Absolutely not. They are still the workhorse for industry on many industrial plants.
It is a common misconception that replacing DC systems with AC systems is the way forward and this is pushed heavily by suppliers who are not familiar with DC motor technology.
Here are 13 key reasons why:
- History: DC motor control in some form has existed from the 19th Century. DC control has been achieved with numerous control methods including Mercury Arc Rectifiers, Face Plate Starters, Motor Generator (Ward-Leonard) sets, and latterly thyristor DC drives. Some DC motors from these early times are still operational to this very day.
- Speed control: DC motors do offer precise speed control, making them suitable for applications where speed regulation is critical. On winding applications field weakening techniques can be employed which allow constant power operation and as such can reduce motor frame sizing and widen the speed range of a given machine.
- Starting torque: DC motors can provide high starting torque, which is crucial in applications that require high start-up torques or quick acceleration like a Steel Rolling Mill or a Rubber Mixer.
- Direction: DC motors can easily be reversed by polarity swapping or utilising a 4- quadrant thyristor drive although some DC compound-wound motors may require additional switching.
- Control dynamics: DC motors do offer excellent control dynamics, allowing for quick and accurate responses to control signals. These characteristic makes them well-suited for applications such as rubber mixers, extruders, wire drawing and rope manufacture, steel and metals processing, elevators, and lifts etc.
- Size and weight: DC motors, especially square framed machine tend to be smaller and lighter than their equivalent standard AC motor equivalent, making them advantageous in space-constrained applications. This characteristic is valuable in industries where size and weight limitations are important.
- Regenerative braking: DC motors can act as generators when decelerating or braking, allowing them to convert mechanical energy back into electrical energy. This regenerative braking feature can enhance overall energy efficiency and reduce power consumption in applications that involve frequent deceleration or braking.
- Thyristor Converters: Thyristor converters are much simpler electronic devices and much cheaper than their equivalent AC solution, especially when it comes to regenerative applications. The electronic component count and physical size of a DC converter is significantly less power for power than its AC equivalent therefore less to go wrong and less to maintain.
- Harmonics: Like AC drives, DC drives do have a fairly heavy harmonic content in their 6 Pulse standard form, typically 25% 5th harmonics, 15% 7th harmonics and 10% 11th harmonic content. These harmonics can be mitigated by using a phase shifted (12, 18 or 24 pulse) transformer supply configuration and thyristor converters arranged as series or parallel configurations.
- Maintenance: DC motors do have a requirement for regular maintenance. Brushes do wear out and need replacing after a recommended number of hours and commutators require skimming periodically. Motor brushes are still available from several suppliers and not hard to obtain. With DC motors it is a little like your car, if you don’t maintain it, it may break down and with DC motors, when looked after correctly they can operate for many years without giving any problems or machine downtime.
- DC Motor Availability: DC motors are still manufactured and available on reasonable lead times. They can easily be manufactured to produce bespoke speed and torque ratings to match an application.
- DC Converter Availability: It was claimed 25 years ago that DC was dead however DC drive manufacturers are still developing and manufacturing new products to satisfy the current steady demand. DC Thyristor drives are still being developed to this day with manufacturers constantly updating their products to meet with client expectations. These new developments include safety features, high-level, high-speed communications platforms and physical size reductions.
- Legacy DC Drive v New: The technology involved in a typical thyristor stack has not changed markedly in the last 30 years or more. Most smaller DC drives use modular thyristor packs with plug in gate leads, on larger drives a ‘hockey puck’ thyristor tends to be used. As such its usually the control board that requires upgrading to incorporate safety, modern communication techniques and allow for future digitisation. There are many solutions in the marketplace that can allow the use of an existing thyristor stack with a modern control board, thus future proofing the control system yet retaining the basic thyristor stack. On much larger drives this method of control can be very cost effective.
The iconsys solution
It is important to note that the choice between DC and AC motors depends on the specific requirements of the application.
We have a wealth of knowledge on single and multi-drive DC drive control systems on most industrial applications.
With our platform partner supply network, we have access to a range of all the latest products and support from all the major manufactures.
We have a great service support team, ServConnect™, that are ready prepared to support clients on legacy plant and recommend upgrades and replacement converters if required.
AC motors, particularly three-phase induction motors, are generally more commonly used in industrial applications due to their cost-effectiveness, reliability, and widespread availability. However, DC motors do excel in many applications and there is still a vast number of DC machines currently in operation worldwide which if retained and the drive controller replaced can provide many years of service.
With our decades of system integration and automation solutions expertise, coupled with our platform independence, means you will always get the right drive choice for your next automation project. It’s not just about finding a DC or AC solution; it’s about finding the RIGHT solution for your needs.
Have a DC application? Or a need to upgrade your current legacy DC drive system to prepare for Industry 4? Then please contact iconsys and myself or one of our engineers will contact you.
Please also read our blog on the rise of the AC motor for a balanced view as to which may be better for your business. We’re here to help.
Author: Ian Pickersgill, Heavy Industries Business Development Manager, iconsys