DC MOTORCreated and Presented by Doren Nedrick

DefinitionA motor is a rotary machine, which converts electrical energy into mechanical energy. The primary function of a motor is to drive a load.

Difference Between Generator and MotorA dc motor is very similar in construction to a dc generator. The major differences are:(a) Electrical energy is applied to the brushes of the motor rather than collected from it.(b) Motors often operate in locations, which expose them to mechanical damage, dust, moisture or explosive fumes. The motor may therefore have a frame or additional designed to protect it from these conditions.A machine that runs well as a generator will operate satisfactorily as a motor.

OPERATION OF DC MOTORSWhen a current-carrying conductor is placed in a magnetic field interaction takes place between the main field and the field due to the current flowing in the armature conductors.

The lines of force due to the current flowing in the armature conductors strengthen one part of the main field and weaken the other part.

This effect produces a concentration of lines of force on one side of the conductor (in this instance, the top) and, as these lines of force tend to straighten out, a force is placed on the conductor. The lines of force act like stretched elastic bands which, when bent, attempt to straighten themselves out.

This ' motor effect' is increased when a current-carrying loop is placed in a magnetic field (Fig. 11.12).

Under these conditions, the interaction between the two fields (the main field and the field due to the current flowing in the armature conductors) tends to make the loop rotate. The armature in a motor is made up of a series of loops.Note the use of commutator to supply d.c. to the rotating loop. This ensures that the direction of the armature field remains fixed although the armature conductors rotate

Construction The d.c. motor is similar in construction to the d.c. generator, the only difference being in the provision made for ventilation: motors are built for conditions where water and inflammable gases exist.

Direction of Rotation The direction of rotation of the armature may be found by using Fleming's left-hand rule (right hand for generators):Index finger -Main field (N to S)Second finger -Direction of current through armature conductorsThumb - Direction of rotation

Reversal of Armature RotationReversal of the motor can be achieved by interchanging the connections; between either; the field and armature windings relative to each other. If both are changed the rotation is unchanged.

Armature Reaction Armature reaction, that is, the distortion of the main field due to the field of the current flowing in the armature conductors, also takes place in a motor, but it is in the opposite direction (Fig. 11.14) to that of the generator. The magnetic neutral axis is behind the direction of rotation, and is an angle of 'lag'.

Perfect (or sparkless) commutation is attained when the brushes are placed on the magnetic neutral axis. But since interpoles are used, the brushes may be placed between the main poles.NOTE. The polarity of the interpoles in a motor (Fig. 11.15) are of the same polarity as the main pole behind it, in the direction of rotation.

Back E.M.F This is the e.m.f. which is generated in the armature conductors as they cut the lines of force of the main field. The back e.m.f. (b.e.m.f.) is in opposition to the applied e.m.f. (Lenz's Law).

Speed of D.C. Motor The speed of a d.c. motor is dependent on four factors:1. Strength of the main field. This is an inverse relationship: Strong field - slow speed; weak field - increased speed.2. Number of poles in the main field.3. Number of armature conductors.4. Voltage across the armature.

Since the number of poles and the number of armature conductors cannot be varied the speed must be controlled by varying the strength of the main field. Speed may also be varied by placing resistance in the armature circuit (point 4 above) but this is extremely inefficient because of the large currents involved.

Torque in D.C. motor Torque (or twisting power in pound-feet) is proportional to armature current. Increased armature current means increased torque.

D.C. Motor Field SystemsThree types of field systems are used in d.c. motors: (1) shunt; (2) series; (3) compound.Shunt Motor: The shunt motor (Fig. 11.16a) has the field (connections Z-ZZ) connected in parallel with (i.e., shunted across) the armature (A-AA). The field is made up of many turns of thin wire.

The current through the field is constant, since the voltage across it is constant, giving a relatively level speed on all loads: the speed drops approximately 5 per cent from no-load to full-load (Fig. 11.16b).

Starting and Speed ControlWhen a d.c. motor armature is run up to speed a back e.m.f. is induced into the armature conductors, which opposes the applied e.m.f. and so limits the current flowing. During the starting period, additional resistance must be placed in the low-resistance armature circuit (which is generally less than 1) to limit armature current until the speed (and the b.e.m.f.) builds up.

Starter The purpose of the starter is to reduce armature current until the b.e.m.f. builds up. This can be illustrated by calculating the current through a 500V d.c. motor having an armature resistance of 0.1.

If the supply voltage is applied across the stationary armature (b.e.m.f. being zero) the current will be 5000 A. (The field current is negligible and can be ignored.) This high current would damage both the motor and the control equipment.

Calculation of Back E.M.F Since the voltages dropped in a series circuit are equal to the applied voltage:VT (applied voltage)= E (back e.m.f.) + IaRa (voltage drop across the armature)Where VT = 500, Ia = 40 A, and Ra = 0.1 . note: Ia (in a motor) = IL (line current) - If (field current)500V = E + 40A x 0.l E = 500V- 4V E = 496Vwhere volt drop across brushgear is considered:V = E + IARA + Vb (brush drop)

Face Plate Starter A shunt motor is started by placing a variable resistance in the armature circuit, thus decreasing the armature current (Ia), until the b.e.m.f. builds up.

Fig. 11.18a shows the basic circuit: when the resistance is taken out of the armature it is inserted in the field, this weakens the field and increases the speed.

Face Plate Starter

Fig. 11.186 shows the practical circuit complete with protection. There are two types of protection included in the face plate starter:No-volt Protection. The purpose of this protection is to ensure that the starting handle is replaced to the off position when the supply is cut off. The no-volt coil, which is usually connected in series with the field, forms an electromagnet which holds the spring-loaded starter handle in the 'run' position. When the supply is cut-off the no-volt coil is de-magnetized and the spring-loaded handle moves to the 'off' position. The no-volt coil also allows the motor to be stopped from a distance by connecting a stop-button which, when pressed, shorts the no-volt coil.

Over-current Protection: The purpose of this protection is to guard the motor circuit against excess current. The line current to the motor flows through a coil (the current coil) which attracts a soft-iron armature when the motor is being overloaded. This soft-iron armature carries a contact which shorts out the no-volt coil, thus releasing the starter handle to the 'off position.

The no-volt coil has many turns of fine wire. The current coil has a few turns of heavy wire, and is usually set to operate at 150 per cent of full-load current.

Speed Control Speed control of a d.c. shunt motor is achieved by varying the field strength (Fig. 11.18b). A variable resistance is inserted in the field circuit. When the resistance is increased, speed is increased. A decrease in field strength will lead to a decrease in b.e.m.f. because the armature conductors are cutting fewer lines of force. As b.e.m.f. decreases armature current increases, increasing torque and, if the load remains constant, the speed will increase.NOTE: The variable resistor must always be set at zero resistance (minimum speed position) before the motor is started.

Series Motor In this machine, the field (Y-YY) and the armature (A-AA) are connected in series (Fig. 11.19).

On light loads the armature current is small and, since the field is connected in series with the armature, the main field is weak. The result is high speeds at light loads and dangerously high speeds on no-load (i.e., very weak field).

The speed of the y-series motor varies with the load (Fig. 11.19b) and some form of mechanical load must always be connected to the shaft or the motor will damage itself. The series motor acts somewhat like an automatic gear box - high speeds on no-load and low speeds on full-load.

Starting and Speed Control The motor is started by connecting a variable resistance in series with the motor (Fig. 11.20). Speed control is obtained by connecting a 'diverter' resistance in parallel with the field.

In this way the current is diverted from the field, giving a weak field and therefore increased speed. At the same time torque is increased in the armature by increasing armature current.

Compound Motor The compound motor (Fig. 11.21a) contains both a series field (Y-YY) and a shunt field (Z-ZZ). The output of the compound motor depends on the method in which the fields are interconnected. There are two types of compounding:1. Differential compounding: where the series field opposes the shunt field (Fig. 11.21b).2. Cumulative compounding: where the series field assists the shunt field (Fig. 11.21b).

Starting and Speed Control The face plate starter can be used for the compound motor. Speed variation can be achieved by placing a variable resistance in series with the shunt field.

Methods of Cooling MotorsElectric motors are often classified according to the type of enclosure used. The type of enclosure used is determined by the conditions under which the motor is to be used and also the degree of ventilation required.

Totally Enclosed Type This type of motor is generally cooled by the conduction of heat through the motor case. All joints are a machine fit, as the motor, particularly the commutator enclosure, must be gas tight to limit the risks from inflammable gases. It is used in hot and humid conditions.

Screen-protected Type Ventilation is achieved by fitting a fan internally. The openings at the end of the motor are fitted with a wire mesh or a perforated sheet. This is the most common type of protection.

Drip-proof Type This type is similar to the screen-protected type, but the additional precaution of a cover over the screen is used to stop drips of water from entering the motor. This type is not waterproof.

Pipe-ventilated Type Air is brought from a dust-free area and piped into the motor. The internally fitted fan ensures circulation of cooling air.

Troubleshooting

Losses in D.C. MachinesThere are three types of loss in a d.c. machine:1. Iron losses. These are losses arising in the magnetic circuit of the machine and are of two types: (a) eddy currents; and (b) hysteresis losses.2. Copper losses. These losses are due to the flow of current through the copper conductors of the motor and are sometimes termed I2R losses'.3. Mechanical losses. These are made up of: (a) bearing friction; and (b) windage. This is a friction loss due to the movement of the armature through air.

Petersfield High Created by D. Nedrick June 23, 2010