In the past, applications operating at less than 300rpm, and rated from 500 kW to 5000 kW, have been controlledusing a medium-voltage constant speed synchronous motors. Commonapplications include crushers and various mills used inmetals, cement or mining.Replacing a constant speed with variable speed drive control brings substantial operating and maintenanceimprovements. Variable-speed drives provide soft starting, precise speed andtorque control and low stress on the electrical network.Soft starting lets the motor get up to speed gradually, therebyavoiding high inrush currents that can prevail when direct-onlinestarting is used. As such, the stress on the motor and allcomponents in the transmission train are much reduced. Thisavoids failure of key components while reducing maintenancecosts and prolonging motor and transmission life time.

ABB industrial drives feature the motor control platform, and direct torque control (DTC). DTC provides the highly accuratespeed and torque control often needed in slow speedapplications.With a power factor approaching unity, the variable-speeddrive has low reactive power consumption; thereby loweringvoltage variations on the supply network. The variable speed drive and motorcan be connected to weaker supply networks without theneed for power factor correction equipment. An advantage of using new synchronous motors with ABB industrial drives is that, for most cases, the existingfoundations can be re-used.A key benefit of variable-speed control is that the new motorhas fewer pole pairs and a smaller shaft height. This is becausea constant speed synchronous motor needs to be designed forits maximum speed. So a 2500 kW motor requiring 150 rpmhas 40 poles and typically a shaft height of 2000 mm. However,using variable-speed drive, the motor has between 8 to 24poles and a typical shaft height between 1250 to 1600 mm.This substantially lowers the motor costs and even when thedrive and transformer are considered, the investment will beless expensive than a 40-pole motor.

Benefits Of Replacing Constant Speed With Variable Speed:

• Soft starting capability reduces stress on motor andtransmission train
• Optimum speed according to process requirements
• Lower maintenance costs and prolonged motor life
• Low stress on the electrical network through unity powerfactor and low reactive power consumption
• Choice of smaller traditional synchronous motor
• No need to alter foundations thereby avoiding any loss ofproduction and costly civil works
• Typically lower total cost than a new constant speedsynchronous motor

To learn more about replacing constant speed with variable speed, or forvariable speed drive repair and replacement quotes, contact Precision Electric, Inc.

Permanent magnet servo motors are also known as brushless servo motors and/or PM (permanent magnet) servo motors. Permanent magnet servo motors are used when an industrial application requires high performance and precise speed regulation. Permanent magnet servo motors are manufactured with a designated controller known as a servo drive or servo driver. Permanent magnet servo motors use position feedback to precisely regulate motor control by being paired with an encoder. Encoders measure a brushless servo motor’soutput shaft position which is compared to the command position. When the servo motor output shaft doesn’t match the demand signal, an error signal is sent to the servo motor controller; the controller adjusts accordingly and sends a signal back to the servo motor until the demand position is met. PM servo controllers (servo drive, driver, etc.) are never included in the base price of the servo motor. Servo motor systems can be expensive but the cost is justified with optimal manufacturing processes via precise speed regulation.

Servo motors output more torque than brushed direct current motors. Brushless servo motors aren’t as loud during operation than brushed direct current servo motors. The lifespan of brushless servo motors exceeds than that of a brushed direct current motor because there aren’t brushes or a commutator to maintain. The idea behind brushless servo motors was to be able to integrate control function of the servo motor with industrial electronic components and personal computers; and to precisely position machines in manufacturing processes. Some manufacturers use brushless servo motors in explosive and dirty environments because brushless servo motors are entirely closed and sealed to prevent dust or sparks to reach the servo motor parts.

Permanent Magnet Servo Motor Design Process

This process comprises threemain steps: Electromagnetic, structural and thermal designs. Electromagnetic design startswith magnetic circuit modeling and parameter optimization with a given set of designspecifications. A series of optimizations such as pole number, loading, current density,dimensional limits etc. have to be performed to find the optimum parameters of the motorbefore proceeding further. When a design is obtained that meets the technical spec, a quickmotor simulation and the influence of parameter variation must be carried out usingsimulation software such as SPEED (PC-BDC Manual, 2002). A detailed electromagneticfinite element analysis (FEA) either in 2D or 3D is the next step to verify that the designmeets the specified torque-speed characteristics and performance. After an electromagneticdesign is finalized, structural and thermal analyses (MotorCAD Manual 2004) have to becompleted. Structural analysis is not a necessity for design on low speed permanent magnet servo motors. If motor does not meet the structural or thermal tests, then theelectromagnetic design study should be repeated for a better design.

Permanent magnet servo motors are widely used in many industrial applications for theirsmall size, higher efficiency, noise-free operation, high speed range and better control. Thismakes quality of their torque an important issue in wide range of applications includingservo applications. For example, permanent magnet servo motors used in defense applications, robotics, servosystems, electric vehicles all require smooth torque operation.One of the most important issues in PM servo motors is the pulsating torque componentwhich is inherent in motor design. If a quality work is not completed during the designstage, this component can lead to mechanical vibrations, acoustic noise, shorter life anddrive system problems. In addition, if precautions are not taken, it can lead to seriouscontrol issues especially at low speeds. Minimization of the pulsating torque components isof great importance in the design of permanent magnet servo motors.In general, calculation of torque quality is a demanding task since the torque qualitycalculation does not only consider the torque density of the motor but also consider thepulsating torque component. Therefore, a mathematical approach about torque qualityshould include harmonic analysis of electric drive system rather than a simple sizing of the motor.

To learn more about permanent magnet servo motors or for servo motor repair and replacement quotes, contact Precision Electric, Inc.

The first electric motors were designed and built for operation from direct current power.The following should be considered when choosing DC variable speed drives for production:

• Wide speed range
• Good speed regulation
• Compact size and light weight
• Ease of control
• Low maintenance
• Low cost

In order to determine if DC variable speed drives have the capability to provide the above characteristics, the DC drive has to be analyzed as two elements that make up the package. These two elements are the electric motor and the control.General purpose DC motors used on nearly all packaged drives have a very simple performance characteristic; the motor shaft turns at a speed directly proportional to the voltage applied to the armature.The Armature amperage of a DC motor is almost directly proportional to output torque regardless of speed. This characteristic indicates that a small fixed amount of current is required to turn the motor even when there is no output torque. This is due to the friction of the bearings, electrical losses in the motor materials and load imposed by the air in the motor winding.

DC motor speed is primarily determined by the applied armature voltage and DC motor torque is controlled by armature current.Understanding these concepts of DC motors provides the key to understanding the performance of DC variable speed drives. DC variable speed drives use a control module. This control module rectifies alternating current power and converts it to direct current for DC motor control. The control module also controls the DC output voltage and amperage in response to various control and feedback signals. The feedback signals then regulate the DC motor performance via speed, and torque.

The regulating function of DC variable speed drives provide an electronic circuit that monitors a number of inputs and sums these signals to produce an error signal. This error signal is processed and transformed into precisely timed pulses that are applied to the gates of the Silicon Controlled Rectifier(SCR) in the power bridge, thereby regulating the power output to the DC motor.In order to control DC motor speed accurately it is necessary to provide DC variable speed drives with a feedback signal related to motor speed. The standard method of doing this in a simple control is by monitoring the armature voltage and feeding it back into the regulator for comparison with the input set point signal.When armature voltage becomes high, relative to the set point, established by the speed potentiometer setting, an error is detected and the output voltage from the power bridge is reduced to lower the motor’s speed back to the “set point”. Similarly when the armature voltage drops an error of opposite polarity is sensed and the control output voltage is automatically increased in an attempt to re-establish the desired speed. The “Armature Voltage Feedback System” which is standard in most packaged drives is generally called a “Voltage Regulated Drive”.

A second and more accurate method of obtaining the motor speed feedback information is called “Tachometer Feedback”. In this case the speed feedback signal is obtained from a motor mounted tachometer. The output of this tachometer is directly related to the speed of the motor. Using Tachometer Feedback gives a DC variable speed drive improved regulation characteristics. When “tach feedback” is used the drive is referred to as a “Speed Regulated Drive”. Most DC drives are capable of being modified to accept tachometer signals for operation in the tachometer feedback mode.In some newer high performance “digital drives” the feedback can come from a motor mounted encoder that feeds back voltage pulses at a rate related to motor speed. These (counts) are processed digitally being compared to the “set point” and error signals are produced to regulate the armature voltage and speed.

In addition to the normal external adjustment such as the speed pot, there are anumber of common internal adjustments that are used on simple small analog type DC variable speed drives.The following is a description of the functions that these individual adjustments serve and theirtypical use.

Adjustments Of DC Variable Speed Drives & Motors

In most cases when DC variable speed drives are initially installed, the speed potentiometer can be turneddown to its lowest point and the output voltage from the drive will go to zero, then the motor will stop.There are many situations where this is not desirable, because there are some machinesthat need to be kept running at a minimum speed and accelerated up to operating speed asnecessary. There is also a possibility that an operator may use the speed potentiometer tostop the motor to work on the machine. This can be a dangerous situation since the motor hasonly been brought to a stop by zeroing the input signal voltage. A more desirable situation iswhen the motor is stopped by opening the circuit to the motor or power to the control usingthe on/off switch. By adjusting the minimum speed up to some point where the motorcontinues to run even with the speed potentiometer set to its lowest point, the operator mustshut the control off to stop the motor. This adds safety into the system. The typicalminimum speed adjustment is from 0 to 30% of motor base speed.

The maximum speed adjustment sets the maximum speed attainable either by raising the inputsignal to its maximum point or turning the potentiometer to the maximum point. On a typicalDC motor the rated speed of the motor might 1750 RPM but the control might be capable ofrunning it up to 1850 or 1900 RPM. In some cases it’s desirable to limit the motor (and machinespeed) to something less than would be available at this maximum setting. The maximumadjustment allows this to be done. By turning the internal potentiometer to a lower point themaximum output voltage from the control is limited. This limits the maximum speed availablefrom the motor.

Digital DC variable speed drives are capable of constantly monitoring the DC motor current. The current drawn by the armature of theDC motor is related to the torque that is required by the load. Since this monitoring and controlis available an adjustment is provided in the drive that limits the output current to amaximum value.This function can be used to set a threshold point that will cause the motor to stall ratherthan putting out an excessive amount of torque. This capability gives the motor/drive combination the ability to prevent damage that might otherwise occur if higher values oftorque were available. This is handy on machines that might become jammed or otherwisestalled. It can also be used where the drive is operating a device such as the center winderwhere the important thing becomes torque rather than the speed. In this case the current limitis set and the speed goes up or down to hold the tension 0f the material being wound. Thecurrent limit is normally factory set at 150% of the motor’s rated current. This allows the motorto produce enough torque to start and accelerate the load and yet will not let the current(and torque) exceed 150% of its rated value when running. The range of adjustment istypically from 0 to 200% of the motor rated current.

IR compensation is a method used to adjust for the droop in a motor’s speed due to armature resistance. As mentioned previously, IR compensation is positive feedback that causes thecontrol output voltage to rise slightly with increasing output current. This will help stabilize the motor’s speedfrom a no load to full load condition. If the motor happens to be driving a load where the torque isconstant or nearly so, then this adjustment is usually unnecessary. However, if the motor is driving a loadwith a widely fluctuating torque requirement, and speed regulation is critical, then IR compensationcan be adjusted to stabilize the speed from the light load to full load condition. One caution isthat when IR compensation is adjusted too high it results in an increasing speed characteristic. This meansthat as the load is applied the motor is actually going to be forced to run faster. When thishappens it increases the voltage and current to the motor which in turn increases the motor speed further. If thisadjustment is set too high an unstable “hunting” or oscillating condition occurs that is undesirable.

The Acceleration Time adjustment performs the function that is indicated by its name. It willextend or shorten the amount of time for the motor to go from zero speed up to the set speed. It alsoregulates the time it takes to change speeds from one setting (say 50%) to another setting(perhaps 100%). So this setting has the ability to moderate the acceleration rate on the DC variable frequency drive.If an acceleration time that’s too rapid is called for, “accelerationtime” will be overridden by the current limit. Acceleration will only occur at a rate that is allowed by theamount of current the control passes through to the motor. On most small DC variable speed drives, the acceleration time is not linear. What this means is that a change of 50 RPM may occur more rapidly when the motor is at low speed than it does when the motor is approaching the set pointspeed. This is important to know but usually not critical on simple applications where these drives are used.This is an adjustment that allows loads to be slowed over an extended period of time. Forexample, if power is removed from the motor and the load stops in 3 seconds, then the deceleration timeadjustment would allow an increased time and “power down” the load over a period of 4, 5, 6 ormore seconds.

To learn more about DC variable speed drives or for DC variable speed drive repair and replacement quotes, contact Precision Electric, Inc.

Information References:

http://www.fincor.net

AC variable frequency drives are also known as VFDs, VSDs (variable speed drives), AFDs (adjustable speed drives), inverters, and micro drives. AC variable frequency drives are used in AC Servo Systems, Air Compressors, Conveyor Systems, Lathes, Mills, Plastic Extrusion, Slitter Lines, Food Processing, Waste Water Treatment Systems, Submersible Pumps, HVAC Fans and Blowers, and many more applications.

Many manufacturers apply AC variable frequency drives to rotating equipment because variable frequency drives reduce amperage spikes upon start up of large electric motors.Choosing the right AC variable frequency drive for an application will benefit rotating equipment by providing less wear on the electric motors where applied.Adjusting the acceleration and deceleration time of electric motors can extend an electric motor’s lifespan. AC variable frequency drives provide the ability to control the frequency of starting and stopping of an AC electric motor.This ability provides a means by which an AC electric motor is only operating when needed for the equipment it’s rotating, and electric motors have a longer lifespan if they are not continuously operating when they don’t need to be.

Approximately one third of the world’s electrical energy is supplied by electric motors in fixed-speed centrifugal pump, fan, and air compressor applications. These fixed-speed applications hardly ever require the full load speed (RPM) of the electric motor in which they’re operating. By installing AC variable frequency drives to these applications, the motor speeds are reduced, and power costs can be reduced by 50% or more. Technology has reduced cost and physical size of AC variable frequency drives, and has improved performance through advances in semiconductor switching devices, simulation, control techniques, and control hardware and software.

Power Savings Via AC Variable Frequency Drives

AC variable frequency drives are used to reduce power savings in food processing, plastic extrusion, waste water treatment, and many other applications.AC variable frequency drives operate as load controls within applications that may accomplish up to 50% reduction in energy costs. Electric motors will turn at a rate proportional to the frequency of the alternating current (AC) applied to it.

The majority of AC variable frequency drives in the market today contain electronic circuitry that converts 60 Hertz Line power into direct current. The variable frequency drive converts this line power into a pulsed output voltage that duplicates varying alternating current to a desired frequency (speed).A properly applied AC variable frequency drive when paired with an AC electric motor, will significantly reduce operating costs. This is particularly true for variable torque loads such asFans,Blowers, andPumps.Blowers, for example, are often used with dampers to control air flow. These dampers may operate either manually or automatically. When dampers are closed, 50% of the electric motor current will drop to approximately 60% of Full Load nameplate current. By utilizing an AC variable frequency drive in this application, current draw in the motor will be reduced 30% for every 10% drop in speed. The same electric motor operating froman AC variable frequency drive at 50% speed, will draw approximately 20% of the full load current.

Example Application: AC Variable Frequency Drives

A 10 horsepower AC electric motor, rated 90% efficient, operating across the line without an AC variable frequency drive, with the dampers operating between 50 70%, for 2000 hours per year will require 11,996 KWH. If the KWH charge is $.08 per KWH, the cost to run this motor will be: $1,248.00 annually.The same 10 horsepower electric motor operating from an AC variable frequency drive, between 50 70% speed for 2000 hours per year will require 4,676 KWH. Operating cost at the same KWH rate will be: $432.00 per year. This represents a savings of $816.00 per year and should be enough to pay for the AC variable frequency drives investment and installation costs, within the first 12 months of operation.If any electric motor application operates more hours than in the above example, and/orthe KWH charge is higher, the savings will quickly compound.

Manufacturers across the globe capitalize on AC variable frequency drives to save money on power consumption, increase quality control, decrease production downtime, and improve overall efficiency. To learn more about AC variable frequency drives or for repair and replacement quotes, contact Precision Electric, Inc.