But

Thus the torque produced by a D.C. Motor is directly proportional to main flux F as well as armature current Ia.

The 3 pole 6 way switch is kept in start mode. This will connect terminal R', Y' B' of stator winding to each other. This act as star point. The supply is connected to R,Y,B terminal of the stator winding. Thus in start mode the stator winding is connected to form the star.

The 3 pole 6 way switch is thrown in run position once the motor accelerate. This will connect the terminals in following manner:

The construction of universal motor is similar to that of DC Machine. The field poles are mounted on stator winding. Field winding is wound on the field poles. Both the stator field poles and armature is laminated to minimize the Eddy current losses while working on a.c. supply. The armature has straight or skewed slots and the commutator and brush arrangement is resting on it. 

Such motors develop unidirectional torque regardless of whether they operate on A.C. or D.C. supply. The motor runs on the same principle as d.c. motor i.e., force between the main pole flux and the current carrying armature conductor. The universal motor produces the electric torque proportional to the quadrate of the supply current. Since the same current flows through the field winding and the armature, it follows that ac reversals from positive to negative, or from negative to positive, will simultaneously affect both the field flux polarity and the current direction through the armature. This means that the direction of the developed torque will remain positive, and rotation will continue in the same direction. Thus, a universal motor can run both on dc and ac. So the electric torque has the same torque direction at any current polarity and in addition at AC power. The starting torque of a universal motor is determined by the current that flows through the armature and field windings. Due to the inductive reactance of these windings the AC starting current will always be less than the DC starting current.Consequently, the starting torque on AC power will be lower than the starting torque on DC power. The characteristics of universal motor are very much similar to those of D.C. series motors, but the series motor develops less torque when operating from an A.C. supply than when working from an equivalent D.C. supply.

Torque-Speed Characteristics

These machines can function as either a motor or a generator

Two related physical principles underlie the operation of generators and motors. The first is the principle of electromagnetic induction discovered by the British scientist Michael Faraday in 1831. If a conductor is moved through a magnetic field, or if the strength of a stationary conducting loop is made to vary, a current is set up or induced in the conductor. The converse of this principle is that of electromagnetic reaction, first observed by the French physicist Andr Marie Ampere in 1820. If a current is passed through a conductor located in a magnetic field, the field exerts a mechanical force on it.

DC motors and generators have the same basic construction, differing primarily in the energy conversion.

The basic requirements are a conductor, flux and relative motion between them. The physical structure of the machine consists of 2 parts: the stator and the rotor. The stationary part consists of the frame, and the pole pieces, which project inward and provide a path for the magnetic flux.

Yoke: Made of cast iron or fabricated rolled steel or steel casting. It act as magnetic return path, holds the poles in position and provides mechanical protection

Pole and pole shoe: They are made of MS sheets or laminated soft steel sheets punched to suitable size and stacked and riveted together. Field winding is placed on the poles. The pole shoe serves the purpose of spreading the magnetic field uniformly in the air gap.

Field coils: It is used to produce magnetic field. It is wound on a former and then placed on the pole. Insulated copper wires are used for winding. In the case of compound machine both series and shunt winding are done on the same pole.

Armature core: The primary function is to provide magnetic path of low reluctance. It is made of stamping of circular sheet steel discs of approximately 0.5 mm thickness. The slots are cut on the armature core. Slots may be open or semi closed.

Armature winding: Winding is done through the slots on the armature core and is connected in succession to a Commutator segment. There are mainly two types of winding namely Lap winding and Wave winding.

Commutator and brush: It is a extension of armature to which the connections of armature winding is soldered. A brush touching the Commutator collects the current as it rotates. They are made of segments of copper bars arranged to from a cylinder and each segment is insulated from each other. The type of brush used depends on the speed of rotation. Normally carbon brushes are used. Other brushes used are, Graphite, Metal Carbon, etc.

One can be aware of the fact that emf induced into the rotating conductor or winding is always sinusoidal or alternating in nature. If the machine is of DC nature that is DC Generator then it is bounding for someone to convert it (sinusoidal emf) into a unidirectional emf. This process of converting AC voltage into a DC is commonly called as commutation. Hence the commutator is a device which normally acts like a rectifier and perform the process of rectification. Coil(s) current is constant and unidirectional so long as the coil is under the influence of given pole pair(s), while it reverses when the coil passes onto the next pole pair as the armature rotates. Commutation take place when the coil is passing through the interpolar region and during this period the coil is shorted via the commutator segments by the brushes located (electrically) in the interpolar region. Commutation takes place simultaneously for P coils in a lap wound machines and two coil sets of  ^P/₂ coils each in a wave wound machine.
Various symbols used in the figure are:

Resistance Commutation

E.m.f. Commutation