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机械外文翻译外文文献英文文献直流电动机调速控制
Speed Control of DC Motor
AbstractConditioning system is characterized in that output power to maintain
stability. Different speed control system can use a different brake system, high starting
and braking torque, quick response and quick adjustment range of degree requirements
of DC drive system, the use of the electric braking mode. Depends on the speed control of
DC motor armature voltage and flux. To zero speed, or U = 0 or Φ = ∞. The latter is
impossible, it only changes through the armature voltage to reduce speed. To speed to a
higher value can increase or decrease the U Φ.
KeywordDC SpeedFeedbackBrake
Regulator Systems
A regulator system is one which normally provides output power in its steady-state
operation.
For example, a motor speed regulator maintains the motor speed at a constant value
despite variations in load torque. Even if the load torque is removed, the motor must
provide sufficient torque to overcome the viscous friction effect of the bearings. Other
forms of regulator also provide output power; A temperature regulator must maintain the
temperature of, say, an oven constant despite the heat loss in the oven. A voltage
regulator must also maintain the output voltage constant despite variation in the load
current. For any system to provide an output, e.g., speed, temperature, voltage, etc., an
error signal must exist under steady-state conditions.
Electrical Braking
In many speed control systems, e.g., rolling mills, mine winders, etc., the load has to
be frequently brought to a standstill and reversed. The rate at which the speed reduces
following a reduced speed demand is dependent on the stored energy and the braking
system used. A small speed control system (sometimes known as a velodyne) can employ
mechanical braking, but this is not feasible with large speed controllers since it is difficult
and costly to remove the heat generated.
The various methods of electrical braking available are:
(1) Regenerative braking.
(2) Eddy current braking.
(3) Dynamic braking.
(4) Reverse current braking(plugging)
Regenerative braking is the best method, though not necessarily the most economic.
The stored energy in the load is converted into electrical energy by the work motor
(acting temporarily as a generator) and is returned to the power supply system. The
supply system thus acts as a”sink”into which the unwanted energy is delivered. Providing
the supply system has adequate capacity, the consequent rise in terminal voltage will be
small during the short periods of regeneration. In the Ward-Leonard method of speed
control of DC motors, regenerative braking is inherent, but thyristor drives have to be
arranged to invert to regenerate. Induction motor drives can regenerate if the rotor shaft
is driven faster than speed of the rotating field. The advent of low-cost variable-frequency
supplies from thyristor inverters have brought about considerable changes in the use of
induction motors in variable speed drives.
Eddy current braking can be applied to any machine, simply by mounting a copper
or aluminum disc on the shaft and rotating it in a magnetic field. The problem of
removing the heat generated is severe in large system as the temperature of the shaft,
bearings, and motor will be raised if prolonged braking is applied.
In dynamic braking, the stored energy is dissipated in a resistor in the circuit. When
applied to small DC machines, the armature supply is disconnected and a resistor is
connected across the armature (usually by a relay, contactor, or thyristor).The field
voltage is maintained, and braking is applied down to the lowest speed. Induction motors
require a somewhat more complex arrangement, the stator windings being disconnected
from the AC supply and reconnected to a DC supply. The electrical energy generated is
then dissipated in the rotor circuit. Dynamic braking is applied to many large AC hoist
systems where the braking duty is both severe and prolonged.
DC Motor Speed Control
The basis of all methods of DC motor speed control is derived from the equations:
E ∝ Φω
U = E + I R
a
a
the terms having their usual meanings. If the IaRa drop is small, the equations
approximate toU ∝ Φω or ω = U Φ 。
Thus, control of armature voltage and field
flux influences the motor speed. To reduce the speed to zero, either U=0 orΦ=∞.The
latter is inadmissible; hence control at low speed is by armature voltage variation. To
increase the speed to a high value, eith er U is made very large or Φis reduced. The latter
is the most practical way and is known as field weakening. Combinations of the two are
used where a wide range of speed is required.
A Single-Quadrant Speed Control System Using Thyristors
A single-quadrant thyristor converter system is shown in Fig.1.For the moment the
reader should ignore the rectifier BR2 and its associated circuitry (including resistor R in
the AC circuit), since this is needed only as a protective feature and is described in next
section.
Fig.1 Thyristor speed control system with current limitation on the AC side
Since the circuit is a single-quadrant converter, the speed of the motor shaft (which
is the output from the system) can be controlled in one direction of rotation only.
Moreover, regenerative braking cannot be applied to the motor; in this type of system, the
motor armature can suddenly be brought to rest by dynamic braking (i.e. when the
thyristor gate pulses are phased back to 180o, a resister can be connected across the
armature by a relay or some other means).
Rectifier BR1 provides a constant voltage across the shunt field winding, giving a
constant field flux. The armature current is controlled by a thyristor which is, in turn,
controlled by the pulses applied to its gate. The armature speed increases as the pulses
are phased forward (which reduces the delay angle of firing), and the armature speed
reduces as the gate pulses are phased back.
The speed reference signal is derived from a manually operated potentiometer
(shown at the right-hand side of Fig.23.1), and the feedback signal or output speed signal
is derived from the resistor chain R1 R2, which is connected across the armature. (Strictly
speaking, the feedback signal in the system in Fig.23.1 is proportional to the armature
voltage, which is proportional to the shaft speed only if the armature resistance drop,
IaRa, is small. Methods used to compensate for the IaRa drop are discussed in Reading
Material.)Since the armature voltage is obtained from a thyristor, the voltage consists of a
series of pulses; these pulses are smoothed by capacitor C. The speed reference signal is
of the opposite polarity to the armature voltage signal to ensure that overall negative
feedback is applied.
A feature of DC motor drives is that the load presented to the supply is a mixture of
resistance, inductance, and back EMF Diode D in Fig.1 ensures that the thyristor current
commutates to zero when its anode potential falls below the potential of the upper
armature connection, in the manner outlined before. In the drive shown, the potential of
the thyristor cathode is equal to the back EMF of the motor while it is in a blocking state.
Conduction can only take place during the time interval when the instantaneous supply
voltage is greater than the back EMF.Inspection of Fig.2 shows that when the motor is
running, the peak inverse voltage applied to the thyristor is mush greater than the peak
forward voltage. By connecting a diode in series with the thyristor, as shown, the reverse
blocking capability of the circuit is increased to allow low-voltage thyristor to be used.
References:
Fig.2 Illustrating the effect of motor back EMF on the
Peak inverse voltage applied to the thyristor
Fig.3 Armature voltage waveforms
The waveforms shown in Fig.2 are idealized waveforms as much as they ignore the
effectsofarmatureinductance,commutatorripple,etc.Typicalarmaturevoltage
waveforms are shown in Fig.3.In this waveform the thyristor is triggered at point A, and
conduction continues to point B when the supply voltage falls below the armature back
EMF.The effect of armature inductance is to force the thyristor to continue to conduct
until point C,when the fly-wheel diode prevents the armature voltage from reversing.
When the inductive energy has dissipated (point D), the armature current is zero and the
voltage returns to its normal level, the transients having settled out by point E.The
undulations on the waveform between E and F are due to commentator ripple.
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