SPEED CONTROL &VARIABLE FREQUENCY DRIVES (VFD)

1.3.1 Introduction to motor operation:

AC induction, squirrel cage elevator motors may be two speeds single speed or variable speed (VVVF). Traditionally a DC motion, AC Variable Frequency is today state of the art prime mover for most elevators. Low slip, closed loop Vector ducty motors are avaliable from 5to 200 horspower4, 6,8or10 poles .Electrical and mechanical designs are ideally suited for demanding low noise Levels and positioning requirements required for this application.

1.3.2TYPES OF MOTORS USED IN Elevators:

·    DC Shunt Field 

·    AC Induction 

·    AC Permanent Magnet (PM)

1.3.3 Why using AC induction motor?

·      High Speed Geared, 1–75 HP

·      Few Low Speed Designs for Gearless  85–94% Efficient

·      Many with Single or 2 Speed Starters

·      Can be Variable Speed by Inverter Control
of Frequency

·      Torque Strength Derived from Out of Phase
Excitation Current

·      Requires Flux Vector Control for Wide
Operating Speed Range

·      Can Act Like a Generator

1.3.4 Types of Control

AC drives that use PWM techniques have varying levels of

Performance based on control algorithms. There are four basic types of control for AC drive today. These are Volts per Hertz, Sensor less Vector Control, Flux Vector Control, and Field Oriented Control.

·        Volts/Hertz control is a basic control method, providing a variable    frequency drives for applications like fan and pump. It provides fair speed and starting torque, at a reasonable cost.

·        Sensorless Vector control provides better speed regulation and the ability to produce a high starting torque.

·        flux Vector control provides more precise speed and torque control with dynamic response.

·         Field Oriented Control drives provide the best speed and torque regulation available for AC motors. It provides DC like performance for AC motors, and is well suited for typical DC applications