§61. The effect of a magnetic field on a current-carrying conductor. Electric motor
Questions
1. How to show that a magnetic field acts on a current-carrying conductor located in this field?
1. If you hang a conductor on thin flexible wires in the magnetic field of a permanent magnet, then when an electric current is turned on in the network with the conductor, it will deviate, demonstrating the interaction of the magnetic fields of the conductor and the magnet.
2. Using Figure 117, explain what determines the direction of movement of a conductor carrying current in a magnetic field.
2. The direction of movement of a conductor carrying current in a magnetic field depends on the direction of the current and on the location of the magnet poles.
3. What device can be used to rotate a current-carrying conductor in a magnetic field? What device in the frame is used to change the direction of the current every half turn?
3. You can rotate a current-carrying conductor in a magnetic field using the device shown in Fig. 115, in which a frame with an insulated winding is connected to the network through conductive half-rings and brushes, which allows you to change the direction of the current in the winding through half a turn. As a result, the frame rotates in one direction all the time.
4. Describe the structure of a technical electric motor.
4. A technical electric motor includes an armature - this is an iron cylinder with slots along the side surface into which the winding turns are placed. The armature itself rotates in a magnetic field created by a strong electromagnet. The motor shaft, running along the central axis of the iron cylinder, is connected to a device that is driven by the motor to rotate.
5. Where are electric motors used? What are their advantages over thermal ones?
5. DC motors have found particularly wide application in transport (trams, trolleybuses, electric locomotives), in industry (for pumping oil from a well) in everyday life (in electric shavers). Electric motors are smaller in size compared to thermal ones, as well as much higher efficiency; in addition, they do not emit gases, smoke and steam, i.e. they are more environmentally friendly.
6. Who and when invented the first electric motor suitable for practical use?
6. The first electric motor suitable for practical use was invented by the Russian scientist Boris Semenovich Jacobi in 1834. Task 11
1. In Fig. 117 shows a diagram of an electrical measuring device. In it, the frame with the winding in the disconnected state is held by springs in a horizontal position, while an arrow rigidly connected to the frame points to the zero value of the scale. The entire frame with the core is placed between the poles of a permanent magnet. When the device is connected to the network, the current in the frame interacts with the field of the magnet, the frame with the winding rotates and the arrow rotates along the scale, in different directions, depending on the direction of the current, and the angle depends on the magnitude of the current.
2. In Fig. 118 shows an automatic device for turning on the bell if the temperature exceeds the permissible level. It consists of two networks. The first contains a special mercury thermometer, which serves to close this circuit when the mercury in the thermometer rises above a preset value, a power source, an electromagnet, the armature of which closes the second circuit, which, in addition to the armature, contains a bell and a power source. Such an automatic machine can be used in greenhouses and incubators, where it is very important to ensure that the required temperature is maintained.
Solar radiation meter (lux meter)
To help technical and scientific workers, many measuring instruments have been developed to ensure accuracy, convenience and efficiency of work. At the same time, for most people the names of these devices, and even more so the principle of their operation, are often unfamiliar. In this article we will briefly explain the purpose of the most common measuring instruments. The website of one of the measuring instrument suppliers shared information and images of the instruments with us.
Spectrum Analyzer is a measuring device that serves to observe and measure the relative distribution of energy of electrical (electromagnetic) vibrations in a frequency band.
Anemometer– a device designed to measure the speed and volume of air flow in a room. An anemometer is used for sanitary and hygienic analysis of territories.
Balometer– a measuring device for direct measurement of volumetric air flow on large supply and exhaust ventilation grilles.
Voltmeter- This is a device that measures voltage.
Gas analyzer- a measuring device for determining the qualitative and quantitative composition of gas mixtures. Gas analyzers can be manual or automatic. Examples of gas analyzers: freon leak detector, hydrocarbon fuel leak detector, soot number analyzer, flue gas analyzer, oxygen meter, hydrogen meter.
Hygrometer is a measuring device that is used to measure and control air humidity.
Rangefinder- a device that measures distance. The rangefinder also allows you to calculate the area and volume of an object.
Dosimeter– a device designed to detect and measure radioactive radiation.
RLC meter– a radio measuring device used to determine the total conductivity of an electrical circuit and impedance parameters. RLC in the name is an abbreviation of the circuit names of elements whose parameters can be measured by this device: R - Resistance, C - Capacitance, L - Inductance.
Power meter– a device that is used to measure the power of electromagnetic oscillations of generators, amplifiers, radio transmitters and other devices operating in the high-frequency, microwave and optical ranges. Types of meters: absorbed power meters and transmitted power meters.
Harmonic distortion meter– a device designed to measure the coefficient of nonlinear distortion (harmonic distortion) of signals in radio devices.
Calibrator– a special standard measure that is used for verification, calibration or calibration of measuring instruments.
Ohmmeter or resistance meter is an instrument used to measure resistance to electric current in ohms. Types of ohmmeters depending on sensitivity: megohmmeters, gigaohmmeters, teraohmmeters, milliohmmeters, microohmmeters.
Current clamps- an instrument that is designed to measure the amount of current flowing in a conductor. Current clamps allow you to take measurements without breaking the electrical circuit and without disrupting its operation.
Thickness gauge is a device with which you can, with high accuracy and without compromising the integrity of the coating, measure its thickness on a metal surface (for example, a layer of paint or varnish, a layer of rust, primer, or any other non-metallic coating applied to a metal surface).
Luxmeter is a device for measuring the degree of illumination in the visible region of the spectrum. Light meters are digital, highly sensitive instruments such as lux meter, brightness meter, pulse meter, UV radiometer.
Pressure gauge– a device that measures the pressure of liquids and gases. Types of pressure gauges: general technical, corrosion-resistant, pressure meters, electrical contact.
Multimeter is a portable voltmeter that performs several functions simultaneously. The multimeter is designed to measure DC and AC voltage, current, resistance, frequency, temperature, and also allows for continuity testing and diode testing.
Oscilloscope is a measuring device that allows you to observe, record, and measure the amplitude and time parameters of an electrical signal. Types of oscilloscopes: analog and digital, portable and desktop
Pyrometer is a device for non-contact measurement of the temperature of an object. The principle of operation of the pyrometer is based on measuring the power of thermal radiation of the measured object in the range of infrared radiation and visible light. The accuracy of temperature measurement at a distance depends on the optical resolution.
Tachometer is a device that allows you to measure the rotation speed and number of revolutions of rotating mechanisms. Types of tachometers: contact and non-contact.
Thermal imager is a device designed to observe heated objects by their own thermal radiation. A thermal imager allows you to convert infrared radiation into electrical signals, which then, in turn, after amplification and automatic processing, are converted into a visible image of objects.
Thermohygrometer is a measuring device that simultaneously performs the functions of measuring temperature and humidity.
Line defect detector is a universal measuring device that allows you to determine the location and direction of cable lines and metal pipelines on the ground, as well as determine the location and nature of their damage.
pH meter is a measuring device designed to measure the hydrogen index (pH indicator).
Frequency meter– a measuring device for determining the frequency of a periodic process or the frequencies of the harmonic components of the signal spectrum.
Sound level meter– a device for measuring sound vibrations.
Table: Units of measurement and designations of some physical quantities.
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During the operation of buildings, situations inevitably arise in which it is necessary to search for the locations of hidden wires and cables. These situations may include replacements, repairs to wiring faults, the need to refurbish or remodel the premises, or the need to install hanging furniture or equipment. A hidden wiring finder helps you quickly find wires without destroying walls. What is such a device, and what types of finders exist?
Hidden wiring
With a hidden installation method, detecting wiring under thick brick or concrete is not an easy task for a person who is encountering such a problem for the first time. Therefore, large volumes of search work are performed by qualified electricians.
However, anyone who is sufficiently versed in electricity can independently carry out searches and further repairs. A device for finding wires will help him. At its core, it is a detector or device for locating cables that cannot be detected visually. Using this device is not difficult at all, just read the operating instructions carefully.
Operating principle
The operation of devices for searching hidden electrical wiring is based on the following principles:
In the first case, the device will react to the metal structure of the conductor and signal the presence of metal in one of the ways provided for by the detector design (usually a light or sound alarm, but options with liquid crystal displays are possible).
The disadvantage of this type of device is the very low detection accuracy. The result of examining a reinforced concrete panel, for example, can be very distorted due to the fact that the device, along with wires, will also show the presence of reinforcement and mounting loops.
In the second case, a sensor built into the device will determine the presence of a conductor by the propagated magnetic field. The number of “false positives” will be minimal, but for positive search results the wiring must be energized. And some devices will be able to detect a magnetic field only if there is also a fairly high power load in the network.
But what if the wiring is damaged and no current flows through it, for example, when searching for a cable break? For this purpose, there are devices that have the properties of both types. With their help, it is easy to identify the wiring in the wall without the fear of bumping into a reinforcing bar instead.
Overview of detector models
Currently, the most common devices for searching for hidden wiring in walls are several devices from different manufacturers.
Woodpecker
E-121 or “Woodpecker” is an inexpensive device that can, with fairly high accuracy, determine not only the location of hidden wiring at a distance of up to 7 cm from the surface of the walls, but also find the location of a break due to mechanical damage to the wire. Using this tester, you can completely test the wiring in your apartment if an unknown and unexpected malfunction occurs. The country of manufacture of the device is Ukraine.
MS-258A
The MS-258A MEET tester is a budget device made in China. Determines the presence of metal in a structure according to the manufacturer at a distance of up to 18 cm; it also works by the presence of a magnetic field. The result is indicated in two ways - by turning on the control lamp and by sounding a sound signal. The design has a variable resistor that allows you to adjust the sensitivity of the device. The disadvantage of this model is the low result when it is necessary to detect a shielded or foil cable.
BOSCH DMF
The next BOSCH DMF 10 zoom detector is a high-quality device from a well-known brand. Determines, depending on the settings, the presence of metal, wood, plastic hidden in building structures. The device has a multifunctional liquid crystal display, which displays the setup process and displays the results.
Wall Scanner
Model Wall Scanner 80 is a device similar in properties to its predecessor in the review. Produced mainly in China by ADA enterprises. Depending on the settings, it can be used to find various materials in building structures. The device is quite compact and light in weight.
Microphone, radio receiver and thermal imager
In the absence of a device to detect hidden wiring, the search can be carried out in many different ways. In most cases, detectors are replaced with electrical devices for other purposes.
As a finder, you can successfully use a regular audio microphone connected to an amplifier with a loudspeaker (speaker). As the microphone approaches the intended location of the electrical wiring, it should produce an increasing background sound. And the closer the microphone is to the wiring, the stronger and louder the sound should be. Obviously, this search method works when there is voltage in the hidden wiring. The device will not detect de-energized wiring.
Instead of a microphone, you can use a portable radio with frequency control for searching. Having tuned it to a frequency of about 100 kHz, you need to use smooth movements along the wall to examine the location where the cables are supposed to be located. When the radio receiver approaches a conductor hidden in the wall, the device's speaker should emit an increasing crackling and hissing sound - a consequence of interference created by the electric current.
It is worth paying attention to the possibility of using a device such as a thermal imager to search for hidden wiring and the presence of faults. It will quickly and accurately show not only the presence and location of cables in the walls, but also the locations of breaks or short circuits. Its use is based on the property of a conductor to emit a certain amount of heat when passing an electric current.
De-energized conductors with a break will appear on the screen of a thermal imager as cold, and when shorted, on the contrary, they will glow very brightly.
Application of the scheme
In the case where none of the detectors are at hand, you can determine the location of hidden wiring absolutely without instruments. To do this, it is enough to know that according to established rules, wires and cables are laid strictly vertically or horizontally in the walls. Along ceilings, wires run in straight lines connecting lighting fixtures to distribution boxes or switches, parallel to the walls of the room and are located in ceiling voids or in pipes behind the suspended ceiling structure. All wire connections are made in junction boxes.
How does this knowledge help in your search? You can draw a diagram of existing hidden wiring or a section of it on the walls and ceilings, and then use this diagram in the future without having expensive devices. First you need to draw straight lines vertically upward from sockets and switches. Distribution boxes should be located on the wall, at a height of 150-250 mm from the ceiling.
You can determine their location by tapping the walls. Based on the changed sound, the boxes are marked and connected with straight lines, which will indicate the location of the cables. The connection of boxes and distribution board also occurs along straight vertical or horizontal lines. Of course, all these rules are valid for hidden wiring, and it is recommended to use them only when searching for fault locations due to the very low accuracy of determination. In the case of open wiring, obviously, you can do without the device and tapping.
How to find a cliff
To first, you need to determine the location where the break or short circuit supposedly occurred. The search algorithm is simple.
If there is no voltage in individual sockets or lamps within one group, there is a break in one of the sections of the wire. Here you need to cut off the non-working sockets with a mental line. A distribution box will immediately be detected, after which there is no current in the conductors. All that remains is to check the presence of voltage in this junction box using such a well-known device as an indicator screwdriver or a multimeter. If there is no voltage, you need to look for a break in the area preceding this node on the side of the switchboard.
If there is no voltage in the entire group, and the circuit breaker protecting it is triggered, then with a high degree of probability a short circuit has occurred in one of the electrical wiring sections. It can be diagnosed by measuring the resistance of each section, disconnecting it from the box and removing all the load from it.
To obtain an accurate result, each section must be tested. A short circuit is detected where the resistance is zero. You can use a regular tester for these purposes.
You can search for the location of the short circuit by sequentially disconnecting sections in the boxes, starting from the side of the furthest circuit from the distribution board. After disconnecting each individual section, it is necessary to check the functionality of the circuit by applying voltage until the circuit breaker stops switching off. This search method must be used with great care to protect yourself and other workers from electric shock.
It should be noted that the above methods of searching for hidden wiring become irrelevant if there is a technical passport, which reflects all the information on the location of electrical wiring in the room. If there is no technical certificate, it is strongly recommended that after discovering the wiring and replacing it, draw up a diagram in order to avoid labor-intensive work in the future.
What is the effect of a magnetic field on a current-carrying conductor?
A magnetic field acts with some force on any current-carrying conductor located in this field.
1. How to show that a magnetic field acts on a current-carrying conductor located in this field?
It is necessary to suspend the conductor on flexible wires connected to the current source.
When this conductor with current is placed between the poles of a permanent arc-shaped magnet, it will begin to move.
This proves that a magnetic field acts on a current-carrying conductor.
2. What determines the direction of movement of a conductor carrying current in a magnetic field?
The direction of movement of a conductor carrying current in a magnetic field depends on the direction of the current in the conductor and on the location of the magnet poles.
3. What device can be used to rotate a current-carrying conductor in a magnetic field?
The device, which can be used to rotate a current-carrying conductor in a magnetic field, consists of a rectangular frame mounted on a vertical axis.
A winding consisting of several dozen turns of wire coated with insulation is laid on the frame.
Since the current in the circuit is directed from the positive pole of the source to the negative, in opposite parts of the frame the current has the opposite direction.
Therefore, the magnetic field forces will also act on these sides of the frame in opposite directions.
As a result, the frame will begin to rotate.
4. What device in the frame is used to change the direction of the current every half turn?
The frame with the winding is connected to the electrical circuit through half rings and brushes, which allows you to change the direction of the current in the winding every half turn:
- one end of the winding is connected to one metal half-ring, the other - to the other;
- half rings rotate in place with the frame;
- each half-ring is pressed against a metal brush plate and slides along it when rotated;
- one brush is always connected to the positive pole of the source, and the other to the negative pole;
- when you turn the frame, the half rings will turn with it and each will press against another brush;
- as a result, the current in the frame will change direction to the opposite;
In this design, the frame rotates in one direction all the time.
5. How does a technical electric motor work?
The rotation of a coil with current in a magnetic field is used in the design of an electric motor.
In electric motors, the winding consists of a large number of turns of wire.
They are placed in slots on the side surface of the iron cylinder.
This cylinder is needed to enhance the magnetic field.
The cylinder with the winding is called the motor armature.
The magnetic field in which the armature of such a motor rotates is created by a strong electromagnet.
The electromagnet and the armature winding are powered by the same current source.
The motor shaft (the axis of the iron cylinder) transmits rotation to the payload.
We know that conductors carrying currents interact with each other with some force (§ 37). This is explained by the fact that each current-carrying conductor is affected by the magnetic field of the current of the other conductor.
At all a magnetic field acts with some force on any current-carrying conductor located in this field.
Figure 117, a shows a conductor AB suspended on flexible wires that are connected to a current source. The conductor AB is placed between the poles of an arc-shaped magnet, i.e. it is in a magnetic field. When the electrical circuit is closed, the conductor begins to move (Fig. 117, b).
Rice. 117. The effect of a magnetic field on a current-carrying conductor
The direction of movement of the conductor depends on the direction of the current in it and on the location of the poles of the magnet. In this case, the current is directed from A to B, and the conductor deviates to the left. When the direction of current is reversed, the conductor will move to the right. In the same way, the conductor will change the direction of movement when the location of the magnet poles changes.
The rotation of a current-carrying conductor in a magnetic field is of practical importance.
Figure 118 shows a device that can be used to demonstrate such a movement. In this device, a lightweight rectangular ABCD frame is mounted on a vertical axis. A winding consisting of several dozen turns of wire coated with insulation is laid on the frame. The ends of the winding are connected to metal half-rings 2: one end of the winding is connected to one half-ring, the other to the other.
Rice. 118. Rotation of a frame with current in a magnetic field
Each half-ring is pressed against a metal plate - brush 1. The brushes serve to supply current from the source to the frame. One brush is always connected to the positive pole of the source, and the other to the negative pole.
We know that the current in the circuit is directed from the positive pole of the source to the negative, therefore, in parts of the frame AB and DC it has the opposite direction, so these parts of the conductor will move in opposite directions and the frame will rotate. When the frame is rotated, the half rings attached to its ends will turn with it and each will press against the other brush, so the current in the frame will change direction to the opposite. This is necessary so that the frame continues to rotate in the same direction.
Rotation of a coil with current in a magnetic field is used in the device electric motor.
In technical electric motors, the winding consists of a large number of turns of wire. These turns are placed in grooves (slots) made along the side surface of the iron cylinder. This cylinder is needed to enhance the magnetic field. Figure 119 shows a diagram of such a device, it is called engine anchor. In the diagram (it is shown in a perpendicular section), the turns of the wire are shown in circles.
Rice. 119. Engine armature diagram
The magnetic field in which the armature of such a motor rotates is created by a strong electromagnet. The electromagnet is supplied with current from the same current source as the armature winding. The motor shaft, running along the central axis of the iron cylinder, is connected to a device that is driven by the motor to rotate.
DC motors have found particularly wide application in transport (electric locomotives, trams, trolleybuses).
There are special non-sparking electric motors that are used in pumps for pumping oil out of wells.
In industry, AC motors are used (you will study these in high school).
Electric motors have a number of advantages. With the same power, they are smaller than heat engines. During operation, they do not emit gases, smoke or steam, which means they do not pollute the air. They do not need a supply of fuel and water. Electric motors can be installed in a convenient place: on a machine, under the floor of a tram, on the bogie of an electric locomotive. It is possible to produce an electric motor of any power: from a few watts (in electric shavers) to hundreds and thousands of kilowatts (in excavators, rolling mills, ships).
The efficiency of powerful electric motors reaches 98%. No other engine has such high efficiency.
Jacobi Boris Semyonovich (1801-1874)
Russian physicist. He became famous for the discovery of electroplating. He built the first electric motor and a telegraph machine that printed letters.
One of the world's first electric motors suitable for practical use was invented by the Russian scientist Boris Semenovich Jacobi in 1834.
Questions
- How to show that a magnetic field acts on a current-carrying conductor located in this field?
- Using Figure 117, explain what determines the direction of movement of a conductor carrying current in a magnetic field.
- What device can be used to rotate a current-carrying conductor in a magnetic field? What device in the frame is used to change the direction of the current every half turn?
- Describe the structure of a technical electric motor.
- Where are electric motors used? What are their advantages over thermal ones?
- Who and when invented the first electric motor suitable for practical use?
Exercise
