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Regarding my previous question diagram, I now want to size a solid-state relay(safety relay) for the driver of a servo motor as illustrated below:

enter image description here

Relay will activate when there is fault.

But since the load is inductive in this case i.e the load is a motor; Im worried if I use a electromagnetic relay or a random fire SSR there may occur inductive spikes during relay action(?). Is it better to use a relay with zero-crossing relay for this kind of application?

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  • \$\begingroup\$ Your diagram seems wrong. The SSR would have to be ON all the time to have the driver work, and switch OFF to drop the drive on a fault. \$\endgroup\$ – Jack Creasey May 27 '18 at 21:52
  • \$\begingroup\$ Thanks of for the comment. Why is wrong? Do you mean SSR should be normally ON? \$\endgroup\$ – panic attack May 27 '18 at 21:59
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The load is not inductive. The input power is connected to a rectifier with a capacitor bank on the DC side. The product documentation indicates that the inrush current current is only about 20% over the rated operating current.

A "normally open" relay is a relay that is open when the control signal is low and closed when the control signal is high. The control signal for the input power signal is intended to be high whenever the drive is running or ready to run. The control signal is cut off by a fault and the equipment is disconnected from mains power.

I don't believe there is really much safety advantage in this scheme. If you are concerned by only electronic circuitry preventing the motor from running when it is not supposed to be running, what advantage is there in shutting the power off with an electronic relay?

I would say that the most important piece of safety equipment would guards or housings that prevent personnel from coming in contact with the live electrical parts or moving mechanical parts of the driven machine.

The second most important safety feature might be a manual power disconnect switch that can be locked off with a padlock.

Another thing to consider might be a mechanical brake that is set by a spring and released by an electric solenoid.

The manual advises:

Perform a risk assessment!

The manufacturer of the machine must generate a risk assessment for the machine, and take appropriate measures to ensure that unforeseen movements cannot cause injury or dam- age to any person or property.

Asking questions here is not a proper risk assessment.

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I now want to size a solid-state relay(safety relay) for the driver of a servo motor as illustrated below:

Solid state relays are generally not suitable for safety applications. Semiconductors can fail short circuit leaving your circuit energised and unsafe.

Your illustration concerns me. There is no indication that you understand circuit theory at all.

Relay will activate when there is fault.

This is an unsafe practice. Safety circuits require the safety rated relay to be active when there is no fault. Failure of the relay should result in the circuit going into a safe mode (stopped in this case). In most industrial applications dual-redundancy circuits with fault monitoring are used. A single failure will still allow the safe de-energisation of the circuit and monitoring will prevent reset until the fault has been cleared.

But since the load is inductive in this case i.e the load is a motor;

The load appears to be a variable speed drive, not direct connection to a motor.

Im worried if I use a electromagnetic relay or a random fire SSR there may occur inductive spikes during relay action(?).

You need to study the inrush characteristics of the controller.

Is it better to use a relay with zero-crossing relay for this kind of application?

Relays are mechanical and cannot provide zero-crossing capability. Solid-state relays provide zero-cross capability but are not suitable for safety applications.


You are out of your depth in this project and I recommend you get professional help, carry out a risk assessment, document the safety performance level required by the application and document its implementation.

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  • \$\begingroup\$ 1-) Oh I see but Im not that blank in electrical theory. I think I was misunderstood when I wrote "relay activates when there is fault". What I meant was "when there is fault the relay acts and becomes open and does not let current to flow". I should have written "relay de-activates". 2-) I really didn't know that SSR was not trustable for safety applications. Thanks. \$\endgroup\$ – panic attack May 27 '18 at 23:54

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