Inverter drives: No sense in fiddling as the world 'burns'
20 Nov 2007
Eddie Kirk explains the variable speed drives market, which has undergone a cathartic revolution since their original introduction
The first volume production drives were launched on the market by Danfoss in 1968 and in the 40 years since the variable speed drives market has undergone a cathartic revolution. In 1968, the drives market lead was held by DC drives although eddy-current couplings remained popular and Ward-Leonard sets were still used widely in the steel and water industries, writes Eddie Kirk.
The raison d'etre of inverter drives was, and still is, that they provide variable speed control of standard, off-the-shelf, cage induction motors. All the other variable speed alternatives incurred significant maintenance commitments and serious production disruption if a failure occurred. However, the uptake of inverter drives was slowed by the significant cost and size of the inverters themselves, not to mention the inherent unreliability of early power components within the drives.
The rectifier front-end of early quasi-square wave inverter drives was basically a complete DC drive, and some manufacturers, rather than design a dedicated variable voltage rectifier for the purpose, employed a standard DC drive. To this then had to be added the output inverter and its controls, so effectively doubling the cost per kW of inverters over DC drives. This was partly offset by the lower price of the cage motor versus the DC motor.
Had life-cost been considered, the reliability and negligible maintenance costs of the induction machine versus those of the DC motor would have more than balanced the equation in favour of the inverter drive.
Where inverter drives really scored was when multiple motors had to be driven together, as in roller table drives in the steel industry or fibre spinning in the synthetic fibres industry. There, the multiplicity of the motors and the simplicity of driving them in parallel, dictated their selection and sponsored further development. Without this advantage the development of AC drives might have been significantly slowed.
These early drives offered fairly basic speed control over a limited speed range. Speed reduction to around 10% of top speed was the general rule as torque dropped off badly at low speed and the increased fluxing current to overcome the hysteresis loss within the motor created overheating. This, along with limited starting torque, necessitated the application of a derating factor to both drive and motor — increasing costs further.
However, applications like centrifugal fans and pumps, where a limited speed reduction is sufficient and where torque drops off with speed, suited inverter application admirably. For this reason, they are well suited to the needs of the water industry for pump drives. This is more to do with a preference for the simplicity, high reliability, low maintenance requirement and standard IP54 enclosures of the cage motor than the characteristics of the drive electronics. And, despite 40 years of development, this remains the prime benefit of AC drives.
If we skip the details of the development, the prime mover for selecting AC drives is still the fact that they facilitate the use of standard squirrel-cage motors. Along the way most of the early alternatives have become redundant and, of them, only the DC drive remains; selected only for a narrow range of applications, holding a minute percentage of the drives market and a cash-cow for the drives companies still manufacturing them.
For a brief period, the switched reluctance drive looked set to challenge the inverter drive but its requirement for a special, albeit very simple, motor made it unattractive for general industrial applications. That drive found its niche for low power drive sets for high volume quasi-domestic machinery, such as laundry equipment.
Having become part of the digital revolution and now employing fifth generation IGBT switching devices, the AC drive dominates the drives market and has widened its range of applicability to even eat into the lower end of the servo market. Whereas early analogue drives provided very crude control of motor fluxing at reduced speed, modern microprocessor-controlled drives control the motor flux continually, ensuring the motor runs at its most efficient point at all speeds and loads, to the point where, with a feedback loop, it is possible to generate full load torque at standstill.
As the standard cage machine has a shaft-mounted fan to provide cooling, and since this becomes less effective as the motor slows, motor rating has to be considered for the speed and torque characteristics of the application. For this reason fan and pump applications still suit the inverter drive as, generally, torque and thus current and thus motor heat drop off faster than the speed, owing to the affinity laws of centrifugal devices.
It's in fan and pump applications that the inverter drive promises the most significant energy and carbon savings, particularly as the design is easily applied to existing induction motors. In the past, the failure to consider whole-life costs of electro-mechanical units led to the selection of lower-cost, but inefficient, control devices, such as dampers for fan installations and control valves for pumps. Contractors walked away on project completion, leaving system owners to bear the costs of maintenance and unnecessarily higher energy bills. awareness of the implications of energy inefficiency, the uptake of inverter control for fan and pump installations, particularly in the HVAC sector, is sparking the highest rate of growth in the inverter drive industry, not least because of the benefits of the induction motor, which are further enhanced with the newer more efficient EFF1 machines.
So despite the significant advances of the technology, now offering in-built software controls, PLC functions, higher efficiencies, fieldbus communications and a whole host of useful control functionality giving it faster response and more accurate speed control, inverter drives are still really all about the motor.
The induction motor is still the workhorse of industry, and that includes fans and pumps in HVAC applications, and looks set to remain so for the foreseeable future. For some reason, however, perhaps because of the apparent complexity of modern drives, potential users often seem to feel unsure about how to apply drives and err by selecting overrated, and thus inefficient, drives 'just to be on the safe side'.
In fact it couldn't be simpler. As stated earlier, it's all about the drive motor. Consider the load to be driven, the starting torque requirement, the speed range, the torque characteristics and then pick the nearest larger standard frame size to suit to application. Include any derating required for any of the foregoing characteristics, bearing in mind that, for instance, an 11kW motor offers a significant service factor for an 8kW load requirement.
It's not usually necessary to select the 11kW motor and then derate and go for the next bigger 15kW frame size. Select the inverter size to match the motor and the starting requirement. Some inverters offer an economical 120% starting capability for fan and pump applications, but care should be taken in this area by discussing the starting torque requirement with the fan or pump manufacturer and selecting the inverter to suit.
So it's still all about the induction motor, even though there is some talk about permanent magnet motors for the future, as potentially these offer efficiency and size benefits. For the moment, though, the inverter/induction motor is king, and over the coming years there are going to very many more of them in use than there are today, not least because their contribution to the fight against global warming is arguably the most significant to be offered by the electrical industry.
If only industry could be convinced to invest in real energy-saving technology instead of fiddling while the world burns — well, at least gets warmer.
The raison d'etre of inverter drives was, and still is, that they provide variable speed control of standard, off-the-shelf, cage induction motors. All the other variable speed alternatives incurred significant maintenance commitments and serious production disruption if a failure occurred. However, the uptake of inverter drives was slowed by the significant cost and size of the inverters themselves, not to mention the inherent unreliability of early power components within the drives.
The rectifier front-end of early quasi-square wave inverter drives was basically a complete DC drive, and some manufacturers, rather than design a dedicated variable voltage rectifier for the purpose, employed a standard DC drive. To this then had to be added the output inverter and its controls, so effectively doubling the cost per kW of inverters over DC drives. This was partly offset by the lower price of the cage motor versus the DC motor.
Had life-cost been considered, the reliability and negligible maintenance costs of the induction machine versus those of the DC motor would have more than balanced the equation in favour of the inverter drive.
Where inverter drives really scored was when multiple motors had to be driven together, as in roller table drives in the steel industry or fibre spinning in the synthetic fibres industry. There, the multiplicity of the motors and the simplicity of driving them in parallel, dictated their selection and sponsored further development. Without this advantage the development of AC drives might have been significantly slowed.
These early drives offered fairly basic speed control over a limited speed range. Speed reduction to around 10% of top speed was the general rule as torque dropped off badly at low speed and the increased fluxing current to overcome the hysteresis loss within the motor created overheating. This, along with limited starting torque, necessitated the application of a derating factor to both drive and motor — increasing costs further.
However, applications like centrifugal fans and pumps, where a limited speed reduction is sufficient and where torque drops off with speed, suited inverter application admirably. For this reason, they are well suited to the needs of the water industry for pump drives. This is more to do with a preference for the simplicity, high reliability, low maintenance requirement and standard IP54 enclosures of the cage motor than the characteristics of the drive electronics. And, despite 40 years of development, this remains the prime benefit of AC drives.
If we skip the details of the development, the prime mover for selecting AC drives is still the fact that they facilitate the use of standard squirrel-cage motors. Along the way most of the early alternatives have become redundant and, of them, only the DC drive remains; selected only for a narrow range of applications, holding a minute percentage of the drives market and a cash-cow for the drives companies still manufacturing them.
For a brief period, the switched reluctance drive looked set to challenge the inverter drive but its requirement for a special, albeit very simple, motor made it unattractive for general industrial applications. That drive found its niche for low power drive sets for high volume quasi-domestic machinery, such as laundry equipment.
Having become part of the digital revolution and now employing fifth generation IGBT switching devices, the AC drive dominates the drives market and has widened its range of applicability to even eat into the lower end of the servo market. Whereas early analogue drives provided very crude control of motor fluxing at reduced speed, modern microprocessor-controlled drives control the motor flux continually, ensuring the motor runs at its most efficient point at all speeds and loads, to the point where, with a feedback loop, it is possible to generate full load torque at standstill.
As the standard cage machine has a shaft-mounted fan to provide cooling, and since this becomes less effective as the motor slows, motor rating has to be considered for the speed and torque characteristics of the application. For this reason fan and pump applications still suit the inverter drive as, generally, torque and thus current and thus motor heat drop off faster than the speed, owing to the affinity laws of centrifugal devices.
It's in fan and pump applications that the inverter drive promises the most significant energy and carbon savings, particularly as the design is easily applied to existing induction motors. In the past, the failure to consider whole-life costs of electro-mechanical units led to the selection of lower-cost, but inefficient, control devices, such as dampers for fan installations and control valves for pumps. Contractors walked away on project completion, leaving system owners to bear the costs of maintenance and unnecessarily higher energy bills. awareness of the implications of energy inefficiency, the uptake of inverter control for fan and pump installations, particularly in the HVAC sector, is sparking the highest rate of growth in the inverter drive industry, not least because of the benefits of the induction motor, which are further enhanced with the newer more efficient EFF1 machines.
So despite the significant advances of the technology, now offering in-built software controls, PLC functions, higher efficiencies, fieldbus communications and a whole host of useful control functionality giving it faster response and more accurate speed control, inverter drives are still really all about the motor.
The induction motor is still the workhorse of industry, and that includes fans and pumps in HVAC applications, and looks set to remain so for the foreseeable future. For some reason, however, perhaps because of the apparent complexity of modern drives, potential users often seem to feel unsure about how to apply drives and err by selecting overrated, and thus inefficient, drives 'just to be on the safe side'.
In fact it couldn't be simpler. As stated earlier, it's all about the drive motor. Consider the load to be driven, the starting torque requirement, the speed range, the torque characteristics and then pick the nearest larger standard frame size to suit to application. Include any derating required for any of the foregoing characteristics, bearing in mind that, for instance, an 11kW motor offers a significant service factor for an 8kW load requirement.
It's not usually necessary to select the 11kW motor and then derate and go for the next bigger 15kW frame size. Select the inverter size to match the motor and the starting requirement. Some inverters offer an economical 120% starting capability for fan and pump applications, but care should be taken in this area by discussing the starting torque requirement with the fan or pump manufacturer and selecting the inverter to suit.
So it's still all about the induction motor, even though there is some talk about permanent magnet motors for the future, as potentially these offer efficiency and size benefits. For the moment, though, the inverter/induction motor is king, and over the coming years there are going to very many more of them in use than there are today, not least because their contribution to the fight against global warming is arguably the most significant to be offered by the electrical industry.
If only industry could be convinced to invest in real energy-saving technology instead of fiddling while the world burns — well, at least gets warmer.
