Voltage/Current rating of Relay for Inductive load

Thread Starter

Manjesh Gowda

Joined Oct 19, 2020
83
Hey,
I selected a relay for my application by seeing the maximum and current rating of Relays but I am confused if the relays works fine with the given rating for all kind of loads.
In my circuit the relay is being used in the primary side of the LLC resonant converter which is working under 40A rms and 400VAC(Load is primary side of transformer,inductive)
The relay which I selected has rating of max 50A and 480VAC. I could not really understand the contacts rating for different loads. Im really confused it I can use this relay for my application. Please let me know if u have any leads to this. I attach the relay datasheet for your reference.
Thanks in advance

PS: I do not do live switching
 

Attachments

Last edited:

Tonyr1084

Joined Sep 24, 2015
6,000
The switch contacts inside the relay can handle up to 50A. The voltage (isolation) should not exceed 480VAC. Switching a transformer on and off comes with inductive kickback spikes that should be snubbed. Look for snubber circuits for AC. Also, whatever is controlling your relay - that, too, may need protection from kickback. If you're controlling your relay with a DC voltage then a diode will provide sufficient protection against kickback, also called flyback and Back EMF (BEMF).

How are you controlling the relay?
 

Thread Starter

Manjesh Gowda

Joined Oct 19, 2020
83
The switch contacts inside the relay can handle up to 50A. The voltage (isolation) should not exceed 480VAC. Switching a transformer on and off comes with inductive kickback spikes that should be snubbed. Look for snubber circuits for AC. Also, whatever is controlling your relay - that, too, may need protection from kickback. If you're controlling your relay with a DC voltage then a diode will provide sufficient protection against kickback, also called flyback and Back EMF (BEMF).

How are you controlling the relay?
Thank you for your relply. I am controlling a relay using toggle switch(SPST). Here I am not going to do any live switching. I just want to know if i can use this relay without doing any live switching. Since they have mentioned ratings for switching and not for without live switching. Please let me know.

The RMS current is 40A, the relay can handle upto 50A. if the relay works without living switching in this case I will be needing the relay which can handle the current upto 40* square root 2 i.e. 56A right? or this relay will do the job? Please let me know. Thank you
 

Tonyr1084

Joined Sep 24, 2015
6,000
Two things: First, I don't understand your use of the term "Live Switching". Please clarify. Second, if you're switching AC then why are you applying the √2?

If the relay is rated for 480VAC at 50A, I would assume (sometimes a dangerous thing to do - assume) that the engineers have already taken this into account. You ARE switching AC, not DC. AC current and voltages are all understood to be RMS values.
 

Thread Starter

Manjesh Gowda

Joined Oct 19, 2020
83
Two things: First, I don't understand your use of the term "Live Switching". Please clarify. Second, if you're switching AC then why are you applying the √2?

If the relay is rated for 480VAC at 50A, I would assume (sometimes a dangerous thing to do - assume) that the engineers have already taken this into account. You ARE switching AC, not DC. AC current and voltages are all understood to be RMS values.
Yea even I assumed it right, Just wanted to confirm with it since I am new to relays. Live switching in the sense I wont do any switching during the operation for ex: I will not change the relay position when it is operating.
 

Tonyr1084

Joined Sep 24, 2015
6,000
Live switching in the sense I wont do any switching during the operation for ex: I will not change the relay position when it is operating.
Then what's the purpose of the relay?

Also, you said you're using a switch to control the relay. What voltage? 5VDC? AC? 12V? 24V? 120, 240?
 

Thread Starter

Manjesh Gowda

Joined Oct 19, 2020
83
Then what's the purpose of the relay?

Also, you said you're using a switch to control the relay. What voltage? 5VDC? AC? 12V? 24V? 120, 240?
Yes I am using the switch to turn on or off the relay ( 12V DC). I could not find any toggle switch or any normal switch which is rated at 50A. I found some online but it was huge and does not fit for PCB. In my case purpose of the relay is to short or open the line.
 

eetech00

Joined Jun 8, 2013
2,376
Hey,
I selected a relay for my application by seeing the maximum and current rating of Relays but I am confused if the relays works fine with the given rating for all kind of loads.
In my circuit the relay is being used in the primary side of the LLC resonant converter which is working under 40A rms and 400VAC(Load is primary side of transformer,inductive)
The relay which I selected has rating of max 50A and 480VAC. I could not really understand the contacts rating for different loads. Im really confused it I can use this relay for my application. Please let me know if u have any leads to this. I attach the relay datasheet for your reference.
Thanks in advance

PS: I do not do live switching
50A is the rating for a resistive load, like an incandescent lamp.
The relay contacts are spec'd for 20A, 277VAC with a ballast type load (Inductive). They'll work, but not very long, and will burnt or weld after a short while. I don't recommend this relay with a 40A load.
 

Tonyr1084

Joined Sep 24, 2015
6,000
Really, to give you the best possible answer, we need to know all the details. Please elaborate as thoroughly as possible.
 

Thread Starter

Manjesh Gowda

Joined Oct 19, 2020
83
Really, to give you the best possible answer, we need to know all the details. Please elaborate as thoroughly as possible.
Yea sure. I am using Relays in the primary side of the LLC resonant converter (I attach the figure). 2 taps are coming from the inverter and each tap is divided into two lines. I want to use the RELAY to switch between the two lines. I looked for a SPDT switch but I could not find on with my ratings, I found few but it was expensive and very big so I decided to use two relays for two lines (SPST). I just want to control the power flow between the two lines. I am thinking to CLOSE one line of the relay before powering the inverter circuit so that I achieve the power flow by opening or closing the line using relays. If i want power flow in another line then I turn off the whole circuit then I turn on another relay. This can also done by using connecters like jumpers.
Please let me know if u need more info on thisIMG_20201214_133919__01__01.jpg
 

Thread Starter

Manjesh Gowda

Joined Oct 19, 2020
83
50A is the rating for a resistive load, like an incandescent lamp.
The relay contacts are spec'd for 20A, 277VAC with a ballast type load (Inductive). They'll work, but not very long, and will burnt or weld after a short while. I don't recommend this relay with a 40A load.
Did u consider the fact that I am not doing any live switching of the relays?
 

Tonyr1084

Joined Sep 24, 2015
6,000
I'm seeing a 400V 40A square wave. Assuming you've already figured out that part of the circuitry you have six coils, two marked L1, two marked L2 and two marked L3. The way you've drawn them gives the sense that L1(s) are NOT part of a same device. Maybe you intend them to be - I just can't tell for sure. Next, you have three switches (just looking at the top row) all configured in an AND configuration. None of them will be in circuit (ON) unless all three of them are. Then there's the bottom row of similarly marked coils. Assuming (and I don't like doing that) but assuming there is some unknown load (and someone just posted while I'm posting), NONE of the circuit will be on unless ALL of the switches are closed. So nothing (for me) matches what you're describing; or the image I'm developing in my mind.

I'm sorry, this information doesn't help me. Maybe it helps someone else, but - - - .
 

Thread Starter

Manjesh Gowda

Joined Oct 19, 2020
83
I'm seeing a 400V 40A square wave. Assuming you've already figured out that part of the circuitry you have six coils, two marked L1, two marked L2 and two marked L3. The way you've drawn them gives the sense that L1(s) are NOT part of a same device. Maybe you intend them to be - I just can't tell for sure. Next, you have three switches (just looking at the top row) all configured in an AND configuration. None of them will be in circuit (ON) unless all three of them are. Then there's the bottom row of similarly marked coils. Assuming (and I don't like doing that) but assuming there is some unknown load (and someone just posted while I'm posting), NONE of the circuit will be on unless ALL of the switches are closed. So nothing (for me) matches what you're describing; or the image I'm developing in my mind.

I'm sorry, this information doesn't help me. Maybe it helps someone else, but - - - .
Hey, The load is inductor which is attached to both side(primary side of LLC converter). L1 and L2 and L3 are the parts of the circuit. Here I am making matching circuit and getting the required inductance value. lets say when i dont need L1 then I short it. I drawn the switch assuming of using SPDT but I could not find the Relay with my ratings. Whatever I get the inductance values in the top row has to be same in the bottom row. Only because I could not find the relay(SPDT) I am using SPST for each line just to divert power flow. Please let me know
 

Attachments

Top