The role of protective devices in improving the reliability of electric motors

The reliability of a technical device is understood as its ability to perform its functions for a certain period of time.

One of the most important indicators of reliability is MTBF, which is measured by the number of hours of operation until the first failure. The higher this number, the higher the reliability of the product.

Distinguish between structural and operational reliability of an electric motor.

Structural reliability of the electric motor depends on the quality of the materials used in the machine, on the quality of the production of individual units and elements, on the improvement of the assembly technology and other factors.

The operational reliability of the electric motor is influenced by the quality of the machine's manufacture, the environmental conditions during operation, the conformity of the characteristics of the electric motor with the requirements of the working machine and the technological process, the level of maintenance.

The economic efficiency of using electric motors is determined not only by their initial cost, but also by operating costs.

The production of unreliable electric motors requires high costs to keep them in good working order. Improper use and lack of proper maintenance result in quality products that do not provide trouble-free operation. Thus, to effectively use all the possibilities inherent in the electric motor, a set of measures is needed, starting with the correct design of the electric drive and ending with the timely support and quality repair. Violation of one of the links in this chain does not allow the desired effect to be achieved.

There are three typical types of failures that are inherent in electric motors.

1. Breakthroughs in electric motor accidents that occurred during the early period of operation. Their appearance related to defects in the production process in factories. Remaining unnoticed, they manifest themselves in the first period of work.

2. Sudden failures of electric motors during normal operation.

3. Malfunctions caused by wear of individual parts of electric motors. They occur either due to the development of resource parts or improper use or maintenance. Timely repair or replacement of worn parts of the electric motor prevents this type of damage.

The above types of failure correspond to three periods of the "life" of the electric motor: leakage period, normal operation period and aging period.

V period of expiration failure rate electric motors are higher than in normal operation. Most manufacturing defects are identified and corrected during testing.However, in mass production it is impossible to test every piece. Some of the machines may have hidden defects that cause damage during the first period of operation.

The duration of the drain time is important during which reliability corresponding to normal operation is achieved. Malfunctions of the first period do not further affect the reliability of the device in subsequent periods of its use.

During normal operation, malfunctions in the operation of electric motors are usually random. Their appearance largely depends on the operating conditions of the device. Frequent overloads, deviations from the operating modes for which the electric motor is designed, increase the probability of failure. During this period, the maintenance and timely removal of deviations from normal working conditions are of primary importance. The task of the service personnel is to ensure that the period of normal operation does not decrease below the standard time.

High reliability means a low rate of failure in operation and therefore a longer period of operation. If systematic preventive maintenance of the electric motor is established in practice, then the duration of the period of its normal operation reaches the design value — 8 years.

The third period of the "life" of the electric motor — the aging period — is characterized by a rapid increase in the degree of failure. Replacing or repairing individual parts has no effect, the whole machine wears out. Its further use becomes unprofitable. The wear of the entire machine is of primary theoretical importance.It is seldom possible to design and operate a machine in such a way that all its parts wear evenly. Usually its individual parts and units fail. In electric motors, the weakest point is the winding.

The most important indicator on which the reliability of the operation of a technical device depends is its maintainability, which is understood as the ability to detect and eliminate damage and malfunctions during maintenance and repair. Repairability is quantified by the time and labor costs required to restore a technical device to serviceability.

Engine failure patterns can be different. It takes different times to regain full functionality. However, observations show that the average recovery time for a given level of maintenance is common to all installations. This value is considered a maintainability characteristic.

MTBF does not fully characterize the reliability of a technical device, but only determines the period of time during which the device works flawlessly. After the occurrence of a failure, it takes time to restore its performance.

A generalizing indicator that evaluates the readiness of the device to perform its functions at the right time is the availability coefficient, which is determined by the formula

kT = tcr / (tcr + tv)

where tcr is the mean time between failures; tв — mean recovery time.

Thus, kT — the ratio of the average duration of work to the sum of work time and recovery time.

The low reliability of the device can be compensated by reducing the recovery time.

Low MTBF and long recovery time can be the cause of low device availability. The first of these values ​​depends on the reliability of the product and the level of its technical operation. The higher its quality, the longer the average time between failures. However, if recovery and maintenance take a long time, equipment availability does not increase. In other words, the use of high-quality equipment must be complemented with a high level of maintenance and repair… Only in this case is it possible to achieve continuous operation.

From the point of view of production, it is important to have ready-to-use and trouble-free equipment in general. The readiness of the main power unit (electric motor) also depends on the reliability of the operation of the starting equipment for protection and control.

Protection cannot prevent engine damage, as it cannot affect those factors that create an emergency situation.

Role overload protection devices is to prevent damage to the electric motor by turning it off in time. This can significantly reduce the recovery time of electrical equipment. It takes less time to eliminate the cause that caused the emergency mode than to repair or replace a damaged engine.

On the other hand, unjustified premature shutdown of the electric motor should not be allowed, as this reduces the reliability of the equipment as a whole. Whatever the reason, the trip is a failure. Inadequate safeguards reduce MTBF and therefore availability.

In some cases, it is advisable not to turn off the electrical installation, but to signal an emergency mode.

Using the terminology of reliability theory, we can say that the general purpose of protection is to reduce the recovery time of the electrical installation as a whole by preventing damage to the electric motor. The protection must respond to the same overloads that actually pose a risk of damage to the electric motor.

Some types of congestion must be overcome with power reserve. False shutdowns reduce equipment reliability and cause production damage. They should not be allowed.