The ability to control both the speed and torque of motors was an evolution of power electronic control systems, which has been labeled as Industrial Drive. This term however, is misleading as it refers to a system that contains the motor and all associated electronic controls. The current term for motor controls is Variable Frequency Drive (VFD).
The VFD consists of three parts: the rectifier, the DC link, and the inverter. The rectifier takes the incoming AC power and transforms it to DC power. The DC link has several capacitors to store the energy from the output of the rectifier. The inverter creates a 2-20 kHz signal that is used to create the output waveform using pulse width modulation (PWM).
By manipulating the output waveform, the VFD can adjust both the torque and the rotor speed. This PWM technique uses a three-phase bridge made of Insulated Gate Bipolar Transistors (IGBT) that draws the rectified energy and transforms it into high frequency AC voltage by switching the IGBTs at a high rate. By varying the time the IGBTs are either on or off, the VFD makes a rough outline of the output waveform. Thus, the VFD can faithfully reproduce all the frequencies necessary to meet the needs of the motor.
A brushless motor with an electronic ballast can be controlled by a VFD from zero to 6000 RPM by varying the frequency of the VFD’s output. The frequency of the VFD’s output determines the rotational speed of the motor. This has two distinct advantages. First, the speed of the motor can be controlled directly with the VFD and secondly, the torque can be controlled. This can be used to maintain control of the motor. For example, if a motor suddenly reduces its load, the VFD can react by reducing the torque to the motor to prevent a motor over-speed.
The concept of motor control can be taken further by adding an outer ring to measure the position of the motor shaft. Now, the controller has a reference for controlling the rotational position of a motor. This type of controller, called a Servo Amplifier, is usually entrusted to a numerically controlled external drive, or for simple applications is delegated to the drive itself.
Most VFDs use IGBT for power transistors. The switching speeds of the IGBTs can be higher than 700Volts/milli-second. This speed can reduce the power loss during switching. However, this creates electromagnetic interference (EMI) in the rest of the machine. The high frequency coupled with the large voltage switching potentially creates two adverse effects. First, the cables between the VFD and the motor become antennas with noise for a signal. The radiated effects can reach 200Volts/meter. If any other cables are placed near the motor cables, the two will couple and the noise will infiltrate the system. Second, the noise will propagate back through the power input to the drive. This will not have the same energy as the motor leads but is definitely high enough to disturb other components in the system. This necessitates the use of Enerdoor's EMI/RFI filters.
Asynchronous Motor and Synchronous Motor
An asynchronous motor is any motor directly supplied by a 3-phase power network. The inner portion, called the rotor, rotates at a frequency less than the supplied power, 50/60Hz. The fixed outer portion is called the stator. An asynchronous motor will turn at slightly less than the power supply frequency. The difference in frequency between the supplied power and the rotational speed of the rotor is called scroll.
The synchronous motor is a motor that is also supplied by a 3-phase power network. As the motor turns at exactly the same speed as the supplied power, the motor must be pre-spun to the proper frequency before being coupled to the power supply. In either case, the speed of the motor cannot be adjusted once spinning. The asynchronous motor will spin at just below the power frequency and the synchronous motor will spin at exactly the power frequency.
Enerdoor has several filters to mitigate the EMI associated with VFDs. Each series has multiple filters with different current capabilities to match the system in question. These series are: FIN538S1, FIN1200, FIN1500, FIN1900, and FIN1900S. Furthermore, Enerdoor has two parallel filter series for three-phase applications; FIN230SP and FIN730. All filters in these series were designed to mitigate EMI-RFI issues in automation machines.
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