sir
what could be the reasons for welding of contacts in contactor

we have a DOl starter for screw compressor(7.5kw/1440 rpm/14 amps fla)which is being controlled with pressure switch.the number of start/stops are five per minute.
The starting current is 41 amps and the normal load current is 10 amps.

we had problem with the weldingof contacts.The contactor earlier we are using wasof AC3 rating 25 amps

.now we have replaced with rating of 40 amps ac3.but still we found mechanical damage in the contactor moving parts.

what could be the reason for welding of contacts/mechanical damage in moving arm

ram

Since it appears as though sizing is not the issue and you have changed out the contactor to eliminate a faulty mechanical component, the next probable reason would be duty cycling, particularly rapid cycling. I would investigate your pressure switch settings. You want to be sure that there are no times when the switch is "bouncing" the contactor in and out. In pressure switches, there should be a "cut-in" and a "cut out" pressure setting. Sometimes the pressure switch will have what is called a "hysteresis" setting which is a dead-band that you set instead of the separate cut-in/cut-out settings. Either way the on and off points should be far enough apart to ensure that the compressor is not being cycled too rapidly. Watch the entire cycle for a while during normal operation and see if perhaps one of the settings is wrong, or maybe even the switch itself is chattering.

The next issue is to see if you have adequate control voltage. Make sure you measure it while the contactor is pulling in because sometimes if you are right on the edge of acceptable, the inrush of the contactor coil might be enough to cause the control voltage drop, right when you need it most! This can be especially true if there is a long distance from the contactor to the control device, in this case the pressure switch. Inadequate wire size for that control circuit would contribute to the problem as well. In this case, increasing the contactor size would just make it worse!

Hello ram

I would suggest that you check out the coil voltage when the motor is starting.
If there is a voltage drop due to the start current, (common) then the pressure on the contacts will be reduced and this can cause them to weld.
A start current of 41 amps for a 7.5KW motor sounds very low suggesting a severe voltage drop.

It is important that the contact pressure is high during start.

Best regards,

Hi ram, I agree with both jraef and marke, you should check supply conditions during start as voltage drop sounds very likely. I would also check on the pressure switch settings as sugested by jraef as five starts/minute sound a lot. This will be putting a lot of stress on all componants in the system. I suggest you also look at the size of the air receiver as it may not be adequate.

Ken

Thanks everybody . we have seperate control voltage of 110vac for the contactors and is being fed from the control transformer . it is in the same MCC.

This air compressor has pressure settings of 5.8 bar(for start) and 7.0bar(for stop).we have checked the operation of the pressure switch and found ok. I will definitely look in to the air receiver.

One important thing i forgot to add up. in the firt case the motor failed due to single phasing and contactor got welded. Inthe second case also the motor failed due to ground fault.Then we replaced the contactor with higher rating 40amps in which mechanical damage occured.

I would like to know what could be the reason for contactor welding . Is it during making condition or opening condition.
ram

Hello ram


It is possible that the contactor welded on two phases and this caused the motor to fail on single phasing?


It is highly likely that the frequent start stop operation caused the insulation in the motor to fail and this caused the ground fault.



What is the rated operations of this contactor? some contactor are rated for only 1M operations. If you are operating at 5 starts per minute, 24/7 you will clock up 1M operations in 138 days which is less than 5 months.

Many motors are rated for ten starts per hour maximum. I would suggest that you are really pushing the envelope of all components, not to mention the compressor mechanical constraints.

I would suggest that rather than starting and stopping at such frequency, you should consider a much larger pressure vessel so that the starts can be limited to about one tenth of the current rate, or you use a variable speed controller to slow the compressor down and run it at a speed which delivers the correct amount of air. This can be easily achieved by using a pressure transducer and the PID (integral part of most modern drives) to run the compressor at a constant pressure.


Most damage is done during the start where there is a very high power dissipated in the contacts. If the contact pressure is light, (due to low control voltage) the contact resistance increases and this increases the power dissipated in the contacts. The increased starting frequency (starts per hour) increases the contact temperature and reduces the contact life. To overcome this, larger contacts need to be used.

Best regards,

thanks everybody
we have now replaced the contactor with higher rating and more number of operations per hour.
Let me see the perfomance
ram