Motion System Design
The best small-motor drive: ac or dc?

The best small-motor drive: ac or dc?

Small motor applicantions have tradictionally been the realm of single-phase, dc drives. But now, ac drives are winning acceptance. This will help you decide which is best for various applications.

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.

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Q: What will be the effect on utility power factor penalty?

A: A dc drive with its SCR section has a power factor that decreases at light loads and low speeds, its measurable range is about 20% to 95%. But, an ac drive with a fixed-diode converter section has a displacement power factor of 95%, regardless of load or speed. AC is best.

Q: How much faster can I run my process?

A: This is a function of the motor. DC motor speed is proportional to applied armature voltage, which is limited by the input supply. AC motor speed is a function of applied frequency from the ac drive, which is typically capable of 400 Hz operation (12,000 rpm on a 4-pole motor). Although a good rule of thumb is to limit a dc motor to 2,500 rpm and an ac motor to 6,000 rpm, you should always check with the motor manufacturer when operating above nameplate speed, regardless of whether it’s an ac or dc motor. Again, ac is best.

Q: What happens if I only have a single- phase supply?

A: DC drives in this size range are designed and rated for single-phase operation. Many ac drives are designed for single or three-phase power for low-horsepower applications. Some, when operating from single-phase power, require derating the output current by about 50%. Other ac drives incorporate an oversized dc bus capacitor to filter the higher current ripple to allow for singlephase operation without derating. But remember, the output of an ac drive is always three phase, regardless of input power supply. This one’s a tie.

Q: Can I run several motors from one drive?

A: With dc drives, you must have one controller per motor. But, as long as the motors all run at the same speed, it is easy to use a common ac drive. If the motors all start at the same time, size the drive rated current for the combined motor nameplate full-load amperes. If the motors do not all start at the same time, consult your drive supplier. In this case, choose ac.

Q: I need to bring the load to a quick stop, how do I do it?

A: Bringing a motor and load to a quick stop usually turns your motor into a generator, which means the regenerative energy developed must be dumped somewhere. That somewhere is usually a dynamic-braking resistor bank, although dc drives (even small, single-phase units) can easily be made to regenerate back to the line, Figure 2. An ac drive requires an additional transistor and logic to monitor the dc bus-voltage level to turn the transistor on at the proper point to dump the regenerative energy into an external dynamic-braking resistor. DC is the simpler choice.

Q: With a 460 V power supply, do I need a transformer?

A: DC drives of this size are typically limited to 115- V or 230-V input and, therefore, require a step-down transformer to operate on a 460-V power supply. However, 230-V and 460-V ac drives are available, so no transformer is necessary. Pick ac.

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