Variable Speed Drive

The speed of the motor depends on the applied frequency, as well as the winding arrangement and the load. Therefore in order to control the motor speed it is easy to control the frequency of the supply. Therefore in order to control the speed of a standard AC motor, the applied frequency and voltage can be controlled using inverters. Although it is difficult to control voltage and frequencies at these high powers, the use of a standard induction motor allows a cost effective speed control system to be built. For reason of energy savings, reduction of operating costs, reduction of machine down time, maximizing productivity and the ability to provide adjustable speed control with standard squirrel cage induction motors, Variable Speed Drives are becoming the essential components in every modern automation system.
In earlier time DC motors are used in applications were speed control is necessary. Now a days Induction motors are used because of above advantages than DC motors. VSD not only facilitates the variation of speed, but also provides several other facilities. The acceleration, Deceleration, High speed, Low speed, Forward/reverse operation, Preset speed, jogging can be programmed by the VSD.
VSD is an intelligent machine in now a days, it has feedback path and also PLC use to control. Now come inbuilt PLC has VSD. Normally, we use Telemecanique band under this Altiva type VSD. 0.18kw-500kw motors can run using this machine. Most of VSD can control remote operate. And they have communication cable (RS485) we can connect the PLC and run. Then can run slave mode. Basically, VSD consist of following sources.

• Power control board: Power board is used to in put & out put power handling.
• Control card and display unit: Control card used mainly measuring the in put &
out put current and voltage, calculation, the heart, RPM, etc., display unit,
inbuilt memory, and other controlling buttons are in the card.

• IGBT unit (Inter Grated Bipolar Transistor): IGBT is the main control part and
important part of the VSD. This IGBT has two type of models single phase & there
phase one. It has diode unit in the left side and IGBT unit in the right side.
Diodes are uses to convert alternative current (A/C) into direct current (D/C),
which act as a converter. The IGBT unit is use to convert direct current (D/C)
into alternative current (A/C), which act as an inverter. It can be shown the
diagram of control card as follows. When it works as an inverter, it changes the
frequency of the out put line’s current into design value using IGBT firing system.
And also, there can be use thyrister unit to the inverter.

The Inverters

An electronic converter which converts Direct Current (DC) into Alternating Current (AC) is known as an inverter. Electronic speed controllers for AC motors usually convert the AC supply into DC using a rectifier unit, and then convert it back to a variable frequency, variable voltage AC supply using an inverter unit. The connection between the rectifier and inverter is called the DC link. The diagram of a speed controller, (normally called an inverter) can also be shown as above.
The supply, which can be single phase (usually at low power) or three phases is fed to a full wave rectifier which supplies the DC link capacitors. The capacitors reduce the voltage ripple (especially on single phase supplies) and supply energy for short mains breaks. The voltage on the capacitors is uncontrolled and depends on the peak AC supply voltage. The DC voltage is converted back to AC using Pulse Width Modulation (PWM). The desired waveform is built up by switching the IGBT (Insulated Gate Bipolar Transistors) or Thyristers. By varying the firing time of the IGBT or Thyristers the desired current can be generated. But the output voltage is still a series of square wave pulses. Pulse Width Modulation (PWM) wave form can be shown as follows.
The control system to calculate the PWM requirements is very complex and specially designed integrated circuits (ASICs) are needed. The output current must be carefully monitored to protect the inverter and the motor during overload and short circuit. At first switch on the DC link capacitors are discharged, and the inrush current must be limited, usually using a resistor which is bypassed by a relay after a few seconds. The inverter, especially the IGBT or Thyristers and rectifier diodes, produce heat which must be dissipated using a fan and heat sink.