Inverter Enable Function

Functional Description

With the Inverter Enable function (IE), you can bring drives to a defined safe stop.

In the sense of the relevant standards, the requirements of the stop category 0 (Safe Torque Off, STO) and stop category 1 (Safe Stop 1, SS1) can be met. Both categories lead to a torque-free motor while SS1 takes this state after a predefined time. As a result of the hazard and risk analysis, it may be necessary to choose an additional brake as a safety-related option (for example, for hanging loads).

Scope of Operation (Designated Safety Function)

The following circuit is a suggestion for the Lexium 52 system. In the circuit suggestions given, the "Safe Torque OFF, STO" safety-related function is carried out by switching off the trigger signals of the power stage. They transmit the PWM signals to the power stage (pulse pattern lock) using the “InverterEnable” signal.

Operating Principle

The safety function Inverter Enable is triggered via 2 redundant inputs. In order to maintain the dual-channel design, both inputs must be switched individually. The switching operation must take place for both inputs simultaneously (time offset <1 s). The power stage is being disabled. The motor can no longer generate a torque. If only one of both inputs is disabled or the time offset is too long, the power stage is disabled and a detected error message is issued.

oAfter the emergency stop device is activated, a controlled ramp down takes place for the drive.

oIn the process, the DC bus voltage increases until the braking resistor is switched on.

oIn the braking resistor, the energy which is fed back from the motor is converted to heat.

oThe power circuit breaker and/or the Inverter Enable signal must remain energized until the drive stops.

oAt the latest after the normal braking time, the Inverter Enable signal is switched off by the delayed contacts of the safety POU PREVENTA XPS-AV.

oAfter this, the drive is in a defined safe stop.

Example stop category 1 with external EMERGENCY STOP safety POU Preventa XPS-AV

 

 

Defined Safe State

Inverter Enable is synonymous with "Safe Torque Off (STO)" according to IEC 61800-5-2:2007. This torque-free state is automatically entered when errors are detected and is therefore the defined safe state of the drive.

Mode of Operation

The safety-related circuit with InverterEnable was developed to minimize wear on the mains contactor. When the stop or the emergency stop button is activated, the mains contactor is not switched off. The defined safe stop is achieved by removing the “InverterEnable” for the opto-couple in the power stage. Thus, the PWM signals cannot control the power stage so that a startup of the drives is prevented (pulse pattern lock).

You can use the Inverter Enable function to implement the control function “Stopping in case of emergency” (EN 60204-1) for stop categories 0 and 1. Use an appropriate external safety-related circuit to prevent the unintended restart of the drive after a stop, as required in the machine directive.

Stop Category 0

In stop category 0 (Safe Torque Off, STO), the drive coasts to a stop (provided there are no external forces operating to the contrary). The STO safety-related function is intended to help prevent an unintended start-up, not stop a motor, and therefore corresponds to an unassisted stop in accordance with IEC 60204-1.

In circumstances where external influences are present, the coast down time depends on physical properties of the components used (such as weight, torque, friction, etc.), and additional measures such as mechanical brakes may be necessary to help prevent any hazard from materializing. That is to say, if this means a hazard to your personnel or equipment, you must take appropriate measures (refer to Hazard and Risk Analysis).

Stop Category 1

For stops of category 1 (Safe Stop 1, SS1) you can request a controlled stop via the PacDrive LMC. The controlled stop by the PacDrive LMC is not safety-relevant, nor monitored, and does not perform as defined in the case of a power outage or if an error is detected. The final switch off in the defined safe state is accomplished by switching off the "Inverter Enable" input. This has to be implemented by means of an external safety-related switching device with safety-related delay (see application proposal).

Independent of the safety function, the detectable errors not affecting the safety function are recognized by the controller, thus avoiding the drive from starting by switching off the mains contactor. Contactor K2 prevents the mains contactor from being switched on.

Execute Muting

To execute muting, determine the muting reaction time for switching off, that is, without the Inverter Enable function, within the application. Should a response time be required because of the risk assessment of the machine, the total response time of the machine has to be taken into account. That is to say, the components related to the safety functions from the sensor to the drive shaft or the driven mechanics have to be considered. The determined reaction time must correspond to the results of the hazard and risk analysis.

Proceed as follows to disable the Inverter Enable function:

Supply the /STO_A or /STO_B input with 24 Vdc to deactivate the Inverter Enable function.

The axes without Inverter Enable function become torque-free via the mains contactor and come to a stop. For more information, refer to Stop Category 0.

Validity of the Safety Case

The safety case for the Inverter Enable function of the Lexium 52 system is identified and defined by the standards listed in Safety standards. The safety case of the Lexium 52 system Inverter Enable function applies to the following firmware (FW) versions and hardware codes, which can be found examining the appropriate software object in EcoStruxure Machine Expert:

For questions on this, contact your Schneider Electric representative.

Interface and Control

The Inverter Enable function is operated via the switching thresholds of the Inverter Enable inputs /STO_A and /STO_B.

oSTO active: -3 V ≤ U_IE ≤ 5 V

oPower stage active: 15 V ≤ U_IE ≤ 30 V

For information on the technical data and electrical connections, refer to the chapter Technical Data.