AC or DC - the choice is yours

May 2002 feature: AC or DC - the choice is yours While recent advances in AC variable speed drives have improved their performance, the inherent strengths of DC drives mean they still provide a perfectly viable and, in some cases, the best solution for many industrial applications. For simple, high-volume applications such as the control of fans, pumps and compressors, AC variable speed drives represent an excellent, energy-efficient solution. At the other end of the application scale, they also offer large bandwidths due to their high carrier modulating frequency and the forced commutation of the IGBTs. If, therefore, the application requires a very rapid, transient response - similar to that offered by servo or motion control drives - the AC drive might be a more appropriate solution. However, DC drive systems remain a highly effective solution for a great many motor speed control applications. Here, higher bandwidth controllers can cause mechanical resonances, creating problems which counteract the benefits offered by their increased speed of response. It is not uncommon for a perceived drive bandwidth problem to be a design, wiring or installation defect, and a DC drive, equipped with a good quality current controller, can easily satisfy the requirements of most industrial applications. Many engineers wrongly regard DC systems as outdated, partly because of the myths surrounding them, whereas, in reality, modern DC drives are at the forefront of variable speed drive technology. DC motors are also perceived to be more expensive to install and maintain than a standard squirrel-cage induction motor, because of their brushgear and commutators. In fact, evidence suggests that brush wear in DC motors is no longer a major problem, with the latest generation of DC motors offering brush service life in excess of 20,000 hours - similar to the life of AC motor bearings. Adding to the cost While off-the-shelf AC induction motors may have a lower purchase cost, they will often require special cooling if they are to achieve low-speed control performance on a par with that offered by DC. This is often vital in applications such as wire drawing where the drive is required to maintain tension on a winder or drum. And, where precise control of starting as well as running torque is often necessary, a position transducer (encoder) is also required on the shaft of the induction motor, adding to the cost of the 'low-cost, standard AC motor'. A DC drive is capable of operating with in-built armature voltage feedback - albeit at reduced accuracy and bandwidth - eliminating the cost of external speed transducers (tachos, encoders, etc) and also increasing the robustness and reliability of the installation. Furthermore, it can revert automatically to armature voltage feedback in the event of a failure in the primary speed transducer (tacho or encoder). The AC drive relies on a position encoder for flux control and some means of speed measurement for speed control, normally combined inside the same housing. It can offer an equivalent mode to the DC armature voltage control by employing sensorless algorithms, but the quality of control can be inferior and is dependent on the sophistication and the tuning of these algorithms. In comparison, DC motors are very competitively priced, especially in the middle power range - between 10 and 250 kW - and, taking into account the lower cost of the drive itself, the total DC package can comfortably beat the AC motor-plus-inverter package on price. In addition, a standard four-quadrant DC drive is able to motor and brake in both directions of rotation, with the energy generated under braking returned into the mains. This regenerative braking is achieved without the need for intermediate storage, resistive dumping or an additional power bridge, resulting in reduced energy consumption, faster response to speed transients and no drive trips due to over-voltage. This makes DC the most cost effective and safely controllable solution for applications with overhauling loads such as cranes, where the motor's ability to hold full load at zero speed means mechanical brakes are not required. Ahead of the alternative For general applications, the accurate motor control across the speed range, ease of motor tuning and energy efficiency of the DC drive puts it ahead of the more complex AC inverter. This is because, in a DC system, torque is generated by the linear interaction of the two magnetic fields of the armature winding and the field winding. The commutator ensures that the axes of these magnetic fields are constantly perpendicular to each other - the optimum torque producing position. The resultant torque is practically a linear function of the two DC armature and field currents, and the heat dissipation in the windings, at a given torque, will be constant at any speed (including zero), so special cooling arrangements are not required. In contrast, the AC induction motor develops torque by exciting the stator winding which, in turn, induces slip frequency currents in the rotor cage. The two magnetic field axes are at a variable angle dependent on the shaft and slip angles. Hence, the resultant torque becomes a function of applied voltage, frequency, rotor resistance and slip. Torque can only be produced as long as there is slip (s), ie the difference between the synchronous and the shaft speeds. The proportion of the total power transferred across the air-gap from the stator that is converted into mechanical power is 1-s, with the remaining s dissipated as rotor-circuit copper loss. At or near zero speed, a disproportionate amount of power is dissipated as heat as the slip approaches unity, hence the need for costly cooling arrangements for induction motors driven by vector controllers. The tuning of a DC drive is very straightforward compared with the procedures required for a flux vector AC unit. DC drive tuning is achieved with the motor stationary, without the need to decouple the gearbox or load. It is a one-off procedure without the need for iteration. The accuracy of the tuning process only affects the optimum performance of the drive, and the sensitivity of the drive settings is relatively low. The drive will turn a motor shaft safely under control even with the default power-up control gains - one of the main user-friendly features of the DC drive. In contrast, the complexity of AC vector control is orders of magnitude higher than the DC control and, as a result, the tuning process is far more complicated and parameter sensitive. In order to implement the vector control calculations it is necessary to have knowledge of the motor magnetising current at different speeds and the rotor time constant. These are the main parameters that the autotune process tries to derive along with some other motor impedances such as stator resistance and total leakage inductance. In order to estimate the magnetising characteristic, the motor has to be rotated up to its maximum speed setting and it also needs to be decoupled from the gearbox and load. Furthermore, it is an iterative process since the value of the rotor resistance has a direct effect on the magnetisation values and vice versa - it also varies with temperature. In general, both DC and AC drives provide a good solution for many variable speed control applications. But rather than automatically going down the AC route, proper consideration should be given at the outset to the strengths of DC drives.