Electrical Systems / Industrial Motors / Energy Management

DC motors: Why are they still used?

The reasons come from the user base, R&D and the application.

By Bob Simon M.Sc., P.E.

DC motors were first developed in the early 19th century and continue to be used today. Ányos Jedlik is credited as being the first to experiment with DC motors in 1827. William Sturgeon (1832) and Thomas Davenport (1837) are credited with taking Jedlik’s laboratory instrument and trying to commercialize it. It wasn’t until 1871 when Zénobe Gramme’s design of a dynamo was accidentally connected to a second dynamo that was producing a voltage that the DC motor we think of today start to turn and do work.

The DC motor reigned alone in the factory for only 11 years. In 1888, Nicola Tesla stepped into the factory with today’s well known three-phase electric system and the AC induction motor has been taking work away from the DC motor ever since.

So, the question remains — why has the DC motor continued to be used from 1888 until today? A primary reason is the motor’s variable speed characteristic. When the voltage to a DC motor is increased from zero to some base voltage, the motor’s speed increases from zero to a corresponding base speed. An induction motor, on the other hand, always runs at full speed. If a speed other then this is desired, it must be achieved via belts and pulleys, hydraulic pumps and motors, or gear boxes and clutches. These devices provide for rotation at a speed something less (or greater) then the design speed, but adds mechanical complexity.

 Figure 1. Torque comparison of DC and AC motors. Motor speed in per unit values is located on the horizontal and torque developed by the motor in per unit values on the vertical axis (1 = 100%). The green line is the nominal developed DC motor torque and shows that a DC motor can develop 100% torque from 0-100% speed. Neither the AC self-ventilated nor the forced ventilated motors can match the torque development at very low rotational speeds.
Figure 1. Torque comparison of DC and AC motors. Motor speed in per unit values is located on the horizontal and torque developed by the motor in per unit values on the vertical axis (1 = 100%). The green line is the nominal developed DC motor torque and shows that a DC motor can develop 100% torque from 0-100% speed. Neither the AC self-ventilated nor the forced ventilated motors can match the torque development at very low rotational speeds.

A DC motor can develop full torque within the operational speed range from zero to base speed (Figure 1). This allows the DC motor to be used on constant-torque loads such as conveyor belts, elevators, cranes, ski lifts, extruders and mixers. These applications can be stopped when fully loaded and will require full torque to get them moving again.

Getting a variable DC voltage to a DC motor was done in several ways. The easiest was with a large carbon rheostat that either increased or decreased the voltage supplied to the motor. It also was done with motor-generator (MG) sets, which used a constant-speed AC motor directly coupled to a DC generator. The generator’s field was then increased or decreased. This resulted in an increase or decrease in the generator’s terminal voltage. As terminal voltage increases or decreases, the speed of the connected DC motor also increases or decreases.

Static inverters were developed later and the rectification of AC to DC was done using vacuum tubes. Semiconductors were developed and the analog converter replaced the rectifiers. Finally, the microprocessor was developed and the converter went digital. That’s where the technology stands today with respect to providing an AC-to-DC conversion.

As the development of semiconductors continued, the development of the digital DC converter also continued. More importantly, this lead to the development of the AC inverter. The AC inverter is the bit of engineering technology that was going to push the DC motor down the same path as the Pickett slide rule and the Post draftsman’s compass. The AC inverter allows a standard induction motor to be operated at any speed, just like the DC motor. And, it does this without brushes. Brushes are the primary maintenance headache when using a DC motor.

Performance characteristics

DC motors have three operating regions (Figure 1). The first is from zero to the base speed and is called the called the constant-torque range. As motor voltage is increased from zero to base voltage, the ability to develop full torque remains constant. Motor power increases from zero to rated power as the voltage changes. Often, this region is labeled VP/CT for variable power/constant torque. This characteristic of a DC motor lent itself well to applications that had to operate at various speeds while fully loaded.

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 Figure 2. Power developed by a DC motor. In the B region the DC motor develops constant torque and the power varies with speed. In the F1 region power remains constant and torque varies. In the F2 region both power and torque varies.
Figure 2. Power developed by a DC motor. In the B region the DC motor develops constant torque and the power varies with speed. In the F1 region power remains constant and torque varies. In the F2 region both power and torque varies.

The second region is called the field-weakening (FW) operational range or constant-power range (Figure 2). This operating range normally ranges from the base speed to a speed that is about two or three times the base speed. When at base speed (full voltage) and the field current is reduced, the motor increases in speed. In this region, the power remains constant as speed increases. The increase in speed comes at the expense of a reduction in the torque available to turn the load. Often this region is labeled CP/VT for constant power/variable torque.

The take up rolls at the end of a paper machine operate using this field-weakening range. Paper comes off the machine at a fixed speed. When a new roll is started, the load on the spindle is the lightest (no paper), but must rotate fastest because it is at its smallest diameter. At this point, the DC motor is in its full field-weakened mode — torque is at a minimum but speed is at its greatest. As the roll fills with paper, it requires more torque to turn the spindle — the load is increasing. The paper comes off the machine at a fixed speed — as the paper roll builds, the roll diameter increases, and the spindle needs to turn slower to keep the roll’s linear surface speed the same as the paper machine. When operating in the field-weakening range, the field is strengthened as the roll builds, which increases torque and decreases spindle speed. In the paper industry, DC motors were used on more or less all of the machines that did some type of work with paper rolls. It was the field-weakening characteristic that allowed this to be the case.

The third operating range is an extension of the field-weakening range. This extended field-weakening range ranges from about four to five times the base speed. As the field is further weakened for even greater speed, it gets more difficult for the current to move between the brush and the commutator. If too much current is flowing, there’s an excess of sparking at the bush-commutator junction, which damages both components. Damage can be prevented at these higher speeds by limiting the current flowing to the brushes. This region is defined as a third area because now both power and torque are dependent on speed. Often, this region is labeled VP/VT for variable power/variable torque.

The application to which this third operating range is applied is a harbor crane that unloads containers from a ship. As anyone that was in the Navy knows, ships are built to be at sea. A cargo vessel tied to a pier isn’t making money. As the harbor crane is picking up the container and lifting it out of the hold, the DC motor is operating in the first region, which allows full torque from zero to base speed. Once the container is placed on the pier and off the hook, the torque needed to lift and get the hook back into the hold for the next lift is a fraction of the lifting torque. During this time, the DC motor operates in the third region, cutting the cycle time between lifts to a minimum. The quicker the hook returns to the hold, the more containers that can be unloaded (or loaded) in a given time period and the quicker the ship gets back to making money.

For almost 100 years, the industry was using one electrical technology to get a variable-speed shaft.

– Bob Simon M.Sc., P.E.

Traditionally, DC motors have had a smaller power density then the conventional induction motor. That is to say, for a given power, the physical size of the DC motor is smaller then the physical size of an equivalent AC induction motor. Smaller is better, and when thinking about footprint, traditionally DC has a smaller one. This also is true for the DC converter as compared to an AC inverter. An AC inverter normally needs two bridges — one to perform a rectification and another to do the inversion to the needed frequency. The DC converter needs only a rectification bridge and is, therefore, smaller in size, has less heat losses and is less complex.

A smaller motor will have a smaller rotor. A smaller rotor means less inertia. DC motors are used in applications with an operating cycle that includes acceleration and deceleration. With less rotor inertia, it takes less time and power to accelerate or decelerate. This allows for quicker reversals, shorter cycle times and faster production.

Because of the potential to have a high power density, DC motors can push well into the 2,000 hp, 3,000 hp, 4,000 hp and greater ranges. Standard low-voltage induction motor power ranges end around 800 hp, 1,000 hp or 1,200 hp. If an application requires both more power and an AC induction motor, the voltage jumps into the medium-voltage ranges of 2,300 V or 4,160 V and even in the high-voltage range of 11 kV. Having a facility with these voltages requires a different level of equipment capabilities and a knowledge and skill level not found in the average trade electrician.

Current state of the technology

Getting back to the original question: DC motors, why are they still used? There are two reasons. The first can be summed up in two words: installed base. Let's remember that the DC motor was the primary variable-speed shaft-turning device since 1888. When AC inverters and AC motors started to replace DC in machines can be debated, so let’s put a stake in the ground and call it 1987. For almost 100 years, the industry was using one electrical technology to get a variable-speed shaft. It takes a good number of acres of ocean to get an aircraft carrier running at a full bell turned around and headed in the other direction.

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Engineers, machine builders and maintenance staffs had and have knowledge of DC. DC converters are simpler in design than AC inverters, lower in cost and easier to repair. DC motors can be repaired repeatedly. If a piece of machinery is powered by a DC converter and motor, and if either one should fail, it’s easier (and cheaper) to replace the failed item then to convert the machine to AC. If a plant has 10 machines using DC and wants to order an 11th, there’ll be a strong bias to purchase what has worked before.

During past several years, DC motor manufacturers’ ongoing R&D has concentrated on redesigning the most maintenance-intensive section of the DC motor, which is the commutator and brushes.

As design engineers continue to increase the power density for a given frame size, the motor’s commutator gets smaller. As the circumference of the commutator shrinks, there’s less brush wear with every turn of the rotor. Reduced brush wear results in extended intervals between brush changes. Engineers also have redesigned brush blocks, pressure fingers and springs to allow for longer brushes. With longer brushes, the interval between brush changes extends further, providing for longer periods of operation without a maintenance shutdown. DC motors can be purchased with brush wear sensors, which warn that a brush is worn down to its lowest level and requires changing. Brush wear sensors often prevent commutator damage from a worn brush being left in too long and resulting in costly repairs.

Active research and development

With the DC motor being one of the oldest technologies, you’d think R&D has ended. Many motor companies continue to offer their older designs and there are some that have dropped the product completely. But, there are motor companies that continue to invest in developing the technology. Using software modeling tools, engineers can get a better understanding of both the magnetic flux and thermal flows in the motor laminations. Companies with active R&D programs are incorporating developments in insulating materials into their designs. Slight changes in lamination geometries, metallurgy, and insulating materials allow for increased power density and smaller motors.

Companies with active R&D also are helping to reduce maintenance costs by extending brush life. This can be done by designing smaller commutators, lengthening the brushes, adding brush wear sensors and making it easier to replace brushes. Studying the brush/commutator junction is a never ending activity. There are groups using the latest sensor and control technology to determine what is the best environment (temperature, humidity, pressures) that leads to optimum junction performance. They’re also asking what can be done to ensure the junction environment is optimum at the locations and ambient environments in which the motor operates.

Everyone has heard the story that in 1899, the head of the U.S. Patent Office sent his resignation to President McKinley because, he said, “Everything that could be invented has been invented.” This turned out to be untrue and so is the tale that DC motors are no longer being used and no one is investing in research and development. The applications available for the DC motor are fewer than in the past. However, the operational characteristics of higher power density, low inertia and higher speed ranges continue to make the DC motor the preferred choice for many machine builders. Also, the magnitudes of the installed and knowledge bases cause users to request DC motors as prime movers even on new equipment.

Bob Simon M.Sc., P.E. is a DC motor specialist at ABB in New Berlin, Wisc. Contact him at bob.simon@us.abb.com and (262) 785-8592