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DC motors will not die or be ditched

02 December 2019

For 30 years, variable speed drive manufacturers have been planning to run down production of DC drives so that they could concentrate solely on the new AC technology. Neill Drennan defends DC

Many automation applications require accurate variable control of both speed and current, as well as full torque capability at very low (or even zero) speeds. Further, 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.

If the application is a standard pump or 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 constant torque throughout the full speed range, such as in extruders, wire and cable making, steel and paper mills, film and plastic manufacture, in winding/unwinding, and in printing presses, then below 5Hz (or 150rpm on a 4 pole AC motor) AC drives struggle to match the performance of DC.

If the process requires keep a constant tension then the current loop of the drive needs to be accurate and repeatable. The 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.

As the motor decreases in speed the cooling is also reduced

DC motors are typically through-blown with a separate blower, which remains effective even at 100:1 speed reduction. 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 drives use thyristors to control the motor, a technology that is well proven and reasonably simple. AC drives require 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. AC drives also switch at a much higher rate than 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 setting up 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.

A programmable DC drive, such as 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.

The move to digital technology has accelerated integration of related machines into sophisticated multi-function plants. Sprint Electric is launching the Anybus CompactCom option module for its PL/PLX drive range, enabling serial communications using almost any networking protocol.

Neill Drennan is BDM at Sprint Electric

Key Points

  • Controlling the speed performance and torque profile of the motor in fan mode allows energy saving if the motor speed can be reduced
  • If the application requires constant torque throughout the full speed range, then below 5Hz AC drives struggle to match DC
  • If the process requires keep a constant tension then the current loop of the drive needs to be accurate and repeatable

 
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