Typical of installations that use both large and small drives is this coal barge loader on the Ohio River.

Because of their many advantages over dc motors, ac motors are attractive for applications in the fractional to 5-hp range. These ac-motor advantages include lower cost, smaller size, higher speed capability, dirty-environment tolerance, and offthe- shelf availability. With the emergence of new power-device technology (such as IGBTs, MOSFETs, and IPMs) and microprocessors, ac drives are rapidly gaining acceptance in this low-end, dc stronghold.

Through a series of typical user questions, we’ll explore the pros and cons of both types of drive packages (motors and controllers). The advantages of each are summarized in the table.

Q: How about cost?

A: Although ac-drive packages are closing the gap, general- purpose dc packages are still less expensive than ac controllers due to their lower level of component complexity and fewer power controlling devices. An ac drive, Figure 1, has a rectifier section to convert ac to dc, and an inverter section to invert dc to controlled ac. By contrast, the dc controller, Figure 2, uses SCRs to both rectify and control the power. In this diagram, there are two sets of SCRs, one for forward and a second for reverse. Although, an ac motor is less expensive than a dc motor (shunt wound or PM), a dc package has the cost advantage.

Q: How easy is it to customize the controller for my application?

A: A dc controller is often an analog drive with analog components. All adjustments are made by on-board potentiometers or option cards. On the other hand, an ac controller is usually a digital device with easy customization via its on-board microprocessor, EPROM (Electrically Programmable Read Only Memory), or ASIC (Application Specific Integrated Circuit). AC generally gets the nod.

Q: What is the package speed regulation?

A: A dc package will generally provide 5% speed regulation by using armature voltage feedback or 1 to 3% with IR compensation. With simple slip compensation (similar to IR compensation on a dc drive), an ac package is capable of 1% speed regulation based on a standard NEMA Design B squirrel cage induction motor. Again, ac is usually best.

Q: How easy is it to upgrade speed regulation?

A: Improved speed regulation for either package requires adding a tachometer- generator or encoder on the motor and an appropriate feedback loop to the drive. Although this option can be accomplished on most drives of this size, it is easier on a dc drive.

Q: How about speed range when powering a constant-torque load?

A: Speed range is a function of the motor’s ability to dissipate heat and to operate without cogging at low speeds. To operate below about 50% of base speed and deliver full torque, both types of motors must be have more heat dissipating capability than is required for operation at base speed. A totally enclosed, nonventilated (TENV) dc motor is often selected for low-speed application. For ac motors, the common choice is to oversize the motor or add a separately powered blower. However, oversizing a motor may require selecting a larger drive controller.

From the cogging aspect, high-efficiency ac motors can usually operate over a 6:1 speed range, and dc over at least 20:1 without using a speed feedback device.

DC wins this one.

Continue on page 2