Original subtitles

The conveyor belt is driven by three

phase AC motors. How is it possible that

this line can start and stop so smoothly?

Let's have a look at the control - it's

all because of the frequency converter.

Other common terms for this electronic

device used in drive technology are VFD,

known as variable frequency drive,

adjustable frequency drive or AC drive

converter. With this device you can

control speed torque and direction of an

AC motor at the same time. So let's know

this device better. This block diagram

illustrates a typical three-phase AC

variable speed drive system with its

main components: First the incoming AC

voltage enters a bridge rectifier which

consists here of 6 diodes. In this stage

the DC voltage has ripples. Therefore a

capacitor and a filter helps to smooth

out the ripple. Now the DC bus system can

take its place. The filter DC bus voltage

passes through to a DC three-phase AC

converter: The DC/AC converter consists

of an array of fast-acting switches,

which convert the DC voltage into

voltage pulses of square shape and constant

magnitude with varying width. The inverter

output is not a truth sine wave but an

approximation based on pwm or pulse

width modulation. This pulse width

modulation scheme works because the

motor coils are large inductors, which

does not allow current to pulse like the

voltage.

To ensure the efficient operation of the

motor, the sine waves produced by each

pair of inverted switches are 120

degrees out of phase. The longer the

switches are on the higher the output

voltage. This way you can control the

torque of this motor regarding the speed

of the motor by increasing the frequency

of the output signal. You get a higher

number of revolutions but now you have a

problem: The inductive reactance of the

motor coils rises by the signal

frequency. That means you would get less

torque. So what is the solution to

overcome this obstacle? You would adapt

the voltage to the frequency when

acceleration and deceleration. The

parameter called ramp describes the

voltage frequency ratio. The other

parameter boost also known as the torque

compensation is to compensate the torque

decrease in low speed because of the

stator winding resistance of the

electric motor and the starting torque

caused by friction. The base edge

frequency is achieved when the voltage

supplied to the motor reaches the main

supply voltage. Raising the frequency

above the edge frequency leads to

magnetic saturation which cannot be

compensated anymore. As a result the

torque of the motor decreases. Concerning

the technical implementation of the

inverter: As electronic switches

transistors are used. In this case IGBTs.

In order to avoid a short-circuit their

control connections are interlocked. The

diodes serve as a freewheeling diode

since the motor coil is an inductance.