DC/AC: the comeback tour

Despite predictions of the demise of DC variable speed drives at the hands of the AC vector control drive, DC systems still account for a substantial proportion of the European industrial drives market, says Dr Aris Potamianos of Sprint Electric. And it’s still ‘horses for courses’ when it comes to deciding which system most suits a particular application.

There are several myths surrounding DC drives that do not stand up under close scrutiny. These include the suggestion that DC motors are more expensive than standard AC squirrel-cage induction motors, a disadvantage made worse when maintenance of the brushgear or commutators is added, and that the DC thyristor converter offers much lower bandwidth than an AC inverter.

In fact, evidence suggests that brush wear in DC motors is not a major problem, with the latest generation of DC motors offering much improved brush life and advance warning when their replacement is required.

Bandwidth issues

DC motors are very competitively priced, especially in the middle power range between 10 and 250kW and, adding the lower cost of the converter, the total DC package comfortably beats the AC motor plus inverter package on price.

Certainly, the bandwidth of a DC drive is limited by the 300/360Hz natural commutation frequency, while the AC drive is capable of higher bandwidth due to its higher carrier modulating frequency and the forced commutation of the IGBTs. In that respect AC drives compete with servo systems, if maximum bandwidth is paramount for the application.

However, in the general industrial arena, higher bandwidth controllers can cause mechanical resonance and create more problems than their increased speed of response can solve. It is not uncommon for a perceived drive bandwidth problem to be actually caused by poor system design, wiring or installation, and a DC drive equipped with a good quality current controller can easily satisfy the requirements of most industrial applications.

Soft start

When it comes to the key issues of smooth control and ease of tuning, DC offers great advantages over AC.

The torque at start and near zero speed offered by DC is far superior to that of any AC system, and is achieved with inherently simple control techniques rather than the complex algorithms, shaft encoders and ventilation fans required by AC drives.This is because torque is generated in the DC motor 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, thereby in the optimum torque producing position. The resultant torque is practically a linear function of the two DC armature and field currents.

The heat dissipation in the windings at a given torque will be constant at any speed – including zero – and therefore motor cooling is not a problem. The converter can also independently set the required current levels in each winding to meet a certain load requirement without the need for complicated algorithms, as the interaction between the two is practically zero.

On the other hand an 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, so the resultant torque is a complicated function of applied voltage, frequency, rotor resistance and slip.

High heat produced

In addition, as speed approaches zero, a higher percentage of the available air- gap power is lost as heat in the rotor winding. If, however, rotor resistance is reduced to cut these losses, the available starting torque is also reduced, because torque developed is proportional to rotor resistance.

So, at low speeds, AC motors dissipate high levels of heat through the windings, and the need for special fan-cooling arrangements for induction motors driven by vector controllers partly defeats the notional advantage of the ‘inexpensive standard induction motor’.

The requirement for a position transducer (encoder) on the shaft of the induction motor if precise control of starting torque is required further undermines this idea of the standard AC motor.

The simplicity of DC drives also makes tuning a straightforward process when compared with the complexity of setting up an AC vector control system. Because the torque is the linear product of the armature and field currents, the armature and field current loops can be tuned separately to optimise torque response.

DC tuning

This can be achieved with the motor stationary, without the need for decoupling of the load or exposing operators to the hazards of uncontrolled rotating machinery before the plant has been fully commissioned. It is also a one-off procedure, with no need for repeated iteration, and can be done by auto-tuning or a simple manual process.

The armature current loop is a proportional + integral (P + I) controller with some adaptive correction for the discontinuous current mode of operation. The reason for the simplicity of this algorithm is the simple transfer function of the armature circuit, ie a fixed resistance (R) and a fixed inductance (L) in series.

In the continuous current mode, the P and I gains dictate the transient response. The integral gain is increased from a low value upwards to give a slightly overshooting response and then the proportional gain is increased until the damping of the transient response returns to nearly critical, i.e. settling without appreciable overshoot or oscillation.

If optimum response is to be achieved from zero current through the entire range, then the discontinuity boundary ‘DISC’ (the percentage of full load current where the current starts to be continuous) needs to be established. This is easily achieved manually, by gradually increasing the armature current from zero until continuity is observed on an oscilloscope, or by auto-tuning.

The three parameters P, I and DISC completely define the performance of the armature current loop and therefore the torque bandwidth.

The field current loop controller is similarly a P + I controller, which operates in the continuous current mode because of the large inductance of the field winding. Only the P and I gains need to be set for the field current, and these are of less importance from the torque transient response point of view, as normally only very small perturbations occur around the full field current reference point.

It is important to note that the accuracy in the setting of the above parameters only affects the optimum level of the drive’s performance, and the sensitivity of these settings is relatively low. Straight from the box, a DC drive will turn motors with a wide range of horsepower ratings safely and under full control even with the default control gains, and this is one of the main user-friendly features of the DC drive.

AC tuning

Compared with the simple model of the DC drive, the complexity of the AC vector control is orders of magnitude higher. As a result, the tuning process is far more complicated and parameter sensitive.

To implement the vector control calculations it is necessary to determine the motor magnetising current at different speeds and the rotor time constant. These are the main parameters that the auto-tune process tries to derive, along with other motor impedance values 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 whilst decoupled from the load.

This can be inconvenient, and creates the potential hazard of uncontrolled shaft rotation – which may be in either direction – at the beginning of the commissioning process.

It is also an iterative process, as the value of the rotor resistance (which is also affected by temperature) has a direct effect on the magnetisation values and vice versa. It is not uncommon to have to repeat the process several times, not necessarily with the same results, and vector tuning tends to be something of a ‘black art’ – even for skilled drive commissioning engineers.

In conclusion, both DC and AC drives can provide a good solution to variable speed control applications, but in terms of simplicity and smoothness of control, DC drives are still out in front.