The start of any engine is accompanied by certain switchings in the power circuit and control circuit. In this case, relay-contactor and contactless devices are used. For engines direct current for the purpose of limitation, starting resistors are switched on in the circuit of the rotors and armatures of the motors, which are switched off when the motors accelerate in stages. When the start is complete, the starting resistors are completely bypassed.
The engine braking process can also be automated. After the braking command, the necessary switching is carried out in the power circuits using relay contactor equipment. When approaching a speed close to zero, the engine is disconnected from the network. During the start-up process, the stages are switched off at certain time intervals or depending on other parameters. This changes the current and speed of the motor.
Motor starting is controlled as a function of EMF (or speed), current, time and path.
Typical components and circuits for automatic control of the start of DC motors
Starting a DC motor of parallel or independent excitation is carried out with a resistor inserted into the armature circuit. A resistor is needed to limit the inrush current. As the engine accelerates, the starting resistor is removed in stages. When the start is complete, the resistor will be completely bypassed, and the engine will operate at its natural mechanical characteristic (Fig. 1). When starting, the engine accelerates according to artificial characteristic 1, then 2, and after shunting the resistor - according to natural characteristic 3.
Rice. 1. Mechanical and electromechanical characteristics DC motor of parallel excitation (ω - angular speed of rotation; I1 M1 - peak current and engine torque; I2 M2 - current and switching torque)
Let's consider the node of the DC motor starting circuit (DCM) in the EMF function (Fig. 2).
Rice. 2. Unit of the parallel excitation DC motor starting circuit in the EMF function
Control in the EMF (or speed) function is carried out by relays, voltages and contactors. The voltage relays are configured to operate at different values of the armature emf. When the KM1 contactor is turned on, the voltage on the KV relay at the moment of starting is not enough to operate. As the engine accelerates (due to an increase in the motor EMF), relay KV1 is activated, then KV2 (the relay operating voltages are corresponding values); they include acceleration contactors KM2, KMZ, and the resistors in the armature circuit are shunted (the circuits for switching on the contactors are not shown in the diagram; LM is the excitation winding).
Let's consider the circuit for starting a DC motor as a function of EMF (Fig. 3). The angular velocity of the engine is often fixed indirectly, i.e. measuring quantities related to speed. Fordc motor This quantity is the emf. Starting is carried out as follows. Turns on circuit breaker QF, the motor field winding is connected to the power source. The KA relay is activated and closes its contact.
The remaining devices of the circuit remain in their original position. To start the engine, SB1 “Start” is required, after which the contactor KM1 is activated and connects the engine to the power source. Contactor KM1 becomes self-powered. The DC motor is accelerated by the motor armature circuit resistor R.
As the engine speed increases, its EMF and the voltage on the relay coils KV1 and KV2 increase. At speed ω1 (see Fig. 1.), relay KV1 is activated. It closes its contact in the circuit of the KM2 contactor, which operates and short-circuits the first stage of the starting resistor with its contact. At speed ω2, relay KV2 is activated. With its contact it closes the power supply circuit of the KMZ contactor, which, when activated, short-circuits the second starting stage of the starting resistor with the contact. The engine reaches its natural mechanical characteristics and finishes its takeoff run.
Rice. 3. Circuit for starting a parallel excitation DC motor as a function of EMF
For proper operation circuit, it is necessary to configure the voltage relay KV1 to operate at an EMF corresponding to the speed ω1, and the relay KV2 to operate at a speed ω2.
To stop the engine, press the SB2 “Stop” button. To de-energize the circuit, you must turn off the QF circuit breaker.
Control in the current function is carried out using a current relay. Let's consider the starting circuit nodedc motor as a function of current. In the diagram shown in Fig. 4, overcurrent relays are used, which operate at the starting current I1 and fall off at the minimum current I2 (see Fig. 1). The intrinsic response time of the current relays must be less than the intrinsic response time of the contactor.
Rice. 4. Parallel excitation DPT starting circuit unit as a function of current
Engine acceleration begins with the resistor fully inserted into the armature circuit. As the engine accelerates, the current decreases; at current I2, relay KA1 disappears and with its contact closes the power circuit of contactor KM2, which with its contact bypasses the first starting stage of the resistor. The second starting stage of the resistor (relay KA2, contactor KMZ) is short-circuited in the same way. The contactor power circuits are not shown in the diagram. After the engine starts, the resistor in the armature circuit will be bypassed.
Let's consider the starting circuitdc motor as a function of current (Fig. 5). The resistances of the resistor stages are selected in such a way that at the moment the engine is turned on and the stages are shunted, the current I1 in the armature circuit and the torque M1 do not exceed the permissible level.
This is done by turning on the QF circuit breaker and pressing the SB1 “Start” button. In this case, the contactor KM1 is activated and closes its contacts. The starting current I1 passes through the engine power circuit, under the influence of which the maximum current relay KA1 is activated. Its contact opens and the KM2 contactor does not receive power.
Rice. 5. Starting circuit for parallel-excitation DC motors as a function of current
When the current decreases to the minimum value I2, the maximum current relay KA1 drops out and closes its contact. Contactor KM2 is activated and with its main contact bypasses the first section of the starting resistor and relay KA1. When switching, the current increases to the value I1.
When the current increases again to the value of I1, the contactor KM1 does not turn on, since its coil is shunted by the contact KM2. Under the influence of current I1, relay KA2 is activated and opens its contact. When, during the acceleration process, the current again decreases to the value I2, relay KA2 drops out and the KMZ contactor turns on. The start ends and the engine operates at its natural mechanical characteristic.
For proper operation of the circuit, it is necessary that the response time of relays KA1 and KA2 be less than the response time of the contactors. To stop the engine, you must press the SB2 “Stop” button and turn off the QF circuit breaker to de-energize the circuit.
Control in the time function is carried out using time relays and corresponding contactors, which short-circuit the resistor stages with their contacts.
Let's consider the starting circuit nodedc motor as a function of time (Fig. 6). The KT time relay operates immediately when voltage appears in the control circuit through the KM1 break contact. After the KM1 contact opens, the KT time relay loses power and closes its contact with a time delay. Contactor KM2, after a period of time equal to the delay time of the time relay, receives power, closes its contact and bypasses the resistance in the armature circuit.
Rice. 6. Parallel excitation DC motor starting circuit as a function of time
The advantages of control as a function of time include ease of control, stability of the acceleration and deceleration process, and no delay of the electric drive at intermediate speeds.
Let's consider the starting circuitdc motor parallel excitation as a function of time. In Fig. 7 shows a diagram of non-reversible startingdc motor parallel excitation. Starting occurs in two stages. The circuit uses buttons SB1 “Start” and SB2 “Stop”, contactors KM1...KMZ, electromagnetic time relays KT1, KT2. The QF circuit breaker turns on. In this case, the KT1 time relay coil receives power and opens its contact in the KM2 contactor circuit. The engine is started by pressing the SB1 “Start” button. Contactor KM1 receives power and, with its main contact, connects the motor to a power source with a resistor in the armature circuit.
Rice. 7. Scheme of irreversible start-up of a DC motor as a function of time
The minimum current relay KA serves to protect the engine from open circuit excitation. During normal operation, the KA relay is activated and its contact in the KM1 contactor circuit closes, preparing the KM1 contactor for operation. When the excitation circuit breaks, the KA relay is de-energized, opens its contact, then the KM1 contactor is de-energized and the engine stops. When the KM1 contactor is triggered, its blocking contact closes and the KM1 contact opens in the KT1 relay circuit, which is de-energized and closes its contact with a time delay.
After a period of time equal to the time delay of relay KT1, the power circuit of the acceleration contactor KM2 is closed, which is activated and, with its main contact, short-circuits one stage of the starting resistor. At the same time, time relay KT2 receives power. The engine accelerates. After a period of time equal to the time delay of relay KT2, contact KT2 closes, the KMZ acceleration contactor is activated and, with its main contact, short-circuits the second stage of the starting resistor in the armature circuit. The start ends and the engine returns to its natural mechanical characteristic.
Typical components of DC motor braking control circuits
Automatic DC motor control systems use dynamic braking, counter-brake braking and regenerative braking.
During dynamic braking, it is necessary to short-circuit the motor armature winding to an additional resistance, and leave the field winding energized. Such braking can be carried out as a function of speed and as a function of time.
Control as a function of speed (EMF) during dynamic braking can be performed according to the scheme shown in Fig. 8. When the KM1 contactor is disconnected, the motor armature is disconnected from the network, but there is voltage at its terminals at the moment of disconnection. The KV voltage relay operates and closes its contact in the circuit of the KM2 contactor, which with its contact closes the motor armature to resistor R.
At speeds close to zero, the KV relay loses power. Further braking from minimum speed to a complete stop occurs under the influence of a static moment of resistance. To increase braking efficiency, you can apply two or three stages of braking.
Rice. 8. Unit of the automatic control circuit for dynamic braking in the EMF function: a - power circuit; b - control circuit
Dynamic brakingdc motor independent excitation as a function of time is carried out according to the scheme shown in Fig. 9.
Rice. 9. Dynamic braking circuit unit of independent excitation DDC as a function of time
When the engine is running, the KT time relay is turned on, but the KM2 braking contactor circuit is open. To brake, you must press the SB2 “Stop” button. Contactor KM1 and time relay KT lose power; contactor KM2 is triggered, since contact KM1 in the circuit of contactor KM2 closes, and the contact of the time relay KT opens with a time delay.
During the delay time of the time relay, contactor KM2 receives power, closes its contact and connects the motor armature to the additional resistor R. Dynamic braking of the motor is carried out. At the end of it, the KT relay, after a time delay, opens its contact and disconnects the KM2 contactor from the network. Further braking to a complete stop is carried out under the influence of the moment of resistance Mc.
When braking by back switching, the EMF of the motor and the mains voltage act in accordance. To limit the current, a resistor is introduced into the power circuit.
Excitation control of DC motors
The motor field winding has significant inductance, and if the motor is quickly turned off, a large voltage may arise on it, which will lead to breakdown of the winding insulation. To prevent this, you can use the circuit nodes shown in Fig. 10. The quenching resistance is connected in parallel to the excitation winding through a diode (Fig. 10,b). Consequently, after switching off, the current passes through the resistance for a short time (Fig. 10, a).
Rice. 10. Units of circuits for connecting quenching resistances: a - quenching resistance is connected in parallel; b - the quenching resistance is switched on through a diode.
Protection against open excitation circuit is carried out using a minimum current relay according to the diagram shown in Fig. eleven.
Rice. 11. Protection against open excitation circuit: a - power excitation circuit; b- control circuit
If the field winding breaks, the KA relay loses power and turns off the KM contactor circuit.
An electric motor is a machine that converts electrical energy into mechanical energy. The first electric motors appeared in the mid-19th century. Success in their development is associated with the names of such outstanding physicists and engineers as N. Tesla, B. Jacobi, G. Ferraris, V. Siemens.
There are electric motors of direct and alternating current. The advantage of the former is the possibility of economical and smooth regulation of the shaft speed. The advantage of the latter is the high power density per unit weight. In microcontroller practice, low-voltage DC motors are often used, used in household and computer fans (Table 2.13). There are also designs with network motors.
Table 2.13. Parameters of Sunon fans
The motor winding should be considered as a coil with high inductance, so it can be switched with conventional transistor switches (Fig. 2.78, a...t). The main thing is not to forget about protection against self-induction EMF.
In DC motors, it is possible to change the direction of rotation of the rotor depending on the polarity of the operating voltage. In such cases, “H-bridge” bridge circuits are widely used (Fig. 2.79, a...i).
(Start):
a) regulation of the air flow speed of fan M1. Capacitor C/ reduces RF interference. Diode VD1 protects transistor VT1 from voltage surges. Resistor R1 determines the degree of saturation of transistor G77, and resistor R2 closes it when MK is restarted. The PWM pulse frequency at the MK output must be at least 30 kHz, i.e. outside the audio range to eliminate unpleasant “whistle”. Elements C/ and R2 may be absent;
b) smooth control of the rotation speed of the motor shaft M1 through the PWM channel. Capacitor C/ is the primary, and capacitor C2 is the secondary filter of PWM signals; ABOUT
Rice. 2.78. Connection diagrams for electric motors via transistor switches
(continuation):
c) transistors VT1, VT2 are connected in parallel to increase the total collector current. Resistors R1, R2 provide a uniform power load on both transistors, which is due to the spread of their coefficients I2]E and the current-voltage characteristics of the base-emitter junctions;
d) the M1 engine (Airtronics) has a “digital” control input, which allows you to connect the MK directly to it. Transistor switches (drivers) are located inside the engine;
e) two separate power supplies can significantly reduce the impact on MK of electrical noise generated by the M1 motor. The system will work more stable. GB1 is a low-power lithium battery, GB2, GB3 are finger-type galvanic cells with a total voltage of 3.2 V and sufficient power to start and operate the M1 motor\
f) parallel resistors R2, R3 serve as limiters of the current flowing through the motor M1. In addition, they stabilize the current in the load if transistor VT1 is in active mode or on the verge of entering saturation mode;
g) MK turns on/off motor M1. Resistor R3 adjusts the speed of its shaft. The stabilizer is a “tape recorder” chip DA1 from Panasonic. With its help, constant parameters are maintained at the M1 motor terminals, which are practically independent of fluctuations in temperature and supply voltage;
h) chokes L7, L2 and capacitors C7, C2 filter radio interference emitted by the engine. For the same purpose, the motor is placed in a grounded shielded housing;
Rice. 2.78. Connection diagrams for electric motors via transistor switches
(continuation):
i) vibration motor M1 is a source of powerful electromagnetic and radio frequency interference. Elements L/, L2, C1 serve as filters. Resistor R2 limits the starting current through two open transistors VT1. Diodes VD1, UA2 cut off the peaks of pulse noise;
j) elements VD1, C1 and VD2, &2 filter the power supply noise generated by the M1 motor in the direction of MK. The speed of the motor shaft can be smoothly adjusted through the PWM channel MK, while a separate low-pass filter is not required, since the motor has a large inertia and itself smoothes out the HF current pulses passing through it;
l) the use of a switch on a field-effect transistor VT1 increases efficiency compared to a switch on bipolar transistor, due to the lower drain-source resistance. Resistor R1 limits the amplitude of interference that can “leak” from the running motor M1 into the internal circuits of MK through the gate-drain capacitance of transistor VT1;
l) transistor VT2 is a powerful power switch that supplies power to the ML motor, and transistor VT1 is a damper that quickly slows down the rotation of the shaft after turning off. Resistor R1 reduces the load on the MK output when charging the gate capacitances field effect transistors VT1, VT2. Resistor R2 turns off motor M1 when MK restarts;
m) the switch on transistors VT1, VT2 is assembled according to the Darlington circuit and has a high gain. To regulate the rotation speed of the motor shaft M1, the PWM method or pulse-phase control can be used. The system does not have feedback, therefore, when the rotation speed decreases due to external braking, the operating power on the shaft will decrease;
Rice. 2.78. Connection diagrams for electric motors via transistor switches
(continuation):
m) embedding MK into the already existing path for controlling the speed of rotation of the motor shaft Ml. This path includes all circuit elements except resistor R2. Resistor R4 sets the “rough” rotation speed. Fine adjustment is carried out by pulses from the MK output. It is possible to organize feedback when the MK monitors any parameter and dynamically adjusts the rotation speed depending on the supply voltage or temperature;
o) the rotation speed of the motor shaft M1 is determined by the duty cycle of pulses in the PWM channel generated from the lower output of MK. The main switching switch is transistor VT2.2, the remaining transistor switches are involved in quickly stopping the engine M1 by a HIGH level signal from the upper output of MK;
n) smooth regulation of the speed of the motor shaft M1 is carried out by resistor R8. The op-amp TS serves as a voltage stabilizer with double feedback through elements R1, R8, C2 and R9, R10, C1. By using a combination of levels from the three MK outputs (DAC), you can stepwise change the rotation speed of the motor shaft M1 (precise selection with resistors R2…R4). MK lines can be switched to input mode without a pull-up resistor to increase the number of DAC “steps”;
Rice. 2.78. Schemes for connecting electric motors via transistor switches (end):
p) phase-pulse control of AC motor M1. The longer the period of mains voltage transistor VT1 is open, the faster the motor shaft rotates;
c) the powerful alternating current motor Ml is switched on through an optothyristor KS7, which provides galvanic isolation from the MK circuits;
t) similar to Fig. 2.78, p, but with one feedback ring through elements C7, R6, R8. Resistor R4 regulates the speed of the motor shaft Ml smoothly, and MK - discretely.
Rice. 2.79. Bridge circuits for connecting electric motors to MK (beginning):
a) the direction of rotation of the motor shaft Ml is changed by a bridge “mechanical” circuit on two groups of relay contacts KL1, K1.2. The switching frequency of the relay contacts should be low so that the resource does not quickly run out. Chokes L7, L2 reduce switching currents when switching relays and, accordingly, the level of radiated electromagnetic interference;
b) at HIGH level at the upper and LOW levels at the lower output of the MK, transistors K77...to TZ open, and transistors KG4...KG6 close, and vice versa. When the polarity of the motor supply Ml is reversed, its rotor rotates in the opposite direction. The signals from the two outputs of the MK should be antiphase, but with a short pause LOW level between pulses to close both arms (elimination of through currents). Diodes VD1..VD4 reduce voltage surges, thereby protecting transistors from breakdown;
c) similar to Fig. 2.79, b, but with different element ratings, as well as with hardware protection against simultaneous opening of transistors of one arm using diodes VD3, VD4. Diodes VD1, KD2 increase noise immunity when long distance to MK. Capacitor C/ reduces the “spark” pulsed radio interference generated by the engine Ml;
Rice. 2.79. Bridge circuits for connecting electric motors to MK (continued):
d) similar to Fig. 2.79, b, but with the absence of “blocking” resistors in the base circuits of transistors VT2, VT4. It is calculated that the motor winding L//is quite low-resistance, therefore, when restarting the MK, external noise on the “hanging in the air” bases of transistors VT1 VT2, VT4, VT6 will not be able to open their collector junctions;
e) similar to Fig. 2.79, b, but with maximum simplification of the diagram. Recommended for devices performing secondary functions. The supply voltage is +E and must correspond to the operating voltage of the motor M1\
f) unlike previous circuits, transistors VT1...VT4 are connected according to a common emitter circuit and are controlled by HIGH/LOW levels directly from the MK outputs. Motor M1 must be designed for an operating voltage of 3...3.5 V. Diodes VD1...VD4 reduce voltage surges. The LL C1 filter reduces impulse noise in the power supply from the M1 motor, which can lead to malfunctions of the MK. Replacement parts found: VT1 VT3- KT972; VT2, VT4- KT973; VD1…VD4- KD522B, R x = 3.3 kOhm; R 2 = 3.3 kOhm;
g) bridge circuit with four control transistors VT1 VT2, VT4, VT5 p-p-p structures. Trimmer resistor R4 regulates the voltage on the motor Ml, and therefore the speed for two directions of rotor rotation at once;
Rice. 2.79. Bridge circuits for connecting electric motors to MK (end):
h) bridge circuit for controlling a powerful motor Ml (24 V, 30 A). Changing the polarity of the voltage on the motor is carried out by antiphase levels at the middle outputs of the MK, and the rotation speed is carried out by the PWM method at the upper and lower outputs of the MK;
i) transistors VT2, VT5 supply power to the bridge motor control circuit Ml. Paralleling them allows you to connect another similar circuit to the VD1 diode.
DC motors are not used as often as AC motors. Below are their advantages and disadvantages.
In everyday life, DC motors are used in children's toys, since they are powered by batteries. They are used in transport: in the subway, trams and trolleybuses, and cars. In industrial enterprises, DC electric motors are used to drive units that use batteries for uninterrupted power supply.
DC Motor Design and Maintenance
The main winding of a DC motor is anchor, connected to the power source via brush apparatus. The armature rotates in the magnetic field created by stator poles (field windings). The end parts of the stator are covered with shields with bearings in which the motor armature shaft rotates. On one side, mounted on the same shaft fan cooling, driving a flow of air through the internal cavities of the engine during operation.
The brush apparatus is a vulnerable element in the engine design. The brushes are ground to the commutator in order to repeat its shape as accurately as possible, and are pressed against it with constant force. During operation, the brushes wear out, conductive dust from them settles on the stationary parts, and must be removed periodically. The brushes themselves must sometimes be moved in the grooves, otherwise they get stuck in them under the influence of the same dust and “hang” above the commutator. The characteristics of the motor also depend on the position of the brushes in space in the plane of rotation of the armature.
Over time, brushes wear out and need to be replaced. The commutator at the points of contact with the brushes also wears out. Periodically, the armature is dismantled and the commutator is turned on a lathe. After grinding, the insulation between the commutator lamellas is cut to a certain depth, since it is stronger than the commutator material and will destroy the brushes during further processing.
DC motor connection circuits
Presence of field windings – distinctive feature DC machines. The electrical and mechanical properties of the electric motor depend on the way they are connected to the network.
Independent excitation
The excitation winding is connected to an independent source. The characteristics of the motor are the same as those of a permanent magnet motor. The rotation speed is controlled by the resistance in the armature circuit. It is also regulated by a rheostat (adjusting resistance) in the excitation winding circuit, but if its value decreases excessively or if it breaks, the armature current increases to dangerous values. Motors with independent excitation cannot be started at idle speed or with a low load on the shaft. The rotation speed will increase sharply and the motor will be damaged.
The remaining circuits are called self-excited circuits.
Parallel excitation
The rotor and excitation windings are connected in parallel to one power source. With this connection, the current through the excitation winding is several times less than through the rotor. The characteristics of electric motors are rigid, allowing them to be used to drive machines and fans.
Regulation of the rotation speed is ensured by the inclusion of rheostats in the rotor circuit or in series with the excitation winding.
Sequential excitation
The field winding is connected in series with the armature winding, and the same current flows through them. The speed of such an engine depends on its load; it cannot be turned on at idle. But it has good starting characteristics, so a series excitation circuit is used in electrified vehicles.
Mixed excitement
With this scheme, two excitation windings are used, located in pairs on each of the poles of the electric motor. They can be connected so that their flows are either added or subtracted. As a result, the motor can have characteristics similar to a series or parallel excitation circuit.
DC motors are rarely found in households. But they are always present in all children's toys powered by batteries that walk, run, drive, fly, etc. Direct current motors (DC motors) are installed in cars: in fans and various drives. They are almost always used in electric vehicles and less frequently in manufacturing.
Advantages of DPT compared to asynchronous motors:
- Well adjustable.
- Excellent starting properties.
- Rotation speeds can be more than 3000 rpm.
Disadvantages of DBT:
- Low reliability.
- Difficulty of manufacturing.
- High price.
- High maintenance and repair costs.
Operating principle of a DC motor
The motor design is similar to synchronous AC motors. I won’t repeat myself, if you don’t know, then look in this one of ours.
Any modern electric motor works based on Faraday's law of magnetic induction and the "Left Hand Rule". If an electric current is connected to the lower part of the armature winding in one direction, and to the upper part in the opposite direction, it will begin to rotate. According to the left-hand rule, conductors laid in the armature slots will be pushed out by the magnetic field of the windings of the DPT housing or stator.
The lower part will push to the right, and the top one to the left, so the anchor will begin to rotate until the parts of the anchor change places. To create continuous rotation, it is necessary to constantly reverse the polarity of the armature winding. This is what the commutator does, which, when rotating, switches the armature windings. Voltage from the current source is supplied to the collector using a pair of pressing graphite brushes.
Schematic diagrams of a DC motor
If AC motors are quite simple connect, then with DPT everything is more complicated. You need to know the brand of the motor, and then find out about its connection circuit on the Internet.
More often for medium and powerful engines DC there are separate terminals in the terminal box from the armature and from the field winding (OB). As a rule, the full power supply voltage is supplied to the armature, and the current is regulated by a rheostat or alternating voltage to the excitation winding. The speed of the DC motor will depend on the magnitude of the OB current. The higher it is, the faster speed rotation.
Depending on how the armature and OB are connected, electric motors come with independent excitation from a separate current source and with self-excitation, which can be parallel, series and mixed.
Used in production motors with independent excitation, which is connected to a power source separate from the armature. 
There is no electrical connection between the field and armature windings.
Connection diagram with parallel excitation in essence it is similar to a circuit with independent excitation of the OB. The only difference is that there is no need to use a separate power source. 
Motors, when switched on according to both of these schemes, have the same rigid characteristics, therefore they are used in machine tools, fans, etc.
Series-wound motors used when high starting current and soft characteristic are required. They are used in trams, trolleybuses and electric locomotives. According to this scheme, the field and armature windings are connected to each other in series. 
When voltage is applied, the currents in both windings will be the same. The main disadvantage is that when the load on the shaft decreases to less than 25% of the nominal value, there is a sharp increase in the rotation speed, reaching values dangerous for the DPT. Therefore, for trouble-free operation, a constant load on the shaft is necessary.
Sometimes used DBT with mixed arousal, in which one OB winding is connected in series to the armature circuit, and the other in parallel. 
Rarely occurs in life.
Reversing DC Motors
To change the direction of rotation DPT with series excitation requires changing the direction of the current in the OB or armature winding. In practice, this is done by changing the polarity: we swap the plus and minus positions. If you change the polarity in the excitation and armature circuits at the same time, then the direction of rotation will not change. The reverse is done in a similar way for motors running on alternating current.
Reversing DPT with parallel or mixed excitation it is better to change direction electric current in the armature winding. When the excitation winding breaks, the EMF reaches dangerous values and a breakdown of the wire insulation is possible.
Regulating the speed of DC motors
DPT with sequential excitation The easiest way to regulate is by variable resistance in the armature circuit. It can only be adjusted to reduce the speed in a ratio of 2:1 or 3:1. In this case, large losses occur in the control rheostat (R reg). This method used in cranes and electric trolleys that have frequent interruptions in operation. 
In other cases, the speed is adjusted upward from the nominal value using a rheostat in the field winding circuit, as shown in the right figure.
DPT with parallel excitation You can also regulate the speed of revolutions downwards using resistance in the armature circuit, but not more than 50 percent of the nominal value. Again, the resistance will heat up due to losses of electrical energy in it.
Increase the speed by a maximum of 4 times allows a rheostat in the OB circuit. The simplest and most common method of adjusting the rotation speed.
In practice, in modern electric motors these control methods are rarely used due to their shortcomings and limited control range. Various are used electronic circuits management.
Similar materials.
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