AC drives control motor speed by converting AC line voltage to a DC bus voltage, and then inverting that DC bus voltage to a controllable pulse width modulated (PWM) AC. Varying the fundamental frequency varies the speed at which a motor shaft turns. The drive inverter creates or synthesises the desired fundamental voltage characteristics by switching the constant level DC bus voltage based on a particular mathematical allgorithm, or modulation scheme. The frequency at which modulation occurs is called the carrier frequency, a term derived from work in the area of radio transmission in which the carrier "carries" the pertinent information. In the case of PWM drives, the carrier "carries" motor speed- torque information and the shaft responds accordingly.
Due to physical limitations in power electronics devices, switching speeds have practical limits, but are continually increasing with technological advancement. Therefore, maximum carrier frequencies are also limited but are increasing. Bipolar junction transistors (BJT’s) used in early AC drives operated with carrier frequencies in the 500-
My first work with this type of filter took place over ten years ago. It was the first effective output filter for drives operating large HVAC air handling systems that created objectionable acoustic noise levels. It was a big filter, and the cost was prohibitive for universal use on PWM drives. Thus, the initial concept of a low pass high performance output filter for "suppressing" carrier frequency was only used to control audible noise in a limited number of applications in motor/drive systems.
Today, carrier suppression has a new place in special duty motor drive systems.
Insulated gate bipolar transistors (IGBT’s) allow operation at much higher carrier frequencies, along with much faster switching speeds. Thus, noise from mechanical and magnetic responses in motors have essentially been eliminated. Sense the adult human ear is less sensitive to sound frequencies above 10 KHz. The mechanical resonance points of the motor laminations at 2x the carrier frequency, will usually be less objectionable or beyond detection of the human ear. In addition to quieter systems, drives now tout smaller size, lower motor temperatures, precise shaftcontrol and faster response with a carrier frequency range of 3 kHz to 30 kHz.
Today’s IGBT’s switch at speeds up to 50 nanoseconds (ns). The high frequency components contained in those steep edges of each pulse create new applications challenges. We will not review the much discussed over-voltage "reflected wave phenomena" related to fast switching speeds and long motor leads. Instead, we will discuss when it is economically sound to filter PWM voltage utilizing similar technologies to those developed many years ago, but for far different reasons.
One area that has shown economic justification for equipment that filters the carrier from the PWM drive output, is in the oil pumping industry where extremely long leads and extremely expensive maintenance costs are associated with well-pump or cable failure. A well shutdown can cause high cost in the loss of product generation, and removal and replacement of damaged equipment. Mid-horsepower drive/motor pumping systems (75-
The criticality of the load is of primary concern for engineers designing these systems. Stated another way, the load is located in a remote or untouchable place, and that, combined with the fact that the process is so critical, makes downtime and frequent maintenance shutdowns intolerable.
So why do we wish to reduce or suppress the energies contained at the carrier frequency?
In less critical applications, off the shelf filters that have a break frequency in the sub-20 kHz range reduce the effective dV/dT of the PWM pulses sufficiently to protect motors with leads up to 1000 feet, depending on dV/dT. But, they are limited in the amount of reduction that they can provide. They will typically try to hold dV/dT below 1000 volts/m s. However, the sheer length of the leads in oil wells is too great for higher frequency dV/dT filters to operate reliably.
So-called "sine-wave" filters that highly attenuate the carrier energy are required to protect leads and motors. Low voltage, pre-transformer filtering is the most economical. It is always best to mitigate a problem at the root cause. In addition, stepped-up fast waveform voltages are much more difficult to control, making medium voltage filters complex, much more costly and fewer competent sources. Therefore, many wells now have low voltage "sine-wave" filters installed close to the inverter output and the
Carrier attenuating sine wave filters have an insertion loss and a phase shifting component. Serious consideration has to be given to the drive topology in order to be applied successfully. All vector drives may have to be reconfigured to accept the insertion of inductive impedance and the possible high peak current spike associated with the filter capacitor. If not, the drive may not perform reliably. Some volts/Hz drives may experience stability issues due to phase shifting and the feedback loop bandwidth modification. Make sure to coordinate all system variables with the drive manufacturer’s application engineering group, and the filter provider.
Last and perhaps most importantly: filter installation has to be coordinated to include fault-monitoring mechanisms. Although the filter is a simple L-R-C circuit that is easy to construct and install, failure could be catastrophic, since the loss of the filter, the capacitor in particular could be unknown to the system. The cable and motors would be vulnerable to the same stresses as they were prior to the installation of the "sine-wave" filter. Therefore, considerations must be made to monitor the effectiveness and integrity of the filter. Monitoring of the output waveform is the preferred method. The protective device should be capable of continuously monitoring for dV/dT and peak voltage. An alarm signal or automatic shutdown method should be incorporated.
About The Author: Dean Mehlberg is the Vice President of Engineering at TCI (Trans-Coil, Inc.), Milwaukee, Wisconsin. He holds a BSET from the Milwaukee School of Engineering, is a member of IEEE and has worked in the area of low-pass filters since 1991. His recent work in adapting low pass filtering techniques for V/Hz drives control submersible pumps is highlighted here.