DC motors: Why are they still used?

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.

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.