HowTo: How to Safely Use a Variable Frequency Inverter Drive

This is intended to provide general guidance covering the main points for Customers arranging their control circuits for the safe use of Inverter Drives.

It is possible to control an Inverter Drive for; Variable Speed, Soft Start, Soft Stop, Jog, Direction and Fast Stop with just simple low cost switches and pushbuttons directly connected to the Inverter Drive terminals.  The drive usually powers its own control circuits with low Voltage signals and includes provision for these functions and well as many others in some products.  This method is only considered safe for a fully guarded machine and other components are necessary to safely control a machine where personnel have access to rotating or moving parts. 

For Industry, the Machinery Directive requires a ‘Risk Assessment’ to be completed and documented by a competent person at the user site to show the level of risk considered and how this has been covered by the control system design.  This is necessary to minimise the possibility of an accident and the human cost of the same.  Another aspect is the legal implications in case of personal injury.

Home use Lathes are no different to those in Industry, in respect of the potential danger to Operators.  The legal position is not much different from Industry, as someone is responsible for the safe design of the control system and its installation.  The responsibility lies with the Designer, Installer, or with the Machine Owner to ensure the equipment is safe for himself and anyone else to use.

The Inverter Drive Manufacturers go to great lengths to ensure their equipment is as reliable as possible and flexible enough to be adapted into a great many applications.  The reliability is dependent on its correct use, application and on its environment.  There is usually a safety statement or a section covering safety and general use of the product in its manual.  The Inverter Drives are components to be correctly applied to suitable applications and included in machine controls by others.  We as a Distributor of Industrial Products cannot take Engineering responsibility for Customers own designs.  The suggested control circuits and ‘Macros’ shown in the manuals are examples of the minimum required for the Inverter Drive to control speed and torque of a motor with various control methods.  It is the Customers responsibility to ensure the machine controls include other components as may be required for the safe use of Inverter Drives fitted to their particular machine.

Run and Stop – The run and stop function is usually controlled by 0V to 24V internal or external supply being applied to one terminal allowing the drive to accelerate up its ‘Ramp’ to the speed set by other functions.  Opening the circuit initiates a stop, with either a ramp to zero speed or coast to rest according to how the Inverter Drive is set-up.  Once stopped the power supply remains connected to the Inverter Drive terminals and the motor is only stopped by there being 0V applied to a terminal where 24V will initiate a start.  There is no air break in the supply and therefore this stop condition is not safe for an un-guarded machine.

Supply Contactor – A switch or a contactor in the supply to the Inverter Drive will render the stop condition safe when open, as long as it cannot be accidentally turned on or closed.  Often an Emergency Stop pushbutton and reset button is all that is required to control a main supply contactor, subject to the Customers own review of the safety requirements and the need for redundancy.  Where a stop button is required to initiate a ramp down of motor speed before disconnecting the supply, a start relay will be required in addition to the contactor and its function is to maintain the supply until zero speed is reached or a timer is included to allow the same when timed out.  The suitability of such circuits is dependent on use of the machine being controlled and the level of protection and redundancy afforded by the Customers design of the control system.

Safe Torque Off – Some Inverter Drive products include a ‘Safe Torque Off’ or ‘STO’ function.  These products will meet an Industry Standard (SIL Rating) and will have been tested and approved before being released for sale.  The ‘STO’ function has more than one method of disabling the Drive internally and malfunction of either one will prevent the drive from applying torque to the load.  As the drive is diabled with this function, it will coast to rest.  Details and suggested minimum circuits are shown in the product manuals.  This is not a recommendation, but is another option to consider in a risk assessment.  It still does not provide an air break in the main supply, but is a more secure method of preventing an unexpected start.

Safe Stop - Also known as an 'SS1' function, this is a sequence where the drive stops quickly with its dynamic braking before 'Safe Torque Off' is activated.  Usually this function requires an additional 'safety stop relay' to activate a 'fast stop' input and the two 'safe torque off' inputs.  A suitable 'emergency stop' pushbutton with two normally closed contacts and a 'reset' pushbutton are required to be included in the Operator controls. This is another option to consider in a risk assessment and is safer than an 'STO' alone, as it will allow the drive to brake to zero speed before activating the 'STO'. A suitable safety stop relay is linked below this acticle. 'SS1' functionality is being added to some Inverter Drive products, but it is important to understand the extent of the function, as it may still be necessary to add an external safety relay to provide the 'reset' function.  

Emergency Stop – This is usually provided by a suitable safety relay designed for the purpose (see the products linked below), or made up from standard relays with timers and other additions to suit the level of protection and redundancy required for the safe use of the machine.  The circuit includes the normally closed contacts of Emergency Stop pushbuttons and other devices such as trip wires or tight line pull cords.  The purpose designed emergency stop relay (safety relay) we offer as 34148 is suitable for a single drive application.  Other products are available from companies such as Pilz, where the choice of relay is dependent on the level of risk, stopping method and level of protection required.  There is a great deal of information available from the website.

Dynamic Braking – This is a feature of many products offered by the Inverter Drive Supermarket.  By adding an external resistor to an Inverter Drive having this facility, the braking energy from stopping a rotating mass can be dumped and the braking time significantly reduced.  The instructions for sizing a brake resistor are included below the calculation schedule under the ‘Which Resistor’ button at the bottom right of your screen.  This is often included as part of an Emergency Stop function and some Inverter Drive products have a separate stop ramp or ‘Fast Stop’ function for this purpose.

Set Speed Potentiometer – Motor speed is usually set via rotation of a simple potentiometer.  The potentiometer divides the 10V available from the drive terminals with a wiper to provide an infinitely variable signal from 0V to 10V equalling zero to maximum speed.  The security of the speed set point signal is dependent on the condition of the potentiometer and on all three wires remaining connected.  It is not advisable to access the rotating parts of a machine, when the motor is stopped just by turning the potentiometer to zero.  Better to set a minimum speed in the Inverter Drive to show the drive is running even when the potentiometer is set to zero.  Other methods of providing a speed set-point resident in most Inverter Drive products include provision for ‘Raise Speed, and ‘Lower Speed’ pushbuttons connected to two digital inputs, a  umber of pre-set speeds via 2, 3 or 4 digital inputs, or via a communications port.

Reverse – All Inverter Drives will reverse by static means, usually controlled by 0V to 24V internal or external supply being applied to one terminal or the use of separate ‘Run Forward’ and ‘Run Reverse’ inputs.  The speed will follow the ramp down and ramp up profiles set in the parameters when changing direction.

Jog – Many Inverter Drives include a Jog or Inch function to rotate the motor at slow speed for the duration a 24V signal is applied to a terminal.  This can be combined with a reverse signal or arranged as ‘Jog Forward’ and ‘Jog reverse’ if available in the Drive firmware.

Digital output – This can be a volt free relay contact or transistor output, used for sequencing, fault or drive status annunciation.

Analogue output – Generally used for speed or current indication, as with Analogue inputs this can be in various forms ie 0-10V, +/- 10V, 4-20ma etc

Earth Leakage Circuit Breakers – Some Inverter products will trip earth leakage circuit breakers (ELCB or RCD) via their EMC Filter circuits.  The level of filtering will affect the leakage to earth through the small filter capacitors.  This does not affect the safe use of Inverter Drives, as long as the Manufacturer’s instructions are followed and the product if permanently earthed.  Some products include filters with leakage levels below the 30mA level of a standard RCD and a facility to remove the filtering altogether, if tripping becomes a nuisance.

Summary - Whilst we do not explain how to connect Inverter Drive to machines, more importantly we offer explanations of the usual control features of many of our Inverter Drive products.  How far the safety circuits are taken for a machine, is a decision for Customers and End Users to make, based on their own perception of the risk and responsibility.  This document is intended to inform Customers of the features available and Industry standards to better make that decision. 

We do not undertake Design or Engineering contracts, so do not offer control system designs for any individual machines.  We therefore take no responsibility for the safe use of any of the equipment we supply.  The responsibility for the safe design of a Machine control circuit and its safe use lies with the Customer and their Technical Advisor on site.  Customers are always advised to employ the services of a qualified Electrician or other Professional, to ensure the safe use of the equipment.

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