Page 205 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Speed control of 3 phase induction motor by VVVF/AC
drive

Objectives: At the end of this lesson you shall be able to
• state about AC drives (VFD/VVFD) and changing of speed of AC motor by AC drive
• explain the operation of AC drive with block diagram
• list out the advantages and disadvantages of AC drive
• explain the components / parts and power and control terminals of AC drive
• state the parameter setting - speed control changes of direction of AC & DC drives / VFD/VVVFD (variable
frequency drive/ variable voltage variable frequency drive)
• state the speed control of universal motor.

Variable Voltage Variable Frequency Drive (VVVFD)
The AC drive industry is growing rapidly and it is now more important than ever for technicians and maintenance
personnel to keep AC drive installations running smoothly. AC drives change the speed of AC motor by changing
voltage and frequency of the power supplied to the AC motor. In order to maintain proper power factor and reduce
excessive heating of the motor, the name plate volts / hertz ratio must be maintained. This is the main task of VFD
(Variable frequency drive).

Applications of AC drives
1 AC drives are used to stepless speed control of squirrel cage induction motors mostly used in process plants
due to its ruggedness and maintenance free long life.

2 AC drives control the speed of AC motor by varying output voltage and frequency through sophisticated
microprocessor controlled electronics device.
3 AC drive consists of rectifier and inverter units. Rectifier converts AC to DC voltage and inverter converts DC
voltage back to AC voltage.
Changing of speed of AC motors by using AC drive
From the AC motor working principle, that the synchronous speed of motor Ns in rpm, is dependent upon frequency.
Therefore by varying the frequency of the power supply through AC drive, it can control the synchronous speed.
Speed (rpm) = Frequency (Hertz) x 120 / No. of poles.
Where
Frequency = Electrical frequency of the power supply in Hz., No. of poles = Number of electrical poles in the
motor stator. Thus the speed of AC motor can conveniently be adjusted by changing the frequency applied to the
motor. There is also another way to make the AC motor work on different speed by changing the no. of poles, but
this change would be a physical change of the motor. The VFD provides the controls over frequency and voltage
of motor input to change the speed of a motor. Since the frequency is easily variable as compared with the poles
variation of the motor. AC drives are frequently used.
Constant V/F ratio operation
If the same voltage is applied at the reduced frequency, the magnetic flux would increase and saturate the
magnetic core, significantly distorting the motor performance. The magnetic saturation can be avoided by keeping
the fm constant.
All AC drives maintain the voltage -to- frequency (V/f) ratio constant at all speeds for the reason that follows. The
phase voltage V, frequency F and the magnetic flux f of motor are related by the equation.
V=4.444 f N f m
or
V/f = 4.444x N f
m
Where N = number of turns per phase
f = magnetic flux
m

192

CITS : Power - Electrician & Wireman - Lesson 93-100