The imminent demise of DC drives technology has been predicted for at least 30 years. Yet it is still going strong and in many applications outperforms AC. Neill Drennan of British drives maker Sprint Electric explains where the DC out-performs AC wand should be used.
As a style conscious teenager I dreamt of one day driving a sporty car, but to my despair the automotive majors wanted to rationalise their manufacturing lines and instead produce hot hatch versions of their family runabouts. Unfortunately we car buyers did not have the purchasing power to stop this lamentable trend, so by the time I could afford a Sierra Cosworth they were already consigned to history.
At about the same time variable speed drive makers were planning to run down production of DC drives so that they could concentrate solely on the new AC technology. They told users that AC would be more energy efficient, more reliable, easier to maintain and therefore have lower lifetime costs.
But the users protested, saying DC had benefits of its own. They encouraged the manufacturers to keep up with their DC product lines – and the more astute drives companies even kept investing to further develop DC technology.
So why were drive users successful where car owners failed?
Well fundamentally automation applications require accurate variable control of both speed and current. Many applications also require full torque capability at very low (or even zero) speeds. Further, the drives need to be able to recover from shock loads and to compensate for these with minimal effect on the motor performance, and to be able keep accurate speed regardless of whether the motor is being driven or overhauled.
Beyond this most types of applications have additional requirements, some of which are best met with AC drives, others with DC. So let’s look at some examples.
If the application is a standard pump or a fan then AC is a sensible option. Being able to control the speed performance and torque profile of the motor in fan mode (or to give it its technical name quadratic mode) does allow good energy saving if the motor speed can be reduced.
However, if the application requires torque throughout the full speed range then this “energy saving” mode cannot be used. In these constant torque applications below 5Hz (or 150RPM on a 4 pole AC motor) AC drives struggles to match the torque performance of DC.
Typical constant torque performance applications include extruders, wire and cable making, steel and paper mills, film and plastic manufacture, winding and/or unwinding as in printing presses. In fact, nearly all continuous process applications require constant torque throughout the speed range. So, a lot of our traditional industries fit this DC remit.
Considering a continuous process line, the line might have multiple drives and there is likely to be a high level of interaction between them. If you consider the middle of the process you may have a nip, a press, a coater etc and it is not unusual for each section to be either driven or overhauled by the adjacent sections.
A standard AC inverter has a DC link between the supply and the voltage output and as a result has very limited braking capability. Once the DC link is saturated the drive must dump the energy (usually through resistors), but this is not continuous. In contrast a four quadrant DC drive does this all with ease, allowing the line to maintain accurate speed control.
If the process requires the product to keep a constant tension then the current loop of the drive needs to be accurate and repeatable. Typical accuracy of an AC drives current loop is +/-20% (this can be improved with additional equipment but at added cost and complexity). In contrast, the DC drive current loop accuracy is typically +/-2%.
Keeping cool
AC motor design usually incorporates air overblown cooling, typically an impeller fan mounted on the non-drive end of the motor shaft. As the motor decreases in speed the cooling is also reduced, thus a standard AC motor requires additional cooling if the motor speed is reduced more than 3:1.
DC motors are typically through-blown with a separate blower, which remains effective even at 100:1 speed reduction. Additionally the constant blow means the motor is positively purged against ingress from its environment and DC motors are also physically more compact than AC, especially over 37kW.
DC motors are based on a brushed design, which means that the angle between the magnetic flux and the magnetic field are mechanically kept at 90Degees regardless of speed. Thus full torque is guaranteed throughout the speed range, including zero speed. In contrast AC motors have an induced rotor, so suffer from slip and therefore torque drop-off at lower speeds.
Looking specifically at the motor, an AC motor needs to be force ventilated and fitted with an encoder to get close to the performance of a standard DC unit. Significantly, a correctly specified AC motor is typically three times the cost of standard AC motor.
The DC brushes do require changing occasionally during the motor’s life to maintain performance and the air filter on the blower motor will also need to be serviced, however neither of these tasks are arduous or costly.
DC drives use thyristors to control the motor, a technology that is well proven and reasonably simple – this should not be confused with crude! AC drives requires a bank of capacitors to maintain the DC link and these are charged and discharged every time the power is cycled, accelerating the ageing process. Further, AC drives switch at a much higher rate than the DC drives and as a result generate more electrical noise, which then requires more filtering.
The control capabilities of DC drives easily match that of AC. The DC drive control circuit gives full access to both the current and the speed loop as they are controlled independently. Full control of torque and speed is available to match the application needs. An AC drive uses the current loop to maintain the speed loop, which can make set up and performance of an AC drive far more complicated.
Quality DC drives control the main contactor, so power is removed from the armature of the motor when the drive is stopped; in this condition it is obviously impossible to produce any torque in the motor as it has no supply. The AC drive requires STO to replicate the same performance, complicating the control circuit.
In today’s high tech world we can say that digital DC drives have all the features of an advanced AC drive and a programmable DC drives (like the Sprint PLX range) can be configured to match advanced applications such as winder control, PID process control and full four quadrant speed and current control. All comms options are available including additional process control with simple expansion of the control circuit on Ethernet IP or Modbus TCP/IP networks for multi drive process control.
It is a fact of life that there are many old DC motors still in operation in the field. Replacing these with AC would be expensive and complicated, but updating them to the latest digital spec keeps the machine or process at its most efficient. It is worth making a balanced analysis of keeping the DC motor and updating the control logic rather than replacing the DC drive with AC and having to introduce a complex control strategy to give similar performance.
Company Information:
Sprint Electric Ltd., based in Arundel, West Sussex and founded in 1987, offers a wide range of DC motor control, with over fifty models covering both single phase and three phase, regenerative and non-regenerative applications. From low voltage servo performance controller to highly sophisticated fully digital three phase DC variable speed drives, Sprint Electric products meet the demands of countless industrial applications around the globe. Sprint Electric is committed to providing innovative products backed up by a high level of customer service to the worldwide industrial market.
Website: www.sprint-electric.com
