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Emergency Brake of Permanent Magnet Servo Motors

Emergency Stops

The Back EMF (BEMF) of a Permanent Magnet Servo motor is commonly used to quickly stop a mechanism as a result of a power failure. There are different category of stop modes and this one will address a safe, reliable and less threatening method of safely stopping a motor during this event.


  • For the safety of the machine, we want to prevent a crash of moving components
  • We need to quickly decelerate a motor in the event the interruption occurs at full speed
  • For the safety of personnel (Operator or Maintenance), motion from the system must be halted by redundant means and no motion possible. No single silicon failure should endanger personnel so depending on one switch would not be considered safe.
  • Although this circuit was used with an S200, this applies to any drive type using linear or rotary permanent magnet servo motors.


Servo motors are permanent magnet generators in runaway mode. Disconnecting the motor from the drive will encourage arc events that can damage the motor, the drive, or the safety relay mechanisms. An alternate means is needed.

Ideal Circumstances

  • Remove the Enable of the drive.
  • Remove the Safe Torque Off (STO)
  • Remove the Bus Power from the drive
  • Maintain the control voltage
  • Stop the motion of the motor as quickly as possible without damage
  • Use internationally acceptable components

Pilz PNOZ Relay

The Pilz PNOZ relay will be the central figure of this circuit. Using this method or other methods described in the Pilz manuals, different category of safety can be met. The following method is a recommendation for the stopping of a linear motor in the event of a runaway or power failure using this relay.

The Relay


Components within the PILZ

  • Force guided contacts in the event of welded contacts
  • Single shot action requiring reset
  • Normally Open and Normally Closed Contact array within the unit.
  • Redundant relays ensuring engagement and disengagement.

The Circuitry



  • KR-1 is a contactor type relay. It doesn’t disengage the motor but simply shorts two dynamic brake resistors across the windings when engaged. This is a normally closed contact so it will engage whenever the 24 VDC is removed from it either by power outage or control.
  • Only two resistors are needed in this setup.
  • The motor is dynamically braking via BEMF to itself, not to ground and thus would be safe in ground current sensitive applications.
  • KR-2 removes the three phase or single phase power to the drive.
  • The STO and Enable can be routed through the PILZ PNOZ relay assembly to ensure that at no time can the resistors be engaged on the motor and current flowing to them. This is controlled by series redundant relays within the PILZ.
  • Routing both STO and Enable through the PILZ ensures redundancy, a sometimes necessary compliance requirement.