Safety Features of VFD

 

6 Sept, 2020.

In this post, we will understand various safety features that are provided in a VFD for a secure and efficient work environment. (Click on the images for a more zoomed view)

Hello guys. I considered this topic to discuss as one of the most critical requirements of industrial automation is safety. Nothing is more valuable than the ensuring a secure environment for the machinery and the people working around. As you know, most of the automation products need to meet the international safety standards to operate. Now, one of the most used products since the revolution of industrial automation is a Variable Frequency Drive (VFD). You must be familiar with this item. What more do you need when you are able to control a motor according to your needs and technical specifications with this tool. From a small fan motor to a large blower in thermal power plant, almost every motor operated nowadays uses a VFD for it’s control.

They reduce the power usage of a plant to a great extent; as they replaced the direct mains-supplied motors which consume a large amount of power. A smooth start and stop is provided to the motor, which increases it’s lifetime and efficiency. Also, they provide data of the motor to the PLC or any other programmed equipment; which helps in better troubleshooting and controlling.  

Now, if you consider some critical machines with heavy moving parts, like conveyors, fan blades, trolleys, hoists etc.; they can sometimes pose to be a potential hazard if not operated properly. Also, in case of maintenance or inspection of these systems, it is necessary that these machines be turned off safely. Earlier, electrical or numerous hardware interlocks like limit switches, encoders, safety inputs, relays etc. were interfaced with VFD or PLC system for interruption and a secure stop of the machine; which overall meant for a safe control of the system. However, now almost all these safety controls are enacted by a VFD itself. All these features are integrated in it’s programming. VFD-based systems are easy to manage, less-complicated and time consuming than the traditional hardware systems. They reduce wiring to a great extent and are less vulnerable to parts failure. Simpler and faster reset procedures inside it reduce the downtime to a great limit.

Safety Functions associated with VFD:

Be it current, speed, torque etc.; you get to know almost all the data of a motor from VFD. So, we will have a look at some of the typical in-built safety features in a VFD; which uses all these basic parameters and which are programmed using these data for a safe operation.

·         Safe torque off (STO):

As the name implies, it simply blocks the motor movement by safely and instantly reducing motor torque to zero. It reduces the torque generating energy to the motor immediately; which causes the motor to stop using natural inertia and friction from the load. So, how a motor stops depends on these two factors. You can consider like an emergency stop situation. The motor can then not restart unexpectedly. You will have to disable STO manually and stop the output command from the controller to restart it. For example, you can implement this function by installing a proximity sensor near the critical mechanical moving part and interfacing it with VFD and PLC. The sensor when sensed will activate this function and disallow the motor to move as it’s torque energy has been blocked by the VFD.  

·         Safe Stop(SS):

In applications where instant stoppage of the motor can damage it’s movement in lifetime run, this function can help. The VFD ramps down the speed of the motor rapidly in a controlled manner; depending on it’s specifications. STO can then be activated. When triggered, this function is enacted by continuously monitoring the deceleration ramp of the motor depending upon it’s parameters set in the VFD; until the motor reaches the standstill state.  

·         Safely Limited Speed(SLS):

This function limits the speed of the motor in a controlled manner. The speed will reduce to either the actual rated speed of the motor or the parameter set in the VFD. It is achieved by continuously monitoring the current stator frequency of the motor. When the speed limit exceeds, this function is triggered and rather stopping the motor, it will just reduce it’s speed and run in the specified range; until the operator finishes it’s work and stops the machine. His production rate will reduce, but he can be safe working there. If still the speed does not reduce, then STO or SS will be activated.  

 

·         DC Injection Braking:

It is a method used to apply quick stop to the motor for immediate braking and safety reasons; depending on the application used. As the name implies, DC voltage is injected in the motor winding by cutting off AC supply. AC supply to the windings is first disconnected internally and the DC supply is then passed on to it. When DC current is applied, a fixed magnetic field is produced rather than rotating one. Braking action is thus produced by the rotor working to match with this stationary field. The braking force is dependent on DC current; higher the current, higher the force. The regenerative energy generated during braking is dissipated as heat by the motor; so limited braking must be applied to the motor for thermal protection. The circuitry inside the VFD controls the braking current to it’s rated one by controlling the amount of voltage applied. This safety method is mostly in hoist/crane applications; where DC current is injected to the motor if the electrical supply to mechanical brakes fails to hold the hoist rope motor and which can cause damage to equipment or any person standing below the hoist. The logic used here is simple; if you are not supplying any output frequency to the motor and if the electrical supply through brake contactor or mechanical brake fails, then hoist will become free to move and will start coming down due to gravity. That means, the motor is moving without any frequency given and encoder counts are coming in VFD; which will activate this function to hold the motor. One thing to remember is that motor sizing and design is the main concern here for implementing this feature, as it has to undergo a high temperature operation.    

·         Safety Function Guard Door Locking (GDL):

This is a very simple function in tandem with it’s programming in VFD. By the name, imagine a critical machine inside a safe guard door lock. This lock output is controlled by the digital output from VFD; which means lock will open only when motor is in stop condition. So, when the VFD senses that the motor power is turned off by sensing current, torque, encoder, STO or any other parameter; it will then only turn the digital output as on and release the lock connected to this output.

Many of us are not familiar with these features inside a VFD. It will require initially, a very good understanding and training of the system. Once you get familiar, you can safely design and implement such type of critical systems.  

I have covered the general theory related to the safety features in a VFD. I have also not attempted to cover every type of features deeply; you can learn it easily once you get familiar with the basic parameters inside it. I have just given you an insight of these types of features. Every manufacturer has it’s own way of programming such facilities; so there can be more facilities too inside the VFD. Learn the basics and explore a new type of study in this type of automation.

 

Thank you guys; I hope you enjoyed reading the practices normally used for this type of study in industrial automation.