Written by Harvey Ursaki, January 28, 2020
While some may claim that direct-current (DC) motors are no longer relevant, that is definitely not the case.
DC motors and DC converters/drives are alive and well in industry, driven by many applications in which they are the best option.
Alternating-current (AC) motors have certainly decreased DC motor sales, and they do have advantages in some applications.
Understanding the differences between AC and DC motors is the key in determing where each works best and helps guide selection and specification.
Standard DC Motor Designs
DC motor designs include:
- Permanent magnet
- Compound wound or stabilized shunt
The basic operation of all these designs is similar. A current-carrying conductor is placed in a magnetic field and applying power through these conductors causes motor rotation. The difference among the designs is how the electromagnetic fields are generated and where – in either the rotor or stator.
In a permanent-magnet motor, the stator is stationary and mounted to the motor frame. It holds permanent magnets mounted in proximity to the spinning current-carrying conductors in the rotor. Applying a voltage through brushes contacting the armature on the rotor induces the current needed to produce mechanical force, which is rotation.
Connecting two wires to the motor and supplying the proper DC voltage will cause the motor to run.
Shunt, series, and compound-wound or stabilized-shunt motor designs have a rotor with electrical connections through a brush and commutator arrangement.
The brush/commutator acts as a switch to apply voltage to different coil segments of the rotor as it spins.
Brushing up on DC Motors
AC motors and DC brushless motors are popular and dominate many applications formerly occupied by standard DC motors.
Although many reasons explain this change, one of the most notable is that AC motors require less maintenance.
All motors require at least some minimal maintenance such as keeping the fan and motor clean or greasing non-sealed bearings. However, DC motors also require monitored and scheduled replacement of the internal brushes.
This is simple to perform on small motors. However, on higher horsepower (hp) DC motors, brush installation procedures are more complex and must be carefully followed.
On smaller, permanent-magnet DC motors, brushes easily and quickly can be changed. They are inexpensive and only take minutes to replace. A good rule of thumb is to replace the brushes once they reach one-third of their original length or every 2,500 hours of use, whichever comes first. This will ensure the brushes are always within specification.
Although brush maintenance is often seen as a disadvantage compared to AC motors, brushes in DC motors continue to improve. Designs that reduce brush wear, such as smaller diameter commutators, extend motor operating time between brush replacements. The design of the brush – including the surface area, shape and contact pressure – can also extend brush change intervals.
DC motors are often selected instead of AC motors for many reasons.
DC motors and controllers are often the low-cost option when compared to inverter-duty AC motors and drives. This is especially true for fractional hp applications.
DC motors have been around for more than 140 years, so they have a large installed base and corresponding widespread familiarity with their operation and maintenance.
For existing installations, replacing a DC motor with a new one – as opposed to redesigning the motor circuit to use an AC motor and drive – is almost always less expensive, quicker and easier.
Along the same lines, the simple design of DC motors makes service, maintenance and control well understood and easily supportable. Field excitation is not required, and brush replacement and motor service are well understood by the typical industrial electrician.
Even speed control is simple: Just adjust the terminal voltage, often using a local potentiometer.
Additionally, until the late 1980s, when the variable frequency drive (VFD) was fully developed, DC motors were the best choice for variable speed control, and this remains a well-supported option.
Torque at Low Speed
While the ease of controlling motor speed was a big part of its early success, several other DC motor characteristics make them the best choice in certain applications.
DC motors develop full torque at low speed and across the full operating range from zero to base speed.
This makes DC motors a good choice for driving constant-torque loads – such as conveyor belts, elevators, cranes, ski lifts, extruders and mixers. These applications are often stopped when fully loaded, and the full torque of the DC motor at zero speed gets them moving again without the need for oversizing.
Just a few reasons why DC Motors are still relevant in today’s industrial world. So, making the right choices when it comes to motor selection is an important part of the choosing the right motor in a specific motor application.
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