Dc motors are increasing in popularity due to their performance advantages over ac motors for applications ranging from high-speed automation to electric motorbikes. Jon Severn reports on a selection of recent developments in dc motors and drives.
Three-phase ac induction motors are the most common type of motor used in industrial application, due largely to their simple design, low purchase cost, reliability and broad choice of sizes and specifications. However, for applications where speed needs to be varied or torque needs to be controlled with high accuracy, brushed dc motors are popular. Where even higher performance and/or reliability are required, brushless dc motors overcome the drawbacks often associated with brushless dc motors.
As well as industrial applications, brushed and brushless dc motors are also employed in smaller machines and equipment where speed control is critical - such as computer hard disk drives - or in battery-powered, automotive and other applications where a dc supply is readily available. It should also be remembered that brushless dc motors are non-sparking and therefore do not generate ozone, which makes them suitable for use in medical applications.
Compared with brushed dc motors, brushless types have the further advantage of producing less electrical noise.
Today there are several factors behind an increasing demand for dc motors. For example, cars are being offered with more powered devices, plus designers are seeking to improve the performance of the equipment they design, which often entails upgrading from an ac motor to a dc motor. In particular, coreless (ironless) dc motors have low inertia so are useful where high accelerations are required.
Alternative energy sources
Another important point is that there is a growing interest in alternative energy sources, which often results in a high-efficiency dc motor being used for the final drive. For example, the prototype ENV motorbike (Fig.1) from Intelligent Energy features a fuel cell that generates electrical energy from hydrogen and oxygen. To make best use of the limited supply of electrical energy, the bike is equipped with a high-efficiency Lynch axial-gap dc motor. This type of low-voltage, high-torque, permanent-magnet dc motor has the added advantage of a high power density. Lynch motors are today manufactured in the UK by the Lynch Motor Company in three frame sizes and with numerous options.
Alternatively two motors can be coupled together to drive a single output shaft, thereby increasing the power available.
To illustrate the motors' capability, the LEM 130 model 95s has a rated power of 3.02 kW, rated torque of 4.35 Nm and rated speed of 6624 rpm. Peak efficiency is 87 per cent. For more demanding applications, the LEM 200 model D135 has a rated power of 14.39 kW, rated torque of 36.4 Nm and rated speed of 3780 rpm. Peak efficiency for this model is 90 per cent.
In the USA, Briggs & Stratton has licensed the Lynch motor technology to manufacture the Etek motor for use primarily in electric outboard motors for boats. Once again, however, this type of motor has been selected for use in a motorbike. This time the company is Electric Motorsport of the USA, though the Electric Motorsport GPR-S (Fig.2) uses 3.3 kWh Lithium batteries rather than the fuel cell of the ENV.
The GPR-S is claimed to have a maximum speed of 112 km/h (70 mph) and a range of 80 km (50 miles). While the bike is equipped with the Etek-RT motor, the company says that the industry-standard motor mounting would make it straightforward to retrofit a motor (and controller) to take advantage of regenerative braking.
An alternative to the Lynch and Etek motors is the PMG 132 high-performance disc armature motors (Fig.3) from Perm Motor in Germany, which operate from 24-72 V and have a rated power output of 2.2 to 7.2kW. Depending on the operating conditions, these can achieve a peak efficiency of 90 per cent or more.
In-wheel motors
PML Flightlink has been manufacturing printed armature motors (or 'pancake motors') for some 40 years. In this type of motor, the flat armature is essentially an ironless disc with coils punched from sheet copper and formed onto non-magnetic disc insulators. Brushes contact on two or more points to create a path through the copper, thereby creating a magnetic field that interacts with the permanent magnets to cause motion. With no magnetic material present in the armature, 'cogging' is eliminated, resulting in smooth operation and continuous torque down to standstill, with minimal torque ripple. Torque is proportional to current and not limited by saturation, and speed is directly proportional to voltage.
Furthermore, the ironless armature has virtually zero inductance; the advantage of this is that when the magnetic field collapses, energy does not discharge by arcing to the brushes. Consequently the printed armature exhibits low EMC properties and brush wear is minimal.
Indeed, PML claims to have achieved over 10000 hours working life expectancy on a high-volume product.
Recent developments from PML Flightlink include the Hi-Pa Drive in-wheel unit that incorporates the motor and drive electronics within a common housing for automotive drive applications. This has already been applied to the Mini QED and the Volvo ReCharge. Scaling this concept down, the company has also created the Nano Wheel that incorporates a brushless motor within a wheel hub (Fig.4).
In addition, the wheel utilises patented integral suspension that is injection moulded in the same operation as the hub.
The first application for this unit is a power-assisted suitcase, and a powered golf trolley is currently in development. Compared with conventional motor technologies, the Nano Wheel is claimed to offer a more compact, simpler mechanical design, with fewer moving parts and less friction.
Products that fold for storage or transit are therefore easier to design, and the energy efficiency and low friction help to maximise battery life.
Dc motors are also being manufactured in ever-increasing volumes for even smaller applications, such as solar-powered devices, toys and mobile telephone handsets - for the vibration function.
Dc drives
Apart from those systems in which the dc motor is connected directly to a switched power supply, the motor is only one half of the dc powertrain.
Industrial applications and those calling for precise control of speed or torque require a dc drive. Given the increased demand for dc motors, it is not surprising that there have also been developments recently in the field of dc drive technology.
One of the companies most active in this area is Sprint Electric. This company's 340XRi, 680XRi and 1220XRi DC four-quadrant dc drives, for motors up to 1.8 kW (2HP), are compact and energy-efficient, being designed to regenerate energy back into the mains ac supply under braking without the need for intermediate energy storage, resistive dumping or additional power bridges.
At the 2008 SPS/IPC/Drives show in Nuremberg, Germany, Sprint unveiled two- and four-quadrant PLX digital drives with current ratings up to 1650 A (Fig.5), which is a major step up from the previous maximum rating of 1050 A.
Parker Hannifin's Parker SSD Drives division has also introduced new dc drives, such as the Frame 6 version of its DC590+ family.
This new drive is much smaller than previous models, with the company claiming that it requires less than 50 per cent of the cabinet space compared with other dc ratings of similar current ratings.
The DC590+ Frame 6 high-power drive is available in a choice of three current ratings of 1250, 1600 and 1950 A, with voltage ranges from 380 to 690 V AC and power ranges from 600 to 900 kW (750 to 1250HP).
According to Parker SSD Drives, the DC590+ Frame 6 DC drive has been designed to provide optimum reliability and efficiency with a range of features to minimise wear and save energy. For example, cooling fans are only switched on when required and are constantly monitored for rotation, and built-in snubbers ensure reliable operation even in the event of mains disturbance.
Operational flexibility
To improve operational flexibility still further, option cards enable the new drive to communicate with popular industrial protocols, including Ethernet, Profibus, Controlnet, Devicenet and CANopen.
The DC590+ Frame 6 is also supplied with a full set of function blocks dedicated to section control, including winder/unwinder, dancer (tension) control, PID and load control. In some applications this eliminates the need for a separate logic controller.
The foregoing presents some of the latest developments in dc motors and drives, though there will be some applications for which low-specification dc motors, ac induction motors or other motor types will be more appropriate.
However, designers seeking to improve the performance of the products they are creating, or who are working with alternative energy sources, certainly need to be aware of the recent technological advances in DC motors and drives.