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Implementing safety relay with unitronics servo motor & drive


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Hello everyone,


I'm working on an application using a unitronics servo and drive for the first time. 

I need the motor to stop every time the  light curtain is broken as a safety requirement.  This will happen several times per hour

As these drives don't have STO  I would have to use a safety relay to interrupt the supply to the motor. 

Having just read the manual it says:

"Frequently turning the power ON and OFF causes the internal circuit elements to deteriorate, resulting in unexpected problems. Always start or stop the Servo motor by using reference pulses."

Obviously I'm going to be violating this by using the safety relay to interrupt power.

Has anyone else encountered this when implementing safety to theses motors?

Would be great to hear from other people using these servo's as I will probably have more question over the next few weeks.



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Paying attention to servo motors, we must take into account that they are usually used in positioning mode.  This mode involves setting the final coordinate in advance.

That is, the activation of the safety system must change the task of the servo drive so that it stops as soon as possible.

Disconnecting the motor from the servo drive via safety relay is generally unacceptable. This will probably cause the driver to crash.

Disconnecting power from the servo driver will not provide the desired instant stop.

Therefore, after studying manual for the Unitronics servo driver, I came to the conclusion that the most effective in your case is the use of discrete input signals P-OT and N-OT. This two signal can be selected via Parameters (p.64-65). This two signal can be used as a command for an unconditional stop. Using the zero clamp settings to work out the P-OT and N-OT mode, you can create safe application.

Additionally, you can add  Unitronics motor with a brake and create a configuration for guaranteed holding of the motor in the stop position.

Safety relay has two ot more contact - that can be used for generate P-OT and N-OT signal simultaneously.

For additional information please contat support@unitronics.com.


P.S. I must draw your attention to the need for careful study of legal norms - national standards that regulate the operation of the safety system for the equipment where the operation of the servo drive is planned. I have studied these documents for many types of equipment, so all your steps and schematics should follow these requirements.

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Hi Kratmel,

Thanks for getting back to me.

I have chosen a motor with a brake, I assumed cutting power then activating the brake would be all that I could do in the absence of a safety rated stop like STO, SS1 and SS2.

There is no mention of the P-OT & N-OT being safety rated so I cant assume that they are. 

I will follow this up with support


Thanks again!

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Hello Hoops,

In order to implement safety, place a safety contactor between the grid and the drive power input (not the control board), so once the safety contactor disconnects the grid from the power, no power supply to the motor but still, the PLC communicates with the drive, once the contactor back to a normal state, you'll be able to recover easily.

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Hi Igeny,

Thanks for your reply, that does make sense but this will still involve cutting power to the motor pretty frequently which the manual says will damage the internal circuitry over time.  Even if the safety relay simultaneously sends a command to the plc to halt the current motion task the power cut will take effect first.



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Unfortunately, you did not say anything about the application itself, for which the servo motor is used.

There are probably safety requirements and standards for this application. Usually, the standards are based on the current state of technology development. Special safety inputs for converters have appeared quite recently and have accordingly been added to existing standards as additions.

On the one hand, you want to stop the engine quickly, and on the other hand, you say that the fastest way to do this is to turn off the power.

So I think only following the safety standard is correct. If it involves turning off the power supply of the power unit, then it should be done that way. The frequency of turning on and off the power will not be high anyway.

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Its going to be used in the UK so the standards will be the same as the current EU directives so the safety system will be have to comply with BS EN 60204.

For my question the application is irrelevant. All I am asking is how to achieved a safety rated (> PLc or Sil2) stop when the drive doesn't have built in safety functions like STO, SS1 or SS2.  Sending a command via a PLC would not be safety rated as its not a safety plc and the drive doesn't have the required safety rated stop features. So It looks like the only option is to cut power via a safety relay.  If this does damage the drive then I wont be able to use this hardware in future as all machine will require a safety function.

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Hi Hoops,

Before STO arrived to the market as embedded drive safety functionality, engineers implemented safety using safety contactor at the way I described previously.

As I wrote, in order to avoid any damage, if your motor stopped and MC_Power is off, no current towards the motor and the contactor can cut safely the motor power loop.

Also, even cutting during operation is fine and the motor will freely run until stopped, and that's fine for emergency.

If you really need Safe Speed 1 or Safe Speed 2 safety functionality, you should use drives that provide that safety functionality.

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Hi Ivgeny,

At no point did I say that I really needed STO,SS1 or SS2. After 20 years of machine design, if I really needed them I would have bought a different system. 

I had a brief meeting with David Shimol about 3 years ago when he was in the UK. We spoke about safety and due to the work involved in getting things like STO approved and certified that this would come much later and using safety contacts and safety relays would be the way to go.

Stopping the motor via contact and safety relay is therefore the only way that safety can be implemented. 

My concern then came from reading this line in the manual, which I stated clearly in my first post:

"Frequently turning the power ON and OFF causes the internal circuit elements to deteriorate, resulting in unexpected problems. Always start or stop the Servo motor by using reference pulses."

One application that I was looking at would require a safety rated stop every time a light curtain was broken which would be 7 or 8 time an hour. 

You said that "cutting during operation is fine" but the line from the manual that I quoted seems to disagree.

I have a ticket open so hopefully this will be cleared up. If it is the case that just cutting the L1 & L2 lines to the drive but keeping L1C & L2C live, like you stated, does not cause internal circuit elements to deteriorate then happy days. Obviously before I start buying loads of these motor and drives for my projects I needs this to be 100% clear and verified.


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  • 1 year later...

I think there was confusion between what the  person that posted the problem and the advice that was given.

The advice was to use a contactor to cut the mains supply to the controller when the safety relay was activated.

The poster was referring to the manual, which stated that the power should not be cut regularly as this would damage the circuits.

My reading of this is that it is OK to cut the power supply to stop the motor, but leave the control power still on the servo.

It is the frequent switching the control power that causes problems with the circuitry.

Someone will correct me on this if i have interpreted it wrongly.


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  • 2 weeks later...

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