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Capabilities of electrical adjustable-speed drives are more advanced now than they were just a year ago. New concepts — including the increasing prominence of the switched-reluctance drive — combined with variations of more proven designs are expanding your options. All this progress may make last year’s guidelines passé.

This first part of the two-part series covers various types of drives and their general capabilities. The second part discusses the most common applications of A-S drives and the drive type best suited for an application. For some, more than one drive type will fill the application requirements. Also, various drive manufacturers offer different capabilities for the same type of A-S drive.

Understanding these variables and the following basics will help you journey wisely down the decision path.

DC drives

Long the mainstay for adjustablespeed drives, general-purpose dc drives use thyristors (also called SCRs for silicon controlled rectifiers) to both rectify the incoming ac and produce controlled dc, which powers a dc the motor, Figure 1.

Thyristor dc drives offer:
• Cost savings over other methods, because thyristors are less expensive for comparable ratings and are easier to control than power transistors. Generally, in the fractional and low-horsepower ratings, thyristor dc drives are the lowestcost A-S drives when supplied with general- purpose motors.
• Analog or digital regulator and firing circuits.
• Option of using drive with or without speed feedback by a tachometer or encoder mounted on the motor. Such feedback gives 1% or better speed regulation, even when considering other variables (see box).
• Regeneration provided economically.
• Excellent starting torque and intermittent overload capability, typically 150% of continuous torque rating for 1 minute. With the added exception of vector drives, the starting torque of most other A-S drives is less than that delivered by an ac motor started across-theline.

On the other side of the fence, dc drives, both thyristor and transistor:
• Use motors with brushes, which require some maintenance.
• Are costly if waterproof, explosion proof, or other special motor enclosures are required.
• Have top speed limitations imposed by armature windings, commutator, and brushes. General purpose dc motors above 5 hp are usually limited to 3,000 rpm. Special purpose, and more costly, dc servomotors are designed for higher speeds.

Brushless dc drives

Frequently termed BLDC, these drives are a hybrid between conventional dc drives and ac inverters. They are often marketed as functional replacements for dc drives but are “electronically commutated” so they avoid the brush and commutator concerns.

A BLDC motor is built with windings on the stator and permanent magnets on the rotor. A rotor-position feedback device, which is usually an encoder or Hall effect device, provides rotor speed information to the controller so it can handle the electronic commutation.

The power circuit, Figure 2 is almost identical to an ac inverter.

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