance the armature MMF.

Zc = Number of compensating conductors per pole face
Za = Number of active armature conductors per pole
Ia = Total armature current
Ia/A = Current passes through each armature conductor

Number of armature conductors = \frac{Z}{P}

Number of armature turns per pole = \frac{Z}{2P}

Therefore,

Number of armature turns immediately under one pole = \frac{Z}{2P} \times \frac{pole arc}{pole pitch} = \frac{Z}{2P} \times 0.7 (approx.)

Hence,

Number of armatures turns per pole for compensating windings;

    \[ Z_C = 0.7 \times \frac{Z}{2P} = 0.7 \times Armature \, Amp-turns/pole \]

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