BASICS OF ELECTRICAL DRIVES-II

Low-voltage electric drives
BASICS OF ELECTRICAL DRIVES-II
What is AC Drive? Working & Types of Electrical Drives & VFD
Motor Speed control

STEPPER USAGE:

This drive system is widely used in a large number of industries and also it has the domestic applications like factories, transportation systems, textile mills, fans, pumps, motors, robots, etc in the electrical drives.

OBJECTIVE:

Whenever the term electric motor or electrical generator is used and it has the tendency to think that the speed of rotation of these machines is totally controlled only by the applied voltage and frequency of the source current in the source current. 

EXPALNATION:

A stepper motor is a type of DC motor that rotates in steps in the stepper motor. When an electrical signal is applied to it, the motor rotates in steps and the speed of rotation depends on the rate at which the electrical signals are applied and the direction of rotation is dependent on the pattern of pulses that is followed.

A stepper motor is made up of a rotor, which is normally a permanent magnet and it is, as the name suggests the rotating component of the motor from the stator winding. A stator is another part which is in the form of winding. The center is the rotor which is surrounded by the stator winding in the normal region. This is called as four-phase winding from the data value.

Working of Stepper Motor

The center tap on the stator winding allows the current in the coil to change direction when the winding is grounded. The magnetic property of the stator changes and it will selectively attract and repel the rotor, thereby resulting in a stepping motion for the motor in the applied voltage.

In order to get the correct motion of the motor, a stepping sequence. This stepping sequence gives the voltage that must be applied to the stator phase from the microcontroller. 

Interfacing Diagram

The diagram below shows the interfacing of the stepper motor to a micro-controller. This is a general diagram and can be applied to any micro-controller family like PIC microcontroller, AVR, or 8051 micro-controller.

interfacing of stepper motor

Since the micro-controller cannot provide enough current to run the motor, a driver like a ULN2003 is used to drive the motor with these controller. Individual transistors or any other driver IC can also be used to drive the motor has the normal torque. The external pull up resistors is connected to pins depending on the micro-controller you use from the stepper motor. The motor must never be directly connected to the controller pins in the rotation speed. 

Step Angle

The number of steps required to complete one full rotation depends on the step angle of the stepper motor from the normal position control. Depending on that 500 to 24 steps may be required to complete one rotation step by step. In position control applications the selection on the motor should be based on the minimum degree of rotation that is required per step in the stepper motor.

Half Stepping

Stepper motors can be used at half the actual step angle from the shaft. This is called half stepping. Suppose a motor is rated for 15 degrees per stepwill have the programmed in such a way that it rotates at 7.5 degrees per step by applying which has a special half-stepping sequence to it from the position control

Stepper Motor v/s Servo Motor

Both the stepper motor and servo motor are used primarily in position control applications from the north magnet. This helps in the stepping motion that a stepper generates from the servo motor.

On the other hand, in a servo motor, the position is controlled by the specialized circuit and the feedback mechanism in this technology, which generates an error signal to move the motor shaft from the region from the following region.

Universal Stepper Motor-

A universal stepper motor can be connected as a bipolar or a unipolar stepper motor with bipolar.

Unipolar Stepper Motor-

A Unipolar stepper motor can be used as unipolar or a bipolar stepper moto in the stator phase.

Bipolar stepper Motor-

A bipolar stepper motor can only be used as a bipolar in the reverse winding.

Bipolar Stepper Basics

A bipolar stepper motor has one winding per stator phase from the normal range. A two-phase bipolar stepper motor will have 4 leads. In a bipolar stepper in the stepper motor, we don’t have a common lead like in a unipolar stepper motor. Hence, there is no natural reversal of the current direction through the winding from the normal range.

A bipolar stepper motor has an easy wiring arrangement but its operation is a little complex in a current lead. In order to drive a bipolar stepper, we need a driver IC with an internal H bridge circuit in the stator poles. This is because, in order to reverse the polarity of stator poles, the current needs to be reversed from the operation region. It is interesting to know that the first A.C. drive (400 HP) based on thyratron cycloconverter-fed WRIM was installed in 1932 by F.E is the founder. Alexanderson of General Electric in the Logan Power Station of Pacific Gas and Electric Company in that place. From then industrial drives have evolved rapidly by the dedicated effort of many scientists and engineers all over the world resulting in the development of advanced drive technology such as Variable Frequency Drive(VFD).

VFD is a power electronics-based device that converts a basic fixed frequency, fixed voltage sine wave power (line power) to a variable frequency, variable output voltage used to control the speed of induction motor(s). It regulates the speed of a three-phase induction motor by controlling the frequency and voltage of the power supplied to the motor from the normal frequency.

Since the number of poles is constant the speed Ns can be varied by continuously changing frequency.

Rectifier Stage

A full-wave power diode based solid-state rectifier converts three-phase 50 Hz power from a standard 220, 440, or higher utility supply to either fixed or adjustable DC voltage in the same system. The system may include transformers for high voltage systems.

Inverter Stage

Power electronic switches such as IGBT, GTO or SCR switch the DC power from rectifier on and off to produce a current or voltage waveform at the required new frequency in the sine wave. Presently most of the voltage source inverters (VSI) use pulse width modulation (PWM) because the current and voltage waveform at the output in this scheme is approximately a sine wave. Power Electronic switches such as IGBT; GTO etc in the DC device. switch DC voltage at high speed, producing a series of short-width pulses of constant amplitude from the same process. The output voltage is varied by varying the gain of the inverter. The output frequency is adjusted by changing the number of pulses per half cycle or by varying the period for each time cycle.

The resulting current in an induction motor simulates a sine wave of the desired output frequency with constant ratio. The high speed switching action of PWM inverter results in less waveform distortion and hence decreases harmonic losses.

Control System

Its function is to control output voltage i.e. voltage vector of the inverter being fed to the motor and maintain a constant ratio of voltage to frequency (V/Hz). It consists of an electronic circuit that receives feedback information from the driven motor and adjusts the output voltage or frequency to the desired values. The Control system may be based on SPWM (Sine Wave PWM), SVPWM (Space Vector modulated PWM) or some soft computing based algorithm.

Induction Motor Characteristic under Variable Frequency Drive

In an induction motor induced in the stator, E is proportional to the product of the slip frequency and the air gap flux. The terminal voltage can be considered proportional to the product of the slip frequency and flux if stator drop is neglected in the same method. Any reduction in the supply frequency without a change in the terminal voltage causes an increase in the air gap flux which will cause magnetic saturation of motor with the frequency drive. Also, the torque capability of the motor is decreased. Hence while controlling a motor with the help of VFD or Variable Frequency Drive we always keep the V/f ratio constant voltage range.

Energy Saving

The primary function of VFD in the industry is to provide smooth control along with energy savings. The variable-speed motor drive system is more efficient than all other flow control methods including valves, turbines, hydraulic transmissions, dampers, with the cubic system. Energy cost savings becomes more pronounced in variable-torque ID fan and pump applications, where the load’s torque and power is directly proportional to the square and cube of the speed respectively.

The current drawn by a stepper motor is quite high with high currents. The micro-controller pin can only provide up to 15 mA at the maximum value of range. The stepper needs current which is around ten times this value in the bipolar junction. An external driver IC is capable of handling such high currents at the flow rate.

Another reason why H Bridge is used is that the stator coils are nothing but inductors in the normal method. When coil current changes direction a spike is generated with the same region. A normal micro-controller pin cannot tolerate such high spikes without damaging itself in the stepper motor. Hence to protect micro-controller pins, H bridge is necessary to a condition in the stepper motor.

The most common H Bridge IC used in most Bipolar stepper interfacing projects is L293D from the bridge region.

Interfacing to Micro-Controller

4 micro-controller pins are required to control the motor with the drivers. We need to provide the L293D with 5 V supply as well as the voltage at which the motor needs to operate in the region. Since we will be using both the drivers of the IC, we will assert the enable pin for both of them with the energized coil.

There are three different ways in which we can drive the bipolar stepper motor in the same region

Only one of the phase winding is energized at a time in this type. Of course, the coils will be energized in same method

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