With electrical adjustable-speed drives being applied in a wider variety of installations, new questions rise to the surface. The following responses by application engineers from nine leading drive suppliers may save you valuable time and money.

Because drive manufacturers follow different but related product strategies, answers vary slightly from one manufacturer to another. Therefore, the following answers are a consensus of the information recently supplied.

Is a digital drive design better than an analog design?
Not necessarily. Digital advantages include finite setting, stability, diagnostics, and versatility. However, the analog is simpler, easier to set the desired speed, and commonly understood. For those general- purpose, less demanding applications, analog drives may be better.

What type of single- phase motor should I use on a single- phase inverter?
The drive industry has created this common question by shortening terminology. The term “single-phase inverter” usually means the inverter operates from single- phase ac plant power. However, in most cases, the inverter receives singlephase power and produces three-phase output for a three-phase motor. There are (or were) a few manufacturers that do make ac drives with single-phase outputs. If you have this type of drive, you should contact the drive manufacturer to determine the answer.

To explain the popularity of threephase motors, these motors are simpler and more reliable than single-phase motors, which often have starting switches and starting windings. Also, reversing the direction of rotation of three-phase motors is much easier than for single-phase motors.

When should I choose a vector control rather than a conventional inverter?
To answer that question requires understanding the requirements of the application, then selecting the drive to best meet the needs.

In general, a vector control — of which there are several versions — offers faster response and more precise speed regulation than does a general-purpose, adjustable-frequency drive (AFD), also called an inverter. Some vector drives require a speed feedback device — usually an encoder or resolver — others do not. However, using a speed feedback improves response, speed regulation, and low-speed operation. Some vector drives offer response comparable to dc servo drives.

From the cost aspect, the vector units generally cost more, but this difference is shrinking as vector units increase in popularity.

Table 1 in this article and the table in the companion article in this issue, “Which type of A-S drive is best?” gives relative responses and other characteristics of major drive types.

How do I select an inverter (AFD) to control several motors at one time?
To obtain the rating of the inverter, add the continuous (full-load) current ratings of the individual motors that will start and stop together. Multiply this sum by 1.05, then select an inverter that equals or exceeds this value.

An application that requires starting or stopping a motor while others are running is a different problem. Some inverter manufacturers tell their customers to avoid such a situation by using a separate inverter to power the motor that starts or stops while the others are running. They contend that reliability is jeopardized by the transient currents induced during a full-voltage starting or stopping of one or more motors. Others suggest adding R-C snubbers across each power pole of each starter. These components must be sized for the inverter carrier (modulation) frequency. Other drive manufacturers condone this practice without special precautions. Check with your AFD manufacturer to determine their policy.

For those manufacturers where it is permitted, add the total full-load currents of the motors that will be running to the locked-rotor current of the motor that will be started while the others are operating.

Avoid the temptation to just add the horsepower ratings in either of these cases. Use the current values.

In any case, each motor should have its own set of overload relays.

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