A scalar is a quantity that has only magnitude , such as mass or temperature. A vector is a quantity that has both magnitude and direction , such as acceleration or force. Scalar methods for VFD control work by optimizing the motor flux and keeping the strength of the magnetic field constant, which ensures constant torque production.
Control tuning is not required but can improve system performance. Vector control — also referred to as field oriented control FOC — controls the speed or torque of an AC motor by controlling the stator current space vectors, in manner similar to but more complicated than DC control methods.
Field oriented control uses complex mathematics to transform a three-phase system that depends on time and speed to a two-coordinate d and q time-invariant system. The stator current in an AC motor is made up of two components: the magnetizing component d of the current and the torque-producing component q. This allows the torque-producing component, q, to be kept orthogonal to the rotor flux for maximum torque production, and therefore, optimum speed control.
Like scalar methods, vector VFD control methods can be open-loop or closed-loop. Open-loop vector control also referred to as sensorless vector control uses a mathematic model of the motor operating parameters, rather than using a physical feedback device. The variable speed drives, which can control the speed of A. C motors, are indispensable controlling elements in automation systems.
Depending on the applications, some of them are fixed speed and some of the variable speed drives. The variable speed drives, till a couple of decades back, had various limitations, such as poor efficiencies, larger space, lower speeds, etc.
Direct currents DC motors have been used in variable speed drives for a long time. The versatile characteristics of dc motors can provide high starting torques which is required for traction drives. Control over a wide speed range, both below and above the rated speed can be very easily achieved. The methods of speed control are simpler and less expensive than those of alternating current motors.
There are different techniques available for the speed control of DC motors. The phase control method is widely adopted in which ac to dc converters are used to supply the dc motors, but has certain limitations mainly it generates harmonics on the power line and it also has poor p. The second method is pwm technique, which has got better advantages over the phase control.
In our proposed project, a 5 H. The pulse width modulation can be achieved in several ways. By varying resistor pot only we can control the speed of motor states that simple and easy method.
The project proposed is a real time working project, and this can be further improvised by using more no. This concept of speed control or adjustment should not be taken to include the natural change in speed which occurs due to change in the load on the shaft. Any given piece of industrial equipment may have its speed change or Adjusted mechanically by means of stepped pulleys, sets of change gears, variable speed friction clutch mechanism and other mechanical devices.
Historically it is proved to be the first step in transition from non adjustable speed to adjustable speed drive. The electrical speed control has many economical as well as engineering advantages over mechanical speed control The nature of the speed control requirement for an industrial drive depends upon its type.
Some drives may require continues variation of speed for the whole of the range from zero to full speed or over a portion of this range , while the others may require two or more fixed speeds 1. Field windings connections for the three types Of DC motors have been shown in fig. Hence, name flux or field control method. The flux of DC motor can be changed by changing with help of a shunt field rheostat. Since in relatively small, shunt field rheostat has to carry only a small, so that rheostat is small in size.
The connection diagram for this type of speed control is shown in fig1. Field rheostat V Fig. As the supply voltage is normally constant, the voltage across the armature is varied by inserting a variable rheostat or controller resistance in series with the armature circuit as shown in fig1.
For a load of constant torque, speed is approximately proportional to the potential difference. Across the armature current characteristics in fig. This method is very wasteful, expensive and unsuitable for rapidly changing loads because for a given value of Rt, speed will change with load. A more stable operation can be obtained by using a diverter across the armature in addition to armature control resistance.
Now, the changes in armature current will not be so effective in changing the potential difference across the armature. The connection diagram for this type of speed control arrangement is shown in fig. The armature will be approximately proportional to these different voltages.
The intermediate speeds can be obtained by adjusting the shunt field regulator. The field of the motor M1 is permanently connected across the DC supply lines whose speed control can be done.
The other motor M2 is directly connected to Generator G. The voltage of generator can be varied from zero to upto its maximum value by means of field regulator. It should be remembered that motor set always runs in the same direction. The addition of a flywheel whose function is to reduce fluctuations in the Power demand from the supply circuit.
The chief advantage of system is its overall efficiency especially at right loads. It has the outstanding merit of giving wide speed Control from maximum in one direction through zero to the maximum in the opposite direction and of giving a smooth acceleration.
In view of this, the demand for dc motors would continue undiminished even in figure. A brief discussion regarding the dc motor applications is given below. Speed of the shunt motors may be regulated in two ways: first, by inserting resistance in series with the armature, thus decreasing speed: and second, by inserting resistance in the field circuit, the speed will vary with each change in load: in the latter, the speed is practically constant for any setting of the controller.
This latter is the most generally used for between synchronous motors and dc shunt motors. It is because the construction of high performance poly phase induction motor with large number of poles is difficult. The decreasing speed with increase in load torque or vice versa has only a marginal effect on the power taken by the series motor. However for traction purposes , series motor is the only choice.
Therefore series motors are widely used in all types of electric vehicles, eletrictrains, streetcars, battery powered tools, automotive starter motors etc. With the increase in the load speed of the machine decreases. DC shunt motor maintains almost constant speed from no load to full load.. Therefore such type of compound motors are used for loads requiring heavy starting torque which are likely to be reduced to zero A compound motor with weak series field has its characteristics approaching that of a shunt motor.
Weak series field causes more drooping speed torque characteristics than with an ordinary shunt motors. Such compound motors with steeper characteristics, are used where load fluctuates between wide limits intermittently. They are also used in feedback and the freewheeling functions of converters and snubbers. Fig 2. In the forward biased condition, the diode can be represented by a junction offset drop and a series-equivalent resistance that gives a positive slope in the V-I characteristics.
The typical forward conduction drop is 1. This drop will cause conduction loss, and the device must be cooled by the appropriate heat sink to limit the junction temperature. In the reverse-biased condition, a small leakage current flows due to minority carriers, which gradually increase with voltage. If the reverse voltage exceeds a threshold value, called the breakdown voltage, the device goes through avalanche breakdown, which is when reverse current becomes large and the diode is destroyed by heating due to large power dissipation in the junction.
The modern era of solid- state power electronics started due to the introduction of this device in the late s. Basically, it is a trigger into conduction device that can be turned on by positive gate current pulse but once the device is on, a negative gate pulse cannot turn it off.
The thyristors have been widely used in dc and ac drives, lighting, heating and welding control. The dc current gain of a power transistor is low and varies widely with collector current and temperature. The gain is increased to a high value in the Darlington connection, as shown in Fig2. The shunt resistances and diode in the base-emitter circuit help to reduce collector leakage current and establish base bias voltages. A transistor can block voltage in the forward direction only asymmetric blocking.
The feedback diode, as shown, is an essential element for chopper and voltage-fed converter applications. Double or triple Darlington transistors are available in module form with matched parallel devices for higher power rating.
Power transistors have an important property known as the second breakdown effect. This is in contrast to the avalanche breakdown effect of a junction, which is also known as first breakdown effect.
When the collector current is switched on by the base drive, it tends to crowd on the base-emitter junction periphery, thus constricting the collector current in a narrow area of the reverse-biased collector junction. This tends to create a hot spot and the junction fails by thermal runaway, which is known as second breakdown.
The rise in junction temperature at the hot spot accentuates the current concentration owing to the negative temperature coefficient of the drop, and this regeneration effect causes collapse of the collector voltage, thus destroying the device.
Two stage Darlington transistor with bypass diode 2. If the gate voltage is positive and beyond a threshold value, an N-type conducting channel will be induced that will permit current flow by majority carrier electrons between the drain and the source. Although the gate impedance is extremely high at steady state, the effective gate-source capacitance will demand a pulse current during turn-on and turn-off. The device has asymmetric voltage- blocking capability, and has an integral body diode, as shown, which can carry full current in the reverse direction.
The diode is characterized by slow recovery and is often bypassed by an external fast-recovery diode in high-frequency applications. Many designers view IGBT as a device with MOS input characteristics and bipolar output characteristic that is a voltage-controlled bipolar device. It combines the best attributes of both to achieve optimal device characteristics.
The IGBT is suitable for many applications in power electronics, especially in Pulse Width Modulated PWM servo and three-phase drives requiring high dynamic range control and low noise. IGBT improves dynamic performance and efficiency and reduced the level of audible noise.
It is equally suitable in resonant-mode converter circuits. Optimized IGBT is available for both low conduction loss and low switching loss. It has a very low on-state voltage drop due to conductivity modulation and has superior on-state current density.
So smaller chip size is possible and the cost can be reduced. Low driving power and a simple drive circuit due to the input MOS gate structure. It can be easily controlled as compared to current controlled devices thyristor, BJT in high voltage and high current applications. Wide SOA. It has superior current conduction capability compared with the bipolar transistor. It also has excellent forward and reverse blocking capabilities.
The collector current tailing due to the minority carrier causes the turnoff speed to be slow. There is a possibility of latchup due to the internal PNPN thyristor structure. The IGBT is suitable for scaling up the blocking voltage capability. In case of Power MOSFET, the on-resistance increases sharply with the breakdown voltage due to an increase in the resistively and thickness of the drift region required to support the high operating voltage. In contrast, for the IGBT, the drift region resistance is drastically reduced by the high concentration of injected minority carriers during on-state current conduction.
The forward drop from the drift region becomes dependent upon its thickness and independent of its original resistivity. Thus by varying the pulse-width, we can vary the average voltage across a DC motor and hence its speed. Ymax The circuit of a simple speed controller for a mini DC motor, such as that used in tape recorders and toys, is shown in Fig2. The heat dissipation problem often results in large heat sinks and sometimes forced cooling.
PWM amplifiers greatly reduce this problem because of their much higher power conversion efficiency. The PWM power amplifier is not without disadvantages. The desired signal is not translated to a voltage amplitude but rather the time duration or duty cycle of a pulse.
This is obviously not a linear operation. But with a few assumptions, which are usually valid in motor control, the PWM may be approximated as being linear i. The linear model of the PWM amplifier is based on the average voltage being equal to the integral of the voltage waveform.
The duty cycle must be recalculated at each sampling time. Pulse width modulation technique PWM is a technique for speed control which can overcome the problem of poor starting performance of a motor. PWM for motor speed control works in a very similar way. Instead of supplying a varying voltage to a motor, it is supplied with a fixed voltage value such as 12v which starts it spinning immediately.
The wave forms in the below figure to explain the way in which this method of control operates. In each case the signal has maximum and minimum voltages of 12v and 0v. By varying the mark space ratio of the signal over the full range, it is possible to obtain any desired average output voltage from 0v to12v. The motor will work perfectly well, provided that the frequency of the pulsed signal is set correctly, a suitable frequency being 30Hz. Pulse Width Modulation Waveforms 2.
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