DC motors
Electric motors of various sizes.
DC motor rotation
A simple DC electric motor. When the coil
is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn toward the right, causing rotation.
The armature continues to rotate.
When the armature becomes horizontally aligned, the commutator reverses the direction of
current through the coil, reversing the magnetic field. The process then repeats.
If the shaft of a DC motor is turned by an external force, the motor will act like a generator and produce an Electromotive force (EMF). During normal operation, the spinning of the motor produces a
voltage, known as the counter-EMF (CEMF) or back EMF, because it opposes the applied voltage on the motor. This is the same EMF that is produced when the motor is used as a generator (for example when an electrical load (resistance) is placed across the terminals of the motor and the motor shaft is driven with an external torque). Therefore, the voltage drop across a motor consists of the voltage drop, due to this CEMF, and the parasitic voltage drop resulting from the internal resistance of the armature''s windings. The current through a motor is given by the following equation:
I = (
Vapplied −
Vcemf) /
Rarmature The mechanical power produced by the motor is given by:
P =
I *
Vcemf Mechanism of the DC motors:
when the current passes throught the coil-wounded around a soft iron core-the side of the positive pole acted upon by an upwards force, while the other side is acted upon by a force opposing in direction.So according to "fleming''s left hand rule", the forces cause a turning effect on the coil making it rotate; to make the motor rotate in a constant direction "direct current" commutators make the current reverse in direction every half a cycle thus causing the motor to rotate in the same direction.The problem facing the motor is when the plane of the coil is parallel to the magnetic field;i.e. the turning effect is ZERO-when coil is at 90 degree from its original position-yet, the coil continues to rotate by INERTI.
Since the CEMF is proportional to motor speed, when an electric motor is first started or is completely stalled, there is zero CEMF. Therefore the current through the armature is much higher. This high current will produce a strong magnetic field which will start the motor spinning. As the motor spins, the CEMF increases until it is equal to the applied voltage, minus the parasitic voltage drop. At this point, there will be a smaller current flowing through the motor. Basically, the following three equations can be used to find the speed, current, and back EMF of a motor under a load:
Load =
Vcemf *
I Vapplied =
I *
Rarmature +
Vcemf Vcemf =
speed *
Fluxar mature Speed control
Generally, the rotational speed of a DC motor is proportional to the voltage applied to it, and the torque is proportional to the current. Speed control can be achieved by variable battery tappings, variable supply voltage, resistors or electronic controls. The direction of a wound field DC motor can be changed by reversing either the field or armature connections but not both. This is commonly done with a special set of contactors (direction contactors).
The effective voltage can be variy inserting a series resistor or by an electronically controlled switching device made of thyristors, transistors, or, formerly, mercury arc rectifiers. In a circuit known as a chopper, the average voltage applied to the motor is varied by switching the supply voltage very rapidly. As the "on" to "off" ratio is varied to alter the average applied voltage, the speed of the motor varies. The percentage "on" time multiplied by the supply voltage gives the average voltage applied to the motor. Therefore, with a 100 V supply and a 25% "on" time, the average voltage at the motor will be 25 V. During the "off" time, the armature''s inductance causes the current to continue flowing through a diode called a "flywheel diode", in parallel with the motor. At this point in the cycle, the supply current will be zero, and therefore the average motor current will always be higher than the supply current unless the percentage "on" time is 100%. At 100% "on" time, the supply and motor current are equal. The rapid switching wastes less energy than series resistors. This method is also called pulse width modulation, or PWM, and is often controlled by a microprocessor. An output filter is sometimes installed to smooth the average voltage applied to the motor and reduce motor noise.
Since the series-wound DC motor develops its highest torque at low speed, it is often used in traction applications such as electric locomotives, and trams. Another application is starter motors for petrol and small diesel engines. Series motors must never be used in applications where the drive can fail (such as belt drives). As the motor accelerates, the armature (and hence field) current reduces. The reduction in field causes the motor to speed up (see ''weak field'' in the last section) until it destroys itself. This can also be a problem with railway motors in the event of a loss of adhesion since, unless quickly brought under control, the motors can reach speeds far higher than they would do under normal circumstances. This can not only cause problems for the motors themselves and the gears, but due to the differential speed between the rails and the wheels it can also cause serious damage to the rails and wheel treads as they heat and cool rapidly. Field weakening is used in some electronic controls to increase the top speed of an electric vehicle. The simplest form uses a contactor and field weakening resistor, the electronic control monitors the motor current and switches the field weakening resistor into circuit when the motor current reduces below a preset value (this will be when the motor is at its full design speed). Once the resistor is in circuit, the motor will increase speed above its normal speed at its rated voltage. When motor current increases, the control will disconnect the resistor and low speed torque is made available.
One interesting method of speed control of a DC motor is the Ward-Leonard control. It is a method of controlling a DC motor (usually a shunt or compound wound) and was developed as a method of providing a speed-controlled motor from an AC supply, though it is not without its advantages in DC schemes. The AC supply is used to drive an AC motor, usually an induction motor that drives a DC generator or dynamo. The DC output from the armature is directly connected to the armature of the DC motor (usually of identical construction). The shunt field windings of both DC machines are excited through a variable resistor from the generator''s armature. This variable resistor provides extremely good speed control from standstill to full speed, and consistent torque. This method of control was the de facto method from its development until it was superseded by solid state thyristor systems. It found service in almost any environment where good speed control was required, from passenger lifts through to large mine pit head winding gear and even industrial process machinery and electric cranes. Its pr