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