Minimize EMI in Automation Facilities

The rise in automation and the use of complex electronic systems in factories has optimized operations and output but such systems and controls are more susceptible to electronic interference.  Enerdoor offers a wide range of products engineered to mitigate interference designed specifically for the automation industry.  In addition to our off-the-shelf filters, we tailor EMI filters, power quality, and motor protection devices to satisfy specific environmental requirements.  EMC/EMI filters are ideal for the automation industry as they lower EMI without significantly impacting the cost, size, or function of factory automation systems.  

The ability to control both the speed and torque of motors was an evolution of power electronic control systems that has been labeled "Industrial Drive."  This term 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 or VFDs.  The VFD consists of three parts: the rectifier, the DC link, and the inverter. The rectifier takes the incoming AC power and transforms it into 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 motor needs.  

A brushless motor with an electronic ballast can be controlled by a VFD from 0 to 6000 RPM by varying the frequency of the VFD output. The frequency of the VFD output determines the rotational speed of the motor and 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 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. As such, the controller has a reference for controlling the rotational position of the 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 IGBT can be higher than 700 volts/milli-second and can reduce 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 200 volts/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 high enough to disturb other components in the system and necessitates the use of EMI-RFI filters to mitigate the disturbance.  

Asynchronous and Synchronous Motors

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 of 50/60 Hz.  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.

A synchronous motor is a motor that is also supplied by a 3-phase power network. However, as the motor turns at 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 EMI-RFI filters are ideal solutions for VFDs and mitigate RF issues for automated machinery.  Contact us today for a free confidential consultation and to ensure your equipment is protected and functioning properly. 


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