Motor Overload Protection

[svan]

Hello

I am using an electric hoist to pull a metal mandrel into a metal casing. Daily around 10 mandrels are pulled and pushed. Sometimes 1 out of hundred casing is under size which makes the mandrel tight to pull and makes the motor overloaded, hence the motor either breaks the wire or its winding damaged itself. What i want is an overload protection for the motor, i mean a kind of tripping mechanism which trips the motor mains line when the ampere goes more than say 10 ampere or 8 ampere wherever we set the value..

So what apparatus is to be used for such kind of work?

[Muffy]

overload relays can be purchased form any switchgear dealer. They have 6 terminals , 3 for connecting phase wire from power, or contactor output, and the other 3 to motor. Other than these 6 , there are additional 2 terminals. This must be connected in series with the coil of the motor contactor. When theres an overload, the relay heats, breaks the NC connection between these two terminals. When this breaks the contactor coil circuit opens and the contactor switch opens.

Overload relays are available in a wide range of amperages.

[svan]

Thanks for the help.
I installed a teknika brand Overcurrent relay which is appropriate. It has range of 1 to 5 amps(a rotating knob can be set) and also has a "time to tirp" setting(by a rotating knob). i have set it at 1 sec(its minimum) but i have realized that i need it to trip in .25 secs so as to protect the mechanical system that is attached to the motor. Where to find such sensitive relay in India?

[Muffy]

You will have to think of some other way to protect the mechanical system. with a very short trip time, your motor will fail to start, because the high starting current will trip it before it reaches the required rpm.

[aj_onduty]

Kindly let me know the motor ratings.
And, there are many relays which you can use, for example, the IDMT relay(Inverse definate Minimum Time lag relay). They adjust their tripping time according to the current flow in them. For motors, the initial starting current lasts for a little time. And this relay doesnt trip for that time, but if the starting current lasts for more than expected time, the relay detects it as an abnormal condition and it trips.The overload current is substantially higher compared to the starting current. So, when it flows through this relay, its tripping time is lowered and it trips in a very short time, thus saving your motor. This is usually used in large motors, and in large generator protection circuits. I believe this answer satisfies you.