Detecting oscillations in the variable frequency drives of large industrial blowers
Until recently, the majority of AC variable speed drives has been applied to variable torque, pump, and fan applications. Advances in drive technology have led to the use of induction motors in high performance applications that exceed the capability of motors designed for operation on sine wave power. These applications, which have traditionally been served by DC systems, have created the need for definite purpose AC induction motors designed specifically for operation on Variable Frequency Drive (VFD) controllers.
VFDs control the rotational speed of an AC motor by controlling the frequency of the electrical power supplied to the motor. Commonly used in environments requiring large amounts of airflow, VFDs are typically connected to the fans of these large systems to save energy and costs by allowing the volume of air moved to match the system demand.
Many large industrial blowers – such as blast furnace blowers used to produce molten iron from iron ore, coke, and limestone in the steel industry – are steadily monitored for correct functioning with an advanced data acquisition system over a long period of time in order to reliably and securely capture any anomalies that may occur. The VFD systems can exhibit some inherent instability if not properly adjusted and tuned, which then affect system performance.
The presence of torsional natural frequencies in the electro-mechanical system, somewhere within the operational range, can cause motor speed fluctuations, which in turn gives current fluctuations. That implies that there is positive feedback to the VFD that, if not properly dampened, causes oscillations. In other words, the Proportional-Integral-Derivative (PID) control elements need to be in the right proportions to each other to have a stable system. A PID controller calculates an “error” value as the difference between a measured process variable and a desired set point. The controller attempts to minimize the error by adjusting the process control inputs, which in this case means dynamically tuning the VFDs output.
ABB, a global leader in power and automation technologies that enable utility and industry customers to improve their performance while lowering environmental impact, needs to make sure its VFD systems are not susceptible to these oscillations.
The measurements on a VFD system installed in a blast furnace blower are addressed in the following discussion. The purpose of the measurements is to investigate the root cause of occasionally occurring oscillations in blower revolution speed that can result in potential harmful variations in combustion airflow.
Tools available to field maintenance electricians are typically limited to RMS current and voltage meters, which are insufficient in diagnosing a VFD’s varying operation within the motor-load system. The transient nature of VFDs, together with the motor and load system components, interacts dynamically, limiting the usefulness of standard RMS and voltage/current meter instrumentation. The measurement system should exemplify the VFD diagnostics using modern instrumentation designed to detect system flaws over long periods of time that can occur far in between one another in order to produce the clearest picture of system stability.
Using a multichannel transient recorder where every channel is acquired and stored to disk independently, the unwanted oscillation events are accurately captured, regardless of measurement length or the time interval between two events. For applications where data points are intermittently generated, every channel on the data acquisition system needs to be an active trigger source in an advanced trigger mode, such as interval or slew rate in either “AND” or “OR” trigger logic.
Drive system configuration
Figure 1 shows the block diagram of the frequency converter in a blast furnace blower system. The transient recorder collects the various signals, mostly through the drive system’s control electronics. The control electronic block is the signal conditioning interface, limited in bandwidth to 5-40 kHz, which probes the signals labeled in the diagram and provides access to a normalized voltage signal.
The modular data recording instrument TraNET 404S made by Elsys Instruments was selected to monitoring the VFD’s measurements. This transient recorder provides 16 analog input channels, each with a sample rate of up to 80 MSps and a record length of 16 MS as well as an Event Controlled Recording (ECR) data acquisition mode with three phases and a large memory depth.
Configured in its ECR data acquisition mode, the recorder enables each of the 16 channels to operate as an active trigger source in “OR” trigger logic. There are three different configuration types in the ECR mode: single channel, single channel with associated channels, and multichannel.
In single channel, only the triggered channel captures the event and stores it to the internal hard disk drive whenever one of the input channels meets the trigger conditions. This is a unique data acquisition mode that can be configured to acquire the relevant events on the triggered channel only if required.
If it is also important to capture related signals synchronously at exactly that time, the single channel mode with associated channels accommodates this data capture scheme, whereas multichannel ECR is ideal in the event that all signals need to be acquired if the event occurs.
For measuring the data needed to monitor the blast furnace blower’s VFD, the data recorder is operating as a stand-alone system in the ECR multichannel mode with the dual mode, which acquires two data streams at once, activated. This means that all 16 channels are continuously acquiring the data stream to disk at a sample rate of 100 Sps once the recording is started. Figure 2 shows the data acquisition configuration of the recorder for this application.
As soon as the trigger condition is met on the only trigger source, D4, the instrument acquires on all 16 channels at 100 kS before the trigger event and 1.1 MS after the trigger event at a sample rate of 10 kSps. This procedure continues in a loop until 100 blocks are acquired.
The continuously sampled acquisition of the dual mode stops 10 seconds after the last block was acquired or when the maximum limit is reached. In this configuration, the dual mode part of the data acquisition would end after 115.741 days of streaming all 16 channels at 100 Sps to the hard disk.
Capturing the events from the VFD system over a long period of time and displaying the events as well as the continuous background signal needs to be done in an effective and flexible way. Easy access to the relevant information in the monitored data acquisition system allows an efficient diagnosis of the unwanted phenomena.
To enable better monitoring flexibility on the part of the operator, the ABB VFD Drive System is monitored either from a local computer or remote connection via the transient recorder’s complementary software, TransAS 3, which allows the operator to configure the transient recorder, read and display the captured waveforms, and troubleshoot without physically accessing the unit (Figure 3).
The sample acquisitions in Figures 4, 5, and 6 are a snapshot of a long acquisition over hours where just a few trigger events are displayed. In this specific application, the trigger pulse was generated by the drive system. The vertical white line “T” indicates the trigger time. In total, 6 of the 16 channels are displayed in the waveform graph. The legend in the upper right corner lists channel names in reference to the control panel shown in Figure 1. The waveform graph is arranged with four different vertical axes in order to display the various traces in different scaling.
The unique combination of fast sampled events and the slow sampled continuous waveform in the ECR data acquisition mode enables the user to not only monitor but also troubleshoot an installed customer converter drive system. And because an application engineer can avoid the need to go on-site in many cases, the problem can be resolved much more quickly and with less expense.
Ensuring system reliability
As the electronics within them have advanced, modern VFD controllers and motors require more than conventional measurement tools to diagnose, troubleshoot, and fine-tune the system and ensure overall reliability. The method described in this article uses open, modular transient recorder equipment that addresses the foolproof approach chosen to address the intrinsic issues represented by VFD technology.
Elsys Instruments, LLC
ABB Switzerland Ltd.