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AC Drives, DC Drives-What are they? and how do they work together?
Electric motor converts electrical energy into mechanical power.
Electric motors are designed and manufactured to run at one constant speed when power is applied. Most machinery and equipment requires ability to vary the speed to complete the process.
There are two types of motors, namely AC Motors and DC Motors.
AC Motors require AC power to operate, and DC Motors require DC power to operate.
Electric Motor Drive is an apparatus that can provide variable speed and torque to electric motor.
AC drive (also called as AC motor drive) accepts AC power input and delivers AC power output to the motor. DC drive (also called as DC motor drive) accepts AC power and delivers DC power output to the motor. In both Adjustable, Variable Frequency VFD Electric Motor Drives, output power (torque) and speed is varied by manipulating the frequency of input AC power.
Variable frequency drive must be AC drive, however, variable speed drive can be AC drive or DC drive.
AC Drives, Variable Frequency Drives:
Electric motors operate under the fundamental phenomena, if you move a conductor through magnetic field a voltage is generated in the conductor proportionate to the number of magnetic lines of forces that are cut per second. If the conductor has closed circuit, current will flow.
If the conductor is carrying current, a magnetic field will be induced around that conductor. Magnetic lines of force seek the shortest distance between poles and the interaction between the two magnetic fields produces a repelling force called torque. The strength and direction of the force is dependent upon the field strengths and relative polarities.
Most common industrial motor is a three phase squirrel cage induction motor. It has electromagnetic stator which is wound with pairs of poles for each phase. The motor stator will always have three circuits in a typical wye or delta configuration.
Both rotor and stator iron surfaces are smooth apart from the regular slotting and are separated by small air gap, the flux produced by the stator windings crosses the air gap radially.
The term 4 pole reflects the fact that the flux leaves the stator from two N poles and returns at two S poles. The radial flux density varies sinusoidally in space. There are two N peaks and two S peaks but the transition from N to S occurs in a smooth sinusoidal way., giving rise to the term flux wave.
Total assembly can be considered as a transformer. The primary winding is in the stator and the secondary is formed in the rotor. It is called induction motor because all energy required in the motor for the torque is induced by this transformer action.
The stator winding is arranged so that as the three phase, power is applied, changing phase voltage will establish a rotating magnetic field.
DC Drives, Variable Speed Drives:
DC current was the first successful form of electric motor control. DC motors are more complicated than AC motors. There are two major parts to consider in DC motor. One is the Stator or stationary shell holds the field poles. The rotating member is called Armature. A voltage is produced in a conductor when it cuts through a magnetic field. The voltage is dependent upon how fast the field is cut.
Magnetic poles in small motors and servo units may consist of permanent magnets but in most cases they will be wounded with a shunt winding. This winding is excited separately to allow precise control of the current. The magnetic flux density is related to the ampere-turn applied to the coil.
Armature has many coils, wounded in slots and terminated to a current collector system called commutator. The commutator consists of many insulated copper segments which collect current from the brush assembly, mounted in the end-bell of the motor.
If the stator is wound in a four pole configuration, the armature will also have the same number of poles and brushes. The brushes may be single or multiple depending upon the amount of current that must be conducted through the armature.
Since we can control the power separately in both parts of this motor, we can provide very precise speed and torque control. The field coils provide a fixed north-south relationship at all times, although the current strength may be varied to meet needs of the operation. Current is fed to the armature and because of the unique position of the coils at any given time, magnetic poles are produced.
These two magnetic systems interact to produce torque as the like poles repel and unlike poles attract each other. As the armature turns, it continuously switches armature coils to maintain a constant magnetic relationship between rotor and stator.