VFD software functionality
In continuation of our series to guide users how to choose a medium voltage VFD this post talks about VFD software functionality.
We live in a digital age and there is no doubt that software is very powerful. Don’t worry, we don’t gonna talk about Industry 4.0 on high level missing any substance like you might hear from every corner. Let’s keep focused on the VFD: What software functionality shall your VFD contain? How many parameters shall the software contain and how many of them shall be accessible to a normal user? Where to draw the line between flexibility and complexity / ease of commissioning?
On one hand the user desires easy and fast commissioning that does not require a senior drive expert to make the VFD run. There the philosophy may call for a handful parameters to set and then press the start button. The commissioning goes fast, there is no need for highly skilled service person and the customer might even be able to do the job himself. On the other hand once an unexpected problem appears the user would wish to have some more freedom in parameterization and eventually few extra functions to activate or deactivate and try to solve the issue.
Too little or too much software functionality?
People naturally have different opinions. We are convinced that in case of software too much is better than too few (although we must stress that it is more about quality rather than quantity). Once the equipment is delivered and installed on site there is typically not much to modify. Besides the software there is hardly anything one could play with. Mature software easily contains several hundreds or thousands of parameters. The input parameters shall have limited range to eliminate typing errors, but shall not be too restrictive (otherwise the “special cases” hit the limits). In a good software the parameters are sorted in logical groups. There might be several access levels (e.g. basic and expert) so that some advanced functions are masked and inherently disabled. If the application requires such functionality a service engineer with expert access can enable and parameterize it (deeper knowledge required therefore not accessible for everyone). The VFD software shall fulfill following main tasks:
- Motor control (adjustable speed and torque)
- Monitoring / supervision / predictive maintenance
- Protection and interlocking
Primary function of the VFD software is to control the speed or torque of the motor. Simple applications might use primitive scalar control (also called ‘V/f’). Applications with higher dynamics require vector control (field oriented control) or direct torque control (DTC). Besides these ‘textbook methods’ there are additional control modes for special applications e.g. with long cable transmission between inverter and motor. Multi-motor applications might need a sophisticated master-follower logic. As the range of drive applications is very large and their requirements are very different, the motor control shall be flexible with regards to dynamics. Some applications simply need high dynamics, others might run much smoother if the dynamics is reduced.
Other task that good software does is a supervision of selected signals and their visualization. The user can get an overview about actual motor values (actual speed, torque, shaft power, apparent power), inverter values (output voltage and current), dc bus, cooling system (temperature of cooling medium, pressure etc). The signals may be sent to an overriding control system where the whole process or even whole plant may be visualized. The software shall therefore support the most common communication protocols.
Protection as integral part of VFD software functionality
A mature software shall cover all important protection functions. There will be a separate article about VFD protection so at this point we stress the fact that software plays key role in protection concept. It already starts when energizing the VFD. Larger drives have a circuit to precharge the dc bus and the software shall not allow to close the main circuit breaker until the drive is precharged to given threshold. Liquid cooled drives monitor the parameters of cooling circuit. Conductivity of cooling medium must not exceed certain threshold and if the initial value is higher the software does not allow to start the inverter modulation. In operation numerous abnormal conditions can appear. The software shall always properly react and make sure that the VFD remains in safe operating area. In case of uncritical fault the drive might just stop (block pulses). In case of a severe fault, especially overvoltage or overcurrent, the software must react in a way that prevents hardware damages and makes sure that VFD does not endanger the personnel. Black box feature is very useful when troubleshooting the failures or trips. You should request your VFD to have this ‘post mortem’ recording of signals to easier unveil what happened.
Condition monitoring and condition based maintenance
Condition monitoring gains on importance and the software shall allow remote access and support predictive maintenance by monitoring and post-processing of selected KPIs, creating trends and informing user about the health status of the drive. Abnormal conditions can be noticed early avoiding unplanned shutdowns. Condition monitoring principally reduces the time required for troubleshooting. If the trends are visible for the manufacturer of VFD, he might use it to further improve his products and make them more robust with longer maintenance intervals.
Flexible VFD software functionality
Flexibility of software is very important and you never know when you might need it. Here few examples:
Is the motor temperature rise higher than expected/designed for? Well, that should not happen, but maybe by increasing the switching frequency the problem can be fixed. Is there a speed range with increased vibrations? Good motor control software shall include several functions that either reduce the excitation or add some more damping into the system. Is the motor very noisy? The root cause might be the frequency content of the output voltage or torque resonating with eigenfrequencies of the motor. It would be a great advantage if you could change the modulation scheme and influence the spectrum of output signals. Does the process require high dynamics? Then there should definitely be several functions in the speed or torque control block to support it. Does the supplying grid suffer from voltage dips? Advanced software contains functions to overcome such voltage dips without tripping the drive system. Shall the drive system operate in extreme environmental conditions that however appear very seldom? Then functions with automatic derating might be very useful to optimally utilize the equipment.
Summary
We definitely recommend a flexible software with “some extras” that can be activated and tuned when needed. Many of the advanced functions might be by default deactivated, but there are there as backup. Maybe the system parameters are different than what was assumed or calculated in the engineering phase. Maybe the process requirements change over a period of time. Maybe something unexpected appears. MAYBE… better be ready.
References
[1] VFD control: Introduction, https://mb-drive-services.com/vfd-control-introduction/
[2] How to choose a medium voltage VFD: Protection concept, https://mb-drive-services.com/choosing-mv-vfd-protection/
