AC inverters have developed a long way in recent years to provide improved control of standard induction motors. Selection of these drives has remained largely within the remit of control or electrical engineers, but some of the latest developments open opportunities which fall within the domain of the designer.
It is usual to think of AC drives as discrete devices that are chosen to suit particular motor control applications. The units are supplied as complete units with all the necessary electronics, keypad or other interface and software. For this reason alone, the design engineer has had little more to consider than whether the drive will satisfy the motion control criteria and still fit within his or her cabinet or machine space. Here we address only the use of smaller drives of, say, up to 22kW, since units with greater power are almost always fitted external to the machine being controlled. AC drives have become more powerful in terms of the levels of control provided. The advent of open loop flux vector control (where there are no external reference signals from the motor or its shaft – such as from an encoder) enabled significantly higher torque at low speeds. This development brought AC technology into applications for which closed loop flux vector drives or DC drives were previously employed.
IMPROVEMENTS IN AC
With faster microprocessors, improvements in electronic components, better software and other features, the drives have become able to tune themselves and achieve energy savings. Similarly, inverters have got smaller for any given power rating – to the extent that drives up to 1kW are available in packages little bigger than a couple of cigarette packets. The trend is also very clearly towards plug-and-play devices. This has led to easier installation – which should be worthy of consideration by the designer.
The smaller size of the latest drives has brought benefits to the design engineer in terms of panel size and the possibility of improved airflow for cooling within control cabinets. This also reduces real estate consumption in terms of storage and assembly floor space.
It is the functionality of the modern inverters – particularly the new open loop vector drives – that has created the greatest design considerations to date. In many cases external references are no longer needed. Here, it is not just encoders that may be redundant, but limit switches and sensors – once required to add peace of mind in an otherwise straight forward application – can be obviated thanks to improved motor control.
Where a machine or system needs differential control, many standard AC inverters now have PID control loops as standard. This enables the designer to make use of a 4-20mA or +/-10V signal within their system to feed directly to the drive (or via a PLC if necessary) to create levels of control over motor speed, acceleration and deceleration within a machine which once required far more expensive drive arrangements.
Communications capabilities within certain drives means that remote programming, functions can be designed into a machine. The scope for linking drives, daisy chain fashion, via serial communications ports, presents further scope for integrated control design.
Also on the subject of communications, designers have had to become more aware of the confusing field bus issues. If the equipment design has to accommodate the possibility of field bus compatibility, consideration must be given to whether the selected drive is appropriate. Many drives adopt the `Anybus’ solution (modules through which any number of bus protocols can be emulated) or offer a selection of plug in cards to cover the most widely encountered protocols – such as Profibus, Interbus-S, DeviceNetand CANbus.
While OEMs have appeared to care little about the running costs of the equipment they manufacture, in the after-market the efficiency of systems is gaining recognition. Driven equipment accounts for more than two thirds of all the electrical power consumed by industry and much of that is wasted as motor heat through poor control or no control. Modern inverters have the capability of saving energy and reducing the running costs of AC machines. Some of the latest models have automatic energy saving features. While these bring the greatest advantages in fans and pumps where motors can be controlled according to demand, there are savings to be made in most applications. Most OEMs continue to concern themselves primarily with the cost of making their equipment, but there are indications that one day legislation may enforce the use of energy efficient systems – the USA has already enacted its EpAct standards for electrical products to reduce consumption.
A design consideration for all engineers using AC drives is the selection of the motor. It has been widely acknowledged that inverters have the potential to damage motor windings – tales of motors being burned out after fitting inverters remain common. The reason for this is the speed of the solid state switches or insulated gate bipolar transistors (IGBTs) which are within the intelligent power modules (IPMs) at the heart of all modern inverters. The fast switching effectively enables transients, or spikes, in the electrical supply to pass without impedance through the drive and straight to the motor. Any imperfections in the motor windings, such as thin wire insulation, can cause pitting and subsequently fatal sparking between the windings. There have been developments to minimise this factor, but it is still wise to select inverter duty motors for use with AC drives. Often, an even better option is to fit a line or load guard to prevent transients from ever reaching the motor – clearly this is a decision best made at the design stage.
FUTURE DESIGN OPPORTUNITIES
Design engineers must always compromise when using AC drives, since they have no control over the dimensions or functions of the drive. Selections of products can be made on a variety of technical grounds, but if the drive which meets the criteria of the design happens to be supplied in an exceptionally large case, the design engineer must resolve that problem by adjusting his or her own design.
There are moves by some manufacturers to develop drives which are modular in construction and will give design engineers greater freedom and potentially reduce the cost of using inverters. As electronics have continued to develop, it has become possible to incorporate all of the essential components of an inverter within a single monobloc construction. The resultant device looks rather like a standard IPM module, with an encapsulated unit containing not only the IPM, but also the central processor (CPU), logic circuits, and rectification circuits. In short, the unit is an entire drive, but without the capacitors required to make the DC link or the interface via which to program the drive.
The benefits of such a development are that the unit can be located or packaged by the design engineer to suit the specific machine or application. There is nothing special about the capacitors used within inverters, so these can be sourced in the most cost effective way by the OEM. The interface to the drive can be integrated within the design engineer’s scheme without the need for further communications requirements – for instance, the drive could be controlled directly from the OEM’s own operator panel, or via a simplified control which could be as basic as a pushbutton or switch.
The concept of the modular drive currently relies on the application having sufficient volume to justify the drive maker’s supply. But in time, it is likely there will be sufficient demand to create inherent volume – this will see inverters generally available as component level devices. When that happens, design engineers will truly be able to `design in’ drives.
Prepared for Design Engineering by John Houston of Houston Associates.
INFORMATION: HID HITACHI Tel: 01493 442525