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There are tangible cost savings to be made with a matched

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					D R I V E S & S W I T C H G E A R

T

here are tangible cost savings to be made with a matched motor and variable speed drive (VSD) combination.

Matched motor and VSD combination means cost savings
A matched motor and VSD package usually refers to a combination from the same supplier, but one supplier has redefined the concept of a motor and VSD package with an integrated offering which provides measurable benefits to customers. The compatibility between AC motors and VSDs is an often debated and misunderstood subject. Any normal 3 phase motor can be controlled by any standard VSD, therefore from this perspective they are compatible. However, true compatibility goes way beyond mere operation. Long term reliability and efficiency of energy use should be the result of a truly compatible motor and VSD combination. An optimal combination is the result of two aspects, namely motor design and VSD design. These have to be considered in conjunction with one another and not in isolation. Compatibility issues The three main issues of compatibility are motor thermal condition, peak voltage value (Vpeak) and the rise time of the voltage peaks (dv/dt). Motors driven by VSD are subjected to thermal conditions that are different to that when operated DOL (direct on line). There are three reasons for this. The voltage wave form that the VSD supplies to the motor is not a pure sinusoidal wave form. The distortion of the VSD wave form causes additional heat in the motor. Secondly the motor is normally dependent on the shaft mounted fan for cooling air flow. When operated by VSD and run at speeds below 50 Hz, the air flow is Information from Zest reduced and therefore the motor heat rise will be greater for the same load. If operated at speeds above 50 Hz the motor magnetic flux which provides the torque is reduced. If the motor nominal torque is maintained its heat rise will be greater than nominal. The individual voltage pulses generated by a VSD reach high peak values at the motor terminals. This phenomenon is known as “voltage reflections” or “standing waves”. This effect is common and similar for all standard VSDs. The typical voltage peak value is 2,8 x V AC supply, but much higher values of 4 – 6 x V AC supply are possible. The severity of the effect of each individual pulse V peak is determined by how quickly or sharply the voltage rises. This is known as dv/dt and is given in volts per microsecond (V/µs). The more rapidly the voltage rises, the more potentially damaging it is to motor insulation. A slow voltage rise allows the peak value to be dispersed across several motor winding turns. The result is a relatively low turn-toturn voltage value. A rapid voltage rise results in high turn-to-turn voltage that may be damaging to motor insulation. A typical value of dv/dt is 3000 V/µs. Once again, much higher values are possible. If one contrasts the normal AC supply voltage with that of a VSD output, the difference is obvious. A 525 V 50 Hz supply provides a smooth sine wave with a peak value of 742 V. It takes the voltage t seconds to rise to this
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value. On the same supply a VSD output would typically rise to a peak of 1484 V in a time of t . To equate these on the same scale, the “impact” of the VSD voltage is 10 000 times greater than that of the sine wave. This is still a good value. As a result of poor design or application, this value can easily rise to much higher values. Motor design should cater for these phenomena in the following ways: l	 Motor thermal design and cooling should take into account VSD output waveforms. This means, that in contrast to former years where fixed speed 50 Hz operation was virtually the only possibility, motor design should now take VSD operation into account. l	 Motor insulation must cater for the V peak and dv/dt values that VSD voltage pulses will cause. There are a limited number of industryappointed suppliers of special wire for inverter fed motors known as “spike resistant wire” and because of the relatively high cost of this specialised wire, the majority of manufacturers have continued to install Grade 2 or 3 wire as standard in their motors. Accelerated life tests have shown that the life expectancy of a standard insulation system may be reduced by as much as 75 % when subjected to the high dv/dt levels and voltage spikes generated by modern inverters. To counteract this effect, all Weg 525 V motors are wound with spikeresistant wire rated 1780 V peak and 6500 V/µs.

Quality and reliability are obviously two key elements to be considered when selecting an electric motor for any application. A critical element underlying these qualities is the motor insulation, and possibly the most important component in this system is impregnation. There are a number of impregnation options to consider, depending on the application. These include dipping, flooding, vacuum impregnation (VI), vacuum pressure impregnation (VPI) and continuous resin flow impregnation (CRFI). The process of impregnation consists of adding varnish or resin (varnish is resin with solvent added, resin is an organic polymer) to the motor stator to fill all gaps, increasing motor thermal conductivity as well as mechanical and dielectric strength. Most people believe that VPI is the best impregnation system, and it has been found that this is true for form wound motors due to the high porosity of insulating tapes and fleeces. However, random wound motors are far superior when impregnated by the CRFI system. CRFI is sometimes known as trickle and should not be confused with dripping, which is achieved by pouring varnish on top of the windings, which may or may not be pre-heated. In the CRFI system, the stator is pre-heated to facilitate resin flow through the slots; it is then tilted and rotated. The combination of gravity and the centrifugal force created by the rotating movement ensures that the resin adheres to the windings and slot insulation, filling all gaps. In addition, the use of a specialised resin instead of varnish increases the percentage of retained solids at the end of impregnation from 40 to over 99 %. The result is an overall increase in mechanical and dielectric strength of the insulation, eliminating vibration between turns, which prevents short circuits and improves heat transfer. CRFI impregnation brings high voltage integrity and mechanical endurance to the motor windings, features that are crucial for reliable operation in applications where motors are used with AC inverters. The CRFI impregnated motor also presents a six times lower amplitude discharge than motors using standard dipping systems at an applied voltage of 2600 V. Also, the voltage amplitude threshold from which partial discharges occur is 41% higher for a CRFI system, providing greater immunity against voltage spikes. The VSD design should be such that the output waveform is minimally distorted and the voltage peaks and dv/dt minimised. This requires an in-depth

integrated understanding not only of VSD power electronic technology, but also of motor design and technology. Suppliers of optimised motor/VSD combinations considers this holistic approach as being fundamental. Research and development facilities have been studying the effects of PWM drives on AC electric motors over a number of years. The result has been the incorporation of sophisticated technology. VSD design should cater for these phenomena in the following ways: l	 The VSD software that controls the output pulses must model the motor theoretically and control the pulses practically in such a manner that the VSD output is nearly a pure sine wave. l	 A software based motor protection model that models motor heat rise and cooling based on actual motor data. This motor thermal model must consider the effect of varying speed, not merely the current value. l	 The semi-conductor switching times must be selected to be rapid enough to allow rapid response, but also slow enough to minimise the VSD output dv/dt. The switching time referred to here is the time taken to turn the semi-conductors on from the off state. l	 The VSD must have a nominal switching frequency that is <5 kHz. The switching frequency is the number of pulses at the VSD output for every cycle of the output voltage. If this switching frequency is too low, the output waveform becomes very distorted. The higher the switching frequency, the smoother the output waveform. However, motor insulation failure is proportional to switching frequency below 5 kHz and is proportional to the square of the switching frequency above 5 kHz. A switching frequency in the region of 3 kHz is normally a good compromise. If one use 3 kHz as a basis of comparison, then at 5 kHz motor insulation failure is 1,6 times more probable and at 10 kHz, motor insulation failure is 33 times more probable! l	 Mean time between pulses (MTBP) must be designed to be >6 µs. If the VSD pulses are too close together, the peak values at the motor terminals can be multiplied to 5,6 x V AC supply instead of 2 , 8 x V A C s u p p l y. O n a 525 V supply this results in Vpeak = 2940 V. All the above factors should be considered in the VSD modulation principal. Weg has introduced an optimal flux
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technology. Its CFW09 VSD is used in combination with a premium efficiency motor. A specially developed flux model in the VSD applies a varying V/F ratio to the motor. This varying V/F ratio is designed to minimise motor losses. The result is reduced motor heat rise and increased efficiency. This represents upfront cost savings as well as energy efficiencies which lower operating costs. No specialised knowledge is required to apply this technology. The commissioning procedure for the VSD has been streamlined with the incorporation of a feature known as ‘oriented start-up’. An automatic programming routine guides the operator through a sequence designed for the completion of the minimum number of parameters for the perfect adaptation between the motor and drive combination. The combined features of these electric motors and VSDs have addressed the major drawbacks in motor and drive combinations, such as the increase in motor temperature and fast transient high voltage peaks. The new technology has revolutionised the concept of the matched motor and drive combinations and provides considerable advantages. Contact Chris Chryssoulis, Zest Electric Motors and Drives, Tel 011 723-6000, chrisc@zest.co.za D


				
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