Case Study 2

AC or DC Drives?

There are some key factors that have to be considered when selecting a variable speed drive for your application. Hopefully, after looking at these factors, it will be easier to decide if your drive should be AC or DC.

Let’s look at some of the differences between AC and DC drive systems. The term, system, refers to the combination of the motor and the electronic control (the drive) that supplies power to the motor.

First, to get the horsepower needed, we look at the motor speed range (the base speed of the motor divided by the slowest running speed of the system) and the torque needed across the speed range. HP = (torque x speed)/5250, with torque measured ft lbs., and speed being the base speed of the motor.

In an AC system, the "full torque" range is limited by motor cooling. A totally enclosed fan cooled (TEFC) energy efficient motor can only provide full torque over a 2 to 1 speed range. With forced cooling added, (a fan with its own motor running at full speed), the full torque speed range will increase to 20 to 1. AC drives can provide 125% to 150% (of full load torque) in starting torque.

A DC system will supply full torque from 0 to the base motor speed, making it ideal for operations that have to start slowly and gradually increase to running speed. Also, DC motors above 5 horsepower usually come with forced fan cooling, increasing the full torque speed range. A DC drive will provide 150% starting torque, which makes it well suited for starting fully loaded systems.

Torque generated whether by telling a motor to stop faster than it would by coasting (stopping torque), or by an overhauling load condition like a downhill conveyor, turns the motor into a generator supplying power back to the drive. AC drives in general can’t regenerate this power back to the power line. Dynamic braking resistors can be added, but they’re limited to the amount of heat the resistors can dissipate. A DC regenerative drive can send the extra power back to the power line, making it useful in quick reversing and overhauling load applications. The dynamic braking on a DC drive only works when the drive is told to stop. This disconnects the motor from the drive, connects it to a braking resistor, and the energy in the motor is dissipated as heat through the resistor.

AC and DC VS drives come in a variety of forms for different environmental conditions. You can get an open chassis drive for mounting in your enclosure. NEMA 1 for clean conditions (it has no door gaskets), NEMA 12 for oily, dirty conditions (it has a door gasket), and NEMA 4 or 4x for wash down conditions, like food plants (it has bolts to hold the door tight and keep water out). For explosive environments, you need to call your Meier sales engineer, for custom set-up.

For those different atmospheres, you need the right motor. AC motors are made to handle more severe environments than DC motors. They come ready for tough duty, automotive duty, extra tough duty, chemical duty, and explosion proof, just to name a few. DC motors are limited because the brushes in the motor are sensitive to outside contaminants. The worse the environment, the shorter the brush life.

We’re working in an increasingly energy-conscious world of industry. These variable speed drives and motors will help your operation run more efficiently and more cost effectively for the long run.

Call Meier for any information or help you might need in deciding the best way to drive, or for any of your motion control applications.