Motor Overload Protection

Sparky,

Your answer is concise and to the point. Obviously you could have added much more information, if the original poster requested it. Subsequent posts need a lot of correction but I don't have the time.

But if there is also a thermal OL in the circuit, then most likely that magnetic device is more for mechanical protection from shock loads. Another term for them would an "electric shear pin" because it acts like a mechanical shear pin, which s a soft piece of metal in a drive shaft keyway that breaks under a shock load before something more expensive breaks.

The "mag trip", or magnetic breaker isn't an overload device, that's what the thermals are for, it's for interrupting short circuit and locked rotor currents quickly before they can cause mechanical damage. As the other responder stated, they can be set to a definable trip point unlike fuses which still have a thermal component to the interrupt curve in the manufacturers catalogue. They are also, potentially, extremely quick. You should look at the interrupt curves on mag trips and fuses of the size necessary to accommodate the starting current of the motor in question and see which would open the circuit more quickly in a locked rotor and short circuit situation to get an idea of the relative merits of each.

He didn't say "mag trip" or "magnetic breaker", he said he has fuses, a TOL (Thermal Over Load relay)and a "magnetic overload". A circuit breaker in series with fuses would be redundant and unnecessary.

There are indeed older devices such as those mentioned by Sparkstation which are often used instead of or (as I mentioned) in conjunction with a TOL relay, especially on ol;der crane control systems.

Example: Click here for link to Squre-D Class 9055 magnetic Current Overload Relays

For the most part, these devices are obsolete because the oil they used is now considered a "hazardous substance" and requires special handling when shipping. For the most part, they have been replaced by electronic Current Sensing Relays (CSRs) when used as Electronic Shear Pins or by more accurate TOLs such as Solid State Over Loads (SSOLs).

The motor protection devices are based on temperature sensing,current sensing and voltage sensing. All protective devices fall in these category . The fuse protection come in the thermal and current ( combination) protection where the over current heats the fuse wire to melt it and disconnect the motor from power supply for protection in case of fault current.The rewireable fuses are now replaced with cartridge fuses which are more safe and avoid fire hazard . Now motor protection is given with a back up short circuit protection device ( SCPD ). MCCB come in this category as backup protection instead of fuse and followed by a magnetic contractor and thermal overload protection.Some designers use backup cartridge fuse protection in addition to MCCB which is double protection but makes the panel costlier. Fuse switch combination are also used as back up protection.

For larger motors single phase preventer can also be incorporated to avoid motor burning due to single phasing. Some engineers are of the opinion that use of single phase preventer do more harm than good to motors and thermal overload protection is sufficient to guard against single phasing.In my experience I have noticed burning of motor due to single phasing even after having thermal overload and fuse protection.This is debatable theory as manufacturers of both these devises contradict each other. MCCB with instantaneous release are preferred for motor protection.In practical experience instantaneous overcurrent protection switches off motor before damaging it.

Well, while on this topic, can anyone eleborate the Type-1, Type 2 protection and who are the equipments /elements responsible.

Ok, I think we need to clear some of the posts up.

1. The use of High Rupturing Capacity fuses (cartridges) as well as circuit breakers is for fault current limiting. That is to limit the prospective fault current to a level that can be safely interrupted by the breaker. If an unsafe fault level occurs the fuses clear the fault, it will still be ugly but with proper component selection the switchboard will not blow up.
2. Circuit protection in the form of fuses or breakers is not usually for motor protection but to protect the wiring system. There are however motor protection breakers of various types. Motor start fuses are of a slow blow construction to allow the inrush current to subside but protect the circuit.
3. Single phasing protection is a great motor saver as the motor is tripped as soon as the single phasing situation is detected rather than having to detect the increased current in the remaining phases which in a simple thermal overload may take a long time or not occur at all. To make it clear, single phasing is the loss of current in one phase, some people erroneously call it "two phasing".
4. Thermal elements are meant to model the thermal curve of the motor and trip before the insulation is damaged. They do it with varying degrees of success.
5. Magnetic elements detect fast rise time current events eg., stall, the dashpot provides some damping to reduce nuisance trips.
6. All of the indirect protection has been found wanting to some degree and various other regimes have been employed. See below.
7. Direct temperature protection Bimetals, thermistors, RTDs are all employed in different designs of motor.
8. P&B Golds was once the Gold Star protection regime and it is an electromechanical design using aluminium discs much like an electricity meter. Trip points for the various parameters such as stall time and current, phase loss, timed overload at various multiples could be programmed into this device.
9. Current technology is microprocessor based motor protection relays which connect to some communication bus such as Modbus plus or ethernet. These relays detect and trend a huge range of motor parameters. They are programmable for hot curves, cold curves, system inertia, winding temperature, bearing temperature, phase loss, phase reversal, locked rotor. Many units such as GE Multilin also incorporate control functions.

I hope that helps.

motor protection is very simple and complicated,simple for small size just you put thermal overload and preferred MCCB with about 115% of motor rated "when problem happened you fix it and then turn MCCB ON. no need to change it by new one like fuses".

when you have a big motor rating it will be very different because the big cost of motor.

will see a current, frequncy, power and voltage measured. the protection element differ no fuse and thermal overload, it will be the digital relays "brain" and C.B. "muscles" these devices are very accurate and speedy. your motor is strongly protected.

at the end, there is a general principle when want to protect a device that :

you have to buy a protection devices cost you from 10% to 20 % from the protected device price.

The thermal and magnetic overloads are enough unless you must deal with a high

short circuit current which might be beyond the capacity of the magnetic unit.

Then the fuses come into play.

"The thermal and magnetic overloads are enough..."

Really? How do you know that? Have you seen his equipment and understand the engineering decision used to determine that they were to be installed in the first place? Do you KNOW that there is not some potentially life and limb threatening consequence to a shaft that would break long before a thermal OL relay would trip?

Please don't make potentially dangerous irresponsible statements like that and then hide behind a "Guest" login.

I feel comment no 7 by a person under the name of 'Guest' is incorrect . The person doesn't have idea or knowledge on short ckt and its devastation in the event of fault. Once we see the characteristic of Magnetic and thermal overload relay, it will be clear how they function.

Thanks for the comments. I would like to add one thing on a slightly different note.

Short circuit currents are devistating. I had a situation on a very old installation where a 13.8kv/480v XFMR's differental protection on the secondary side was not placed in the correct spot. this left about two feet of bus unprotected. After thirty years of service we had a fault in this region. Before backup devices could clear the fault, the bus was litteraly vaporized in this area. I mean nothing was there, not even on the floor.

I had a second situation where thanks to Joslyn static trip device on the main secondary breaker (480v) took out a bolted fault from a 350 mcm cable. Even knowing the exact location of the fault we could only find a small deformation at the point of contact.

I have another story where a man actual walked away from a bolted fault he caused with only first and second degree burns to three of his fingers on one hand.

Proper overload selection can save costly down time and more inportantly LIVES.

Thanks again