Efficiency is one of the most important parameters by which users evaluate hydraulic pumps. The demand for higher efficiency has led pump manufacturers to adopt sophisticated new topologies and control techniques in the pump’s motor, and these designs call for new approaches to the pump’s power stage.
This article examines one of the ways in which power semiconductor manufacturers are attempting to meet the power system designer’s needs: the Integrated Power Module (IPM). In theory, a module dramatically simplifies the design process for the pump manufacturer, removing the need to create a complicated power circuit containing tens of discrete components. But is there sufficient availability of off-the-shelf solutions to meet the diverse needs of pump manufacturers?
Authorities Push for Higher Efficiency
Circulator pumps with hydraulic power of <2,500W are in use in tens of millions of homes and offices around the world, chiefly in central heating and water distribution systems. The combined energy use of such a vast number of devices makes a large contribution to the world’s consumption of electricity, and therefore of fossil fuels; it is estimated that these pumps consume approximately 50TWh of energy each year.
With good reason, then, the European Union (EU) sees great scope for energy savings if it can bring about improvements to the operating efficiency of these widely used types of pump. Indeed, a 2009 EU regulation, directive 2005/32/EC, requires pump manufacturers to improve efficiency, reaching an Energy Efficiency Index (EEI) score lower than 0.27 in 2013, and down to 0.23 by 2015.
Pump manufacturers’ main strategy for achieving these targets is the application of advanced electronic control techniques, which offer reductions in energy use of as much as 80% compared to traditional motor designs.
AC Induction Motors are Less Efficient
The designer of a hydraulic system must select a pump that will operate at highest efficiency when the hydraulic load is at its highest. It follows that the pump is less efficient when the hydraulic load falls below maximum. Efficiency is therefore reduced if the pump works at a constant speed appropriate for the maximum load.
Traditionally, hydraulic system designers have therefore specified variable speed asynchronous AC induction motors, which are still in widespread use. This motor type, however, is inherently far less efficient than modern Permanent Magnet Synchronous Motors (PMSMs). These types of motors are also commonly described as electronically commutated motors; they require a complex control scheme to match the current through the stators to the position of the magnets affixed to the rotor, in such a way as to maximize torque.
Since PMSMs can usually run at higher speeds than traditional AC induction motors, a smaller pump is required to support any given hydraulic load. Pump manufacturers also find PMSM systems easier to assemble and more reliable than AC induction motors.
The typical topology of a PMSM’s power stage is shown in Figure 1. An input rectifier and a 3-phase inverter supply the motor. The algorithms implementing the commutation scheme are usually run by a DSP or microcontroller; application specific standard parts for motor control are also available from suppliers such as Fairchild Semiconductor.
Suppliers of DSPs and MCUs for motor control, such as STMicroelectronics, provide software libraries containing the building blocks of a commutation scheme. The use of this code dramatically simplifies the process of developing a motor control system. The control unit of a PMSM should also be powerful enough to run an algorithm that adjusts the speed of the motor automatically in response to changes in hydraulic load, in order to maximize efficiency at all load levels.
A New Generation of IPMs
The heart of a sophisticated PMSM is, then, its control unit, and the control scheme that it executes. This will occupy by far the largest part of the manufacturer’s design effort, and will be the key to its success in fulfilling the requirements of regulations such as 2005/EC/32.
At the same time, however, the power circuit for a PMSM (see Figure 1) is by itself a complex circuit that consists of many individual components.
In an industrial or residential pump, this circuit typically needs to be reliable, robust and compact while meeting a tight price target. Achieving this by designing a circuit from scratch, calls for a huge investment of design, time and effort.
This is why power semiconductor suppliers such as STMicroelectronics, Fairchild and International Rectifier have stepped into the market with ranges of off-the-shelf power supplies suitable for pump motors. These smart power modules usually integrate the following blocks in a single package (see Figure 2):
- A power stage consisting of six IGBTs or MOSFETs and six freewheeling diodes
- A driver network containing three high voltage gate drivers with gate resistors and bootstrap diodes
- Protection circuits (for over-voltage, under-voltage, overcurrent and over-temperature) and additional features such as current sensing and dead time
The SLLIMM™-nano IPM from STMicroelectronics shown in Figure 2 is suitable for applications up to 100W. In fact, most circulator pump applications are specified for applications in the range 50W to 150W. This is the target for IPMs from other suppliers as well, including Fairchild and International Rectifier, which provide IPMs offering a similar level of integration to the SLLIMM-nano devices, Fairchild with its Motion SPM® 5 Series, and International Rectifier with its μIPM™ Integrated Power Modules (see tables).
At first glance, these devices look rather similar to each other. In fact, however, there are important differences between the IPMs that provide a valuable element of choice for the pump manufacturer. Users can distinguish between the modules in particular in terms of:
- Voltage tolerance
- Autonomous protection features
As the tables show, the modules from Fairchild and International Rectifier use a MOSFET as the power switching device; ST’s SLLIMM-nano uses an IGBT. The use of an IGBT enables the SLLIMM-nano to withstand a higher voltage – 600V – which provides greater assurance to the designer that the pump will cope with over-voltage events without failing. A counterbalance to this observation is the lower isolation voltage (1,000V) compared to the 1,500V available in the SPM 5 series and the μIPM modules.
No Heat Sink Required
The modules also have a quite marked difference in size, at the system level. The μIPM devices from IR are produced in a PQFN package, which is just 12 x 12mm. This is already small – but this does not provide the biggest space savings. In fact, the QFN package offers very good thermal performance, and as a result the μIPM modules can drive a motor up to around 100W with no external heat sink (see Figure 3); according to STMicroelectronics, the same is true of the SLLIMM-nano modules.
In compact pumps, the elimination of this bulky component provides a considerable advantage over competing surface mount packages that require a heat sink to be assembled on top of the device.
As the tables show, all the modules integrate a number of protection features, which are essential to reliable operation. There is a difference in the implementation of these protection features, however. Both the SLLIMM-nano and the μIPM devices implement protection independently of the controller (such as an MCU or DSP), and can switch themselves off when necessary. The Fairchild SPM 5 Series, on the other hand, requires an external op amp to provide a feedback signal to the controller. The Fairchild devices also require an op amp for temperature sensing and protection.
STMicroelectronics has a clever approach to this function in its STGIPN3H60. In its Smart Shutdown feature, an internal fault comparator monitors for over-current, short circuit and over-temperature conditions. If a fault occurs, the device will be switched off and a fault signal sent to the controller. This provides for instant shutdown, without suffering from latency due to the cycling of the controller’s processor core.
Table 1: Fairchild Motion SPM 5 Series
|Part Number||Switching Device||Breakdown Voltage [V]||Current [A]||RDS(on) at 25°C [Ω]||Switching Frequency [kHz]||Isolation Voltage [V]||Bootstrap Diode||Protections|
Table 2: International Rectifier μIPM Integrated Power Modules
|Part Number||Switching Device||Breakdown Voltage [V]||Current [A]||RDS(ON) at 25°C [Ω]||Switching frequency [kHz]||Isolation Voltage [V]||Bootstrap Diode||Protections|
Table 3: STMicroelectronics SLLIMM-nano Family
|Part Number||Switching Device||Breakdown Voltage [V]||Current [A]||VCE(SAT) at TJ = 125°C [V]||Switching frequency [kHz]||Isolation Voltage [V]||Bootstrap Diode||Protections|
|STGIPN3H60||IGBT||600||3||1.65||20||1000||Yes||UVLO, interlocking function, smart shutdown, comparator, op amp|
|STGIPN3H60-H||IGBT||600||3||1.65||20||1000||Yes||UVLO, interlocking function, smart shutdown, comparator, op amp|
|STGIPN3H60A||IGBT||600||3||1.65||20||1000||Yes||UVLO, interlocking function|
Saving up to 80% of the 50TWh of energy consumed today by circulator pumps is challenging pump manufacturers and their R&D engineers. The use of modern PMSMs is enabling pump manufacturers to meet and exceed the demanding efficiency requirements laid down by the European authorities. The smart IPMs available today provide a ready-made power stage that well matches the 50W to 150W rating of the circulator pumps most commonly used today. They can simplify the design of the electrical drive circuit, and make it smaller, more robust and efficient. It is important to note they are also supported by comprehensive design resources – datasheets, application notes, reference designs and evaluation boards – enabling manufacturers to reliably find the best device for their application.