AC and DC motors

Drive systems DC motor AC motor Stepper motor

DC motor

Working principle When current passes through the coil, the resulting forces acting on its sides at right angles to the field cause forces to act on those sides to give a rotation. For the rotation to continue, when the coil passes through the vertical position the current direction through the coil has to be reversed.

Parts Stator (permanent or non permanent magnet) Rotor (electromagnet) Armature Commutator Brush

A brush type dc motor is essentially a coil of wire which is free to rotate - termed as rotor in the field of permanent or non-permanent magnet. The magnet termed a stator since it is stationery. For the rotation to continue, when coil passes through vertical position the current direction is reversed which is got by use of brushes making contact with split ring commutator.

Torque produced along the axis of the conductor due to force F is For an armature conductor of length l and carrying a current I, the force resulting from a magnetic flux of density B at right angles to the conductor is given by F = BIL Torque produced along the axis of the conductor due to force F is T = F x b = nBIL x b = KI

Since armature is a rotating magnetic field it will have back emf Vb Since armature is a rotating magnetic field it will have back emf Vb. The back emf depends on rate of flux induced in coil. Back emf is proportional to angular velocity w Vb = Kw Equivalent circuit diagram for D.C motor

Control of brush type DC motor Speed control can be obtained by controlling the voltage applied to the armature. Since fixed voltage supply is often used, a variable voltage is obtained by an electronic circuit. When A.C supply is used a Thyristor can be used to control the average voltage applied to armature. PWM – pulse width modulation Control of d.c motors by means of control signal from microprocessors.

Brush type motor with non-permanent magnet Series wound Shunt wound Compound wound Separately excited

Series wound Armature and field windings are connected in series. Highest starting torque Greatest no load speed Reversing the polarity of supply will not effect the direction of rotation of rotor.

Shunt wound Armature and field coils are in parallel. Lowest starting torque Good speed regulation. Almost constant speed regardless of load. For reversing direction of rotation either armature coil or field coil supply has to be reversed.

Compound wound Two field windings one in series an another in parallel with armature windings. High starting torque with good speed regulation.

Separately excited Separate control of armature and field coils. Speed of these motors can be controlled by separately varying the armature or field current.

Brush less dc motor Its consists of a sequence of stator coils and a permanent magnet rotor. Current carrying conductors are fixed and magnet moves. Rotor is ferrite or permanent magnet. The current to the stator coils are electronically switched by transistor in sequence round the coils. Switching being controlled by position of rotors. Hall effect sensors are used to input signals related to a particular position of rotor.

A.C motors Single phase squirrel cage induction motor Its consists of a squirrel cage rotor, this being copper or aluminum bars that fit into slots in end rings to form a complete circuit. Its consists of a stator having set of windings. Alternating current is passed through stator windings an alternating magnetic field is produced. As a result EMF are induced in conductors in the magnetic field. Initially when rotor is stationery net torque is zero. Motor is not self starting.

3-phase induction motor 3 windings located 120 deg apart each winding being connected to one of the three lines of the supply. 3 phase reach maximum currents at different times, magnetic field rotates round the stator poles completing one rotation is one full cycle. Self starting

Synchronous motors Similar to that of induction motor but rotor will be a permanent magnet. Magnets rotate with the same frequency as that of rotating magnetic field which rotates 360 deg in one cycle of supply. Used when precise speed is required. Not self starting.

Speed control of AC motor Speed control of A.C motor is done by provision of variable frequency supply. Torque is constant when ratio of applied stator voltage to frequency ration is constant. AC is rectified to DC by convertor and inverted back to AC with a selected frequency.