Motor protection is necessary and indispensable to use together with other automation tools. It is intended to protect motors and engines from overheating and overloading. Motor protection is installed quickly and easily to various contactors and provides trouble-free operation of engines. The name „motor“ means that this protection is provided to protect electric motors or engines, it doesn’t mean that it has any motor drive. Overloading can result in electrical and mechanical breakdowns. Electrical failures are caused by over-voltage, sharp voltage drop, drop in one or two of the phases, short circuits in which the current reaches levels that can damage the engine, etc. Mechanical failures are blocked rotor or overloading of the rotor, leading to overheating and increase of the current.
Overloading of electric motors can occur for several reasons: break of one of the phases, excessive increase of the load of the rotor, difficult motion or complete blockage of the rotor due to mechanical damage, the supply voltage and frequency are out of their nominal levels by more than 15%, etc. The damages caused by these failures include production loss, loss of raw material, need to repair a production tool, poor quality of production and delivery delays. These accidents can also lead to dramatic consequences for those who are in direct contact with the engine. Overloading of motors can also be of several types. Depending on the duration, there are short-term overloads and long-term overloads.
For example, when starting the engines we have high inrush current, which leads to a momentary overloading, but this type of overloading is not dangerous and it is neutralized upon reaching the rated engine speed. It is therefore very important that the engine protection does not stop the engine when there are inrush currents. This is achieved by a thermal component in the protection. Such protection is called thermal-current protection. Therefore, each engine must have protection from short circuit, which cuts off the power when it suddenly reaches over 10 times the nominal current. Short circuit protection must be activated as soon as possible.
To protect the motor from short circuit we use fuses. Fuses with fusible plugs are one-off safety guards which require replacement after melting of the plug. Protection of electric motors secured by fusible plugs, although being cheap and widespread, is not always a good choice, because if a fusible plug melts and breaks one of the three phases, the engine will continue operating with two phases and can break down if there isn’t some kind of motor protection. Another way to protect the motor from short circuit is using magnet breakers, which can be restarted.
The majority of accidents lead to overheating of the motor. The reasons for engine overheating are: asymmetric power supply, difference in the currents of the phases or break of one of the phases, high start-up frequency, which may occur due to failure in the system for automatic engine regulation, leading to numerous restarts due to an error which causes frequent inrush currents that lead again to overheating. Another reason for overheating is the instability of the supply voltage, which can arise from the switching of heavy loads or simply when the supplied voltage is different from the nominal voltage, which often happens in Bulgaria. The insertion of parasitic harmonics in the electrical circuit supplying the engine, which is most often caused by malfunctioning frequency regulators, converters and inverters, also leads to increased losses and overheating.
Failure of the rotor windings can lead to a significant reduction in the magnetizing current as a result of which we will have a loss of magnetization of the machine. The consequences are a drop in the power factor and overheating of the rotor. To protect the motor from overheating we can use various relays for overloading, which are also known as motor protection. There are several types of motor protection. One of them is the motor over-current protection.
It functions as an automatic circuit breaker when the motor is overloaded with power. Motor over-current protection can be adjusted for different magnitude of current in a definite range. It can be both single-phase and three-phase. There is electronic and magneto-electric current protection. There are models with automatic or manual recovery of the initial state. There are also many models of motor over-current protection designed for DIN rail mounting.
Other type of motor protection is the motor thermal overload protection. It has bi-metal thermal relays which react to engine overheating. In combination with a contactor, they protect the motor from overloading and may be single-pole and three-pole, AC and DC, the more reliable ones are temperature compensated, in other words, they are designed to be insensitive to changes in ambient temperature. They can be set according to the current of the motor, i.e. it is enough to check the current indicated on the list with technical characteristics of the motor and then set the regulator of thermal overload protection at the same indication.
This protection is useful in the following emergency situations: overloading, blocked rotor, failure of power phases (imbalance of phases due to asymmetry of the currents, reversed phases, which may result from improper connection of the engine), abnormal rise in temperature, blocked rotor bearing, over-increased start-up time, fluctuation of voltage and frequency in a very wide range, a cycle of repeating reboots incurred due to failure in frequency regulators or other automation devices.
Another type of thermal motor protection is called bimetal thermal overload relays. These relays are designed to be installed as additional contact systems on motor controlling contactors. Bimetal thermal relays react only to the thermal ingredient of the current, flowing through their contacts, and break the circuit thanks to the built-in bimetal. It is obvious that they cannot be used alone for the protection of electric motors – there must also be other types of fuses and over-current protection.
As a conclusion we will add that the market is full of a wide variety of motor protections. Some of them are equipped with different probes, placed in specially selected places in the engines, which serve as sensors for the ongoing engine processes. Others have precise and expensive electronics that responds to very small changes in the parameters of the engine. But if the quantity and the price of the protection begin to exceed substantially the value of the protected engine, it may be better to use the well-known and reliable conventional motor protection, together with fusible plugs.