Still relevant: dc drives important in an ac age

For brute force speed control applications where positioning or extremely precise control are not required, it’s hard to beat a dc motor and drive. While the basic design of the brushed dc motor has not changed much, the technology used to control the dc motor’s speed has evolved over the years.


The addition of advanced diagnostics, made possible by digital design, enhances the reliability of the dc drive and makes troubleshooting and setup easier as well. (Image courtesy of Parker Hannafin)They’ve been powering industry since the days of Thomas Edison, but unlike many older power transmission technologies, dc motors and drives are still around. For brute force speed control applications where positioning or extremely precise control are not required, it’s hard to beat a dc motor and drive. While the basic design of the brushed dc motor has not changed much, the technology used to control the dc motor’s speed has evolved over the years—from rheostats to vacuum tube rectifiers, to motor/generator sets, to solid state control using silicon controlled rectifiers (otherwise known as SCRs or thyristors).

dc drives have been used in applications including extruders, wire drawing, cranes and hoists, pulp and paper, mining, and oil and gas. While they have seen a decline in popularity over the last couple of decades (in many cases being replaced by ac drives), dc drives are far from obsolete. Available from fractional to thousands of horsepower, the dc system uses a relatively simple and low-cost drive. While the capital cost of a new dc motor is somewhat high, older existing motors can be effectively re-wound and refurbished many times for a reasonable price. This makes dc drives attractive in retrofit applications where there are viable motors.

Ease of retrofit

When replacing an obsolete or failed dc drive, some users may decide to evaluate moving to an ac drive system. This strategy will not only involve the replacement of the drive, but also require a new motor and the associated wiring between motor and drive. One fact that may be overlooked during the evaluation is that any mechanical power transmission components coupled to the dc motor may have to be replaced if the new ac motor is not mechanically exactly the same as the dc motor it is replacing. For example, these mechanical components could be costly gear reducers. In cases where a low base speed dc motor is used, the ac replacement could be prohibitively expensive or hard to find.

However, updating the drive only to a modern dc drive will allow existing motor and wiring to be used, and typically the drive alone will cost less than an ac drive of equal size. In the case of very high power dc systems, it is even possible to upgrade an existing dc drive without completely replacing it. Some manufacturers offer a digital “front end” that can drive the existing SCR bridges, offering all of the advantages of a modern drive at a very competitive cost.

One particularly attractive retrofit opportunity is the motor/generator set. M/G sets were a popular means to control groups of dc motors before the advent of practical solid state drives. A large dc generator was powered by a constant speed ac motor, and the generator’s output varied by way of field control. This variable output was then fed to the armatures of a series of dc motors, each with its own field control. The M/G set system was rather complicated, maintenance intense and inefficient. In addition, due to the age of many systems, users report difficulty in obtaining repair parts.

Replacing an M/G set system with dc drives is easy, and the existing drive motors can be retained, saving the expense of new ones. In addition to the elimination of high-maintenance and generally obsolete equipment, replacing an M/G set with dc drives also provides the benefit of energy savings. dc drives only use energy while running motors, as opposed to M/G sets which generally idle constantly, whether the driven machinery is being operated or not.

Simplicity of design

One reason for dc drives’ relative low cost and high reliability is their simplicity of design. Where an ac drive includes two stages of conversion, from ac to dc and back to ac, the dc drive converts only once by way of a controlled rectifier bridge. This means less work for the control logic as well, and simpler algorithms are used since the complexity of vector control is not being performed. Less control logic leads to fewer parameters to set, making for easier commissioning compared to ac. Efficiency of the dc drive is surprisingly high because it employs only one stage of conversion.

<< First < Previous Page 1 Page 2 Next > Last >>

The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
Pipe fabrication and IIoT; 2017 Product of the Year finalists
The future of electrical safety; Four keys to RPM success; Picking the right weld fume option
A new approach to the Skills Gap; Community colleges may hold the key for manufacturing; 2017 Engineering Leaders Under 40
Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
The cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Power system design for high-performance buildings; mitigating arc flash hazards
VFDs improving motion control applications; Powering automation and IIoT wirelessly; Connecting the dots
Natural gas engines; New applications for fuel cells; Large engines become more efficient; Extending boiler life

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
This digital report explains how plant engineers and subject matter experts (SME) need support for time series data and its many challenges.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me