Checking three-phase asynchronous motors
Warning!
The following article is aimed at the technically interested layperson and is not intended to be a repair guide. All work on safety-relevant electronic parts may only be carried out by qualified electricians.
The most frequently used electric motors due to their robust construction are three-phase asynchronous motors. Their robust construction ensures that asynchronous motors are particularly low-maintenance. But what to do if they give up their service.
Before we deal with this topic, we want to draw your attention to the dangers that 3-phase alternating current and improper repairs can pose. The supply of "three-phase alternating current", which is also known as three-phase current, has a significant advantage: despite its simple structure, electric motors generate high power (see also the article Construction of an electric motor). However, this means that anyone working on three-phase motors must exercise extreme caution. There is danger to life! The voltage of three-phase alternating current is typically 400V, which is fatal to humans. For this reason, repairs or tests should only be carried out by trained specialists. Improper repairs or changes can endanger anyone who works with the engine. There is also an increased risk of fire. In any case, the following applies: When you work on a three-phase motor, observe the following 5 safety rules:
1. Disconnect: Disconnect all poles of the system from the mains (switch off circuit breaker or main switch)
2. Secure against being switched on again: Accidental switching on of the system must be prevented (e.g. by using special locks)
3. Determine absence of voltage: With a suitable measuring device (two-pole voltage tester) the absence of voltage must be determined on all poles.
4. Earthing and short-circuiting (from 1000V systems): All conductors must be short-circuited and earthed with suitable tools in order to trigger the upstream safety device (if switched on by mistake).
5. Cover or barricade neighboring live parts (from 1000V systems): Neighboring live parts must be protected against contact.
Check three-phase motor, step 1 Are the coils insulated?
A specialist would first check whether the coils are correctly insulated, i.e. that the coils have no electrical connection to the housing. Special insulation measuring devices are used by experts for this purpose. However, if you only have a digital or analog multimeter or a flow meter at hand, you can also use it to measure the first errors.
To do this, one measures in the highest measuring range of the measuring device in the continuity measurement function. One test lead of the measuring device is connected to the contact in the motor connection box, the other test lead is connected to the housing. It would be a desirable result if the measuring device shows nothing, i.e. no continuity. If, however, continuity is displayed, it means that there is an electrical connection between the housing and the coils. In this case the insulation of the coils would be defective. Should this occur, the engine should be repaired by a specialist. If the measuring device does not display anything, this does not mean that the motor is OK, further tests are necessary.
Is there damage inside the coil?
If there was no indication of an insulation fault in the previous step, the next step should be to measure the resistances of the coils. The resistance can be measured with a normal multimeter. The resistors of the coils should be roughly identical. A slight deviation of three to four percent is okay, since the resistance depends on the number of turns and even machine-wound coils have one turn more or less.
Before measurements can be taken, the bridges that are inserted for a delta or star connection must be removed from the terminal board. If you find that there is a clear difference between the individual coils, then there is damage to the coil. Should this occur, a specialist should be consulted.
The measurement method just described with a commercially available multimeter is only possible up to a power of around 5.5 kW. With larger motors, the resistance of the coil is too small to be measured with such a measuring device. If you have such a motor, a specialist with a special measuring device must take it over.
Are the bearings, fans and control electronics defective?
When you test an electric motor, it's not just the winding that needs to be checked. You should definitely also look at the bearings, as well as the fan and fan cover. If, for example, the ventilation grille is very dirty, there is a risk that the engine will get too warm. A common cause of failure for asynchronous motors is bearing damage. So check the bearings for ease of movement and play. A look at the fuses and the control electronics can also be worthwhile. With the appropriate measuring tools and specialist knowledge, you can determine whether, for example, a fuse or a contactor is defective. The following applies to the entire test of a three-phase motor: Trust your senses. If something smells charred, something grinds or does not look right, there may be a serious defect. Then you should definitely get a professional for help.
