Hope someone can shine a light on a box of tricks, buss-bar mounted Merlin Gerin and Telemecanique switch gear. Talking 3-phase also. mostly Ca and D type MCBs to drive compressor pump and smaller motor for cooler system. There is a 3-phase contactor with some sort of piggy-back module underneath. Looks like there is an adjustable timer screw and a reset button. Could this be a thermal overload relay and if so what does it do and why is it required?
Contactors
- Thread starter owspark
- Start date
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It sounds like a current overload device.
These are usually fitted so that if whatever the motor is driving ceases(sp?) the overload will operate, saving the motor from burning its windings out. It also stops the motor getting dangerously hot if it ceases.
These are usually fitted so that if whatever the motor is driving ceases(sp?) the overload will operate, saving the motor from burning its windings out. It also stops the motor getting dangerously hot if it ceases.
Yep, it sounds like an overload unit, more than likely thermal. The screw will adjust the trip setting of the unit, there may also be another screw to set the reset button to auto/manual.
An overload is required as pointed out by RF that a motor needs protection from excess current causing it to overload, either by the motor seizing or just being put under too much load. Overload units are generally used in conjunction with fuses, these fuses may need to be larger than normal to accomodate high starting currents of motors hence the fuse is used to deal with the fault current protection and an overload unit to protect against overload, an overload unit being used as it has a time delay to overcome the initial startup surge.
An overload is required as pointed out by RF that a motor needs protection from excess current causing it to overload, either by the motor seizing or just being put under too much load. Overload units are generally used in conjunction with fuses, these fuses may need to be larger than normal to accomodate high starting currents of motors hence the fuse is used to deal with the fault current protection and an overload unit to protect against overload, an overload unit being used as it has a time delay to overcome the initial startup surge.
Thanks for all the info. Just a couple of things... are these protection device add-ons required for single phase motors as well as 3-phase? i guess the protectors come in various current ratings for different size motors...or?
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It depends on the type, size and application of the motor, I have seen them on single phase before but just using one coil out of the three although this may have just been a fudge. Most the motors I have anything to do with are 3phase, the overload devices come in different ratings and types ranging from thermal, electronic (inc using current transformers) and magnetic-dashpot.
thanks. one more thing.. the coil live was energised via an RS din rail mounted timer module. Looking up the RS part number showed it to be discontinued but it came up with "power module". The contactor energises approx 60seconds after the various breakers are switched to ON. Why is this device required? Any ideas?
There is likely going to be a reason, maybe some sort of an inbibit to stop the motor from re-starting for 60s after its stopped? I think I know what type of module you mean, I fitted one to some lighting to stop them coming on for 1min after the timer switched on as it took approx 30secs for the light-sensor to kick in turning the lights off in daytime.
Spark123 said:
You wanna watch them dashpots.
Full of mineral oil they are.
Gives you cancer.
Yeah, on with the rubber gloves. It is quite strong stuff, takes the writing off the gloves. The only place I have seen them is on slip-ring motors, I think they tend to go for more VSDs and electronic overloads probably incorporated nowadays.
Spark123 said:
not sure. how can i tell that? there is the Amps adjuster and a reset button (which at current configuration is non latching and releases contactor only when button held in) and a screwdriver-worn dial next to the reset button
RF Lighting said:
I served my time working in various furniture factories in the east end of London. We used to have "snowball" fights with wet blue asbestos lagging and the control panels were full of overloads with dash pots
Most overload units have a selector on them which can be switched from auto to manual, in manual when they trip you have to go and press the button on the overload unit to reset it. In auto it just resets itself after it has cooled down, not always a good idea. I don't like the sound of the dial being worn!! What is it on, is the machine seizing up or not being used properly?
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