Variable speed motor controls ("drives") have been around for decades. In the early years, application and budget were the primary decision criteria when choosing between DC motor/drive packages and their AC equivalents. While this practice continues, advancements in AC drive technology have made the decision of where AC or DC meets application requirements more complex. The decision criteria are broader and now include:
Application requirements
Ease of installation and operation
Cost and availability
Maintenance and support
Impact of brushless DC
A quick review of published information on the subject yields numerous articles that tend to favor AC as the better choice, especially with the emergence of lower-cost, more powerful AC variable frequency drives (VFDs) - drives that enable induction motors to produce speed/torque responses previously only found in DC permanent magnet motors. However, these same articles focus on larger motors - the majority of drives and motors sold annually are of 2HP size.
The requirements of a specific application will influence the decision between AC and DC. Does the application require the motor to provide high starting torque? Examples may include centrifugal loads, such as rotating discs or wheels on a pitching machine, or pumping viscous fluids. This used to be a straightforward DC choice. However, even the most basic AC drives now feature a "boost" function, which generates additional torque at lower motor speeds. Moreover, for just a few extra dollars, an AC vector drive can be used, which can also produce greater motor torque at low speeds.
The power supply voltage of the drive/motor package is also a consideration. The cheapest AC frequency converters use three-phase AC power supply voltage – this simplifies the power front-end of the drive circuit. However, as an OEM equipment design engineer, you cannot always rely on the availability of three-phase power from customers. Single-phase power supply voltage is more common both domestically and internationally, but 230VAC single-phase power involves more complex VFD power circuits, and 115VAC is even more so, which increases costs.
Application Requirements (continued)
For DC drives of 2HP and smaller, the power supply voltage is minimally problematic – most drives today are designed to handle 115VAC or 230VAC (single-phase) power supply voltages using the same circuit design.
For applications requiring the drive to be installed in a wet environment (exposed to rain or direct washing), NEMA 4/4X (IP65/66) rated DC drives are readily available. The brand options for washdown AC drives are more limited, although several companies are said to be developing them.
For applications requiring motor braking or reversal, AC drives have the following advantages: they switch the output voltage multiple times to achieve motor braking and/or reversal, using the same solid-state devices (MOSFET/IGBT) that control motor speed. Basic DC drives typically use mechanical relays to reverse the polarity of the motor armature voltage and additional circuits to brake the motor (using power resistors) and confirm that the motor has stopped before allowing the armature voltage polarity to switch – increasing complexity and cost. Regenerative DC drives use solid-state technology to brake and reverse the motor, similar to AC drives – significantly increasing costs due to circuit complexity.
Ease of Installation and Operation
DC drives have an advantage in this category – they only require four wires to connect and can be operated with a toggle switch. Drives of the same model can usually handle a range of motor sizes and voltages – setup is as simple as making some fine adjustments to potentiometers based on the motor voltage and HP rating.
AC drives typically require some parameter configuration or "commissioning" for installation. Progress has been made in simplifying commissioning through the use of factory default parameter settings and "quick start" parameter settings. Although simpler, any number of parameter configurations for installing the drive can put pressure on factory personnel and their associated level of expertise and training. For a large number of customers, ease of installation is a primary factor when choosing a drive.
Cost and Availability
When comparing AC vs. DC, a general rule of thumb regarding the total cost of motors and drives is:
· For AC, as motor horsepower increases, the total cost is lower than that of DC.
· For DC, as motor HP decreases, the total cost is lower than that of AC.
If we collect data and plot the total cost against the HP rating, our curves should intersect at a certain (motor HP) point – let's use some pricing examples to see where that might be.
