VFD safety functions
In this article you will read about VFD safety functions, their importance and principles.
Safety is on top of the agenda for most companies, at least in all developed countries. One top manager calls safety one of the key elements for the “license to operate”. VFDs and drive systems include several safety functions. Keep reading this article to get an overview on VFD safety functions.
Requirements on safety of personnel, machines and equipment are continuously growing. In this context, there is an increased demand for integrated VFD safety functions. Integration does not just increase the safety – it also simplifies the technical solution. There is no need for additional hardware and supervision components which in turn reduces the effort for installation, wiring, commissioning or maintenance. And it helps to reduce space. Moreover, reaction times can be minimized and probability of an error is significantly reduced. All these benefits sound really convincing, don’t they?
Fundamental VFD safety functions
Modern VFDs offer an entire palette of electronic safety functions built directly into them [1]. Such VFD safety functions can replace partly obsolete external electro-mechanical safety systems.
After we got excited about the integrated VFD safety functions in general, let’s explore what functions actually belong to this category.
E-OFF (Emergency Off)
E-OFF – Emergency Off – belongs to stop category 0 according IEC 60204-1. When triggered, the function immediately removes power to the machine actuators. Consequently, the machine coasts down being decelerated by the load and by inherent losses. This approach is suitable for systems with low inertia.
E-STOP (Emergency Stop)
E-STOP – Emergency Stop – is a stop function belonging to stop category 1 according to IEC 60204-1. Unlike E-OFF, E-STOP is a controlled stop with power available to the machine actuators. Once the stop is achieved the power to the actuators is removed. E-STOP is preferred for systems with higher inertia (too long coast stop).
STO (Safe Torque Off)
STO – Safe Torque Off – enables reliable disconnection of torque. It is the most common safety function when motor is not galvanic disconnected and the VFD remains available for diagnostics and supervision. STO is a core safety function of modern VFD. It is stop category 2 according to IEC 60204-1.
SS1 (Safe Stop 1)
If the drive system does not track parameterized ramp and SS1 is active, it will trigger STO function (described above) or eventually SS2 (described below).
SS2 (Safe Stop 2)
This is an additional function, safe stop or safe brake, that acts similar like SS1and brings the drive into SOS state.
SOS (Safe Operation Stop)
This function prevents unwanted motion of a drive that is kept in a specific position. Control system and power supply remain uninterrupted.
SLS (Safely Limited Speed)
In this mode the drive continues spinning at certain speed that must not be exceeded. When accelerating above this speed, an STO function is actived and the inverter pulses get blocked. This function is highly recommended for drives with higher speed and/or larger inertia.
SMS (Safe Maximum Speed)
SMS function monitors the speed of the system and intervenes if the speed operates above the maximum speed setpoint. The purpose of SMS function is mainly to prevent mechanical damages of the load in overspeed situation.
SLA (Safely Limited Acceleration)
SLA function monitors the acceleration rate of motor to ensure that is stays within defined limits.
Besides above safety function a lot of attention is paid to a VFD design eliminating risk of an arc fault [2]. However, that is a topic that deserves its own article.
Standards and regulations with regards to VFD safety functions
⇒ EN ISO 13849-1
⇒ IEC/EN 60204-1
⇒ IEC 61508
⇒ IEC 61800-5
⇒ IEC/EN 62061
⇒ EN P54-1
⇒ Machinery Directive 2006/42/EC
Summary
VFD safety functions are important contributors to increased safety in machinery. Modern VFDs provide a wide range of integrated safety functions. This article describes some of them, but does not aspire to provide a complete list. For more information please see the references below. We recommend to check the article [3] that was published recently.
References
[1] ABB drives with integrated safety functions, https://new.abb.com/drives/functional-safety/abb-drives-with-integrated-safety-functions
[2] How to choose a medium voltage VFD: Protection concept, https://mb-drive-services.com/choosing-mv-vfd-protection/
[3] A crash course in VSD safety functions, https://www.inverterdrivesystems.com/a-crash-course-in-vsd-safety-functions/
[4] Safety-conscious drives meet safety standards, https://search.abb.com/library/Download.aspx?DocumentID=9AKK107045A5361&LanguageCode=en&DocumentPartId=&Action=Launch
[5] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives