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Plenty of bad press has been given to DC drives and DC motors. Energy efficiency, maintenance and reliability have all been used to try and convert the customer over from DC to AC, yet it still survives.
So, why? Well fundamentally automation applications require accurate variable control of both speed and current to match the application requirements and many of these applications require full torque capability at very low speeds and at zero speed. They need to be receptive to shock loads and be able to compensate for these and recover with minimal effect on the motor performance. They also need to be able to keep accurate speed regardless of whether the motor is being driven or overhauled.
First of all, if the application is a standard pump or a fan, then switching to AC is a sensible option. Being able to control the torque performance 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) the AC drive still struggles to match the torque performance of the DC drive.
Typical constant torque performance applications include nearly all continuous process applications which require constant torque throughout the speed range. Several traditional industries fit this remit.
Considering a continuous process line, the line might have multiple drives with 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 next section. A standard AC inverter has a DC link between the supply and the voltage output and as a result it has very limited braking capability. Once the DC link is saturated the drive must dump the energy usually with resistors, but this is not continuous. The 4Q DC drive does this all with ease and as standard allowing the line to maintain accurate speed control. If the process requires the product to keep a constant tension (any process that has any elasticity) then the current loop of the drive needs to be accurate and repeatable. Typical accuracy of an AC drive’s current loop is +/-20%, which is very poor and, in most cases, unsuitable to accurately control torque. There is additional equipment that can be added to the AC drive to improve this performance, but it also adds an additional and unnecessary level of complexity. The DC drive current loop accuracy is typically +/-2%.
AC motor design usually incorporates air overblown cooling (IC0141), in other words an impeller fan on the back of the motor, mounted on the non-drive end of the motor shaft and blowing air over fins on the outside of the motor. This gives the motor IP54 protection but as the motor decreases in speed, the cooling is also reduced. A standard AC motor requires additional cooling if the motor speed is reduced more than 3:1 (500RPM on a 4 pole AC motor), this is all an additional cost. The DC motor is typically through blown (IC06), in other words it has a blower motor that blows through the motor. The IP rating of this motor is typically IP23 however the air is being blown through the motor so is positively purged stopping ingress into the motor from its environment. Typically, this type of DC motor design considerably reduces the physical size of the motor in comparison to AC, especially over 37KW and the DC motor is happy with a 100:1 speed range as standard.
The brushed DC motor design means that the angle between the magnetic flux and the magnetic field are mechanically kept at 90 degrees regardless of speed and so full torque is guaranteed throughout the speed range including zero speed. The AC motor has an induced rotor so suffers from slip. This means that the 90 degree angle is not maintained and torque suffers at low speed.
So, looking specifically at the motor, the AC motor needs to be force vent and fitted with an encoder to get close to the performance of the standard DC motor. A correctly specified AC motor is typically five times the cost of standard AC motor.
Yes, the DC motor does have brushes that do require changing during the motors life to maintain performance and the air filter on the blower motor will also need to be maintained, however this is nowhere near as arduous as some overexuberant salespeople have promoted.
The DC drive uses Thyristors to control the motor, the technology is well proven and reasonably simple, this should not be confused with crude. The AC drive requires a bank of capacitors to maintain the DC link and these are charged and discharged every time the power is cycled. Every time this is done it ages the AC drive. The AC drive devices are also switched at a lot higher rate than the DC drive and as a result it generates higher electrical noise which then requires more filtering.
Front end or control of the DC drive easily matches that of the AC drive. 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 and improve the speed loop so are not always available independently which can make set up and performance of an AC drive far more complicated.
The DC drive controls 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, again overcomplicating the control circuit.
It should not be confused between the power circuit simplicity of the DC drive and the front-end control capability. The digital DC drive has all the features of an advanced AC drive and if a programmable DC drive is selected (such as the Sprint PLX range) it 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.
There are many old DC drives in the market and updating these to the latest digital DC drive 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.
About the author:
Neill Drennan joined Sprint Electric as Business Development Manager in May 2018. His long pedigree of specialist knowledge includes 11+ years at Parker SSD as well as experience working at Oracle Drive Systems, SIEI UK, Eurotherm Drives and Renold. Originally a mechanical engineer apprentice, Neill re-trained after being more attracted to electrical engineering. He systematically built valuable industry knowledge adding a Business Studies Degree (BA Hons) into the mix, enabling him to work on a strategic level as well as offering expert technical sales support.
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