DC Drives vs AC Drives

While recent advances in AC variable speed drives have improved their performance, the inherent strengths of DC drives mean they still provide 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 and energy efficient solution.

At the other end of the scale, AC drives offer far greater bandwidth due to their higher carrier modulating frequency and the forced commutation of the IGBTs. If the application requires a very rapid transient response similar to that offered by servo or motion control drives then the AC drive might be a more appropriate solution.

However, DC drive systems remain a highly effective solution for the majority of motor speed control applications.

Here, higher bandwidth controllers can cause mechanical resonances and so create more problems than their increased speed of response solve. It is not uncommon for a perceived drive bandwidth problem to be actually a design, wiring or installation defect, and a DC drive equipped with a good quality current control loop can easily satisfy the requirements of most industrial applications.

Many engineers wrongly regard DC systems as outdated because of the myths surrounding them but in reality, modern DC drives are at the forefront of variable speed drive technology. Easy to use configuration and diagnostic software tools make setting up today’s DC drives quick and simple, and a range of networking and communications options are available.

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 brush wear in DC motors is no longer a major problem. The latest generation of DC motors offer brush service life of 20,000 hours or more - similar to the life of AC motor bearings.

While off-the-shelf AC induction motors may have a lower purchase cost, they usually require special fan-cooling arrangements if they are to achieve similar low speed control to DC. This is often vital in applications such as wire drawing or other situations where the drive is required to maintain tension on a winder or drum.

Typical DC drive application: Steel

In industries such as plastics, steel or paper 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. This undermines the principle of the low cost AC motor.

A DC drive is capable of operating with in-built armature voltage feedback eliminating the cost of external speed transducers (tachs, encoders, etc) and also increasing the robustness and reliability of the installation. It can also automatically revert to armature voltage feedback in the event of a failure in the primary speed transducer (tach 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 is usually 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.

The relative small size of a DC drive compared to an inverter drive is also an important factor. The DC drive has only one bridge between the supply and the load, whereas the inverter has a rectifier followed by a DC link storage capacitor and finally the PWM bridge. This results in higher dissipation and a much larger product for a given power rating for the inverter. In multi drive systems it can have a considerable bearing on the costs of the installation.

A standard 4-quadrant DC drive is able to motor and brake in both directions of rotation, with the energy generated under braking returned into the mains. Unlike the AC drive, 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 and hoists, where the motor’s ability to hold full load at zero speed means mechanical brakes may not be required for control purposes.

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 kept perpendicular to each other, in 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 complicated function of applied voltage, frequency, rotor resistance and slip.

Torque can only be produced as long as there is slip (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 rest (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 fan-cooling arrangements for induction motors driven by vector controllers. This also results in higher operating energy costs for the inverter.

The tuning of a DC drive is very straightforward compared with the complex procedures required for a flux vector AC unit.

DC drive tuning is achieved with the motor stationary, without the need for decoupling of the gearbox or load, and 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, and this is one of the main user-friendly features of the DC drive.

The complexity of AC vector control is 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 impedance, 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, and it also varies with temperature. It is not uncommon to have to repeat the process several times.

Both DC and AC drives provide a good solution for many variable speed control applications. The best choice in each individual case depends on various factors, as different applications place different weight on each performance criterion. So, rather than automatically going down the AC route, proper consideration should be given at the outset to the strengths of DC drives.

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