How to determine the rating of Solid State Relay?

[ankurgajjar]

Dear all,
I am facing an awkward problem with temperature control using Siemens S7-300 PLC. Now, the problem is that I want to control the temperature of the tank using PWM output from PLC to heater through solid state relay. Now, the problem is that I don't have the rating of the heater. So, if I want to use SSR to control the PWM to heater, how can I determine the rating of SSR if the heater's rating itself is unknown?

P.S. : Heater operates on 230 v AC supply.

Regards,
Ankur Gajjar

 

[realolman]

Dear all,
I am facing an awkward problem with temperature control using Siemens S7-300 PLC. Now, the problem is that I want to control the temperature of the tank using PWM output from PLC to heater through solid state relay. Now, the problem is that I don't have the rating of the heater. So, if I want to use SSR to control the PWM to heater, how can I determine the rating of SSR if the heater's rating itself is unknown?

P.S. : Heater operates on 230 v AC supply.

Regards,
Ankur Gajjar

I don't think you can decide what relay to use without knowing the rating of the heater

 

[rdrast]

Plug the heater in to 230 VAC, and measure the current.

 

[osmanmom]

put MCB also e.g 6A

 

[boneless]

Also, PWM of an heating element is not as straight forward as you would think. It will work, but only with lower frequencies, I believe.

A thyristor control would be better suited IMO.

JUMO

CLICK

 

[keithkyll]

PWM is for DC loads, not AC. For heaters, you use PID. How many watts is the heater? From that, you can calculate current. Otherwise, look at the size of the wire feeding it, and use the maximum current rating for the wire. At some ppint, you must measure the current.
Rate the SSR at least 2.5 times the actual current. If it's over 40 Amps, use a Phase Angle Controller, with analog input.

 

[paulcs]

Meter the heater. 240V / resistance = amps

The solid state relay needs to be rated at least 20% greater than that. Higher is better.

If the application is more than 2 or 3 amps, make sure that you use a solid state relay on a heat sink. The higher the amps, the bigger the heat sink. Many suppliers have SSR+HeatSink units for 25, 40, 50 amps, etc. Google "solid state contactor" for info or pictures.

PID pulsing the heater on/off/on/off is fine. A tank usually has a slow response time.

Standard resistive heaters can use standard solid state relays. Thyristor based solid state relays are more industrial. Triac based solid state relays are cheaper. Analog phase angle units are needed for transformer coupled loads which your tank probably isn't.

 

[keithkyll]

...The solid state relay needs to be rated at least 20% greater...

May be okay for Phase Angle Controllers or other off-the-shelf controllers, but I'm holding to at least 250% for "Hockey Pucks". You have to consider startup current when cold.

 

[Bering C Sparky]

Meter the heater. 240V / resistance = amps

The solid state relay needs to be rated at least 20% greater than that. Higher is better.

Not sure of the code in the OP's country.
But here in the states NEC says that all fixed heaters will be considered contiuous use and have a minimum circuit ampacity of 1.25% FLA.
So all wiring and line controls would need to be at least 25% larger just to meet NEC standards.

Keithkyll may be onto something with sizing solid state relays even larger than this, as the few I have had the pleasure of working with did not stand the test of time.

Keithkyll, I am a bit confused about one thing you said though.
Why would a relay need to be sized larger for start up current for a resistive element? There is nothing additional induced onto the circuit for starting like would be the case with a motor.
(just curious)

BCS

 

[realolman]

I don't know that it would be exactly PWM, but I think lots of AC heaters are controlled by giving them short "shots" of current, when they approach the set point

 

[Davek0974]

I would use phase angle firing, randomly switching loads without it can create loads of RF and other interference.

What size is the fuse or MCB supplying the heater?

That would be an indicator of the load for the SSR.

 

[keithkyll]

...Why would a relay need to be sized larger for start up current for a resistive element? ...

The resistance of a heater element, light bulb, etc. is much lower when cold. The surge current can be many times the operating current.

 

[paulcs]

I would use phase angle firing, randomly switching loads without it can create loads of RF and other interference.

It is actually the opposite. Phase angle or "random firing SSRs" chop the sine wave and creates electronic noise at the leading edge of each half sine wave. For European installations, CE regulations restrict noise, and custom noise filters may be needed. "Zero crossing" control means that each thyristor turns ON exactly at the zero mark of a sine wave. When the control signal is received, the thyristor will turn ON at the next zero mark. When the control signal is removed, the thyristor will wait until the next zero mark, and then it will turn OFF. Zero crossing, when done correctly, is very noise free.

The resistance of a heater element, light bulb, etc. is much lower when cold. The surge current can be many times the operating current.

Depends. Standard resistive heaters don't have inrush. Incandescent light bulbs or some IR heaters can have high (8 - 10 times) inrush. Motors have inrush. Specialized high temperature heaters (Kanthals, etc) can have inrush. The heater used in this tank application is probably a standard resistive heater.

 

[rootboy]

Follow rdrast's advice and hook it up and measure the current. Take that number and multiply that by 1.5. Find yourself an SSR that meets or exceeds this rating. Fuse it to no more than 1.25 times your current rating. That's a good starting point. Your codes may vary...

As for controlling it, don't worry about running it off of PWM, instead do it like the furnace or AC in your house. When your process calls for heat, turn it on, when it is satisfied, turn it off.

This can be modified after observing your process run for awhile to an adjustable duty cycle. Suppose you find that it needs the heater to be on for about 80% of the time. Since it is a heater, it is unlikely to run away on you quickly, so you have the luxury of turning the relay for 8 seconds out of every 10. Or 8 minutes out of every 10 minutes. This is all process dependent.

Ether way, this cycle would be run continuously as long as you needed 80% power to keep things where your process needed them to be. And a PWM circuit isn't going to deliver your power any more efficiently.

A PID loop can be added if you have feedback available. Keep in mind that if your device has an appreciable thermal mass (meaning that it takes a long time to heat up and cool down), then you run the serious risk of overshoot/undershoot in your process.

Meaning that your heater will eventually get to temperature and then merrily go on its way higher and higher. Regardless of the method that you choose to use, you are going to have to watch for this.

In any case, I would stick to the duty cycle method and avoid PWM.

 

[xiaolong]

There's no way to select your SSR without knowing the current of your heating resistance.

So, power it and measure your current.
Select your SSR with a rated current at least 20% over the value you'll find, for AC 230V network.

Key point : you need to defien what will be the ambiant temperature within the cabinet in which you will install your SSR. That's compulsory to define the proper heatsink to use , even if you use an SSR with premounted heatsink.
Otherwise your SSR will not last longer than an electromechanical relay.

You can find a lot of information about your need on the web. I would recommend you to go to http://www.crydom.com/en/.