How to check the electric motor, armature winding with a multimeter, ohmmeter for serviceability – ElektrikExpert.ru

ЭElectricity has firmly entered into all spheres of our life. In everyday life, it is used to solve two main problems: lighting and converting electrical energy into mechanical energy.

The second group of tasks is physically implemented by electric motors. Other household applications of electricity are possible, but they are much less common.

The long period of use of electric motors, whose history goes back almost 200 years, has led to the fact that:

• in practice, there is a wide variety of varieties of such devices;
• modern electric motors are highly reliable.

It is known, however, that even the most advanced technology sometimes fails. Accordingly, there is a problem of accurately diagnosing the cause of the malfunction, on which further actions already depend, the extreme of which boils down to the need to purchase a new device or the whole thing is in a lost contact.

Important limiting factors when performing such checks are:

• the possibility of self-diagnosis without contacting specialized repair organizations or calling a private master for reasons of saving time and money;
• performing a full range of checks for an unambiguous reliable localization of the cause of failure using improvised means, the most difficult of which is a household multimeter.

The principle of operation of the electric motor

Its direction is determined by the left-hand rule known from the school physics course, that is, it depends on the ratio of the directions of current flow and the orientation of the magnetic field lines, and the value depends on the current strength and the value of the magnetic field induction in the area of ​​its interaction with the conductor.

Another means of increasing the force acting on the conductor is to increase its effective length, for which the current flow circuit is formed in the form of a multi-turn winding. Due to this, the force developed by individual turns is summed up.

The type of magnetic field source does not matter. It can be either a permanent magnet or its electromagnetic counterpart.

The efficiency of the electromagnet is increased by the core, which actually concentrates the magnetic field and delivers it to the area that corresponds to the greatest developed force.

Key design features, basic approaches to performing its checks

Any electric motor, regardless of its design, always contains a stationary part, which is traditionally called a stator, and a rotating structural element that interacts with it, which is usually called a rotor.

Sometimes the term anchor is used to refer to the rotor. In the vast majority of motors, the rotor is located inside the stator.

Mechanical work is removed from the rotor, the transformation of rotational motion into rectilinear or other movement is assigned to other external known mechanisms, the consideration of which is beyond the scope of this article.

Similarly, the so-called linear motors are not considered, which provide linear movement of the movable part of its structure without performing an intermediate transformation of rotational motion.

Learn more about how a stepper motor works.

The stator includes one or more stator windings, when current flows through which (s) a rotating magnetic field is formed.

The stator field interacts with the rotor field, resulting in a torque that allows mechanical work to be performed. To reduce useless losses and increase the efficiency of the engine as a whole, the rotor is mounted on bearings.

From the above multiple description, three main provisions immediately follow, which are always carried out in a serviceable electric motor:

• when the rated voltage is applied to the windings, operating currents flow through them, for which the motor design was originally designed;
• the insulation of the conductive parts of the structure does not have mechanical damage and provides the specified resistance value;
• the mechanical part of the rotor-stator system in terms of the state of the bearings, the values ​​of the gaps, the tightening of the nuts, the level of wear of the brushes and similar ones fully comply with the requirements of the standards.

Motor health checks always explicitly or implicitly include the control of these positions, performed in various ways. These include, for example, a visual inspection of the bearings, checking the size of the gaps, the ease of rotation of the rotor, etc.

In the future, we will focus on performing checks on those electrical components of the engine, the malfunctions of which can only be detected using a multimeter.

When constructing a scheme for the corresponding measurements, it is necessary to take into account the design features of the tested electric motor. By default, it is considered that the engine is connected to a 220 or 380 V network.

Additionally, we point out such a feature of the electric motor as its reversibility. The latter is understood as the fact that when the rotor rotates under the influence of an external force, it generates an electric current.

Schemes for constructing electric motors

The functions of the energy source for the engine can be performed by a DC and AC network.

The change in the direction of current flow required to create a rotating magnetic field is provided in various ways. In particular, switches are widespread.

The switch can be:

• internal mechanical (it is used in collector motors of direct and alternating currents);
• internal electronic (the so-called brushless electronic motors);
• external (on this principle, single-phase and three-phase asynchronous AC motors are built.

Brushed and brushless motors

The principle of operation of a collector electric motor is illustrated by the picture below, which schematically shows the interaction of one of the turns of the rotor winding with a magnetic field.

In such a structure, after the rotor completes half a revolution, the direction of the current changes to the opposite (right side of the image) and the magnetic field starts to slow down the rotor instead of accelerating.

To eliminate this undesirable effect, a mechanical or electronic switch is introduced into the motor design, which reverses the direction of the current flowing through the stator winding in the opposite direction every half turn.

As a result, a constant directional torque is maintained.

The supply of voltage to the rotor windings, if necessary, is carried out through slip rings specially designed for this, to which the beginning and end of the corresponding winding are connected.

The current supply in collector motors is controlled by a mechanical switch, in brushless motors, this function is performed by its electronic counterpart.

Asynchronous motors

AC induction motors use a different principle for generating torque. The essence of this scheme is that a rotating magnetic field is formed by the stator, which drags the rotor along with it. In this case, depending on the type of network and the required power, two slightly different schemes are used.

If it is necessary to obtain higher powers, they turn to a 3-phase 380 V network.

If you initially set the current (voltage) shift angle between the individual phases to a third of the period or 120 degrees, then a uniform rotating magnetic field is formed.

A 3-phase network can be considered as a combination of three current sources, interconnected in a special way.

The strength of this configuration is the ability to increase power compared to the case of a single-phase 220-volt network.

For most household consumers, the 3-phase network is too powerful, and they are connected to a more economical 220 V network.

In this case, to obtain a rotating magnetic field, you have to resort to a little engineering trick.

Its essence lies in the fact that the capacitor as a reactive element always has a 90-degree phase shift between the voltage and current vectors.

Thus, using a capacitor as a phase-shifting element, it is possible to artificially turn a single-phase network into a quasi-two-phase network, thereby solving the problem of obtaining a rotating magnetic field. This is shown schematically on the right side of the figure above.

Motor Test Approaches and Controlled Parameters

In the future, it is assumed that the electric motor under test is in good working order from a mechanical point of view: it has no bearing play and proper lubrication, the gaps between the rotor and the stator do not go beyond the allowed tolerances, the brushes and lamellas of the collector system are not worn out, the power supply cable and the like are in good order. .

The main tool here is a visual inspection. It is also useful to make sure that there is no smell of burnt insulation.

Additionally, the disassembly of the structure, if necessary, was carried out carefully, without mechanical damage, using specialized tools.

It is also believed that the type of electric motor used is known: direct or alternating current, collector, etc. For this, the data from the nameplate on the housing and the accompanying documentation are used.

If necessary, relevant information is available on the Internet.

Taking into account the principle of operation of the electric motor, checks are subject to

• the presence of breaks in the windings and short (interturn) circuits in them on the rotor and stator;
• absence of insulation breakdowns on the body and other metal structural elements;
• state of the capacitor of single-phase electric motors.

The general scheme for performing checks for all types of electric motors differs little.

Therefore, it is further considered from a unified standpoint, the nuances arising from the design features, if necessary, are discussed separately.

The health of the stator windings

To perform this test, the multimeter is put into resistance measurement mode with maximum sensitivity (200 ohm range or equivalent).

Three-phase motor

The most difficult case is a 3-phase electric motor, on the body of which there are 6 terminals, each of which is responsible for the beginning and end of a particular winding.

This is shown in schematic form below. The important thing here is that all windings are the same.

Checking procedure:

• first, with a multimeter that shows resistance, the pairs of terminals responsible for a particular winding are determined;
• the resistance of each of them is accurately measured, and the obtained values ​​are compared with each other. The absence of a difference indicates that the windings are in good condition, and also that they do not have interturn short circuits of the corresponding winding.

Single phase motor

Unlike its 3-phase counterpart, in a single-phase one, in addition to reducing the operating voltage to 220 V, the number of windings is also reduced to two: one of them is considered working, and the second is starting.

At the same time, two schemes for their connection are approximately equally popular, which are conditionally shown below and outwardly differ from each other in the number of terminals.

In practice, one of these schemes can be encountered on such a popular household device as a washing machine.

Regardless of the winding connection scheme that the machine designer has chosen, by performing several measurements, you can check the resistance of each of the windings. A more powerful working winding will have less resistance.

The 4-pin circuit will require six measurements to be made (AB, AC, AD, BC, BD, and CD – specifying, for example, AB, it is considered that the multimeter is connected to points A and B).

It is important that:

• changing the position of the probes to the opposite should not change the readings of the multimeter (AB = BA);
• for a serviceable engine, only two measurements will give a final resistance value of a maximum of tens of ohms (for example, AB and CD), the rest will show a gap.

For a three-terminal circuit, three results will be obtained in total. The greatest resistance refers to the series connection of two windings (it is measured between points A and C in the right sketch of the figure shown above), the average is characteristic of the starting winding and the smallest is for the working one.

Checking breakdowns and leaks on the body

The standard instrument for determining the insulation resistance is a megohmmeter. A household multimeter does not implement this function due to the low battery voltage and the relatively low sensitivity of the device itself in terms of low currents.

Therefore, with its help, you can only make sure that there are no breakdowns. For example, for the circuit shown below, any measurement of DA, DB, and DC must show a break.

A more complex circuit is shown in the following figure. The essence of the experiment is to artificially increase the testing voltage, for which a 220-volt network is used.

When assembling the circuit, it is necessary to use a conventional incandescent lamp with a power of approximately 60 W, which takes on the functions of a current-limiting resistor.

The multimeter is used in the ammeter mode, to protect against damage to the device by an excessively high current, measurements begin on the coarsest possible scale, gradually increasing the sensitivity.

The insulation is considered to be serviceable if the measured current I does not exceed I = 1 μA. Taking into account the fact that the lamp resistance is much less than the insulation resistance Riz, the value of the latter is found as Riz u220d XNUMX / I MΩ, and the current is substituted into this formula in μA.

When conducting the described experiment, a voltage of 220 V is used, that is, all electrical safety rules should be observed. Additionally, the motor must be dismantled and placed on a dielectric base.

Checking the health of the electrical circuits of the rotor

Different types of electric motors have different rotor designs. This feature imposes some specifics on the measurement process.

Synchronous motors

The rotor of a synchronous motor contains several windings, the ends of which are normally connected to metal rings.

The rings are mounted on the rotor shaft and are suitably insulated. In schematic form, this electric motor design block is shown below.

The electrical check of the rotor is carried out in the same way as the stator and includes

• resistance measurement of individual windings with additional verification of their identity;
• control of the absence of interturn short circuits;
• insulation testing for the absence of breakdown on the case.

Asynchronous motors

The rotor of an asynchronous motor stands out from the background of others with its structural simplicity and is made in the form of a so-called squirrel wheel.

Checks with a multimeter of this block are practically useless due to its massiveness and extremely low resistance, which the multimeter is often unable to fix due to its relatively low accuracy.

Taking into account this feature, the rotor in this case is checked by visual inspection for the absence of mechanical damage.

Mechanically commutated commutator motors

The rotor of engines of this variety contains several identical windings, the ends of which are brought out to the collector plates.

To eliminate the influence on the measurement accuracy of additional current flow circuits, brushes are removed from the motor, after which the resistance of each winding is determined by a multimeter that is connected to a pair of plates. Equality of readings indicates the health of the windings.

Other schemes for individual winding checks are also possible, but they are difficult to implement and therefore are not considered.

Checking the vacuum cleaner motor

The principle of implementation of this test is based on the reversible nature of the electric motor, which, as noted above, when connected to an external energy source, can operate in generator mode.

To perform this test, in addition to the multimeter, you will need a second serviceable vacuum cleaner, and it is advisable to dismantle the engine under test, together with the centrifugal air compressor impeller.

The picture shows a diagram of the construction of the corresponding configuration.

A working vacuum cleaner creates an air flow in the hose, which rotates the impeller of the centrifugal compressor of the Central Committee and through it spins the rotor of the electric motor under test.

A multimeter operating in the AC voltage measurement mode and connected to the terminals of a serviceable electric motor (EM) should have readings of the order of 150 – 220 V.

After the vacuum cleaner is turned off, the EM rotor speed drops rapidly and the voltage recorded by the multimeter decreases proportionally to this.

Condenser test

The phase-shifting capacitor, installed in single-phase electric motors, is designed to create a rotating magnetic field.

Checking its serviceability can be performed by two different devices according to an identical scheme.

In both cases, preliminary preparation is required, the essence of which is to de-energize and discharge the capacitor.

To do this, the capacitor is disconnected from the engine, for which it is enough to remove one of the terminals, after which its terminals are shorted with a screwdriver or a piece of wire.

The first approach is implemented if the multimeter has a capacitance detection function. The measured actual value should not differ from the nominal value indicated on the capacitor case by more than 15-20% downwards.

Similarly, measurements are made with a specialized RC meter, which manufacturing companies often package in the form of tweezers that are easy to use. An example of the design of such a tester is shown below.

Determination of winding direction

The direction of the magnetic fluxes created during the operation of the electric motor is determined by the direction of the winding of the wires of the individual windings, is set during the design of the motor and cannot be changed.

When checking the correctness of switching, the need for which may arise after repair or maintenance, it should be assumed that the windings interacting through magnetic fluxes can be considered as a transformer.

The latter means that the windings can be connected both in opposite directions and in the same direction.

The essence of the experiment, which makes it possible to determine the mutual direction of the windings, is that a short-term alternating current can be created by simply connecting or breaking the circuit with one voltage source, the functions of which are assigned to an ordinary battery.

The corresponding circuit is shown below. It is based on the property of a modern multimeter to automatically determine the polarity of the measured voltage.

One of the windings (left for both configurations of the figure) is taken as a reference and a battery is connected to it through a key of any design (up to a conventional wire that is connected to the winding terminal and removed from it by hand).

A multimeter switched to voltmeter mode is connected to the terminals of the second winding. If, when the key is closed, the multimeter shows a short-term positive voltage, then the winding directions of the wires are the same. This case is shown on the left.

On the right is shown the case of counter (including in the direction of the generated magnetic field) inclusion, when the voltmeter shows a negative voltage.

The polarity of the voltage is conditionally shown by the signs “+” and “-” next to the image of the voltmeter.

This experiment is somewhat more convenient to carry out with old pointer analog testers, in which the pointer deviation to the right corresponds to a positive voltage, and to the left – negative.

Measurement Safety

The bulk of the measurements described above can be performed without dismantling the electric motor from its regular place. Taking into account this feature, before starting work, you must make sure that the cord plug is disconnected from the outlet (the device is de-energized). If there is a separate grounding of the equipment, it is advisable to leave it in its place.

Conclusion

As you can see, a sufficiently high-quality and comprehensive check of the condition of the electric motor is quite possible without the use of special tools and devices.

The necessary conditions for this are an understanding of the principle of operation of the device under test, the presence of elementary knowledge in the field of electrical engineering, as well as compliance with safety regulations and accuracy in work.

More complex comprehensive checks, such as normal operation under load, will require the use of complex measuring instruments such as current clamps and cannot be recommended for home use.

Fortunately, the need for their implementation arises quite rarely.