## Commutation and Interpole & DC Generator Characteristics

Table of Contents

Commutation and Interpole in Machine, what is Commutation, DC Generator Characteristics

## Commutation and Interpole in Machine

The Commutation process would produce Excessive Sparking in larger machines, which would wear down the brushes and release noxious gases (like ozone) that Encourage corrosion. It is typical to employ Distinct Commutation interpoles in these circumstances. These are independent, Typically slender or visually apparent vestigal pole pieces that transport Armature current. They are set up so that the Interpole’s flux drives the Commutated coil’s current in the right direction.

The Interpole is in the ideal location to affect Commutation because the coil being Commutated is physically Situated between the active poles. The Armature current is wound around the Interpole (it is in series with the main brushes). It is clear that for a current to be Commutated, the Interpole needs to have a flux density Proportional to that current. If the right number of turns are made around the Interpole, Commutation can be made to be quite Accurate because the speed with which the coil must be Commutated is Proportional to Rotational Velocity and the voltage induced by the Interpole.

**DC Generator Characteristics**

The three most important Characteristics or curves of a D.C Generator are:

**1. Open Circuit Characteristic (O.C.C.)**

This curve depicts the relationship between the field current (If) at constant speed and the Generated emf (E0) at no load. Magnetic Characteristic or no-load Saturation curve are other names for it. Regardless of whether it is a standalone or Self-excited Generator, they all have essentially the same shape. The information needed to create the O.C.C. curve is Experimentally obtained by running the generator at a constant speed and no load while observing the change in terminal voltage as the field current was changed.

**2. Internal or Total Characteristic (E/Ia)**

This curve depicts the relationship between the armature current and the generated emf on load (E) (Ia). Due to the Armature Reaction’s Demagnetizing effect, the emf E is lower than E0. The open circuit characteristic will be below this curve as a result (O.C.C.) It cannot be directly obtained through experimentation. It is due to the voltage drop in the armature resistance, which prevents a voltmeter from reading the emf generated on load. If winding resistances are known, the internal characteristic can be determined from the external characteristic since the armature reaction effect is included in both Characteristics.

**3. External Characteristic (V/IL)**

The relationship between the Terminal voltage (V) and load current is Depicted by this curve (IL). Due to the voltage drop in the armature circuit, the terminal voltage V will be lower than E. This curve will therefore be lower than the internal Characteristic. This quality is crucial in Determining whether a Generator is appropriate for a particular use. You can get it by working simultaneously.

**4. No-load Saturation Characteristic (E0/I***f*)

*f*)

It is also known by the names open circuit Characteristic and Magnetic Characteristic (O.C.C). And It illustrates the relationship between the field or exciting current, if at a given fixed speed, and the no-load generated emf in the armature, E0. It is merely the Demagnetization curve for the Electromagnets’ material. For all Generators, whether separately excited or Self-excited, its shape is Essentially the same.

A typical no load saturation curve is shown in Figure. It has generator output voltage plotted against field current. The lower straight line portion of the curve represents the air gap because the magnetic parts are not saturated. When the magnetic parts start to saturate, the curve bends over until complete saturation is reached. Then the curve becomes a straight line again.

**5. Separately-Excited Generator**

A separately excited generator’s No-load saturation curve will look like the figure above. As long as the poles are unsaturated, it is obvious that when it is increased from its initial low value, the flux and consequently the generated emf, for example, increase directly as current. In Figure, the straight portion represents this. However, as the flux density rises, the poles become saturated, requiring a larger increase in If than on the lower part of the curve to produce a given rise in voltage. The upper part of the curve bends as a result.

##### In the figure above, the O.C.C curve for self-excited generators, whether shunt or series wound, is displayed. Even when If = 0, there is still some emf (=OA) produced because of the residual magnetism in the poles. As a result, the curve begins slightly to the up. Due to magnetic inertia, the lower end has a slight curve. It can be seen that the curve’s initial portion is essentially straight. This is because the iron path’s reluctance is so negligible at low flux densities that the air gap’s constant reluctance is what determines total reluctance. As a result, the flux and resulting emf are directly proportional to the exciting current.

However, at high flux densities, where μ is small, iron path reluctance becomes appreciable and straight relation between E and If no longer holds good. In other words, after point B, saturation of pole starts. However, the initial slope of the curve is determined by air-gap width. O.C.C for higher speed would lie above this curve and for lower speed, would lie below it.

Separately-excited Generator Let we consider a separately-excited generator giving its rated no-load voltage of E0 for a certain constant field current. If there were no armature reaction and armature voltage drop, then this voltage would have remained constant as shown in Figure by the horizontal line 1. But when the generator is loaded, the voltage falls due to these two causes, there by giving slightly dropping characteristics. If we subtract from E0 the values of voltage drops due to armature reaction for different loads**,** then we get the value of E-the emf actually induced in the armature under load conditions. Curve 2 is plotted in this way and is known as the internal characteristic.

In this generator, because field windings are in series with the armature, they carry full armature current Ia. As Ia is increased, flux and hence generated emf is also increased as shown by the curve. Curve *Oa *is the *O.C.C*. The extra exciting current necessary to neutralize the weakening effect of armature reaction at full load is given by the horizontal distance *ab*. Hence, point *b* is on the internal characteristic.

**6. External Characteristic (V/I)**

It is also known as a voltage-regulating curve or a performance characteristic. It reveals the relationship between the load current I and the terminal voltage V. Because it accounts for the voltage drop over the armature circuit resistance, this curve is below the internal characteristic. IaRa is subtracted from the corresponding values of E to produce the values of V. In determining whether a generator is suitable for a given purpose, this characteristic is crucial. You can get it in two different ways.

By making simultaneous measurements with a suitable voltmeter and an ammeter on a loaded generator or

Graphically from the *O.C.C* provided the armature and field resistances are known and also if the demagnetizing effect or the armature reaction is known.

The external characteristic curves for generators with various types of excitation are shown in the figure above. The output voltage will decrease with increased load current as shown if a separately excited generator is driven at a constant speed with a fixed field current. The armature resistance and armature reaction effects are to blame for this decrease. The generated voltage would tend to remain constant if the field flux did as well, and the output voltage would be equal to the generated voltage less the IR drop of the armature circuit. However, the demagnetizing element of armature reactions has a tendency to reduce the flux,. Adding another element that lowers the output voltage.