The main types of tests of induction motors
Induction motors come with a squirrel-cage rotor or with a phase rotor. The latter are most often used at start-up with a load on the shaft (cranes, elevators, etc.). There are various tests of asynchronous motors, for example, acceptance tests, which are carried out before commissioning and after repair. If the electric motor was modernized, then it is subjected to type tests, confirming the ability to operate an asynchronous or other motor for its intended purpose and to fix its new load characteristics. In the article below, we told readers of the site Elecroexperthow are the test work and what documents are regulated.
PUE (Electrical Installation Rules) regulate the acceptance tests before putting the electric motor into operation (paragraph 1.8.15). Test programs and the number of devices that will be exposed (from a batch) are established by the standard or technical specifications for a specific type of engine. The following is verified:
- The ability to turn on the electric motor without pre-drying the windings (for electric motors with a rated voltage of up to 1 kV or more).
- Insulation resistance.
- Checking the stator windings by applying an increased voltage of industrial frequency. Each winding is checked separately (with two others connected to the case). If there are no leads from the coils, then it is allowed to check the winding completely.
- Measurement of resistance of windings to a direct current. To identify low-quality connections, inter-turn faults, errors in the connection diagram. Also to remove the parameters necessary for calculating modes, transitions and regulators.
- Electric motor idling (the drive mechanism is not loaded).
- Electric motor operation under load.
After carrying out the work, the ACT and the Test Report of the squirrel-cage induction motor (or another type of motor) are drawn up. The protocol shall indicate the obtained parameters and values, as well as the measures taken, the place and composition of the participants. At the same time, verification should be carried out by specialists with an access group of at least IV and having licenses to conduct such activities.
Tests of electric motors during operation are necessary for the timely detection of malfunctions with their subsequent elimination. Also for the safety of production, the operation of electric motors with a deviation is fraught with negative consequences for people serving electric cars.
The tests are as follows:
- measurement of insulation resistance, in this case, for motors with a voltage of more than 1000V, the absorption coefficient of the stator windings is measured;
- checking the condition of insulation;
- checking the stator windings by applying an increased voltage of industrial frequency;
- measurement of resistance of windings to a direct current;
- measuring the gap between the steel of the rotor and the stator;
- measurement of clearances of plain bearings;
- verification of pathogens;
- measurement of vibration characteristics of bearings;
- measurement of the axial run of the rotor;
- in the presence of an air cooler hydraulic tests are carried out;
- checking engine operation under load;
- check of serviceability of rods (only for HELL with a squirrel-cage rotor);
- check ED in idle mode or with a drive mechanism without load.
In the video below, you can see several verification methods:
The resistance of the windings is given in the table in the catalogs of manufacturers of electric machines, or in references. The permissible insulation resistance is usually 1 MΩ per 1 kV of supply voltage. Machines that are powered by 380V are allowed to work if their insulation resistance is not less than 500 kOhm.
According to the results, an act and protocol are also drawn up. In addition, the parameters of the electric machine are recorded in the test log.
After Repair Tests
Before the start of repairs, pre-repair tests are carried out to accurately detect the components of the induction motor. The goal is to identify serviceable engines that entered the audit by mistake or have a minor malfunction that can be fixed immediately.
During the repair process, operational tests (operational control) are carried out, the purpose of which is the identification of errors, poor-quality materials or spare parts and the timely elimination of identified observations. First of all, the importance of operational control is due to the reduction of the repair time (if it is not carried out, if there is a defective unit, the repair will be delayed), the second reason is the reduction in repair costs.
If the operational control is not carried out, then, for example, when the stator or rotor is rewound (in the presence of a metal or wire defect), the malfunction can be detected already in tests after repair, this will lead to a significant increase in the cost of maintenance. A comprehensive inspection stand will not only shorten the operational control period, but also greatly simplify its implementation.
After overhaul, acceptance tests are carried out (if the electrical and magnetic characteristics have changed, then type tests are carried out).
In the operation, inspection and maintenance of electric motors can be guided by the book N.M. Slonim “Tests of asynchronous motors during repair”, which describes the methods of their implementation. Despite the 1980 issue, the book contains relevant information. Test methods for induction motors are described in GOST 7217-87, it is valid, the text was updated on 04/06/2015, it was reissued in 2003. In addition, the PUE and PTEEP also provide a test program for electric AC machines.
This is where we conclude our discussion of this topic. If you have questions or you just want to add material, write in the comments under the article!