Which type of adjustable speed drive is best?

April 1, 2000
With the increasing number of adjustable-speed drives, it’s hard to know which is best for an application. But this two-part series gives you meaningful background on the major drive types and tips to speed the selection process.

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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Common BLDC advantages include:
• Precise speed regulation without additional cost because the speed feedback device is always present.
• Starting torque and dynamic response comparable to conventional dc drives.
• Ability to operate several BLDC drives from a common dc bus. On the negative side, BLDC drives:
• Require wiring to the speed feedback device even if tight speed regulation is not needed.
• Use power transistors and snubbers, thus preventing the use of inexpensive line regeneration capabilities offered by thyristor dc drives. However, line regeneration modules can be added at a significant cost.
• Have heavy permanent magnets that increase rotor inertia and limit maximum speeds to 2,000 to 6,000 rpm, depending upon magnet material and motor power rating.

Standard inverter drives

Using the same basic power circuit as a BLDC drive, Figure 2, standard PWM inverters enable most standard threephase ac induction and synchronous motors to produce soft starts and adjustablespeed operation. Although the power circuits are similar for both drive types, the control circuits are vastly different.

PWM inverters offer:
• Pulse width modulated (PWM) output that produces current waveforms close to a sine wave.
• Speed regulation of approximately 1% (with slip compensation) of motor base speed for 100% load change without any speed feedback. That is, a motor with a 1,750-rpm base speed will have a 17.5- rpm speed droop regardless of the set speed.
• Simplified motor selection for special applications such as explosive and corrosive environments.
• Typical response of 5 rad/sec, which is adequate for most applications. If higher performance, including acceleration and deceleration times of less than a second, is required, you should consider a vector drive.
• Speed range of 10:1 without tachometer or encoder for speed feedback. Operating in this range produces generally smooth motor operation.
• Starting torque equal to the motor’s full-load rating, which is about two-thirds of an ac motor’s locked rotor capability. Hard-to-start loads may require selecting an oversized drive to assure sufficient starting torque.
• Higher motor-speed capabilities than BLDC or conventional dc drives. Of course, there are limitations, so check with the machinery and motor manufacturer before operating any motor above its base speed.

For proper inverter drive selection, you should also consider these factors:
• All drive types have limitations, and general-purpose inverters are no exception. Therefore, you should use caution for any application more demanding than centrifugal pumps and fans. Consider especially the required starting torque values.
• Operating at speeds below one-tenth of base speed may produce cogging. If this could be a problem, consider a vector drive.

Vector ac drives

Bridging the performance gap between dc drives and standard inverter drives, vector ac drives combine standard motors with microprocessor-based controllers — and in some cases, digital signal processors (DSPs) — to produce performance equivalent to or better than dc drives.

Here are some of the advantages of vector drives:
• Tight speed regulation, usually better than 0.01% when a precise speed feedback signal is used. Such a signal is usually supplied by an encoder or resolver. For less demanding applications, some vector ac drives monitor currents within the controller to eliminate the speed feedback; but the performance is close to general PWM ac drives.
• Wide speed range, including smooth operation from zero to base speed.
• High-starting torque. Vector drives with speed feedback can typically deliver 150% of full-load torque from zero to near rated speed for 1 min.
• Dynamic response of at least 50 rad/sec is common for drives through 50 hp. A few years ago, this response required special drives.
• Ability to use standard ac induction motors, although motors designed especially for vector drives deliver better performance.

Downside considerations are few but do include:
• Hold-back torque is by snubber or optional regeneration module.
• Proper controller operation is required for any electrical braking.
• Motor heating is always a consideration for drives operating with over 50% torque over the speed range.

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Subsets of dc, BLDC, and vector drives, servodrives are designed to deliver high performance with speed-loop dynamic response of more than 500 rad/sec. In operation, servodrives start and stop several times a second.

Servo drives are ideal for applications requiring absolute positioning or position coordination.*

Switched reluctance drive

Developed many years ago, the switched reluctance (SR) drive is gaining recognition. Some consider it a competitor to the technologies discussed earlier. Both the motor and controller are unique to this technology.

The motor is the secret to SR drives. The rotor resembles a laminated gear, Figure 3, and is built without windings or a commutator. This design offers exceptional ruggedness, extended operation at stall, and high-speed operation — more than 10,000 rpm.

The stator windings are wound on discrete poles. Figure 3 shows a 6/4 construction with six stator poles and four rotor poles. This design requires running six power leads to the motor plus wires for a resolver or other combination speed-position feedback device. The drive controller operates from a fixed bus dc supply, similar to that used in BLDC, PWM, and vector drives. In the SR inverter section — which is unique to the SR drive — typically insulated gate bipolar transistors (IGBTs) chop the constant potential dc to produce adjustable-voltage, adjustable-frequency ac for the stator windings, Figure 4.

Common terms

Here are some of the general terms used in many types of drives:

Dynamic braking: An uncontrolled method of braking. After power is removed from a dc motor armature (assuming the field supply is maintained), the motor acts as a generator and turns the kinetic energy of the rotating motor and load into electrical energy. This is dissipated as heat in a resistor. A normally closed contact on the motor starter connects the motor armature circuit to the resistor.

The above describes dynamic braking (DB) in its pure sense, and is frequently used with dc thyristor drives.

However, with the newer drives that have a dc bus, dynamic braking is also used to describe a braking method for drives that dissipate the energy in a resistor connected across the dc bus. (See “Selecting Nonfriction Stopping Methods,” PTD, 11/93, p. 29.)

No form of dynamic braking operates as a holding brake. Holding must be accomplished with a mechanical brake.

Dynamic performance: The ability of a drive to respond to a change in command. Typically measured in radians/sec when the motor is included, the higher the number, the faster the drive can respond. For many applications, a response of 1 to 5 rad/sec is sufficient. High-performance servo applications require response in the hundreds of rad/sec. When the response of only the drive controller is indicated, the response may be indicated in rad/sec or in Hertz (Hz).

“Other” variables: Part of the specifications for A-S drives, other variables include changes in temperature, humidity, line voltage, and frequency, plus drift. Often these “other” factors can cause drives — especially those with analog and hybrid analog-digital circuits — to change motor speed more than do load variations.

Regeneration: A drive capability that controls the deceleration of a load by letting the motor function as a generator and regulating the rate electrical energy returns to the plant power system, or to batteries if they are the main power source. Unlike dynamic braking, regeneration controls the rate of braking.

Snubbers: Installed in drives using power transistors — BLDC drives, dc servos, and ac inverters — snubbers dissipate the power produced while the drive supplies hold-back (negative) torque. Snubbers are usually resistors or large capacitors. Both types are frequently rated for a 10% duty cycle. Thus, they usually lack the continuous capabilities of regeneration, unless large components are added externally.

Safety Notes

In some installations, it may be necessary to install a mechanical brake if a machine must be quickly stopped during a power failure and if the machine friction is insufficient to stop the machine fast enough.

This brake requirement may apply to dc drives with wound fields and to BLDC and ac drives that use snubbers or line regeneration modules to provide holdback torque. During this hold-back phase, these drives require proper regulator operation. Thus, supply power must be maintained. Without supply power,
these drives will let machines coast to rest.

Time will tell if reality will meet the promises of this new technology. Part 2 of this series will discuss some of the common applications for electrical adjustable-speed drives and details of the drive requirements.

*For more information on servo drives, see articles listed in the PTD 1994 Handbook Issue, “Quadrennial Feature Article Index,” p. A29.)

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