How to fix a blown engine

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The most widespread electric motors in professional use are three-phase motors. The so-called asynchronous motors are used most frequently. With a few exceptions, which also work with alternating current, they work with three-phase current. These drives are the workhorses among electric motors and can also start against large counter torques. In this article we address the question of how a three-phase motor can be tested with simple means.

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Since three-phase motors are widespread, they are relatively inexpensive to purchase. The motors are also considered to be very robust. They require comparatively seldom maintenance and still achieve a long service life because there is hardly any wear and tear. But what if the workhorse suddenly refuses to work? Fortunately, three-phase machines are built relatively simply, so that at least gross defects are easy to find when inspecting a three-phase motor.

Take care when handling three-phase machines - risk of death and fire!

The supply of "three-phase alternating current", which is also known as three-phase current and is often equated with heavy current in common parlance, has a great advantage: despite its simple structure, the electric motor generates a lot of power. However, this implies that anyone working on high-voltage motors should be extremely careful. There is danger to life. Three-phase current usually has a voltage of 400 volts, which is lethal for humans. The repair and also the inspection of a three-phase motor should therefore be carried out by a trained specialist. In the event of improper repairs or changes, there is a risk of an operator receiving an electric shock or a fire. In any case, the following applies: When you test a three-phase motor, make sure that the motor is disconnected from the power supply first.

How is a three-phase asynchronous motor constructed?

The simple and robust design of three-phase asynchronous machines has played an important role in their widespread use. There are hardly any parts that wear out easily. As with all electric motors, the essential parts are the fixed part, called the stator, and the rotating part, called the rotor or runner.

In three-phase asynchronous motors, the stator essentially consists of three (or a multiple of three) coils, the bearings and the housing. The coils are each fed by a phase voltage phase of the three-phase current. The three phases of the alternating current are each shifted in phase by 120 degrees. The three coils each induce a magnetic field offset by a third, which ultimately rotates and thus produces a torque.

In most three-phase motors, the rotor is a passive electromagnetic component called a squirrel-cage or squirrel-cage rotor. It is built in such a way that a closed current is induced in it by an external magnetic field. The magnetic field produced in turn by this current interacts with the external rotating magnetic field and sets the rotor in a directional rotation.

As the rotor rotates more slowly than the magnetic field of the outer coils due to the magnetic interactions, it is referred to as an asynchronous machine. The frequency of the rotor lags behind that of the magnetic field. So it does not run synchronously with the frequency of the alternating current.

Other parts of the motor are the bearings in which the rotor is suspended, and usually a fan connected to the rotor, located at the rear end of the motor, which ensures that the motor is air-cooled. In addition, there is the corresponding control electronics, which are important for starting up and regulating the engine.

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How can you check an asynchronous three-phase motor?

So how do we go about testing a three-phase motor? In the case of an asynchronous three-phase motor, the test is facilitated by the fact that there are three (or a multiple of three) identical coils that can be compared with one another when measured.

Before loosening the first screw of the housing, you should make absolutely sure that the motor is disconnected from the mains and that it cannot be switched on accidentally. A 400 volt electric shock is often fatal and almost always causes severe burns or permanent damage.

Check three-phase motor, step 1: Is the coil insulated?

In the professional field, a specialist would check whether the windings of the coils are correctly insulated, i.e. not short-circuited with the housing. For this purpose, experts use special insulation measuring devices or crank inductors. If you only have a digital or analog multimeter or a flow meter at hand, you can also use it to test the three-phase motor.

To do this, we measure in the highest measuring range of the device in the continuity measurement function. The test person places one tip of the test device on the contact in the motor connection box and the other test contact on the housing. It is a good sign if the measuring device does not display anything. If, on the other hand, it indicates continuity, i.e. the test current flows from the coil to the housing, this means that the insulation of the coil is defective and it is in contact with the housing. The engine should be repaired by a professional. However, if the meter shows nothing, it does not mean that everything is OK with the engine. In this case, further tests are useful.

Is there any damage inside the coil?

If the test in the previous step did not give any indication of insulation damage, you can now use a simple measuring device to compare the coils with one another. When you test the three-phase motor, all coils should have approximately the same resistance. A deviation of three to four percent is okay, since the resistance also depends on the number of turns and even machine-wound coils can have one more or less winding.

Before measuring the resistance of a single coil, we need to remove any cross-wirings called bridges. If the ohmmeter shows a clear difference between the coils in comparison, then damage in the coil is obvious and a specialist should take a look at it.

Measuring the resistance with a commercially available multimeter, which only has a weak battery-supported test current, only works with three-phase asynchronous motors that have a low power. For engines with an output of around 5.5 kilowatts or more, a specialist with special measuring devices must carry out the test.

Are the bearings, fans, fuses and control electronics OK?

As with other electric motors, you should also take a look at the fan slots, the fan itself, and the bearings when testing the AC motor. Dirt, too much play in the bearing or a jammed bearing can also cause problems with these motors. 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 these components are still doing their job. As with the entire test of the three-phase motor, the following also applies here: Use your sense of smell! If it smells scorched or burnt, there may be a serious defect and the engine is irreparably broken.

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Other types of engines

In addition to the widespread and simply built three-phase asynchronous machines with squirrel-cage rotors, there are other types of three-phase motors. Special forms such as linear motors that do not drive a rotary movement but a linear movement are rather rare. External rotor motors, in which the stator is on the inside and the rotating part on the outside, are also used comparatively little.

Collector motors, which are also classed as asynchronous motors, and asynchronous machines with slip ring rotors are more widespread. Various synchronous machines in which the rotation frequency is the same as that of the rotating magnetic field are also widespread. The main difference between these motors and squirrel-cage motors is that they have actively powered rotors. Their connections and windings can also be a source of errors that can be checked.

Tags: asynchronous three-phase motor, defect, diagnosis, three-phase motor, check electric motor, electric motors, expert contribution