Relay ratings, DC inductive loads, surge absorbers

[d x l]

I just asked tech support at Phoenix Contact a question and the answer is making me think maybe I have misunderstood relay ratings for years.

What's everyone's opinion on relay ratings for DC inductive loads, *IF* you also install a surge absorber?

Tech support said I could use the relay at the resistive load rating if I use a surge absorber! This had never occurred to me before. I always assumed that if the load was inductive, that rating stands regardless of other measures taken!

 

[goghie]

Let the PC support write you that opinion in email.
PC produces excellent equipment, but is prone to marketing BS, from time to time. Inductive DC load that is wired up with freewheeling diode is far from ideal contact protection (although it is good one).

 

[lfe]

The problem is that the DC coils accumulate energy that is released upon disconnection with a voltage peak that generates a spark in the relay contact and that can ruin it in a short time.
This is how car spark plugs work.

A surge absorber should be placed in the same coil, normally a diode inverted with respect to the polarity of the supply, since the voltage peak occurs with reverse polarity.

It is better to put the surge absorber on the same coil since this prevents it from circulating through the wiring and that can produce noise in nearby cables due to induction.

 

[James Mcquade]

Bad idea in my opinion.

when using a purely resistive load, you have the initial contact arc, much like a stick welder. the supressor will help, but you will need bigger rated contacts to prevent the contacts from welding (sticking). that has been my rule ever since i used the load rating for a resistive load with suppressor way back in the 80's
james

 

[Bit_Bucket_07]

Two informative links:

https://www.weschler.com/blog/relay-contact-protection/

https://www.sprecherschuh.com/conte...oads/tech-docs/TECH_Surge_Suppression_109.pdf

 

[d x l]

Let the PC support write you that opinion in email.
PC produces excellent equipment, but is prone to marketing BS, from time to time. Inductive DC load that is wired up with freewheeling diode is far from ideal contact protection (although it is good one).

In fact, I don't want to use the Phoenix relay. I'd like to use the Idec equivalent, but Idec published a current-voltage graph with an inductive curve that is too low for the calculated amp draw I'm looking for. Phoenix Contact doesn't have any such graph, it just states "2 A (at 24 V, DC13)."

The application is a servomotor holding brake, 24V, rated 0.23A (104.8 ohms). The application guide calls out the relay and the exact MOV to use.

It was the cross-referencing to Idec that started getting me confused about what the inductive load rating means once a MOV is installed.

 

[d x l]

The problem is that the DC coils accumulate energy that is released upon disconnection with a voltage peak that generates a spark in the relay contact and that can ruin it in a short time.
This is how car spark plugs work.

A surge absorber should be placed in the same coil, normally a diode inverted with respect to the polarity of the supply, since the voltage peak occurs with reverse polarity.

It is better to put the surge absorber on the same coil since this prevents it from circulating through the wiring and that can produce noise in nearby cables due to induction.

I do understand what happens in real life. I just want to interpret the ratings on relays better. I'm not an electrical engineer and reading all the different ways manufacturers list relay contact ratings can get confusing. Slim-style relays often have a nice resistive rating and a vague or very tiny inductive rating, so it would be nice to know if they can be used at all if some surge protection is added.

The application guide that I'm trying to follow also says to install the MOV or diode right at the holding brake. This is for a servo motor holding brake.

Of course, it is impossible to do so by their own design. The motor and brake are all in one unit, and only accept a proprietary cable. There is no way to install anything at the brake.

Anyway, I was really asking more about ratings in general, for future reference, than how to carry out my installation. For this I'm going to do what the manufacturer says in the application guide and it will be their fault if something goes bad.

 

[Bit_Bucket_07]

I do understand what happens in real life. I just want to interpret the ratings on relays better. I'm not an electrical engineer and reading all the different ways manufacturers list relay contact ratings can get confusing. Slim-style relays often have a nice resistive rating and a vague or very tiny inductive rating, so it would be nice to know if they can be used at all if some surge protection is added.

The application guide that I'm trying to follow also says to install the MOV or diode right at the holding brake. This is for a servo motor holding brake.

Of course, it is impossible to do so by their own design. The motor and brake are all in one unit, and only accept a proprietary cable. There is no way to install anything at the brake.

Anyway, I was really asking more about ratings in general, for future reference, than how to carry out my installation. For this I'm going to do what the manufacturer says in the application guide and it will be their fault if something goes bad.

It sounds like you might be better off using a power relay rather than a control relay for this application. Nothing wrong with a bit of overkill.

 

[lfe]

I do understand what happens in real life. I just want to interpret the ratings on relays better. I'm not an electrical engineer and reading all the different ways manufacturers list relay contact ratings can get confusing. Slim-style relays often have a nice resistive rating and a vague or very tiny inductive rating, so it would be nice to know if they can be used at all if some surge protection is added.

The application guide that I'm trying to follow also says to install the MOV or diode right at the holding brake. This is for a servo motor holding brake.

Of course, it is impossible to do so by their own design. The motor and brake are all in one unit, and only accept a proprietary cable. There is no way to install anything at the brake.

Anyway, I was really asking more about ratings in general, for future reference, than how to carry out my installation. For this I'm going to do what the manufacturer says in the application guide and it will be their fault if something goes bad.

There is no fixed rule to compare the rating of a contact with resistive or inductive load. Each manufacturer can indicate what they consider.

I think it is not appropriate to use that small relay for an industrial load such as the brake solenoid of a motor. Life expectancy will be short.

I would use a contactor that indicates rating for inductive loads, for example this:
https://ckm-content.se.com/ckmContent/sfc/servlet.shepherd/document/download/0691H00000FJov0QAD

If the diode cannot be connected to the motor itself, it can be connected to the connection box where that proprietary cable goes.

 

[d x l]

Bit_Bucket_07 and Lfe,
This advice to use a power relay or contactor is sound. That is what I would do if A) I had panel space, and B) the servo manufacturer hadn't called out the Phoenix relay. I'm probably going to do what they recommended mainly because I don't have space.

Now, since I'm still trying to learn more about interpreting the ratings, let me ask this: if I find a good contactor or relay rated for DC-23 or a graph like I described previously, then aren't the contacts good without surge protection?

Lfe, there is no connection box, the proprietary cable plugs into the motor on one end, and has flying leads on the other, so the flying leads will end up in a terminal block that can hold a MOV and then be wired to the relay contacts.

 

[lfe]

Even if the diode is installed in the terminals of the relay, it will be a plus to extend the life of the relay.

In the case of the link I posted about Schneider contactors, I assume that the currents indicated would be for loads without surge protection.

 

[d x l]

Even if the diode is installed in the terminals of the relay, it will be a plus to extend the life of the relay.

In the case of the link I posted about Schneider contactors, I assume that the currents indicated would be for loads without surge protection.

Thank you. Assuming others agree with you, that's a good takeaway for me.

Would it be reasonable to say that WITH surge protection, the rating improves, but not improved all the way to the resistive load rating?

 

[Bit_Bucket_07]

Bit_Bucket_07 and Lfe,
This advice to use a power relay or contactor is sound. That is what I would do if A) I had panel space, and B) the servo manufacturer hadn't called out the Phoenix relay. I'm probably going to do what they recommended mainly because I don't have space.

This relay has a small footprint, and a fairly decent Amp rating: LINK

Bit_Bucket_07 and Lfe,
Now, since I'm still trying to learn more about interpreting the ratings, let me ask this: if I find a good contactor or relay rated for DC-23 or a graph like I described previously, then aren't the contacts good without surge protection?

The thing is that no contacts last forever. If you quench the voltage spikes that making and breaking an inductive load presents, your contacts will certainly last longer. If you use a multi-pole relay, then you can parallel multiple poles to increase the ampacity of the relay.

I wouldn't worry too much about installing the diode at a distance from the load. That's better than not having it installed.

 

[lfe]

Would it be reasonable to say that WITH surge protection, the rating improves, but not improved all the way to the resistive load rating?

As I said before there is no fixed rule to compare the rating of a contact with resistive or inductive load.

The only one who can answer that is the device manufacturer and it seems that in your case Phoenix Contact already did it.

 

[d x l]

Thanks Bit_Bucket_07, for the useful insights.