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# Rotor winding

## Introduction

The armature winding is composed of a certain number of armature coils connected according to a certain rule. It is the circuit part of the DC motor, which also induces electromotive force and generates electromagnetic torque for electromechanical The part of energy conversion. The coil is wound with insulated round or rectangular cross-section wires, which are embedded in the armature core slot in two layers. The upper and lower layers and between the coil and the armature core must be properly insulated and compressed with a slot wedge. The end of the armature winding of a large motor is usually tightly tied to the winding support. The armature winding is divided into two categories: DC armature winding and AC armature winding. They are used for DC motors and AC motors respectively. Usually double-layer windings are used. The effective part of the coil contains two effective sides on the left and right. The effective edge placed in the slot and close to the slot is called the upper edge, and the effective edge close to the bottom of the slot is called the lower edge. The upper and lower layers in the same tank are separated by insulating paper. The distance between the upper and lower effective edges of the same coil in the circumferential direction is the span of the coil, which is usually expressed as a multiple of the slot pitch (the distance between two adjacent slots). The span is approximately equal to a pole pitch (the distance between two adjacent magnetic poles is also often expressed as a multiple of the slot pitch).

## Main classification

There are three types of DC armature windings: split winding, wave winding and frog winding. The two outlet ends of each coil are connected to the two commutator segments of the commutator. The distance between the two on the circumferential surface of the commutator is called the commutator pitch, which is expressed as Ys Said. Different types of windings have different commutator pitches.

### Stacked windings

There are single stack windings and cascade windings. Single-stack windings connect adjacent coils under the same magnetic pole in series to form a parallel branch, so there is a parallel branch corresponding to a magnetic pole. The basic feature of a single-stack winding is that the number of parallel branches is equal to the number of magnetic poles. The branches are connected in parallel through brushes. The commutator pitch of the single-stack winding coil is Ys=1. Ys>1 is called cascade winding. The more commonly used is the Ys=2 cascade winding, also known as the double-stack winding. The double-stack winding has two parallel branches under one magnetic pole. For example, when a four-pole DC motor adopts double-stack windings, there are 8 parallel branches in total. The branches are also connected in parallel through brushes. The number of brush groups is equal to the number of poles of the motor. Half of them are positive brushes, and the other half are negative brushes. There are many parallel branches of the stacked winding, which is equal to the number of poles or an integral multiple of the number of poles, so it is also called parallel winding.

### Wave winding

There are single-wave winding and complex-wave winding. The characteristic of single-wave winding is to connect all coils of the same polarity in series according to a certain rule to form a parallel branch. Therefore, the entire armature winding has only two parallel branches. In the commutator pitch formula of the wave winding coil, P is the number of magnetic pole pairs; k is the number of commutating pieces; a is a positive integer that makes Ys equal to an integer, which is equal to the parallel branch of the wave winding Logarithm. The single-wave winding a=1, and the complex-wave winding with a=2 is called a dual-wave winding. It can be regarded as a complex winding composed of two single-wave windings in parallel, so there are 4 parallel branches; a> The two can be analogized, but they are rarely used. From the principle of parallel circuit connection, the wave winding requires only two sets of brushes, namely a set of positive brushes and a set of negative brushes. However, in general, the number of brush groups in the wave winding of a DC motor is still equal to the number of poles. This is to reduce the current load on the contact surface of the brush and the commutator segment, thereby shortening the length of the commutator. In addition, the commutation of the coil current is also beneficial. DC armature windings often cause uneven current distribution in each parallel branch due to some reasons, which increases copper consumption and overheats the armature windings; sometimes harmful sparks under the brushes will be generated, which will adversely affect the operation of the motor. Connecting the theoretical equipotential points inside the armature winding directly with wires can improve the operating conditions of the motor. The connecting wires specially set up for this purpose are called equalizing wires.

### Frog Winding

A DC armature winding that is a mixture of appropriately matched stacked windings and wave windings. The coils of the stacked winding and the wave winding are connected to the same commutator and work in parallel. It is named because its coil combination looks like a frog. Since this kind of winding acts as a voltage equalizing line between the wave winding coil and the stacked winding coil, there is no need to add an additional equalizing line. The DC motor adopting frog winding has good running performance, so its application is becoming more and more extensive. The armature winding is the core part of the DC motor. When the armature rotates in the magnetic field, the electromotive force is induced in the armature winding. When the current flows in the armature winding, the armature magnetomotive force is generated. It interacts with the air gap magnetic field to generate electromagnetic torque and electromotive force. It interacts with the current to absorb or release electromagnetic power, and the electromagnetic torque interacts with the rotor speed to absorb or release mechanical power. Both exist at the same time, forming the mutual conversion of electromagnetic energy and mechanical energy, completing the basic functions of a DC motor. Therefore, the armature winding plays an important role in the DC motor.

## Common faults and treatment methods

### Ground fault

The grounding of the rotor winding is a fault that is more likely to occur in the operation of the generator, and it seriously affects the safe operation of the generator Failure. In normal operation, there is a certain insulation resistance and distributed capacitance between the generator rotor winding and the ground. The insulation resistance is generally greater than 1Ω. Because the water-cooled winding rotor has an insulated water pipe, the insulation resistance is only a few thousand ohms when the water is flowing. When the insulation resistance drops severely or the insulation to the ground is damaged due to some reason, the most common one is a point ground fault. At this time, since no current loop is formed, it has no direct influence on the operation of the motor. However, after a single point of ground fault exists, such as closing the excitation switch and generator outlet circuit breaker, or other operation accidents, when the rotor circuit generates overvoltage, it may lead to the formation of another grounding point, which seriously threatens the safe operation of the generator. Two or more grounding accidents. At this time, the generator will have severe consequences such as increased vibration in varying degrees, magnetization of the main shaft of the unit, and partial burnout of the rotor winding insulation and shaft.

Treatment method:

The method to deal with the ground fault is to replace the insulating material of the relevant part. The specific method is: remove the insulating carbide between the 25 and 26 slots under the steam side guard ring and the insulating carbide between the first turn to the 4th turn of the 25 and 26 slots, and use a vacuum cleaner to repeatedly vacuum 3 to 5 times, and then Clean with cleaning agent for 2 to 3 times and replace with a new insulating layer and insulating pad. Replace and adjust the position of the 6 previously loose insulating pads, and make the tightness appropriate.

### Inter-turn short-circuit fault

Generator rotor winding inter-turn short-circuit fault is a common fault in its operation. In severe cases, it will affect the reactive power output of the generator. If the asymmetric turn-to-turn short circuit will increase the vibration of the generator set, it may further lead to damage to the ground insulation of the rotor winding, and then develop into a ground fault, which is safe and stable for the generator set itself. Operation poses a great threat.

Processing method:

1. Determine the number and location of short-circuit turns

When a short-circuit occurs in the rotor winding, relevant inspections must be carried out to determine the number and location of the short-circuit turns. According to field experience, there are often unstable inter-turn short circuits in the rotor windings. When the rotor is stationary or the guard ring is pulled out, the short circuit between turns disappears due to the bounce of the turns, but when the guard ring is installed or the rotor is running, the short circuit between turns still exists. In order to eliminate this hidden danger, it is necessary to find out the unstable inter-turn short-circuit point. At this time, dozens of pairs of special pressure plates can be used to clamp the ends and corners of the windings. As shown in Figure 3, the windings are pressurized point by point to simulate the thermal sleeve tightening force of the guard ring and the centrifugal force generated during the winding operation, and then pass the voltage Test one by one by descending method to find out the fault point.

2. Deal with the fault point

After finding the short-circuit point between the turns, use an L-shaped tool made of round steel to pry open the short-circuit turns slightly, as shown in Figure 4, clean the damaged insulation, and then Use a mica board brushed with silicone organic paint as the adhesive between the pads, and then flatten the pried turns.

3. Inspection and installation

After all the inter-turn insulation is intact, check the insulation again. After passing the test, clean and check that there are no relics at the ends. Install the end spacers according to the original marks. Spray a layer of oil-proof insulating paint on the surface. Finally, install the guard ring, center ring, fan, etc.