Audio Frequency Transformer

Audio Frequency Transformer: Construction

• Figure (a) shows center taped secondary winding to provide two anti-phase signals. 
• The dot near the winding indicates relative polarity of the signal on different windings. 
• The dot at point P shows secondary winding signal is in phase with primary winding signal whereas the dot at point Q indicates secondary winding signal is antiphase with primary winding signal. 
• Figure (b) shows two audio amplifier is used to couple the power output stage of an audio amplifier to the loud speaker.

Function of Audio Frequency Transformer

• They allow only AC audio signal to the loud speaker whereas preventing DC from the amplifier.
• They provide isolated external connection for the loud speakers.
• They provide impedance matching between low input impedance of loud speaker to higher output impedance of an amplifier. It will allow maximum power transfer from the amplifier to speaker. 

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Application of Audio Frequency Transformer

• The audio frequency transformer works in the frequency range of 20 Hz to 20 kHz. They are used in the audio amplifiers. 
• They are essential in the impedance matching. i.e., matching high impedance output of amplifier to the low impedance of loud speaker. 

Working of Isolation Transformer

In this article, construction, function and application of isolation transformer is given

Function of Isolation Transformer

• The isolation transformer isolates input circuit and output circuit. 
• As there is no electrical connection between input circuit connected to primary and output circuit connected to secondary, it provides protection to users against live equipment. 

Radio Frequency Transformer

 

Introduction

• The range of radio frequency is from 30 kHz to 300 GHz . 
• The radio frequency transformer is classified on the basis of operating frequency.

Microphone Transformer

Function of Microphone Transformer

• The audio transformer can be used for matching microphone to input of amplifier. 
• The main function of the microphone transformer is to matching impedance between microphones, connecting cables and amplifier input.
• It should be noted that there is no signal reduction during impedance mismatching.

Microphone Cable

• Figure shows long microphone cable consists of two conductors twisted together which are surrounded by conducting metal foil or braid. 
• As two conductors are twisted, magnetic fields generated by each conductor tends to cancel out. 
• The earthed conducting foil protects conductor from external magnetic field.

• A microphone is connected to cable through single primary winding and center taped secondary winding transformer. 
• As cable is fed from a center taped transformer, signals on two conductors are in antiphase.
• The difference between two input signals applies at the amplifier input, the output signal is doubled in amplitude. 
• Any noise at the input signal cancel out automatically because two conductors are twisted each other in the opposite direction. 
• The combination of signal at the amplifier can be carried out either by differential amplifier or balun. 
• A balun is a balanced input to unbalanced output device.

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Balun Antenna

• Figure shows microphone transformer that plugs directly into high impedance amplifier input. 
• The XLR socket at the other side allows low impedance microphone to connected via a long lead. 
• A metal screening case in the transformer acts as a balun and impedance matching device.

Types of Resolver

Resolver Control Transmitter & Resolver Control Transformer

• In this article, principle, construction and working of resolver control transmitter and resolver control transformer is given. 
• Synchro is often referred as cousin of resolver. 
• People often interchange term for resolver as motion feedback sensors, transducer sensors, encoders and rotary position sensors. 

Synchro : Construction, Working & Application

 

Principle

• When a magnet material is placed in the magnetic field, it always moves in the direction of strongest magnetic field. 
• A synchro looks like as simple electrical motor and it works as variable transformer. Its operation depends upon electromagnetic induction.

Brushless DC Motor : Construction & Working

The classification of permanent magnet synchronous motor is done based on induced emf in the it.

Brushless AC Motor

• When the waveform of induced emf is sinusoidal, it is called as brushless AC Motor

Brushless DC Motor

• When the waveform of induced emf is trapezoidal, it is called as brushless DC Motor.

Construction of BLDC motor

• The configuration of BLDC motor is done on the basis of supply whether it is single phase, two phase or three phase BLDC motor. 
• Actually, the three phase BLDC motor is used in the industries. The main parts of the BLDC motor are as below.

BLDC Motor: Stator

• The construction of stator of BLDC motor is similar to that of three phase induction motor stator but the stator winding distribution is different to that of three phase induction motor. 
• The stator winding is star connected and each winding is connected through interconnection. According to interconnection of the stator winding, the configuration of BLDC motor is

Sinusoidal BLDC Motor

• The stator waveform of the sinusoidal BLDC motor is sinusoidal type.
• The torque produced in the sinusoidal BLDC motor is smoother than that of trapezoidal BLDC Motor but due to interconnection of stator winding its copper weight and cost increases.

Trapezoidal BLDC Motor

• The stator waveform of the trapezoidal BLDC motor is trapezoidal type.
• The voltage waveform diagram of the three phase BLDC motor is shown in the Figure.

BLDC Motor: Rotor

• The rotor of the BLDC motor is made of permanent magnet. 
• The number of pair of poles in the range of 2 to 8. The ferrite material is used to make permanent magnet. 
• The rotor material is selected based on necessary magnetic field density in the rotor. 
• The cost of ferrite material is less by considering volume of rotor material but at the same time flux density in the rotor is also less.  
• The flux density of the ‘ rare earth alloy ’ is higher than that of ferrite material for same volume of material therefore for the same torque, the size of rotor resulting stator size reduces when using rare earth ferrite material. 
• The neodymium ( Nd ), samarium cobalt ( Smco ), alloy of neodymium and Neodymium magnet ( NdFeB ) is used as rare earth ferrite magnet material.

BLDC Motor: Hall sensor

• The commutation process in the BLDC motor is done by electronic circuit. 
• The stator winding is energized in proper sequence in order to rotate BLDC Motor rotor. 
• There are three sensors placed at the non – driving end of the stator as show in the figure. 
• The function of the hall sensor is to check the position of rotor. 
• The hall sensors are placed at 120 degree to each other and commutation process is done by them. 
• When north or south pole of rotor passes closely to hall sensor, the hall sensor generates low or high signal. 
• This signal indicates north or south pole of rotor passes through hall sensor. 
• The power requires for hall sensor may be provided form 4 Volt or 24 Volts.  
• In order to make DC motor, brushless it is considered as ‘inside out’. 
• It means that the winding is placed on the stator and rotor is made of permanent magnet. 
• The reversal of current in the DC motor is done by commutator and brushes whereas it is done by electronic switch in the BLDC motor.
• When the response of hall sensor is low, electronic amplifier or drive circuit works as commutator in the servo system.

Working of BLDC Motor

• The DC supply to the BLDC motor is not given directly but it is given by reversing through semiconductor switch for fixed rotor position. 
• The torque in the BLDC motor is produced when the stator magnetic field and magnetic field produced by permanent magnet interact. 
• The maximum torque is produced when two magnetic fields are at 90 degrees to each other. 
• The rotor moves and catch with stator magnetic field to continuous rotation of rotor. 

Torque – Speed Characteristics

• Figure shows torque – speed characteristics of BLDC Motor. 
• The BLDC can be loaded up to rated torque during continuous operation. The torque remain constant upto rated speed. 
• When the BLDC motor run at 150% speed of rated speed, the torque starts drooping. 
• The BLDC motor is useful where frequent starts and stops and frequent reversal of rotation with load is necessary.

Comparison between conventional DC Motor and BLDC Motor

Commutation Sequence

• For each commutation sequence, positive supply is given to one winding, negative supply given to another winding and third winding is kept at de-energized position. 
• The rotor position is sensed through hall sensors placed on the stator. 
• The hall sensor changes its state ( low or high ) according to rotor position. One hall sensor changes its state for every 60 degree of rotation of rotor. 
• It takes 6 steps to complete on electrical cycle. However, one electrical cycle may not be equal to one mechanical revolution of rotor. 
• One electrical cycle is completed for each rotor pole pairs therefore the number of electrical cycle is equal to number of pair of poles.

Relation between electrical rotation and mechanical rotation

θ_e = ( P / 2 ) ( θ_m )

Where

θ_e = Electrical rotation

P = Number of poles

θ_m = Mechanical rotation

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Equivalent Circuit of BLDC Motor

• The equivalent circuit of the BLDC motor is shown in the figure. 
• The three-phase supply to the stator winding is given through inverter.
• There are six semiconductor switches in the three-phase inverter circuit, the switching of semiconductor depends upon signal of hall sensors. 
• The pulse width modulation technique is used in order to switching semiconductor device. 
• The frequency of PWM is at least 10 times to that of maximum frequency of motor. 
• The BJT, MOSFET, IGBT or GTO is used as semiconductor switching device.

 

Switching Sequence in the BLDC Motor

• The switching sequence for the position of hall sensor is show in the table. 
• Hall sensor – 1 shows high position whereas hall sensor – 0 shows low position. 
• Figure shows position of hall sensors and current passes through coil R, coil Y and coil B with reference to hall sensors position. 
• The commutation sequence of semiconductor for six position of hall sensor is shown in the figure. 

Sr. No	Hall Sensors	Phase	Semiconductor Switches “Switched ON”
1	101	R ( + ) – Y ( - )	S1 & S4
2	100	R ( + ) – B ( - )	S1 & S6
3	110	Y ( + ) – B ( - )	S3 & S6
4	010	R ( - ) – Y ( + )	S2 & S3
5	011	R ( - ) – B ( + )	S2 & S5
6	001	Y ( - ) – B ( + )	S4 & S5

 

( 1 ) The position of hall sensor H1H2H3 – 101  or electrical 60 degree, phase R gets positive supply, phase Y gets negative supply and phase B is kept in deenergized condition. This will result in semiconductor switch S1 and S4 is switched on.

( 2 ) Similarly, the position of hall sensor H1H2H3 – 100 or electrical 120 degree, phase R gets positive supply, phase B gets negative supply and phase Y is kept in deenergized condition. This will result in semiconductor switch S1 and S6 is switched on. 

Applications & Features of BLDC Motor

Applications

• Constant load: Fan, blower, pump
• Variable load: Compressor, dryer, washers, fuel control, electronic steering control, engine control, robotic arm control, gyroscope control etc.
• Position control: Computer numeric control ( CNC ) machine, process control, conveyor control

Features

• High efficiency
• High power factor
• Silent operation
• Compact
• Reliability
• Low maintenance

Variable Reluctance Stepper Motor : Construction & Working

In this theory, construction and working of full step (single phase ON) and full step (two phase ON) and half step (single phase and two phase ON) in different step mode is given.

Variable Reluctance Stepper Motor

Construction of Variable Reluctance Stepper Motor

• In the VR stepper motor, the stator consists of winding whereas the rotor has salient poles. 

DC Servo Motor : Characteristics

Construction of DC Servo Motor

• The construction of DC Servo motor is similar to that of separately excited DC Motor or Permanent Magnet DC Motor. 
• The speed control of DC Servo motor is done by armature voltage control.  

Permanent Magnet Stepper Motor

In this article, Single Phase ON operation, Two Phase ON operation, Half Step Operation and Features of Permanent magnet ( PM ) Stepper Motor is given.

Permanent magnet ( PM ) Stepper Motor

• The stator of the PM stepper motor consists of two project poles which is wound by separate windings. 
• The rotor is made of permanent magnet. 

Hybrid Stepper Motor

 

The characteristics of Hybrid stepper motor is combined that of variable reluctance ( VR ) stepper motor and permanent magnet ( PM ) stepper motor.

Construction of Hybrid Stepper Motor

• Its stator is similar to that of single stack VR stepper motor. 
• The phase winding of stator is either Mono-filar or Bifilar. 

Multi Stack Variable Reluctance ( VR ) Stepper Motor

The construction, step angle, advantages and disadvantages of variable reluctance stepper motor is given in this theory.

Construction of Variable Reluctance Stepper Motor

• Figure shows four stack variable reluctance stepper motor.  
• We can achieve low step angle by multi stack variable reluctance stepper motor. 
• The stator frame and rotor shaft are common in the multi stack variable reluctance stepper motor. 
• Each stack is parallel to shaft axis and its excitation is given by separate winding. 

Step angle in Variable Reluctance Stepper Motor

• The number of stator poles is equal to number of rotor poles. 
• If there are 20 teeth in the four stack VR stepper motor, the step angle becomes 4.5⁰.
• Step angle ( Θ ) = 360⁰ / S

       Where S = 4 ( Number of stacks ) × 20 ( Number of rotor teeth )

                     = 80

• Therefore, Step angle ( Θ ) = 360⁰ / S = 360⁰ / 80 = 4.5⁰

Advantages and Disadvantages of VR Stepper Motor

Advantages

• Higher stepping rate is possible
• Low inertia of rotor
• Light weight
• Freely rotation of rotor

Disadvantages

• Nil detent torque
• Higher efficiency for low stepping rate

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