Improved Process Control

33.1.3.2 Improved Process Control

Using VSDs to improve process control results in more

33.1.4 Disadvantages of VSD

efficient operating systems. The throughput rates of most industrial processes are functions of many variables. For The cost of VSD is generally space, cooling, and capital cost. example, throughput in continuous metal annealing depends Some of the drawbacks are: on, amongst other factors, the material characteristics, the

• Acoustic noise

cross-sectional area of the material being processed and the

• Motor derating

temperature of one or more heat zones. If constant speed

• Supply harmonics

motors are used to run conveyors on the line, it must either run without material during the time required to change tem-

The PWM voltage source inverter (VSI) drives, equipped perature in a heat zone or produce scrap during this period. with fast switching devices, add other possible problems such Both choices waste energy or material.

as (a) premature motor insulation failures, (b) bearing/earth With VSDs, however, the time needed to change speed is current, and (c) electromagnetic compatibility (EMC). significantly less than the time it takes to change heat-zone temperature. By adjusting the material flow continuously to

33.1.4.1 Acoustic Noise

match the heat zone conditions, a production line can operate In some installations, placing a VSD on a motor increases continuously. The results are less energy use and less scrap the motor’s acoustic noise level. The noise occurs when the metal. drive’s non-sinusoidal (current and voltage) waveforms pro-

duce vibration in the motor’s laminations. The non-sinusoidal

current and voltage waveforms produced by the VSD are the Starting a motor on line-power increases stress on the mechan- result of the transistor switching frequency and modulation in ical system e.g. belts and chains. Direct on-line start-up of an the DC-to-AC inverter. The switching frequency, fixed or vari- induction motor is always associated with high inrush current able determines the audible motor noise. In general, the higher with poor power factor.

33.1.3.3 Reduced Mechanical Stress (Soft Starts)

the carrier frequency, the closer the output waveform is to a VSD can improve the operating conditions for a system by pure sine wave. One method of reducing audible motor noise giving a smooth, controlled start and by saving some energy is full-spectrum switching (random switching frequency). The during starting and running. Smoother start-up operation will VSD manufacturers accomplish full-spectrum switching by an prolong life and reduce maintenance, but it is difficult to do algorithm within the VSD controller. The motor performance more than make an estimate of the cost-advantages of these. is optimized by evaluating motor characteristics, including The benefits of soft start, inherent in VSD, is that it eliminates motor current, voltage, and the desired output frequency. The the uncontrolled inrush of current that occurs when stationary resulting frequency band, though audible to humans, produces motor is connected to full line voltage, and also the inevitable

a family of tones across a wide frequency band. So, the per- suddenly applied high start-up torque. Benefits are that the ceived motor noise is considerably less than it would be with power wasted by current inrush is eliminated and that the life

a single switching frequency.

884 Y. Shakweh Motor noise may not present a problem. Relevant factors adequate stator cooling over the whole speed range. However

include motor locations and the amount of noise produced by the rotor will run hotter than designed as internal airflow other equipment. Traditionally motor noise level is reduced by remains a function of speed. As there are no windings in the adding a LC filter between the VSD and the motor, i.e. reducing rotor, insulation failure is not an issue but bearings may run the high frequency component of the motor voltage wave- hotter and require more frequent lubrication. form. Modern PWM inverter drives run at very high switching

Fan-cooled motors with centrifugal loads present less of a frequency and with random switching frequency thus reduc- problem. Pumps and fans, for example, do not require full ing the noise level too. Various methods have been proposed torque at reduced speeds. So, in these cases, there is less ther- to reduce the magnetically generated noise, which is radiated mal stress on motors at reduced speeds. Centrifugal load does from inverter-fed induction motors.

not cause the motor to exceed thermal limits defined by the insulation system.