Answers on ASDs to save time and money

April 1, 2000
To save you from having to reinvent the wheel, here are the answers to the most frequently asked questions on electrical adjustable-speed drives

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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Will all the motors connected to the same inverter be synchronized?
If induction motors are used, the speed can vary 3% between loaded and unloaded motors. To keep the motors running at the same speed — within less than 1 rpm speed difference — requires using synchronous or synchronous-reluctance motors. The following answer gives another approach.

How can I get two or more motors to run at the same speed?
There are two basic approaches:
• Connect synchronous or synchronous- reluctance motors to a single inverter, as mentioned in the previous answer. The motors should always be started and stopped together to avoid overloading the drive. For motors in the fractional and low-integral horsepower ratings, this approach may be economical. For larger ratings, you should compare the costs with the following approach.
• Use a separate drive for each motor, and include a precision tach or encoder on each motor to give a precise speed feedback signal. Also, use a common speed reference for each drive, similar to that shown in Figure 1. Caution: If the driven sections are directly connected together or indirectly connected, such as by a process web between sections, an engineered system may be needed to prevent machine destruction or web problems.

When should an isolation transformer be installed with a drive?
This is one of the most persistent questions that continues to haunt us. Yet, in some cases neglecting to raise the question can degrade drive performance and contribute to personnel injury. Briefly, an isolation transformer changes voltage, if needed, electrically isolates the secondary from the primary, and adds impedance to the power circuit. Here are some of the details:
• A transformer can reduce a 480-Vac plant supply to, say, 240 V for drives. It can also increase voltage as well. An autotransformer can also increase or decrease voltage, but it does not isolate the primary from the secondary, Figure 2.
• The primary of an isolation transformer is magnetically coupled to the secondary, thereby electrically isolating the two circuits. Thus, if the main plant power system is a grounded system, an isolation transformer enables a drive system to electrically float. This usually requires two faults for a person to receive a serious electrical shock or to damage the drive and connected equipment.
• By adding impedance to the electrical system, an isolation transformer — or line reactors — reduces the rate of change of current (di/dt). This limitation on current often lengthens drive life. Some suggest installing an isolation transformer (or reactors) if the distribution transformer is more than five times a drive’s KVA rating. An isolation transformer also reduces the available short-circuit current if there should be a bolted short circuit.

Line reactors are less expensive than an isolation transformer, and they also add impedance to the power circuit. However, they tie the output directly to the input and avoid the isolation benefit.

Contrary to popular opinion, isolation transformers will not “clean up” a polluted ac power line. If a line has notching and distortion, an isolation transformer will pass these conditions through to the secondary. However, these transformers may reduce notch depth and some harmonics created by an A-S drive.

What is the operating power factor of adjustable-speed drives?
Drives with diode input rectifier sections (PWM ac, brushless dc, and other drives that have constant voltage dc buses) operate at about 90% power factor, regardless of the motor speed and load. By contrast, drives that have thyristor input rectifiers operate at 90% at full speed, but the power factor decreases linearly with speed, Figure 3.

What common types of signals can I use to give a drive speed commands?
Typically, drives accept 4 to 20 mA and 0 to 10 Vdc speed commands. Others include 0 to 1 Vdc and digital signals from a keypad. More advanced drives interface directly with PLC networks and other serial communications ports. These can be drive-to-drive and user-to-drive communications, many of which are configurable to meet specific hardware and application requirements.

Will dc permanent magnet (PM) motors connected in parallel load share (share the load equally)?
No! And achieving load sharing to avoid jerky conveyor operation, for example, is a common question. Unless the PM motors are perfectly matched — and the chances of winning a multimillion-dollar lottery are much greater than having matched motors — the uniqueness of each motor causes it to draw different current and develop different torque than another motor.

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To achieve load sharing, use either:
• Wound-field dc motors and adjust the individual field currents to get equal armature currents.
• High-slip motors, such as NEMA Design D, if they can be obtained.
• Install an inverter and adjust the Volts/Hertz ratio to produce lower than normal voltage for any given frequency. For example, 7.66V/Hz is normal for 460 V motor at 60 Hz. Then adjust the control to produce 6.66V/Hz, which would produce 400 V at 60 Hz. Such reduced voltage enables the motors to better load share, but it also reduces the output power capability.
• An individual controller for each motor and trim each controller for equal motor currents. All the drive controllers must be controlled by a master speed controller, Figure 1. In this latter case, the motors can be ac or dc including dc PM.

I have three blending pumps with mixing valves. How do I retrofit with ac adjustable-speed drives that will follow a master speed controller yet be able to individually set speed ratios for each pump?
As discussed in the previous answer, use a master speed controller and equip each drive with a ratio pot, Figure 1. The drives will go up and down in speed together and still maintain the same relative speed ratios.

Digital drives with encoder feedback can also achieve the same result with improved precision. Such precision can equal the speed-matching capabilities obtainable with mechanical systems.

In HVAC applications, can the supply fan and the return fan operate from the same inverter?
Typically yes. In many HVAC applications this is the preferred connection. Here, both motors (supply and return) operate at relatively the same speed, which maintains a constant CFM for both fans. In many cases, the supply fan has a slightly larger motor than the return fan. This approach also helps eliminate pressurization or a loss of pressure, both of which are undesirable.

I have a long distance between my ac drive controller and my motor. Will this cause problems?
It may. Possible problems include nuisance tripping of the ac drive, low voltage and lack of power at the motor, and highvoltage spikes at the motor. Consult the drive manufacturer for specifics, but most drives will operate properly with output wiring up to 100 feet. Longer lengths may present a problem. Output reactors often reduce the problem encountered with longer lead lengths. Consult your drive supplier for details on the line reactors. Make sure to install adequately sized wire for acceptable voltage drop.

Application engineers at the following drive suppliers contributed the questions and answers for this article. Their inputs and technical assistance is appreciated.

Baldor Electric Co., Ft. Smith, Ark.; Bodine Electric Co., Chicago; Cygnus Inc., Medina, Ohio; Halmar Robicon Group, Pittsburgh; IMO-Boston Gear Div., Quincy, Mass.; IMO-Fincor Electronics Div., York, Pa; Reliance Electric Co., Cleveland; Square D Co., Raleigh, N.C.; Warner Electric, South Beloit, Ill.; and T.B. Wood’s Sons Co., Chambersburg, Pa.

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