A commutator motor is a synchronous electric machine in which the current switch in the winding and the rotor position sensor are made in the form of the same device - a brush-collector assembly. This device comes in many forms.
Varieties
A DC commutator motor usually includes items such as:
- three-pole rotor on sleeve bearings;
- two-pole permanent magnet stator;
- copper plates as brushes of the commutator assembly.
This set is typical for the lowest power solutions usually used in children's toys where high power is not required. More powerful engines include several more structural elements:
- four graphite brushes in the form of a collector assembly;
- multi-pole rotor on rolling bearings;
- permanent magnet stator with four poles.
Most often this type of motor deviceused in modern cars to drive the fan of the cooling and ventilation system, washer pumps, wipers and other elements. There are also more complex aggregates.
The power of an electric motor of several hundred watts involves the use of a four-pole stator made of electromagnets. To connect its windings, one of several methods can be used:
- In series with the rotor. In this case, a large maximum torque is obtained, however, due to the high idle speed, the risk of engine damage is high.
- In parallel with the rotor. In this case, the speed remains stable under changing load conditions, but the maximum torque is noticeably less.
- Mixed excitation, when part of the winding is connected in series and part in parallel. In this case, the advantages of the previous options are combined. This type is used for car starters.
- Independent excitation, which uses a separate power supply. In this case, the characteristics corresponding to the parallel connection are obtained. This option is rarely used.
The commutator motor has certain advantages: they are easy to manufacture, repair, operate, and their service life is quite large. As disadvantages, the following is usually highlighted: effective designs of such devices are usually high-speed and low-torque, so most drives require the installation of gearboxes. This assertion is well foundedsince an electric machine oriented at low speed is characterized by an underestimated efficiency, as well as cooling problems associated with this. The latter are such that it is difficult to find an elegant solution for them.
Universal commutator motor
This variant is a kind of DC commutator machine capable of operating on both DC and AC. The device has become widespread in some types of household appliances and hand tools due to its small size, low weight, low cost and ease of speed control. Quite often found as a traction vehicle on the railroads of the United States and Europe. You can consider the device of the electric motor.
Design Features
For a better understanding of this issue, you should consider in more detail what formed the basis of the presented device. The universal commutator motor type is a direct current device with excitation windings connected in series, optimized for operation on alternating current of a household power supply network. The rotation of the motor occurs in one direction, regardless of polarity. This is due to the fact that the series connection of the stator and rotor windings leads to a simultaneous change in their magnetic poles, and due to this, the resulting torque is directed in one direction.
What is it made of?
The AC commutator motor involves the use of magneticallysoft material with low hysteresis. To reduce eddy current losses, this element is made of stacked plates with insulation. As a subset of AC collector machines, it is customary to single out pulsating current units, which are obtained by rectifying the current of a single-phase circuit without using ripple smoothing.
An AC commutator motor is most often characterized by the following feature: in the low-speed mode, the inductive resistance of the stator windings does not allow current to be consumed more than certain limits, while the maximum motor torque is also limited to 3-5 of the nominal. Approximation of mechanical characteristics is achieved through the use of sectioning of the stator windings - separate outputs are used to connect alternating current.
A rather difficult task involves switching a powerful alternating current collector machine. At the moment when the section passes the neutral, the magnetic field, which is in engagement with the rotor, changes its direction to the opposite, and this causes the generation of reactive EMF in the section. This happens when running on AC power. In alternating current collector machines, reactive EMF also takes place. The transformer EMF is also noted here, since the rotor is in the stator magnetic field, which pulsates in time. A smooth start of the collector motor is not possible, since at this moment the amplitude of the machine will be maximum, and as it approaches the speed of synchronism, it will proportionally decrease. As furtheracceleration, a new increase will be noted. To solve the switching problem in this case, several sequential steps are proposed:
- Single turn section should be preferred in design to reduce clutch flow.
- The active resistance of the section needs to be increased, for which the most promising elements are resistors in the collector plates, where good cooling is observed.
- The commutator must be actively ground with brushes of maximum hardness with the greatest resistance.
- Reactive EMF can be compensated by using additional poles with series windings, and parallel windings are applicable for transformer EMF compensation. Since the value of the latter parameter is a function of the angular velocity of the rotor and the magnetizing current, such windings require the use of slave control systems, which do not yet exist.
- The frequency of the supply circuits should be as low as possible. The most popular options are 16 and 25 Hz.
- The reversal of the UKD is performed by switching the polarity of the stator or rotor windings.
Pros and cons
The following conditions are used for comparison: the devices are connected to a household electrical network with a voltage of 220 volts and a frequency of 50 Hz, while the engine power is the same. The difference in the mechanical characteristics of devices can be a disadvantage or an advantage independing on the requirements of the drive.
So, an AC commutator motor: advantages compared to a DC unit:
- The connection to the network is made directly, there is no need to use additional components. In the case of a DC unit, rectification is required.
- Starting current is much less, which is very important for devices used in everyday life.
- If there is a control circuit, its device is much simpler - a rheostat and a thyristor. If the electronic component fails, then the collector motor, the price of which depends on the power and ranges from 1,400 rubles or more, will remain operational, but will immediately turn on at full power.
There are also certain disadvantages:
- Due to losses due to stator reversal and inductance, the overall efficiency is noticeably reduced.
- The maximum torque has also been reduced.
Single-phase collector electric motors have certain advantages in comparison with asynchronous ones:
- compactness;
- lack of binding to the network frequency and speed;
- significant starting torque;
- proportional decrease and increase in speed in automatic mode, as well as an increase in torque with increasing load, while the supply voltage remains unchanged;
- speed control can be smooth over a fairly wide range by changing the supply voltage.
Disadvantages compared to induction motor
- when the load changes, the speed will be unstable;
- the brush-collector assembly makes the device not very reliable (the need to use the most rigid brushes significantly reduces the resource);
- AC switching causes strong sparking on the collector, resulting in radio interference;
- high noise level during operation;
- the manifold is characterized by a large number of parts, which makes the engine quite massive.
Modern commutator motor is characterized by a resource comparable to the capabilities of mechanical gears and working bodies.
Other comparisons
When comparing collector and asynchronous motors of the same power, regardless of the rated frequency of the latter, a different characteristic is obtained. This will be described in more detail below. The universal collector electric motor implements a "soft" characteristic. In this case, the moment is directly proportional to the load on the shaft, while the revolutions are inversely proportional to it. The rated torque is usually less than the maximum by 3-5 times. Idle speed limitation is characterized exclusively by losses in the engine, while turning on a powerful unit without load, it can collapse.
The characteristic of an asynchronous motor is "fan", that is, the unit maintains a speed close to the nominal, increasing the torque as sharply as possible with a slight decrease in speed. If we are talking about a significant change in this indicator, then the engine torque not only does not increase, but also decreasesto zero, which leads to a complete stop. The idle speed is slightly higher than the nominal, while remaining constant. A characteristic of a single-phase induction motor is an additional set of problems associated with starting, since it does not develop starting torque under normal conditions. The magnetic field of a single-phase stator, pulsating in time, breaks up into two fields with opposite phases, which makes it impossible to start without all sorts of tricks:
- capacitance that creates an artificial phase;
- split groove;
- active resistance forming an artificial phase.
Theoretically, an anti-phase rotating field reduces the maximum efficiency of a single-phase asynchronous unit to 50-60% due to losses in a supersaturated magnetic system and windings loaded with counter-field currents. It turns out that there are two electric machines on the same shaft, while one operates in the motor mode, and the second in the opposition mode. It turns out that single-phase collector electric motors do not know competitors in the respective networks. This is what deserved such high popularity.
The mechanical characteristics of the electric motor provides it with a certain scope of use. Low speeds, limited by the frequency of the AC mains, make asynchronous units of similar power large in weight and size in comparison with universal collectors. However, when included in the power circuit of the inverter with a high frequency, comparable dimensions and weight can be achieved. The rigidity of the mechanical characteristic remainsmotor, to which are added current conversion losses, as well as an increase in frequency, magnetic and inductive losses increase.
Analogues without manifold assembly
An AC commutator motor has an analog that is closest to it in terms of mechanical characteristics - a valve one, where the brush-collector assembly was replaced with an inverter equipped with a rotor position sensor. The following system is used as an electronic analogue of this unit: a rectifier, a synchronous motor with a rotor angular position sensor, combined with an inverter. However, the presence of permanent magnets in the rotor reduces the maximum torque while maintaining the dimensions.
Operation principle
The collector electric motor device demonstrates how the device converts electrical energy into mechanical energy and vice versa. This indicates its ability to be used as a generator. It is worth considering in more detail the collector electric motor, the diagram of which will demonstrate its capabilities.
The laws of physics clearly state that when an electric current is passed through a conductor in a magnetic field, a certain force is exerted on it. In this case, the right-hand rule works, which has a direct impact on the power of the electric motor. The commutator motor works exactly on this basic principle.
Physics teaches us that the basiscreating the right things are small rules. This served as the basis for creating a frame rotating in a magnetic field, which made it possible to create a collector electric motor. The diagram shows that a pair of conductors is placed in a magnetic field, the current of which is directed in opposite directions, and hence the forces too. Their sum gives the required torque. The device of the electric motor is much more complicated, since a whole complex of necessary elements has been added to it, in particular, a collector that provides the same direction of current over the poles. The uneven travel was eliminated by placing more coils on the armature, while the permanent magnets were replaced by coils, which eliminated the need for direct current. This made it possible to give the torque a single direction.
Do-it-yourself electric motor repair
Like any other device, this unit can fail for any reason. If the electric motor, the photo of which you can see in our review, cannot gain the required number of revolutions, or the shaft does not spin when it is started, you need to check if its fuses have blown, if there are breaks in the armature electrical circuit, if the device itself is overloaded. Very often, overloading results in abnormal current consumption. To eliminate this malfunction, it is necessary to carefully inspect the mechanical transmission and brake, and then eliminate the causes of overloads.
The design of the electric motor is such that when it starts, it consumesa certain amount of current. If it is greater than the nominal value, it is required to check the consistency of the connection of the parallel and series windings relative to each other, as well as in relation to the rheostat. When do-it-yourself repair of electric motors is carried out, quite specific mistakes are most often made. In particular, the shunt winding can be connected in series with the electrical resistance of the rheostat, or connected to one pole of the electrical network.
Checking the consistency of the connection of the working excitation winding is carried out by connecting one of the ends of the shunt winding with the anchor end, and the second - with an electrical conductor coming from the rheostat arc. Usually the cross section of this electrical conductor is slightly smaller than the others, so it can be detected without a megohmmeter. After turning on the power switch and shifting the rheostat slider to the middle position, power is supplied to the free ends. By means of a control lamp, a sequential check of all conductive ends is performed. When you touch one of them, the lamp should light up, but not with the other. This is how the entire motor is tested. The price of the work carried out will depend on the type of breakdown of the unit.
If during the operation of the device there is a number of revolutions that is less than the nominal, then the main reasons for this are usually the following: low mains voltage, overload of the device, large exciting current. If an inoperability of the opposite nature is noted, it is required to check the excitation circuit, eliminate all identified defects, after whichyou can set the normal value of the excitation current. In some cases it may be necessary to rewind the motors.
When the reason for the inoperability of the unit is the erroneous pairing of the parallel and series field windings, it is necessary to restore the correct connection order. If it is impossible to eliminate such a problem in a simple way, it may be necessary to rewind the electric motors. It is also necessary to check the magnitude of the voltage in the electrical network, since with an increase in its nominal value, the device's revolutions may increase.
