ac motors and variable speed drives

8/18/2019 AC Motors and Variable Speed Drives

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From the pages of Plant Engineering Magazine

HOW TO MATCH AC MOTORS AND VARIABLE-SPEED DRIVES

Stewart Jackson and Frank Liggett, Rockwell Automation,Greenville, SC -- 7/1/2002

Key concepts

Inverter-duty motorsproduce full-loadtorque withoutexceeding thetemperature rise of theinsulation.

Consider theapplication, load

characteristics, speedrange, environment,and driverequirements.

The available torquefor starting and peakloads is different usingan AFD.

Sections: AC motor selection

Sizing the ac motor Sizing the ac drive General sizing methodfor use with multipleinduction motors Reflected waveconsiderations

Sidebars: Speed range explainedDrive technologiesexplained

Careful consideration must be given when selecting a drive/motor package toensure you get the desired performance. This article presents guidelines forselecting and sizing an adjustable frequency drive (AFD) and ac motor packagefor a given variable speed application.

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AC motor selection

Two general categories of ac induction motors are suitable for operation with acdrives:

•

Fixed speed NEMA design B motors with insulation systems designed forpulse-width modulated (PWM) inverter power• Inverter-duty motors designed specifically for inverter variable speed

operation.

NEMA has defined four standard classes (A, B, C, and D) of squirrel cagemotors. The NEMA design B motor is the most common type of three-phaseindustrial ac motor in use today. It is used in general industrial applications andhas normal across-the-line characteristics (starting torque and current,breakdown torque, and full-load slip).

Generally, inverter-duty motors are designed to produce full-load torque fromzero to base speed without exceeding the temperature rise of the insulationsystem. Inverter-duty motors are often described as motors that have a 1000:1constant torque capability (see the sidebar on "Speed range explained"). Thisdescription is just another way to say the motor is designed to operate from zeroto base speed and produce full-load torque over the entire speed range. Forapplications that require the motor to operate at, or near, zero speed the motorusually has a motor-mounted encoder for feedback to the AFD. Motors thatoperate at a speed range of 10:1 or less usually do not require encoderfeedback, unless the application requires very precise speed regulation of themotor or high response from the AFD and motor. High response means that the

output of the inverter changes very quickly with the slightest change of encoderfeedback.

Before selecting a motor for use with an AFD, it is important to understand thenature of the application in terms of load (torque) characteristics, speed range,environment, and drive requirements. Regarding torque characteristics, themajority of variable-speed ac drive applications fall into either variable torque orconstant torque applications. Centrifugal pumps and fans are variable torqueapplications where most fixed-speed, energy-efficient ac motors can be usedwithout concern of overheating. The horsepower required to operate centrifugalpumps and fans decreases with the cube of the speed. If you reduce the speed

of the ac motor to one-half of base speed, the horsepower required is only one-eighth of rated horsepower. On variable torque applications, the motor insulationsystem must be designed for PWM power. Most of this article addressesconstant-torque applications.

The exact motor performance curve for a given fixed-speed NEMA design Bmotor for operation on an AFD varies and should be supplied by the motormanufacturer. The motor speed/torque performance graph shown in Figure 1 is


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typical for most standard fixed-speed, totally enclosed fan-cooled (TEFC),premium-efficiency motors. The graph shows that these motors can produceconstant full-load torque over a 4:1 speed range, 100% of base speed down to25% of base speed. Many standard TEFC motors can be modified for AFDoperation by installing a larger fan to provide a wider constant torque speed

range.

Fig. 1. This motor speed/torque performance graph is typical for moststandard fixed-speed TEFC premium-efficiency motors. These motors

can produce constant full-load torque from 100% of base speed downto 25% of base speed, after which the full-load torque must start to

decrease.

Most standard ac motors are designed to operate at a fixed, rated frequency andspeed, such as 460 V ac, 60 Hz, 1800 rpm. At this fixed speed, the cooling

system, which is usually a shaft-mounted fan, will keep the motor fromoverheating. However, when operated as an adjustable speed device at slowerspeeds, the motor shaft-mounted fan produces very little cooling action. The full-load torque must start to decrease once the AFD reduces the motor speed to25% of base speed.

Figure 2 shows the capability of an inverter-duty motor designed specifically for AFD variable speed applications. Inverter-duty motors not only have aninsulation system designed for PWM power, but they are also designed tooperate at zero speed with full-load torque and not exceed the temperature riseof the insulation system. Traditionally, these motors were totally enclosed

nonventilated (TENV), totally enclosed air-over blower-cooled (TEAO-BC), ordripproof-guarded force-ventilated (DPG-FV) enclosures (Fig. 3.). Theseenclosures provide a cooling system that does not depend on motor speed.Therefore, these motors can provide full-load torque at any speed below basespeed — including zero speed.


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Fig. 2. Graph indicates the capabilities of inverter-duty motors that aredesigned specifically for AFD variable-speed applications. These

inverter-duty motors have insulation systems designed for PWMpower, and are designed to operate at zero speed with full-load torque

without exceeding the temperature rise of the insulation system.

Recently, some motor manufacturers have developed inverter-duty, (TEFC)motors that can provide full-load torque from zero to base speed withoutoverheating. These are special inverter-duty electrical designs that are notdesigned to meet NEMA across-the-line starting torque or current specifications.The designs provide reduced heat loss at low to zero speeds and higher lossesnear base speed where the fan provides maximum cooling. These specialinverter-duty TEFC motors also provide the performance shown in Fig. 2 in astandard NEMA TEFC frame and enclosure.

AFDs and motors may also operate in the constant horsepower range, shownabove base speed in Fig. 1 and 2. Center winders and machine tool applicationsare ideal for using the constant horsepower range. Constant horsepoweroperation of most standard TEFC motors is limited to 150% of base speed. Oneof the limiting factors is the amount of noise that the shaft-mounted fan producesat speeds above base speed. Inverter-duty motors, TENV, or blower-cooledmotors have a much wider constant horsepower operating range because thecooling is independent of motor speed. For a constant horsepower range greaterthan 150% of base speed, get application assistance from the AFD/motorsupplier.

Fig. 3. Surrounding air is blown through this drip-proof guarded force-


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ventilated (DPG-FV) ac motor, along the rotor, and out of the drip-proof covers on the drive shaft end. A motor such as this one canprovide 1000:1 constant torque.

Sizing the ac motor

Motor users are responsible for ensuring that drive train mechanisms, the drivenmachine, and process materials are capable of safe operation at the maximumspeed of the machine. Failure to observe these precautions could result in bodilyinjury or equipment damage.

The following procedure provides a conservative, engineering-based approachfor sizing and selecting various ac motors for use with an ac drive.

• Determine the required drive/motor output horsepower, starting torque,constant-torque speed range, constant horsepower range (if any), and

total speed range.• Select the type of motor required: standard, fixed-speed, TEFC, energy

efficient, or inverter-duty motor (TENV, TEAO-BC, DPG-FV, or TEFC).• Using graphs from the motor manufacturer, such as the ones shown in

Fig. 1 and 2, confirm that the required torque falls within the "acceptable"region of the graphs.

Sizing the ac drive

The capabilities of the ac drive are determined by its output current rating. Thechosen drive must have a continuous current rating equal to or more than the

maximum motor load current. Be sure to consider all loads including startupacceleration.

Fig. 4. The multipurpose industrial ac drive shown above is appropriate

for 90% of all 1-400 hp variable speed motor applications.


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Also, when applying an AFD with a motor, keep in mind that the available torquefor starting and peak loads is different using an AFD compared to across-the-linemotor operation. Typically, AFDs limit current to 150%, providing about 150%torque for starting and peak loads. An across-the-line motor can have startingtorque and peak load capabilities that exceed 150%. The AFD and motor

combination must be sized to produce the appropriate amount of torque that theload will demand.

General sizing method for use with multiple induction motors

To size the AFD for multiple motor applications or for any application from 6 to -120-Hz operation, use the following procedure:

• Determine the motor full-load amperes at rated line voltage for each motorto be driven.

• Add the full-load current requirements for each motor to determine the

total full-load current.• Add the high currents of any overloads which may exist, such as

acceleration, peak load, etc., to the full-load currents of all the motorsbeing controlled by the AFD, and determine maximum short-term load atline voltage. (Note that motor acceleration is by linear timed-rateacceleration control. Therefore, locked-rotor amperes normally associatedwith across-the-line starting of ac motors are not encountered.)

• Select the ac drive rating with a current capacity that will support therequired currents as calculated in the previous steps.

Once the appropriate drive has been selected based on current rating, verify that

the drive will provide the appropriate speed regulation over the desired speedrange of operation. Most manufacturers indicate speed range and speedregulation capabilities of their drives. The desired speed regulation and speedrange will also dictate the type of drive technology required: V/Hz, sensorlessvector, or flux vector (see sidebar "Drive technologies explained").

Reflected wave considerations

With increased market demand for AFDs and increased use of insulated gatebipolar transistor (IGBT) technology, awareness has been raised regardingcertain application issues surrounding the motor and AFD. Under certain

installation conditions, there exists potential for motor insulation damage whenoperated with PWM inverters. In some cases, high voltage spikes at the motorterminals can produce destructive stress of the motor insulation. Awareness ofthis issue along with the proper selection and application of the motor and AFDtogether will greatly reduce the risk of this type of failure.

Peak voltages seen at the motor input terminals depend on IGBT rise times(dv/dt). Typically IGBTs have rise times in the 50-400 nsec range. This, in


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conjunction with the short duration of pulses to the motor (50 nsec to 1 msec),can result in excessive overvoltage transients at the motor. The charts in Fig. 5show a typical PWM output waveform from the drive terminals.

The cable installed between the drive and the motor is impedance to the PWM

voltage pulses. These cables contain significant values of inductance (L) andcapacitance (C). These L and C values are directly proportional to the length ofthe cable run. When this cable-surge impedance does not match the surgeimpedance of the motor, a reflected wave occurs.

Solutions to reflected wave problems include:

• Specify motors with insulation systems designed for PWM power. Inverter-duty-rated motors have insulation systems designed to withstand theanticipated magnitude and rate-of-rise of the voltage spikes at the motorterminals.

•

On low-horsepower applications, use 240-V ac AFDs and motors, thereflective wave impact is fundamentally reduced to half that of a 460-V acsystem.

• Limit motor cable lengths to that specified by the manufacturer. Limitingmotor cable lengths to the manufacturer-specified limits ensures that thecable impedance matches the impedance of the motor.

• Install a drive output line reactor or filter. The inductance in the reactorinteracts with the fast output rise times of the drive to slow down rise timeand voltage magnitude, thus reducing any reflected waves or ringing. Thespecialized filter eliminates voltage reflection by closely matching cableimpedance with the passive elements in the filter.

•

Select a matching drive/motor package. Matching drive/motor packagesoffer superior design and proven performance because thesecombinations have been tested for dynamic stability. When appliedproperly, motor stress effects and high peak voltage are minimal.

— Edited by Jack Smith, Senior Editor, 630-288-8783, [email protected]

mailto:[email protected]:[email protected]


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Speed range explained

Most standard general-purpose ac motors are designed for operation at a fixedspeed, such as 1800 rpm. For example, the size of the fan (TEFC) is sized toprovide proper cooling at the designed speed. When motors are powered withan inverter, the motors now operate over a speed range, depending on theapplication. Most energy efficient motors can operate with full-load torque(nameplate amps) over a 4:1 speed range (450 to 1800 rpm). By installing alarger diameter fan, many of these motors can operate over a 10:1 speedrange (180 to 1800 rpm) with full load torque.

Traditionally, motors that needed to operate over a 1000:1 speed range (1.8 to1800 rpm) required an external blower motor to cool the motor. Some motormanufacturers have developed special TEFC motors specifically for inverteroperation that can operate over a 1000:1 speed range without a separateblower motor. If a motor can operate with full load torque over a 1000:1 speedrange, it can operate at zero speed with full-load torque.

Drive technologies explained

The following list explains different types of drive technology. Speed regulationand range dictate the type of drive required or an application.

• V/Hz — an open loop means of controlling a motor. The drive varies thevoltage and frequency to the motor to control the speed without anyfeedback.

•

Flux vector — an encoder is added to the motor shaft. It provides actualspeed feedback in a closed loop system, which allows the drive toprecisely control the motor speed.

• Sensorless vector — a way to approach flux vector speed regulationwithout the need for adding an encoder to the motor. The drivecalculates motor speed instead of having an actual speed measurementfrom an encoder.

© 2002, Reed Business Information, a division of Reed Elsevier Inc.

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