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Servo Drive
Outline description of safe torque off (STO)

The safe torque off (STO) function is a safety function that shuts the motor current and turns off motor output torque by forcibly turning off the driving signal of the servo driver internal power transistor. For this purpose, the STO uses safety input signal and hardware (circuit).When STO function operates, the servo driver turns off the servo ready output signal (S-RDY) and enters safety state.

This is an alarm condition and the 7-seg LED on the front panel displays the error code number.

Safety precautions

  • When using the STO function, be sure to perform equipment risk assessment to ensure that the system conforms to the safety requirements.
  • Even while the STO function is working, the following potential safety hazards exist. Check safety in risk assessment.
  • The motor may move when eternal force (e.g. gravity force on vertical axis) is exerted on it. Provide an external brake, etc., as necessary to secure the motor. Note that the purpose of motor with brake is holding and it cannot be used for braking application.
  • When parameter Pr5.10 Sequence at alarm is set to free run (disable dynamic brake), the motor is free run state and requires longer stop distance even if no external force is applied. Make sure that this does not cause any problem.
  • When power transistor, etc., becomes defective, the motor will move to the extent equivalent of 180 electrical angle (max.). Make sure that this does not cause any problem.
  • The STO turns off the current to the motor but does not turn off power to the servo driver and does not
    isolate it. When starting maintenance service on the servo driver, turn off the driver by using a different
    disconnecting device.
  • External device monitor (hereafter EDM) output signal is not a safety signal. Do not use it for an application other than failure monitoring.
  • Dynamic brake and external brake release signal output are not related to safety function. When designing
    the system, make sure that the failure of external brake release during STO condition does not result in
    danger condition.
  • When using STO function, connect equipment conforming to the safety standards.
Power supply Model Frame
Single Phase 100V MADHT1150 A-Frame
MBDHT2110 B-Frame
MCDHT3120 C-Frame
Single/3-Phase 200V MADHT1505 A-Frame
MBDHT2510 B-Frame
MCDHT3520 C-Frame
MDDHT5540 D-Frame
3-Phase 200V MEDHT7364 E-Frame
MFDHTA390 F-Frame
Single/3-Phase 200V MDDHT3530 D-Frame
3-Phase 200V MEDHT7364 E-Frame
MFDHTA390 F-Frame
Single/3-Phase 200V MDDHT5540 D-Frame
3-Phase 200V MFDHTA390 F-Frame
Single/3-Phase 200V MDDHT3530 D-Frame
3-Phase 200V MEDHT7364 E-Frame
MFDHT390 F-Frame

MADHT1105  A5 series Driver: A-frame
MADHT1107 A5 series Driver: A-frame
MADHT1505  A5 series Driver: A-frame
MADHT1507  A5 series Driver: A-frame
MADHT1105E  A5E series Driver: A-frame
MADHT1107E  A5E series Driver: A-frame
MADHT1505E  A5E series Driver: A-frame
MADHT1507E  A5E series Driver: A-frame
Part No. Title
MBDHT2110  A5 series Driver: B-frame
MBDHT2510  A5 series Driver: B-frame
MBDHT2110E  A5E series Driver: B-frame
MBDHT2510E  A5E series Driver: B-frame
Part No. Title
MCDHT3120 A5 series Driver: C-frame
MCDHT3520  A5 series Driver: C-frame
MCDHT3120E  A5E series Driver: C-frame
MCDHT3520E  A5E series Driver: C-frame
Part No. Title
MDDHT3530  A5 series Driver: D-frame
MDDHT5540  A5 series Driver: D-frame
MDDHT3530E  A5E series Driver: D-frame
MDDHT5540E  A5E series Driver: D-frame
Part No Title
MEDHT7364  A5 series Driver: E-frame
MEDHT7364E  A5E series Driver: E-frame
MFDHTA390  A5 series Driver: F-frame
MFDHTB3A2  A5 series Driver: F-frame
MFDHTA390E 5E series Driver: F-frame
MFDHTB3A2E 5E series Driver: F-frame
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