DC motors: Why are they still used?
DC motors were first developed in the early 19(Nineteen)th century and continue to be used today. Ányos Jedlik is credited as being the first to experiment with DC motors in 1827(Eighteen twenty-seven). William Sturgeon (1832(Eighteen thirty-two)) and Thomas Davenport (1837(Eighteen thirty-seven)) are credited with taking Jedlik’s laboratory instrument and trying to commercialize it. It wasn’t until 1871(Eighteen seventy-one) 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(Eleven) years. In 1888(Eighteen eighty-eight), 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(eighteen eighty-eight) 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.
A DC motor can develop full torque within the operational speed range from zero to base speed . 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. 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.
The second region is called the field-weakening (FW) operational range or constant-power range. 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 paaper 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.
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(Two thousand) hp, 3,000(Three thousand) hp, 4,000(Four thousand) hp and greater ranges. Standard low-voltage induction motor power ranges end around 800(Eight hundred) hp, 1,000(One thousand) hp or 1,200(one thousand two hundred) hp. If an application requires both more power and an AC induction motor, the voltage jumps into the medium-voltage ranges of 2,300(One thousand two hundred) V or 4,160(four thousand one hundred and sixty) V and even in the high-voltage range of 11(Eleven) 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.
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D.C. Motors |
Двигатели постоянного тока |
Developed |
Разработан |
Credited |
Приписывают |
Experiment |
Экспериментировал |
Commercialize |
Коммерциализация |
Accidentally |
Случайно |
Reigned |
Царствовал |
Factory |
Завод |
Three-phase electric system |
Трехфазная электрическая система |
Induction motor |
Асинхронный двигатель |
Reason |
Причина |
Speed characteristic |
Скоростная характеристика |
Base voltage |
Базовое напряжение |
Hydraulic pump and motor |
Гидравлических насосов и двигателей |
Full torque |
Полный крутящий момент |
Static inverters |
Статические инверторы |
Rectifiers |
Выпрямитель |
A.C. inverter |
Инвертор переменного тока |
Performance characteristics |
Эксплуатационные характеристики |
Field-weakening characteristic |
Характеристика ослабления поля |
Bush-commutator |
Bтулка-коммутатор |
Field-weakening range |
Диапазон ослабления поля |
Harbor crane |
Портативный кран |
Full torque |
Полный крутящий момент |
Power density |
Плотность мощности |
Inertia |
Инерция |
Acceleration |
Ускорение |
Deceleration |
Замедление |
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