Speed control of 3phase induction motor using microcontroller and pwm F or H y Type of stator connection (for 3-phase only).Class A. and reduction in lower order harmonics. reliable. This method is most suitable for applications without position control requirements or the need for high accuracy of speed control. While there are different methods for control. Induction motor control is complex due to its nonlinear characteristics. The heart of any PWM inverter is the switching strategy used to generate the switching edges of the PWM voltage waveforms. industrial control. star(Y) or delta ( ) . Regular sampled PWM techniques have the following advantages: elimination of even harmonics by using quarter-wave symmetry. Recently. The pulse-width modulation (PWM) inverter can be used to obtain variable voltage with variable frequency. In addition. These electronics not only control the motor¶s speed.. it runs at its rated speed. and automation. Variable Voltage Variable Frequency (VVVF) or V/f is the most common method of speed control in open loop. fans and blowers. Supplying a squirrel-cage induction motor with a variable-frequency variable-voltage source is known to be and efficient method for step less control of the speed from zero to rated speed and beyond. a washing machine may use different speeds for each wash cycle. They are robust. almost linear variation of the inverter output voltage. but can improve the motor¶s dynamic and steady state characteristics. However. the value of the frequency ratio p should be high because the significant value of the voltage harmonic order is then greater. For example. air conditioning. many applications need variable speed operations.INTRODUCTION Induction motors are the most widely used motors for appliances. they are often called the workhorse of the motion industry.e. It also offers a number of advantages over rival techniques for ac motor drives. INDUCTION MOTOR BASICS NAME PLATE PARAMETERS A typical nameplate of an induction motor lists the following parameters: y Rated terminal supply voltage in Volts y Rated frequency of the supply in Hz y Rated current in Amps y Base speed in RPM y Power rating in Watts or Horsepower (HP) y Rated torque in Newton Meters or Pound-Inches y Slip speed in RPM. mechanical gear systems were used to obtain variable speed. For a better (i. electronics can reduce the system¶s average power consumption and noise generation of the motor. or slip frequency in Hz y Winding insulation type . When power is supplied to an induction motor at the recommended specifications. However. more nearly sinusoidal) output voltage waveform. and durable. Examples of these applications include heating. The main switching strategies described in the literature are naturally sampled PWM optimal PWM and regular sampled PWM. electronic power and control systems have matured to allow these components to be used for motor control in place of mechanical gears. hence. restrictions on the value of p are the width of the pulses and the switching losses. B. Historically. the rotor runs slower than the speed of the stator field. induction motors are broadly classified in two categories: squirrel cage motors and slip ring motors. sweeps past the rotor surface and through the stationary rotor conductors. If the slots are semiclosed. Rotor flux rotates slower than synchronous speed by the slip speed. Synchronous speed is directly proportional to the ratio of supply frequency and number of poles in the motor. The frequency of the induced EMF is the same as the supply frequency. This difference in speed is called slip speed. The stator construction is the same in both motors Squirrel cage motor Almost 90% of induction motors are squirrel cage motors. The frequency and speed of the motor. 2: As the load increases. the rotor starts rotating in the same direction as that of flux and tries to catch up with the rotating flux. with respect to the input supply. So. it generates a magnetic flux of constant magnitude. This is because the squirrel cage motor has a simple and rugged construction. the EMF induced produces a current in the rotor conductors. 2: The number of poles is always an even number to balance the current flow. Its magnitude is proportional to the relative velocity between the flux and the conductors. Synchronous speed of an induction motor can be written as. then these bars are inserted from the ends. Note 1: Percentage of slip varies with load on the motor shaft. This speed is called the base speed. The speed listed on the motor nameplate is the base speed. Each slot carries a copper. is called the synchronous frequency and synchronous speed. An electromotive force (EMF) is induced in the rotor conductors due to the relative speed differences between the rotating flux and stationary conductors. The direction of the rotor current opposes the relative velocity between rotating flux produced by stator and stationary rotor conductors (per Lenz's law). To reduce the relative speed. as shown in Figure 1. INDUCTION MOTOR TYPES Based on the construction of the rotor. The rotor consists of a cylindrical laminated core with axially placed parallel slots for carrying the conductors. it draws the rated current (or corresponding power) and runs at base speed and can deliver the rated torque. MOTOR ROTATION When the rated AC supply is applied to the stator windings. These rotor bars are permanently short-circuited at both ends by means of the end rings. Synchronous Speed (Ns) = 120 x F/P where: F = rated frequency of the motor P = number of poles in the motor Note 1: The number of poles is the number of parallel paths for current flow in the stator. or alloy bar. Since the rotor bars are shorted at the ends. rotating at synchronous speed. This slip speed depends upon the mechanical load on the motor shaft.When the rated voltage and frequency are applied to the terminals of an induction motor. But in practice. The flux passes through the air gap. Synchronous speed is the speed at which the stator flux rotates. the rotor never succeeds in 'catching up' to the stator field. 3: 4-pole motors are the most widely used motors. . the slip also increases. aluminium. Rotor teeth tend to remain locked under the stator teeth due to direct magnetic attraction between the two. The X axis shows speed and slip. Instead. This starting torque is also called locked rotor torque (LRT). b) To help reduce the locking tendency of the rotor. 1 Typical squirrel cage motor Slip Ring Motors The windings on the rotor are terminated to three insulated slip rings mounted on the shaft with brushes resting on them. the slip ring motor functions like a squirrel cage motor Speed torque characteristics of induction motor Figure 2 shows the typical speed-torque characteristics of an induction motor. the motor delivers 1. using an external metal collar. When running under normal conditions.This total assembly resembles the look of a squirrel cage. . This high starting current overcomes these components and produces the momentum to rotate the rotor. The external resistor can be used to boost the starting torque of the motor and change the speed-torque characteristic. the current drawn by the motor reduces slightly (see Figure 2). So. At start-up. the slip rings are shortcircuited. This allows an introduction of an external resistor to the rotor winding. During start-up. which gives the motor its name. FIG. The Y axis shows the torque and current.5 times the rated torque of the motor. This happens if the number of stator teeth are equal to the number of rotor teeth. The rotor slots are not exactly parallel to the shaft. they are given a skew for two main reasons: a) To make the motor run quietly by reducing the magnetic hum. and losses in the bearings due to friction. As the speed increases. the losses in the stator and rotor windings. The characteristics are drawn with rated voltage and frequency supplied to the stator. the motor typically draws up to seven times the rated current. This high current is a result of stator and rotor flux. which is pushed along the shaft to connect the rings. in normal conditions. When the load is increased (over-rated load). the motor draws the rated current and delivers the rated torque. In addition. then the motor may burn out.5 times the rated torque. torque is highly nonlinear as the speed varies. This torque is called breakdown torque. At base speed. This makes the flux produced by the stator proportional to the ratio of applied voltage and frequency of supply. the speed needs to be varied. At base speed. them induction motor draws the rated current and delivers the rated torque at the base speed. By varying the . As we have seen in the earlier section. if the load on the motor shaft is increased beyond its rated torque. So.FIG. which makes the torque vary. Motor windings can withstand different temperatures. inherent losses in the windings increase as well.5 times the rated torque with around 20% drop in the speed. the speed drops and the slip increases. the voltage applied to the stator is directly proportional to the product of stator flux and angular velocity. As seen in the speed-torque characteristics. 2 Speed torque characteristics of induction motor The current drops significantly when the motor speed approaches ~80% of the rated speed. Some motor manufacturers provide the data on overload capacity and load over duty cycle. In many applications. it will not be able to take any further load and the motor will stall. while running at base speed. based on the class of insulation used in the windings and cooling system used in the motor. The torque developed by the motor is directly proportional to the magnetic field produced by the stator. when the load is increased beyond the rated load. If the load on the motor is increased further. the motor can take up to 2. Variable Voltage Variable Frequency (VVVF or V/f) in this application note. If the motor is overloaded for longer than recommended. We will discuss a simple open loop method of speed control called. This leads to a higher temperature in the motor windings. the load can increase up to 2. the load current increases following the current characteristic path. Any further increase of load on the shaft can stall the motor. When the motor is running at approximately 80% of the synchronous speed. V/f Control theory As we can see in the speed-torque characteristics. the speed starts dropping and slip increases. Due to this higher current flow in the windings. Therefore. However. At base speed. the torque can be kept constant throughout the speed range. 3 speed torque characteristics with v/f control Speed torque characteristics with v/f control . EQUATION 3 Stator Voltage (V) V x2 f V/f [Stator Flux( )] x [Angular Velocity ( )] This makes constant V/f the most common speed control of an induction motor. Figure 3 shows the relation between the voltage and torque versus frequency. the voltage applied cannot be increased beyond the rated voltage. Figure 3 demonstrates voltage and frequency being increased up to the base speed.frequency. by varying the voltage and frequency by the same ratio. flux and hence. We can drive the motor beyond base speed by increasing the frequency further. since friction and windage losses increase significantly at higher speeds. Fig. the factors governing torque become complex. Above base speed. which results in the field weakening and the torque available being reduced. Hence. the voltage and frequency reach the rated values as listed in the nameplate. only the frequency can be increased. the torque curve becomes nonlinear with respect to speed or frequency. Therefore. the speed of the motor can be varied. Special high voltage pulse transformers are also used to generate high power pulses for radar. The product of the peak pulse voltage and the duration of the pulse (or more accurately. For the same reason. particle accelerator. pulses with fast rise and fall times and a relatively constant amplitude ). Larger power versions are used in the electrical power distribution industry to interface low-voltage control circuitry to the high-voltage gates of power semiconductors. Generally speaking. the larger this product. often for matching logic drivers to transmission lines. the voltage-time integral) is often used to characterise pulse transformers.5. a pulse transformer needs to have low values of leakage inductance and distributed capacitance. To minimise distortion of the pulse shape. A good transient response is necessary to maintain the rectangular pulse shape at the secondary. high insulation resistance and high breakdown voltage are required. and a high open-circuit inductance. the larger and more expensive the transformer. because a pulse with slow edges would create switching losses in the power semiconductors. In power-type pulse transformers. Pulse transformers by definition have a duty cycle of less than 0. a low coupling capacitance (between the primary and secondary) is important to protect the circuitry on the primary side from high-powered transients created by the load. or other high energy pulsed power applications. Medium-sized power versions are used in power-control circuits such as camera flash controllers. whatever energy stored in the coil during the pulse must be "dumped" out before the pulse is fired again . Small versions called signal types are used in digital logic and telecommunications circuits.Pulse transformer A pulse transformer is a transformer that is optimised for transmitting rectangular electrical pulses (that is. Optocouplers have also become essential to modern televisions as the technology and complexity of digital components has grown. An optocoupler's efficiency is measured through its current transfer ratio (CTR). Benefits Optocouplers play integral roles in many common household items. such as telephones. Because of this trait. photocoupler or photoMOS---is a piece of an electric circuit that transfers electricity between two other parts without allowing them to make a direct connection.They are used extensively to separate the parts of the circuit that manage the cable receiver. We used this optocoupler to test the pulse transformer . making it a perfect light source for an optocoupler. as they distort input signals and do not last very long. screen performance and screen settings. cellular phones. internet modems and even the entire house electrical circuit. Most optocouplers work at a CTR between 10 and 50 percent. Considerations Optocouplers are often constructed with light emitting diodes (LEDs) as part of the optical transmitter side.Optocoupler What Do Optocouplers Do? An optocoupler---also referred to as an opto-isolator. Incandescent lamps are also sometimes used in optocouplers. An LED produces light when voltage is added to it. Significance Optocouplers are most often used to separate two circuit elements that are operating on extremely different voltages. optocouplers are best utilized in associated with on/off switches and the transfer of digital data. which is the relationship between the current change on the output side of the barrier and the current change on the input side of the barrier. however they are not as efficient as LEDs. This prevents damage to the part working at a lower voltage.computer circuits. They also work to keep the two elements from being damaged by reverse voltage or power surges. They are commonly found between a transmitter and a receiver in an electric circuit. While optocouplers offer element isolation similar to a relay component. as they are smaller and fit easily into the microcircuit systems used in electronics Features The most important feature of an optocoupler is its electricity transfer efficiency. they are often a better choice for circuit designers. Triggering circuit .
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