1-VFD
Induction motors, the workhorses of industry, rotate at a fixed speed that is determined by the frequency of the supply voltage. Alternating current applied to the stator windings produces a magnetic field that rotates at synchronous speed. This speed may be calculated by dividing line frequency by the number of magnetic pole pairs in the motor winding. A four-pole motor, for example, has two pole pairs, and therefore the magnetic field will rotate 50 Hz / 2 = 25 revolutions per second, or 1500 rpm. The rotor of an induction motor will attempt to follow this rotating magnetic field, and, under load, the rotor speed "slips" slightly behind the rotating field. This small slip speed generates an induced current, and the resulting magnetic field in the rotor produces torque.
Since an induction motor rotates near synchronous speed, the most effective and energy-efficient way to change the motor speed is to change the frequency of the applied voltage. VFDs convert the fixed-frequency supply voltage to a continuously variable frequency, thereby allowing adjustable motor speed.
Variable speed drives are used for two main reasons:
- to improve the efficiency of motor-driven equipment by matching speed to changing load requirements; or
- to allow accurate and continuous process control over a wide range of speeds.
In addition to energy savings and better process control, VFDs can provide other benefits:
-A VFD may be used for control of process temperature, pressure or flow without the use of a separate controller. Suitable sensors and electronics are used to interface the driven equipment with the VFD.
-Maintenance costs can be lower, since lower operating speeds result in longer life for bearings and motors.
-Eliminating the throttling valves and dampers also does away with maintaining these devices and all associated controls.
-A soft starter for the motor is no longer required.
-Controlled ramp-up speed in a liquid system can eliminate water hammer problems.
-The ability of a VFD to limit torque to a user-selected level can protect driven equipment that cannot tolerate excessive torque.
2-Pole changing method:
The principle of pole changing windings and the special case of the Dahlander winding were developed at the end of the 19th century. In the 1950s and 60s, the principles were generalizes and the techniques were improved.
The pole-amplitude modulation (PAM) and the pole-phase modulation (PPM) were developed as such generalized pole changing techniques. In their theory, the PPM is considered the most general winding design approach, of which the PAM is a specialization, and again, of which the Dahlander winding is a specialization. Each specialization limits the choice of the pole ratio p1/p2. For example, the Dahlander winding is only capable of generating fields of pole ratio 2:1, a PAM winding however can generate pole ratios of n : (n − 1) with n as an integer. In addition to the improvements, which were achieved in the development of the winding, the pole-changing winding gained new fields of application with upcoming power electronic devices. In the
1990s pole-changing techniques were used together with an inverter supplied induction motor to extend the speed range for traction applications.
3-Type of Motor and VFD
Motors and VFDs must be compatible. Consult the manufacturers of both the VFD and the motor to make sure that they will work together effectively. VFDs are frequently used with inverter-duty National Electrical Manufacturers Association (NEMA) design B squirrel cage induction motors. (Design B motors have both locked rotor torque and locked rotor current that are normal.) De-rating may be required for other types of motors. VFDs are not usually recommended for NEMA design D motors because of the potential for high harmonic current losses. (Design D motors are those that have high locked rotor torque and high slip.)
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