Solid state contactor

rQx

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Oct 2010
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Hi,
I have never used any solid state relay or contactor.
A customer want us to replace regular 100-c AllenBradley contactors with solid state. I saw that Allen Bdraley have 156-C line that are solid state. Is there anything that I have to think of when using these? The contactor is controlling a motor. It will have 24VDC control.

There is both options with fan and without, is it a problem with solid state that they are getting warm? So I benefit to have one with fan? (and with that an alarm output)

I read in the specs that:

Number of motor starts (x:6, Tx:6 s, F:50%) at 40 ˚C(2) 30 30 30 30 30 30 30 30

(2) Overload current profile definition: x: multiple of AC53a rating, Tx: duration of current surge, F: duty cycle


I can't really understand this. What does 30 represent? Starts per minute?

Any help is appreciated

/Tim
 
In my line of work we have used solid state relays called 'Triac's.

These Triac's are making and breaking AC volts. They can be ordered in a variety
of input and output voltages, currents, etc. etc. For our applications we ensured that
the 'coil voltage' for the triac matched the desired control voltage and that the triac
was rated for much more than the needed current.

Work great. Where a conventional relay contact was getting burnt up, the triac
just ran for years and years and years with no problems.

We have had good luck with Crydom brand solid state relays.

http://www.crydom.com/en/products/

Edit : P.S. Heat has not been a problem for us - but we do tend to overrate the relay - cost is
not much but the peace of mind is priceless. And if it might be an issue, get the type that
mount directly to the plate and put a dab of heat sink compound behind it.

Poet.
 
Last edited:
I can't speak for the AB units, but we've used several Crydom SSR & Contactors, non-reversing and reversing. They work great...BUT...

Be aware that most SS Contactors have the center leg HOT all the time, so if inexperienced people will be changing motors or working near them they must be marked clearly and/or personnel instructed to this fact. On several applications if space allows we have installed a IEC contactor on the line side of the SS contactor that is switched by the E-Stop so if the machine is E-Stopped the power to the SS device is disconnected.

As far as heat is concerned we have several installed in small, very full enclosures with no ventilation and have had no issues with them.
 
Thanks for the answers!
The AB SS contactor is full three phase so that is good.

Any input on the number of motor starts?

And:

Use of AC solid-state contactors may according to the application and the load current, cause conducted radio interferences. Use of mains
filters may be necessary for cases where the user must meet E.M.C requirements. The capacitor values given inside the filtering specification
tables are only indications, the filter attenuation depends on the final application.

Is it common to use filters? I've had hard times finding ready filters for SS contactors
 
Two things to aware of regarding solid state contactors.
First, the device "under the hood" is going to be a Triac. Triacs have a propensity to fail in the on state.
Second, all solid state devices exhibit some level of leakage current in the off state. There will still be a small current flow to the motor after you turn off the 24 VDC signal to the contactor.
 
SS Relays work great if used and rated correctly
As was mentioned before all solid state devices leek when turned off
Actually all SS Relays and contactors are semiconductors they are called that because they are never fully on or fully off. Meaning that when they are turned off the still leek a small current in the ma range. The higher the current rating and voltage the more current will leak. Normally that’s not a problem but if you were to touch the output side with them turned off you will get a shock depending on conditions it could kill. So always consider them to be ON when working around them. I have seen many people get a bad shock from them.
Even when they are turned on they have some resistance through them. That’s where the
heating comes in, Voltage Drop Across them X the Current through them will give the heat loss dissipation. They don’t handle large over current very well and tend to short out when that happens.
The can fail open or closed. You should learn proper to test them if you plan on working with them.
Under the correct conditions they as better them mechanical contactors and will give you years of good service.
 
Slight correction, most solid state CONTACTORS use inverse parallel SCRs, not triacs. Triacs are more common in solid state RELAYS (SSRs). The difference is in how they are controlled and SCRs allow more control capabilities which makes the contactors more useful for 3 phase loads and for doing things like soft starting, PWM control etc. etc. that you can't do as well with triacs.


Another issue with SSCs besides the "leakage current" and potential failure mode is that they produce a lot more heat; 1-1/2W per amp per phase, and that heat MUST be safely directed away from the SCRs themselves, otherwise they fail. So heat sinks are a must, and then if the SSC is in a sealed box, the heat gets trapped inside and they can cook themselves and other components in there. So you either need to have ventilation, which introduces contaminants, or you ALSO need to add back in a "bypass contactor", in which case you are back to square one.


They also need to be protected from line side transients because they can be made to "self commutate", meaning turn themselves on without a command, if there is a high dV/dt event on the line side (delta Voltage / delta time, i.e. a rapidly increasing voltage "spike" on the line).
 
How about using a small soft-starter as the "solid state contactor". Soft-starters have become real inexpensive.
There are still concerns about dissipation of heat, and there must be a way to isolate the motor(s) for access to the terminal box.
But nothing special really.

What is the size of the motor ?
Is it a heavy start ?
How many starts per hour ?
 
We use soft starts for larger motors, this is a 3kW. We then limit the starts to 40/hrs according to the specs. You are right we can always just use the softstarter instead of trying to meet the specific demand from the customer on "SS contactor", didn't think of that.

It is a pump that is started light, it can start around 100/hrs
 
One side note about solid state contactors from a maintenance perspective.

We started installing solid state contactors at a place and saw an increase in calls saying they had failed when it was something else. The ones we used did not have an indicator to let you know that they were "on" so they always assumed that was the problem since they couldn't hear them "click". We added an LED across two of the load legs and that resolved the problem since they had a clear indication when the solid state contactor was on and off.
 
A lot of good feedback has been shared on this thread already. Three phase SS contactors today offer many advantages over mechanical, but not in every application. They are industrial packaged SS contactors with LED indication, such as https://w3.siemens.com/mcms/industr...vices/Pages/default.aspx?tabcardname=products

Some manufactures make what is called a hybrid starter for motor loads, a Softstart falls into this category too. These are devices that use solid-state to start and stop, but over the run (on) period of the device it transitions over to contacts. This provides benefits with the wear portion of starting and stopping with contacts, but gives the add benefit of less heat generation associated with solid-state.

Good luck with your application.
 

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