Written by Harvey Ursaki, January 28, 2020
some may claim that direct-current (DC) motors are no longer relevant, that is
definitely not the case.
motors and DC converters/drives are alive and well in industry, driven by many
applications in which they are the best option.
(AC) motors have certainly decreased DC motor sales, and they do have
advantages in some applications.
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:
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
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
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
DC motors are often selected instead of AC motors for
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
Harvey Ursaki, February 26, 2020
Voltage drop is defined as the amount of voltage loss that occurs through all or part of a circuit due to impedance. Understanding voltage drop is the key to a successful circuit design. A common analogy used to explain voltage, current and voltage drop is a garden hose. Voltage is like the water pressure supplied […]
Harvey Ursaki, February 14, 2020
Contrary to popular opinion, bigger isn’t always better—especially when it comes to electric motors. Plant maintenance and engineering departments like having a little extra power available “just in case,” so they sometimes specify larger motors than applications require. But oversized motors cost more to operate—sometimes a lot more. Fortunately, there’s a simple procedure for determining […]
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 […]
Mr. Harvey Ursaki is the Director of Programming and Operations for Electrical Design Management Software Ltd. He has over 37 years experience in the electrical industry.
Experienced in thermal and hydro generation, radial distribution substations, multi-breaker, ring bus transmission terminals. Along with many years in the oil and gas industry, he has a well-rounded knowledge of the electrical consulting industry.
Prior to forming EDM, Mr. Ursaki was Director of CLA Utility Services Ltd. an electrical consulting service, specializing in developing electrical engineering standards, serving clients in Canada, USA and in the Caribbean.
He also served as a Supervisor of Transmission Engineering for a privately- owned utility in southern British Columbia, Canada.
He now brings his years of experience to EDM company, developing an electronic standards toolbox for Engineers, Technologists and Electricians worldwide.