overload question

[Guest Guest_joe]

What is the reason in a panel you see both fuses and overloads, don’t they do the same work, And what does the overload protects the motor from, is it current? Isn’t that what the fuse does, in panels fuses the overload, then motor starters One more question if the motor is running and I tried to stop it, let say I had my hand on it would the overload trip Thank you

[Guest Guest_joe]

One more question ,For the case where im preventing the motor to rotate I guess my question would the motor die or would the overload trip or is the overload is not related in this case

[93lt1 0]

A typical (legal) industrial motor control has an overload device and an overcurrent device. They are not the same. The overload device is intended to prevent damage to the motor/wiring (and eventually fire) due to a somewhat extended overload condition. The overcurrent protection is intended to shutdown an overcurrent condition such as shorted power wiring or grounded power wiring.

[Gerry 13]

The overload device's function is to protect the motor. It's I^2 t curve is designed to allow short-term overloads without tripping and also to allow starting current without tripping. Time to trip is inversely related to the amount of over-current. The fuses' function is to protect the wiring. Their I^2 t curves are very steep, meaning that once current gets into their operating range, they blow very quickly (under a half cycle of the mains). This prevents the normally huge short circuit capacity of your distribution system from being delivered to shorted motor windings or field wiring and causing fires and bent bus bars. A motor overload would take several cycles to operate, by which time the damage would be done and the starter's contacts may well be welded together. There are MCB's (motor circuit breakers) that combine the two functions.

[JimRowell 1]

To expand a bit on what 93lt1 said... An "overload" is when a circuit or load is drawing more than it can safely handle in the long-term. An "overcurrent" is a very high current flow that is dangerous in the short-term. An overload is generally no more than a few hundred percent at the most (generally much less). How much it can be depends on how long it occurs, etc. An overcurrent situation is usually a short-circuit and can be hundreds or thousands of percent above the proper level. A standard induction motor actually draws an overload amount of current while it is starting (or at any time when its speed is less than its synchronous or "rated" speed). If you chose fuses that would protect the motor from overloads while it was running, you would blow those fuses while starting. So we choose fuses that will protect the wiring against severe currents such as occur during short circuits or very long-term overloads. The fuses don't protect the motor at all. We use an overload device to protect the motor. This is a device which has what is known as "inverse time characteristics". This basically means it will allow a mild overload for a long time or a severe overload for a short time. The device will more or less match the motor's own ability to withstand heating. It protects the motor but it's not able to respond quickly enough to protect the wiring (or even itself) during severe shorts so we still need the fuses. Fuses also offer inverse time characteristics but not to the extent nor with the accuracy of overload units. They basically match the abilities of the wiring which is more robust than the motor. Another feature offered by IEC style overload devices is single-phasing protection where if one of the 3 phases is lost, the unit will trip. The current in this case is only moderately high but the internal heating is huge. Older Nema overloads don't offer this and will allow the motor to burn out. Some devices combine overload/overcurrent features in one package (ie. breaker and overload relay). To answer your other question, if you didn't allow the motor to rotate, the current would be high (around 4 to 6 times too high) and the overload device would trip and shut off the power to the motor. Jim Rowell

[JimRowell 1]

Sorry Gerry, I didn't mean to trounce you by repeating everthing that you said. I was in edit mode when I got called away. Just hit send when I returned without realizing you had posted. :*-(

[Gerry 13]

not a problem!

[Guest Gino]

Regarding fuses and overloads everything was explained. I just want to add DO NOT STOP THE MOTOR WITH YOUR HANDS,

[JimRowell 1]

Isn't that why they put the option on the switch? You know Hand/OFF/Auto??