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Safe Reduced Speed

Valid for S400, S600

This note explains how to project an application for safely reduced speed according to EN 954-1 category 2 or 3.

Possible measurements to fulfill the directives:

  1. Machine builder must do a machine risc analysis
  2. The machine type must be certified according to Appendix 4 of the machine directive. In this case a certificartion by a qualified institution is required.

Standard EN 954 may be used until 30.12.2011 the latest. For safety related control components the standard EN ISO 13849 must be used according to machine directive 2006-42-EG (Performance Level).

Safely reduced speed by reducing the DC bus voltage (category 2)


External wiring of the servo amplifier and parameter adjustments (e.g. with the setup software) are required to get a safely reduced speed function.

Maximum possible speed of the motor must be calculated by multiplying  DC bus voltage X voltage constant (Ke) of the motor. Motors with different Ke values therefore need different voltages for the safely reduced speed. Required torque can be an additional factor for calculation.

The proposed wiring fulfills category 2 according to EN 954-1. This can be reached by using a voltage supervision and a contactor according to the category.

Enable must be wired according to the requirements.

Configuration of the servo amplifier

Standard operation:             NONBTB=0     UVLTMODE=1      PVMAX=zzzz
Safely reduced speed:          NONBTB=1     UVLTMODE=0      PVMAX=zzz

ASCII Commands:

  1. NonBTB 1
  3. Save
  4. Coldstart (~ 30 sec delay, only one's)

The automatical control with digital inputs is not supported today. Parameters (NONBTB=1 UVLTMODE=0) for safe reduced speed must be stored in the servo amplifier (SAVE; COLDSTART). This disables phase loss and undervoltage supervision.

With DigIn1 tghe speed is set to a low value, if low voltage is detected by input 1 (In1mode 30).

ASCII Commands:

     5.     In1mode 30
     6.     InLCMD pvmax zzz (zzz maximum low speed)
     7.     InHCMD pvmax zzzz (zzzz maximum high speed)



  • In principle this wiring can be used with every servo amplifier type (S300 / S400 / S600)
  • the method is nut usable for vertical axes, because the motor torque decreases to zero when the speed reaches the speed limit
  • a special voltage is used for every motor / speed / torque combination because of different motor voltage constants. We recommand test with an adjustable transformer to evaluate the required voltage.


After switching off the supply voltage with key S5 the setup mode can be activated with key S4. In this setting the DC bus link is unloaded via a resistor under an adjustable limit value first. Afterwards a transformer for the reduced tension is added.

In this status the drive can be moved in the range of the low safe speed. In particular also, if the safety device responded.

For shifting back to normal driving the keys S6 and S3 must be pressed.

Single phase voltage supervision S1UM

Contactor K5 e.g. voltage limit switch by PILZ company.

The S1UM supervises the exceeding of a voltage level. With type UP the polarity of the measuring entrances is arbitrary. If the measuring voltage achieves the adjusted response value Uan, auxiliary contact 11-14 switches over and the LED shines. If, with automatic RESET, the measuring voltage falls below the hysteresis value Uab, the auxiliary contact switches over again and the LED expires. The equipment is again ready for use. In case of error storage the equipment is only ready for use again, if an external RESET key is pressed or the supply voltage is switched off and on again.

  • 12 measuring ranges, 0,1 V ... 500 V, selectable
  • Reaction time up to 10 sec adjustable
  • automatical detection of AC/DC voltage values
  • alternatively resting or full load current principle
  • galvanic separation between measuring and supply voltage
Variant UP: arbitrary polarity of the measuring entrances

Safely reduced speed with external components (category 3)

Wiring acc. to EN 954-1 Kat 3



The encoder feedback signal passes the PAD. Only one feedback and one feedback cable is required. Servo amplifier and supervision use the same signals. The supervised speed range can be selected by the PAD type (512,1024,2048).

The operation mode safely reduced speed can be reqached in these steps:

  1. slow down speed to 0
  2. switch to homing mode. Safety doors or similar can be opended
  3. with a hardware switch the safely reduced speed can be enabled
  4. the servo drive can be operated up to the speed limit

To make it impossible to overrun the safe speed limit, the digital inputs of the servo amplifiers must be specially configurated. Please contact the support department for information.

Mult axis system acc. to EN 954-1 (category 3)

We recommend the use of a Pnozmulti device from Pils company in multi axis systems. With a graphical software the axes can be adjusted to the required functionality and safety.

Cost saving

A lot of functionality can be transferred to the Pnozmulti by adding communication or I/O modules. Therefore solutions with small footprint and good price-performance ration are possible.

Required switch off variants


1.) Emergency Stop (z.B. 100ms)
2.) with STO (Cat 3)
1.) Emergency Stop (z.B. 100ms)
2.) with redundant contactor switch off (Cat 1 o. 2)
1.) Emergency Stop (z.B. 100ms)
2.) with Option AS (Cat 1) + contactor switch off (Cat 1 o. 2)
  •  Two contactors must be used with S400 only
  • Contactors must fulfill the required saftey standards
  • If emergency stop enables the required braking, then step 2 will not be executed!