Protection of the electric motor against overload is today one of the main tasks that need to be solved in order to successfully operate this device. These types of engines are used quite widely, and therefore, many ways have been invented to protect them from various negative effects.
Protection levels
There is a wide variety of devices to protect this equipment, however, all of them can be divided into levels.
- External level of protection against short circuit. Most often, various types of relays are used here. These devices and the level of protection are at the official level. In other words, this is a mandatory subject of protection, which should be established in accordance with the safety rules in the Russian Federation.
- The overload protection relay of the electric motor will help to avoid a variety of critical damage during operation, as well as possible damage. These devices also belong to an external level of protection.
- The inner layer of protection prevents possible overheating of engine parts. For this, external switches are sometimes used, and sometimes overload relays.
Causes of Hardware Failures
Today, there is a wide variety of problems due to which the performance of an electric motor may be impaired if it is not equipped with protection devices.
- A low level of electrical voltage or, conversely, too high a supply level can cause failure.
- Breakage is possible due to the fact that the frequency of current supply will change too quickly and often.
- Incorrect installation of the unit or its components can also be dangerous.
- Raising the temperature to a critical value or higher.
- Too weak cooling also leads to breakdowns.
- Highly affected by the increased ambient temperature.
- Few people know that low pressure or the installation of the engine is much higher than sea level, which causes low pressure, also have a negative effect.
- Naturally, it is necessary to protect the motor from overloads that may occur due to malfunctions in the mains.
- Frequent turning the device on and off is a negative defect, which also needs to be eliminated with the help of protective devices.
Fuses
The full name of the protective equipment is a fuse safety switch. This device combines both a circuit breaker and a fuse, which are located in one housing. The circuit breaker can also open or close the circuit manually. A fuse is the protection of the electric motor against overcurrent.
It is worth noting that the design of the emergency switch provides for the presence of a special casing that protects personnel from accidental contact with the device terminals, as well as the contacts themselves from oxidation.
As for the fuse, this device should be able to distinguish current overload from the occurrence of a short circuit in the circuit. This is very important, as short-term overcurrent is quite acceptable. However, the overcurrent protection of the electric motor should work immediately if this parameter continues to grow.
Short circuit fuses
There is a kind of fuse that is designed to protect the unit from short circuit (short circuit). However, it is worth noting here that the quick-blow fuse may fail if a short-time overload occurs during the start-up of the device, that is, an increase in the starting current. For this reason, such devices are usually used in those networks where such a leap is impossible. As for the means of protecting the electric motor against overload, the quick-blow fuse can withstand current that will exceed its rated by 500% if the differential lasts no more than a quarter of a second.
Delayed fuses
The development of technology has led to the fact that it was possible to create a device for protection against both overload and short circuit at the same time. Such a tool was a fuse with a delayed response. The peculiarity is that it is able to withstand a 5-fold increase in current if it lasts no more than 10 seconds. An even stronger increase in the parameter is possible, but for a shorter time before the fuse blows. However, most often an interval of 10 seconds is enough to start the engine, and to prevent the fuse from tripping. Protection of a single-phase electric motor against overloads, against short-circuit, as well as another type of electric motor by such a device is considered one of the most reliable.
It is also worth noting here how the response time of this protection device is determined. The response time of the fuse is the segment over which its fuse (wire) melts. When the wire is completely melted, the circuit opens. If we talk about the dependence of the trip time from overload for these types of protective equipment, then they are inversely proportional. In other words, the current protection of the electric motor against overloads works like this - the higher the current strength, the faster the wire melts, which means that the circuit disconnection time is reduced.
Magnetic and thermal appliances
Today, automatic thermal devices are considered the most reliable and economical devices for protecting the electric motor from thermal overloads. These devices are also capable of withstanding large current amplitudes that may occur during instrument startup. In addition, thermal fuses protect against problems such as rotor blocking, for example.
Protection of asynchronous electric motors against overload can be carried out using automatic magnetic switches. They are characterized by high reliability, accuracy and efficiency. Its peculiarity lies in the fact that its temperature limit is not affected by a change in the ambient temperature, which is very important in some operating conditions. They also differ from thermal themes, they have a more accurately determined response time.
Overload relay
The functions of this device are quite simple, however, and quite important.
- Such a device is able to withstand a short-term current differential during engine start-up without breaking the circuit, which is most important.
- The circuit is opened if the current rises to that value when there is a risk of breakdown of the protected device.
- After the overload has been eliminated, the relay may return to its original position automatically or may be manually reset.
It is worth noting that the current protection of the electric motor against overloads using relays is carried out in accordance with the response characteristics. In other words, depending on the class of the device. The most common are classes 10, 20 and 30. The first group is the relays that operate in the event of an overload within 10 seconds and if the numerical value of the current exceeds 600% of the nominal. The second group is triggered after 20 seconds or less, the third, respectively, after 30 seconds or less.
Fuses and Relays
At present, quite often, two means of protection are combined - fuses and relays. This combination works as follows. A fuse should protect the engine from short circuit, and therefore it must have a sufficiently large capacity. Because of this, he can not protect the device from lower, but still dangerous currents. To eliminate this drawback, relays are introduced into the system that respond to weaker, but still dangerous current fluctuations. The most important thing in this case is to set the fuse so that it trips before any element is damaged.
Outdoor protective equipment
Currently, advanced motor protection systems are often used. They can protect the device from overvoltage, phase imbalance, can eliminate vibrations or limit the number of on and off. In addition, such tools have a built-in thermal sensor that helps control the temperature of the bearings and stator. Another feature of such a device is that it is able to perceive and process a digital signal that creates a temperature sensor.
The main purpose of external protective equipment is to maintain the operability of three-phase motors. Besides the fact that such equipment is able to protect the engine during a power outage, it also has several more advantages.
- An outdoor device can generate and signal a malfunction even before it disrupts the machine’s performance.
- Diagnoses those problems that have already occurred.
- Provides the ability to test relays during maintenance.
Based on the foregoing, it can be argued that there is a wide variety of devices for protecting the motor from overload. In addition, each of them is able to protect the device from certain negative influences, and therefore it is advisable to combine them.