Drive System Control Methods--AC vs. DC

An adequate economic comparison between two types of drives requires an analysis of all of the costs incurred over the entire life cycle of the equipment. In addition to the purchase price of the drives and related equipment, this includes all of the material and labor costs required to obtain and install the equipment. It also requires an analysis of the costs to put the drive into operation, plus all of the costs to operate and maintain the equipment during the entire time it’s expected to be in service.

The best way to determine which is the most economical system: perform a detailed analysis. There are no rules of thumb that will consistently and accurately predict the outcome of an analysis. Since the introduction of AFDs (Adjustable Frequency Drives) in the late 1960s, these drives have been slowly proving to be the most economical choice in an increasing variety of applications, but individual application details can often tip the balance either way.

The following is an outline of elements typically included in total life cycle cost. The items marked with an asterisk are the most significant items.

• Procurement expenses

• Project engineering expenses of selecting and specifying the equipment

• Purchasing department expenses

• Freight and receiving expenses

• Cost of equipment and installation materials

• Controller options and accessories

• Motor, options, and accessories

• Operator interface equipment

• Supervisory control equipment

• Machine interface equipment

• Transformer and other power distribution equipment*

• Power factor and harmonic correction equipment*

• Wire, cable, conduit, etc.

• Installation and commissioning expenses

• Operating expenses

• Electric power

• Periodic maintenance

• Planned downtime

• Unplanned downtime

• Cost of routine or major anticipated repairs

• Spare and/or replacement parts and equipment

In addition to the above, the following factors should be considered when analyzing AC versus DC systems.

Technology

Because it’s relatively easy and economical to control the speed and torque of a DC motor, DC drives have long been the adjustable-speed drive of choice. However most drive users prefer to use VFDs wherever possible because AC motors are much more rugged and reliable than DC motors and they require less maintenance.

For many years, drive manufacturers have been working to develop adjustable-frequency drives that will allow AC motors to be controlled as effectively and economically as DC motors. It’s evident that this development effort would result in an overall shift in drive use from DC drives toward AC drives. The motor is the controlling element of a DC drive sys tem, while the electronic controller is the controlling element of an AC drive system. Since the emphasis on technology advancement is primarily electronic rather than electromechanical, the overall progress in technology has a greater impact on AC drives.

Performance Capabilities

With the introduction of flux vector drives, there are virtually no fundamental performance limitations that would prevent a VFD from being used in any application where DC drives are used. Using the latest control techniques, the performance available from AC motors equals or exceeds the performance available from DC motors.

In areas such as high-speed operation, the inherent capability of AC motors exceeds the capability of DC motors. Several manufacturers now offer inverter duty motors that are specifically designed for use with VFDs.

Inverter-duty motors have speed-range capabilities that are equal to or above the capabilities of DC motors. In addition, DC motors usually require cooling air forced through the interior of the motor to operate over wide speed ranges. Totally enclosed AC motors are also available with wide speed range capabilities.

The only question should be one of availability of models in the required horsepower range or implementation of certain optional capabilities or special functions.

Motor Purchase Price

The price of the motor must be evaluated along with the cost of all of the other drive system equipment. Although DC motors are usually significantly more expensive than AC motors, the motor-drive package price for an VFD is often comparable to the price of a DC drive package. However, if spare motors are required, the package price tends to favor the VFD. Since AC motors are more reliable in a variety of situations and have a longer average life, the DC drive alternative may require a spare motor while the AC drive may not.

Since DC motors tend to be less efficient than AC motors, they generally require more elaborate cooling arrangements. Most AC motors are sup plied in totally enclosed housings that are cooled by blowing air over the exterior surface which is in intimate contact with the stator core, the source of the majority of the losses.

Since cooling air does not enter the interior of the motor, dirt and contaminants in the air don’t usually cause problems. Totally enclosed DC motors are usually very expensive because they must be over-sized to adequately dissipate heat because of losses in the armature.

DC motors are usually cooled by blowing air through the interior of the motor. At a minimum, the motor will be equipped with a blower and filter box. If the atmosphere is particularly dirty or corrosive, clean air must be ducted in from a centralized cooling system. In evaluating the price of the motor, it’s important to consider the cost of a cooling arrangement that is adequate for the application.

Cost of Motor Options

AC motors are available with a wide range of optional electrical and mechanical configurations and accessories. DC motors are generally less flexible and the optional features are generally more expensive. Optional mechanical configurations include various types of enclosures, special shafts, optional conduit box locations, special bearings, and other options.

Mounting options include vertical mounting and several types of flanged end brackets. Motor accessories include separately powered blowers or fans, tachometer generators or encoders, various types of temperature sensing devices, space heaters, friction brakes, and other items. Some con figurations such as explosion-proof enclosures are very expensive options for DC motors compared with AC motors.

A number of AC motor manufacturers have developed motors specifically designed for use with adjustable-frequency drives. As a result, AC motors are readily available with special cooling arrangements or enhanced thermal capacity for wide-speed ranges. Motor-mounted tachometer generators or encoders are also readily available.

A VFD without tach feedback can be used in some applications where DC drives typically require tach feedback. Since a tach generator or encoder adds significantly to the price of the motor, it’s important to carefully consider whether or not it’s required.

Additional System Component Costs

As mentioned earlier, a valid comparison of equipment prices must include the cost of all of the components of the drive system. Most DC drive installations require a drive-isolation transformer or input-line chokes. The transformer or chokes provide impedance, which reduces power line notching caused by the SCRs in the DC controller. Since PWM drives have a diode bridge input section, they don’t cause line notching and therefore have less need for added input impedance.

Large DC drives, with motors rated 1000 HP or higher usually require rather costly armature circuit chokes to provide sufficient commutating reactance to ensure spark-free commutation and acceptable brush life.

Large AC drives require no equivalent expenditure.

The cost of power factor and harmonic-correction equipment must also be considered as part of the total drive package price. As mentioned earlier, DC drives sometimes require a centralized cooling system that provides clean cooling air through ducts to the motor. Although a centralized cooling system is used to supply air to multiple motors, it’s a drive-system component that must be considered in any cost comparison.