damaged relay contact to control Ac motor

Thread Starter

aydi

Joined Feb 28, 2017
4
good morning,
i try to control Ac motor using a relay. i used the attached schema. when i try to start the motor . the varistor was damaged after 4 trial on /off.
also when i used the same schema just without the varistor (i keep the snubber). the contact of the relay was damaged after 50 trial on /off.
could you please tell me where is the problem ?
characteristics of the contact of the relay songle : 10A-230VAC
Ac motor : 230V-50hz-0.7A.
thank you an advance.
 

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MaxHeadRoom

Joined Jul 18, 2013
19,449
What is the nature of the relay?
Sounds like you you may be using a relay that is not suited to motor switching.
There are relays or contactors more suited to motor load inrush and starting.
Max.
 

Thread Starter

aydi

Joined Feb 28, 2017
4
What is the nature of the relay?
Sounds like you you may be using a relay that is not suited to motor switching.
There are relays or contactors more suited to motor load inrush and starting.
Max.
normally the snubber is used to protect the contact from overvoltage
i want to use a relay not a contactor.
the ac motor is a roller shutter motor.

the relay is SRD-112vdc-sl-c

https://www.google.com/search?q=songle+relay+srd-12vdc-sl-c&safe=strict&client=ms-android-samsung&prmd=ivsn&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjD07u19cXiAhWRmIsKHUPjD6MQ_AUoAXoECA0QAQ&biw=360&bih=560#imgrc=NU6ulbMojWaklM
 

MaxHeadRoom

Joined Jul 18, 2013
19,449
Use the correct kind of cap for the snubber AC rated ceramic or similar.
Personally I would use a different relay than that PCT type, they typically over rate the contacts..
Max.
 
You are driving a heavy inductive load that your relay cannot handle. The damage is occurring when the relay opens, the arc is not extinguished quickly enough. The reason this happens is the strength of the spring is too low and the contacts do not open fast enough. You want a relay with a strong spring and magnetic field so that the contacts are driven together hard and fast. You also want the contacts to open fast. A good motor control relay will also have a wider gap when open. The drive signal to the relay coil needs to have good clean edge transitions at the rated coil voltage and current.
All the circuitry around the contacts may actually harm the relay by keeping the arc going. Get rid of your all of the stuff around the contacts.
 

Thread Starter

aydi

Joined Feb 28, 2017
4
A SSR (solid state relay) may be more reliable for your application.
first of all, thank you for your answers.
the capacitor for the snubber that i used is : polypropylene film capacitor X2-334K MPX/MKP 275V
the resistor that i used is: CARBON FILM RESISTOR
to compute the components of the snubber i used the attached excel file. in the input fields can i enter the value 2A in the Nominal load current in order to control a motor with 2A as 0.7A. because i have two types of motor, one which have a nominal current=2A and an other 0.7A so i want to use the same board to control the two AC MOTORS.

for the placement of the components you will find an attached image that show you the components around the relays. is it a good placement?

thank you an advance
 

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MisterBill2

Joined Jan 23, 2018
4,562
What has not been mentioned is how fast the relay is closing and opening. A slow opening or closing will certainly allow lots of time for high powered arcing. Also, it may be that the relay is only rated for a resistive load, such as lights.
Because all induction motors have a higher starting current it may be that the contact ratings are exceeded.

The motor with a 2 amp running current may have a 5 or even 6 amp starting inrush current. I have observed that to be the cause of contact failure in a home appliance.
 
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MaxHeadRoom

Joined Jul 18, 2013
19,449
Also with PCT relays, they often have a vent to avoid internal ionization from arcing.
The vented types usually come with the vent sealed for PCB cleaning after assembly and the vent has to be removed after before putting into operation .
Often over looked when replacing etc.
In any event, for this application a vented type should definitely be used.
Max.
 

Thread Starter

aydi

Joined Feb 28, 2017
4
Also with PCT relays, they often have a vent to avoid internal ionization from arcing.
The vented types usually come with the vent sealed for PCB cleaning after assembly and the vent has to be removed after before putting into operation .
Often over looked when replacing etc.
In any event, for this application a vented type should definitely be used.
Max.
thank you.
i did not hear about vented relay. the sla-12vdc-a is it a vented relay ?
 

MisterBill2

Joined Jan 23, 2018
4,562
It may also be that the relay is operating to slowly. If the contact gap is not large enough and anything slows the release time, then there can be a large heat rise from the arc as the relay opens. And 230 volts is quite close to the 250 volts spec. The picture of a portion of the PCB seems to include some control electronic circuit, and so there may be something to suppress the inductive spike as the coil is switched off. Unfortunately the same thing also slows the release time. A solid state relay may be your very best choice.
 
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