The working temperature of an engine provides a lot of information about its quality and condition. If an engine overheats, the coils deteriorate rapidly. Every 10 ° C increase in the coils of a motor above its rated working temperature shortens the life of the motor by 50%, even if the overheating is only temporary.
If a temperature reading on the motor housing shows an abnormal increase in temperature, it can take a thermal image of the motor and find out more precisely the exact location from which the temperature rise, for example, of the coils, the bearings or couplings. If a coupling heats up, it may indicate that there is bad alignment.
There are three main causes for atypical thermal patterns to occur; normally most of them are the result of high resistance contact points, either in connections or terminals on switches. They usually appear with a higher temperature at the point of high resistance, cooling down as they move away from that point. Load imbalances, whether normal or out of specification, appear equally hot in the phase or in the part of the circuit that is poorly dimensioned or overloaded. Harmonic imbalances create a similar pattern.
If the whole conductor is hot, it may be because it is badly sized or overloaded; Check the nominal and actual load to determine which of the two is the cause. Faulty components usually seem to have a lower temperature than those that usually function. The most common example is that of a blown a fuse. In the control circuit of an engine, this can produce a single-phase condition and, possibly, costly damage to the engine.
About checking insulation resistance
Insulation problems in motors and inverters are usually due to improperly installed installations, environmental contamination, mechanical stress or the age of elements of a part or the entire installation. The insulation check can easily be combined with the mechanical maintenance of the motor to identify the degradation before a fault occurs, or it can be done during the installation procedures to check the safety and performance of the system. When it comes to troubleshooting, the insulation resistance check can be the final link that allows the engine to be put back into operation easily, with the simple operation of changing a cable.
The insulation testers apply DC voltage to the system and measure the resulting current. In this way, they can calculate and show the insulation resistance. Normally, the check verifies the insulation resistance between a conductor and the grounding or insulation resistance between adjacent conductors. Common examples include checking and structure of motor coils and their structure and checking the insulation resistance of phase conductors from connected ducts and housings.
The megohmmeters with insulation measurement combine the functions of insulation resistance described above with the rest of the necessary checks to investigate electrical problems, the motor or the drives, from basic measurements of power to contact temperatures. The main difference between an insulation measurement and the rest of the measures described is that the former is carried out in systems without current, while the electrical (and thermal) measurements are made in systems with voltage and operation.
How to act: electrical measurements and insulation resistance tests in motors
1. Visual inspection
- Disconnect the power to the motor and starter motor (or drive), following proper disconnection procedures and disconnect the motor from the load.
- Perform a visual, olfactory and thermal inspection of the device; and check the rating plate. Check for loose connections on the starter motor and examine all rivets.
- Use a digital megohmmeter to check the supply voltage and the starter motor contacts. Do not run the risk of a fire by causing a short circuit. If the power is correct, it is the motor that has the problem.
2. Checking control contacts
Next, check the quality of the contacts in the control stage:
- Lock the starter motor disconnection.
- Activate the starter motor manually, so that the contacts are closed.
- Select the Ohms function in the insulation tester, designed for the measurement of contact resistances.
- Measure the resistance of each series of contacts.
- The reading should be close to zero. If it is greater than 0.1 ohms, you should check, adjust, and if necessary replace, the terminals or plates that intervene in the connection.
- Lock the starter motor disconnection.
- Select the appropriate test voltage (250, 500, or 1,000 V) in the insulation tester.
- Identify the resistance between these points: the line side of the starter motor to the ground and load side of the earthing motor
- Comparatively low and balanced resistance values in the three phases of the stator.
- High resistance values in the phase-to-ground insulation test.
- Essential resistance problems, for example, that of a phase in a phase short circuit.
- Any imbalance of resistance from the coil to coil.
If the readings differ markedly, the motor cannot operate safely.