and avoid the inefficiencies and capital cost of filtering used by earlier LCI and Cycloconverter solutions. VFD solutions provide high torques down to inching ...
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Evaluation of Synchronous Motors on Grinding Mills George Seggewiss
Jingya Dai
Mark Fanslow
PEng., Senior IEEE Member Phd., Product Design Engineer P.E., Senior Design Engineer Rockwell Automation Rockwell Automation TECO-Westinghouse Motor Co. Abstract -- Synchronous motors continue to be a viable alternative to induction motors because of efficiency advantages. Long used for providing leading kVARs for PF improvement and helping with the device switching in Load Commutated Inverter (LCI) drives, these machines are efficient prime movers for large Mill applications. Both in motor efficiency and in low base speed configurations the synchronous motor has advantages of lower current and higher efficiency. Often used in large Mills in the cement and mining industry, these motors were driven by LCI variablefrequency drives (VFDs) or used with direct start with a clutch system that would synchronize the motor first to overcome low starting torques. This paper reviews various Mill drive configurations and improved synchronous motor characteristics when used with more advanced Current Source Inverter (CSI) drives. It will also review the different synchronous motor excitation types and resulting performance characteristics with VFD control for new or retrofit installations. Index Terms – synchronous motors, VFD, CSI, VSI, LCI, efficiency, grinding mills, motor field excitation
I.
INTRODUCTION
This paper concentrates on modern motor and drive solutions for single and dual pinion driven Ball and SAG Mills and the motors and drives used to power them. It will cover the applications from three vantage points, the motor, VFD and the application characteristics determining the sizing and control requirements for the prime mover. The most widely used motor for Mill applications is the synchronous motor shown in Figure 1 however reasons for its use have evolved with VFD control. Mills are usually one of the largest loads in a cement or mining operation and were used to provide leading reactive power to compensate for many other plant inductive loads. Early drives provided lower inrush currents, less mechanical stress and variable speed control however needed a leading power factor to control switching of the power thyristors. Again the synchronous motor provided a solution and could be used in bypass should the VFD fail. With newer VFD control, increased reliability and low speed torque characteristics along with reduced harmonics and torque pulsations allowed lower frequency operation. Once more this favoured the synchronous motor which has reduced current and higher efficiencies over similar induction motors. Synchronous motors require separate excitation and therefore a VFD control algorithm different from induction motors. Flux is controlled by interaction between a separately controlled rotor current and stator currents. Knowledge of rotor position is important to starting torque with no slip field orientation. Ball and SAG Mills have unique torque requirements. Although induction and wound rotor motors may develop more starting torque when started across line (necessitating the use of clutches for synchronous motors), VFD’s have opened up higher torque starting applications for synchronous motors. The synchronous motor / VFD combination provides an economical solution for many reasons including low operating costs and the avoidance of complex drive train with gearboxes, clutches and inching equipment. II.
AVAILABLE MOTOR SOLUTIONS
Wound rotor motor solutions are used mostly for their higher starting torque capability (achieved by adding external rotor resistance), loadsharing on dual motor arrangements and some limited speed control. Once a unique solution, they are now often encountered when Mills are retrofit with VFD’s. A wound rotor solution, although it he high losses, provides across-line starting capability for mills which require high torque during initial sub
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synchronous speed acceleration [1]. The WRM can be retrofit to VFD control, in most cases the rotor connections are shorted and the motor operates as an induction motor on the VFD. New slip energy recovery drive solutions (although reasonably priced due to the smaller drive size) have limited speed capability and still require separate across line starting equipment. Induction motors have higher currents for the same power levels due to lower efficiency and PF points than synchronous motors. This gap becomes wider with increased pole pair construction and lower base speeds. Synchronous motors operate with higher efficiency, lower currents due to high PF and have usable low speed torque characteristics with a VFD, additional motor details are available in the motor portion of the paper. When operated direct on line, the leading PF that are provided by synchronous motors offer reactive power that compensates for the reactive power used in other parts of a plant. This leading power factor is required to provide a positive biased condition for inverter thyristor switching in LCI type VFDs. When operated with a VFD however the line side leading PF is lost. Induction motor PF depends largely on the base speed or number of poles in the motor design. For example in a 5000HP 20pole synchronous motor design, the current can be 585amps while an induction motor at the same base speed and power requires 820amps, a 40% increase due largely to extra reactive power used with a lower PF in the induction motor design.
Figure 1 SAG Mill and synchronous motor The complexity and initial cost of a low speed synchronous motor and VFD is offset by the simplicity in avoiding initial cost and maintenance associated with a gearbox, couplings, clutches, bearings and support structures. Gearboxes needed for higher speed induction motors typically have losses in the range of 0.5 to 0.8% per step or pair of gears. Additionally, maintenance associated with bearings, gears and lubricants are avoided. Clutches are often used in the starting of synchronous motors due to the low starting torques associated with across line and early LCI control. Clutches allowed the synchronous motor to reach full or near top speed before applying load. With VFD’s now using field oriented vector control and encoders providing position feedback, VFD controlled synchronous motor starting torques will easily handle Mill starting characteristics without the use of clutches. The need for an air clutch requires a machined hole through the center of the shaft on the coupling end to the non-driveend in order for the clutch to be supplied with operating air. Eliminating the manufacturing operation of machining a hole through the center of the shaft reduces motor costs depending on the length of the shaft. Additionally, with a 100:1 speed range, inching equipment for positioning or maintenance is also eliminated. However on retrofits mechanical inching equipment is sometimes kept to reduce the lockout time associated with electrical equipment.
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III.
SYNCHRONOUS MOTOR EXCITER TYPES
Synchronous motors (SMs) require an external DC supply to power the rotor field winding, and can be classified into brush-type or brushless-type based on the exciter structure, as shown in Figure 2. A brush-type synchronous motor is fitted with a static exciter where the current is provided to the rotor through slip rings and brushes. A brushless-type synchronous motor is supplied by a rotating exciter. The exciter stator current, either in AC or DC form, is transferred to the rotor through the rotating transformer. The AC current induced in the rotor is then rectified to a DC current to supply the field winding. The difference in the exciter structure also determines the starting capability and control methods that can be applied. In particular, in a DC-brushless synchronous motor, the induced voltage in the rotor is proportional to the rotor speed, assuming a constant DC supply in the exciter stator. When the rotor is at standstill, there is no field current supplied to the rotor and therefore, the starting torque capability is much more limited as compared to brush-type or ac-brushless-type of synchronous motor where constant torque load can be supported with position sensor feedback. As the motor speeds up, the voltage induced in the exciter rotor winding increases and thus the DC-brushless motor can support high load torque in the medium to high speed range. Motor rotor
Motor stator Rotor Protection circuit
Field winding
DC exciter current supply
Slip ring
Exciter stator
(a) Brush-type exciter
DC or AC exciter current supply
Motor rotor
Motor stator
Rotor Protection circuit
Field winding
Exciter stator
(b) DC or AC brushless exciter Figure 2 Synchronous motor with brush or brushless exciter Static or brush type excitation is the simplest method of providing DC current to the motor field winding. The drawback is that the brush and slip ring setup requires regular maintenance to replace brushes due to brush wear. A brushless synchronous motor that is starting across the line without the use of a VFD will be equipped with a DC-AC brushless excitation system. The stator of the exciter is made from stacked laminations with an opening cut into the I.D. of the stator to allow a copper coil to be inserted. In practice any number of poles can be used based on manufacture preference. The exciter rotor consists of a 3 phase alternating current winding that is wound into the slots of a stack of pressed rotor laminations. DC current is fed to the exciter field winding from an external source which creates positive and negative poles in the exciter stator that induce a 3 phase alternating voltage in the exciter rotor winding. The 3 phase alternating current from the exciter rotor is rectified to DC current in a rectifier bridge
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located on the control wheel. When the motor has accelerated to synchronizing speed the control system will apply the DC current to the motor field causing the rotor to pull into synchronism. For a motor to be started with a VFD the motors field winding must always be energized from zero speed to full speed. The DC-AC exciter cannot provide a rotating magnetic field when the motor is at zero speed because the poles of the stator are also fixed in space and therefore no magnetic lines of flux are cutting the rotors AC winding. To create exciter output at zero speed the input to the exciter stator must be 3 phase AC to create a 3 phase AC output in the rotor winding at zero speed. The AC-AC exciter is more like an induction generator than any other sort of synchronous machine. Figure 4 shows the picture of an AC-AC exciter and its associated control schematic. Notice the control schematic does not require any SCR to block the voltage output of the rotating rectifier since DC current must always be provided to the main field. The schematic diagram in Figure 3 will work for starting the synchronous motor on the VFD only. If the motor is desired to have both VFD starting capability and ATL capability then the configuration shown in Figure 4 must be used as in the DC-AC case.
Figure 3 AC-AC Exciter Schematic For Operation on a Drive Only IV.
Figure 4 AC-AC Exciter Schematic For Starting on a VFD or Across the Line
MOTOR LOSSES AND HEATING AS A FUNCTION OF SPEED
To understand how the load torque-speed characteristic effects motor sizing it is important to understand how motor heating from stator I2R, Rotor I2R, and core losses changes as a function of speed as shown in Figure 5. The motor is thermally limited by the temperature rise of the stator and rotor windings. The heat input to these windings comes primarily from stator I2R, Rotor I2R, and core losses. Stator I2R Losses - As the motor slows down the voltage of the motor must decrease to maintain constant Volts/Hz., therefore if it was desired to maintain constant HP, then the stator current must increase roughly linearly as the voltage drops and the copper I2R losses will increase as the square of stator current. Core Losses - The heating in the stator iron from core losses is only dependent on voltage and frequency levels and not dependent on HP load. Heating from iron loss is not a linear or square relationship with speed but will decrease faster than proportional to the speed decrease. Motor Field I2R Losses - Motor Field losses are proportional to the torque of the motor. So if the motor is running on a constant torque curve then the motor field losses are also constant until very low frequencies are reached (under 5 Hz) at which time the field losses will increase with further decreases in speed.
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Losses Vs Speed for Constant Torque Load 80 70
KW Losses
60 50 40 30 Stator I2R
20
Field I2R Core
10 0 0
10
20
30
40
50
60
70
80
90
100
Percent Speed
Figure 5 Motor Losses V.
SYNCHRONOUS MOTOR ACROSS THE LINE STARTING CHARACTERISTICS
Figure 6 compares the typical across the line speed-torque characteristics of a synchronous motor and mill load when starting the mill disconnected via an air clutch and when starting the mill directly coupled to the motor. The total heat generated in the squirrel cage winding in bringing the load from rest to full speed is equal to the kinetic energy supplied to the rotating parts, if friction and load torque are neglected, which is close to the case when the motor is starting the mill through a clutch. For this case the heat energy generated in the squirrel cage winding can be estimated in equation (1). Speed Torque Curves For Motor and Mill 160
Percent Motor Torque
140 120 100 80 60
Clutch Start Motor
40
Clutch Start Mill Direct Coupled Motor
20
Direct Coupled Mill
0 0
10
20
30
40
50
60
70
80
90
Percent Speed
Figure 6 Mill starting torques
Ε= Where:
1 × ΙM ×ω2 2
(1)
E = Energy dissipated in the squirrel cage winding during acceleration IM = Inertia of the Motor ω = Rotational Speed of Motor at Pull-In
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If the motor is connected to a mill directly then equation (2) is more accurate for calculating the total heat energy input into the squirrel cage winding during starting:
Ε=
TM 1 1 T × × (Ι M + Ι L ) × ω 2 = × M × (Ι M + Ι M ) × ω 2 2 TM − TL 2 TA
Where:
(2)
IL = Inertia of Load T(s)M = Torque of Motor as a Function of Speed Over the Acceleration Period T(s)L = Torque of Load as a Function of speed over the acceleration period T(s)A = Acceleration Torque as a Function of speed over the acceleration period
The Energy of equation (2) is larger than the energy of equation (1) due to the inclusion of the load inertia and a term for the ratio of motor torque to acceleration torque. The combined motor and mill inertia is larger than the motor inertia alone by a factor of 2-4. The energy from the acceleration torque can be estimated from Figure 6 as the ratio of the area underneath the motor torque curve during acceleration to the area between the motor and load torque curve during acceleration. For typical ball mills this starting torque factor multiplier is around 3 – 5 depending on the mill and motor specifics. The result of comparing equations (1) and (2) is that the squirrel cage must absorb anywhere from 6 to 20 times the heat energy when starting directly connected to a grinding mill vs. starting with an air clutch depending on the mill specifics. Starting the synchronous motor on a VFD allows the motor to start a directly connected mill without having to increase the volume of copper alloy in the squirrel cage to match the increase in energy in starting the mill without a clutch. VI.
SYNCHRONOUS MOTOR STARTING ON A VARIABLE FREQUENCY DRIVE
When starting across the line, a synchronous motor is powered by the squirrel cage winding until it reaches about 95% speed at which time the motor’s DC field winding is energized and the motor pulls into synchronism. When a synchronous motor starts on a VFD; the motor operates with the DC field energized continuously from zero speed with the rotor synchronized with the stators’ revolving magnetic field. Since the squirrel cage plays no part in starting the motor it can be reduced in size significantly, although it is still required to mitigate transients during abrupt changes in load angle.
FLT =
0.1016 × Poles × E × eff × FLA freq
Where: FLT = Full load torque
(FT-LBS.)
(3)
FLA = Full load stator amps
Poles = Number of poles
Eff = Motor efficiency
E = Line to Line Voltage
Freq = frequency
Equation (3) demonstrates that as long as the volts per hertz ratio of the drive is constant then rated torque output can be achieved at rated stator current. In practice slightly more than full load stator current is required at low speed since motor efficiency is decreased at lower speeds. Drive starting conditions can have the motor accelerate over a 2-3 minute period at 110% stator current. As far as stator thermal heating is concerned most synchronous motors can run up to 15 minutes as high as 150% stator current before injurious heating occurs. A 2-3 minute acceleration time at 110% rated current will provide no difficulty. A major benefit of using VFDs for synchronous motors is the controlled motor current and torque during startup. Traditional across-line starting method as discussed earlier draws a huge amount of inrush current in the motor, typically 4~4.5pu of rated current or higher for a high-torque-capability motor. This not only impacts the motor rating design but also stresses the grid and possibly causes instability on a weak power system.
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The presence of the VFD mitigates the starting stress of the system. In this case, field current can be provided from the very beginning of the starting process, and the motor operates in synchronous mode from zero speed. Unlike the across-line start where the voltage and frequency are determined by the supply system, the VFD feeds the motor with a reduced voltage and frequency which closely matches the load profile. Close-loop speed control allows the motor to produce proper electrical torque to ramp up the motor according to a defined speed profile. High starting torque capability is guaranteed with reduced initial motor voltage and current. No magnetic saturation and maximized motor current utilization are achieved through VFD control. The field current is also regulated by the VFD so that the motor flux is kept to its optimum value and no over-voltage is generated at the motor terminals. The controlled performance is beneficial to systems with a weak grid or supplied through long transmission lines. VII.
ADVANCED VECTOR CONTROLS
Various control schemes are developed for VFD controlled synchronous motors. The simple open-loop Volt/Hertz (V/F) control [2] has long been employed for low to medium performance drives. The basic idea is to link the stator voltage proportional to the electrical frequency so that the magnetic flux is kept constant. Various methods are devised to address the drawbacks of the design, for example: compensate the motor resistive voltage drop at low speed, or improve dynamic performance through proper motor or load monitoring and adjustment. Advanced vector control schemes [3,4] can also be applied for synchronous motor control to improve the overall performance. Stator flux, air-gap flux, rotor flux or rotor oriented controls have been proposed in the literature. Among them, stator flux oriented control is used for the simplicity of motor stator power factor control. Air-gap flux oriented control is seen more in practice to provide better steady-state and transient performance. The reason is the relatively large time constant in the air-gap flux due to various damping effects in the air gap region. Moreover, accurate field current feedback measurement is difficult in brushless synchronous motors, rendering the often-used rotor flux oriented control in induction motors difficult for these types of motors. The synchronous motor is a complicated control object, considering rotor saliency, damping windings and the angle displacement between the rotor position and flux reference frame. All these lead to variation of equivalent inductances and cross-coupling effect in flux and torque control. To simplify the understanding of the flux construction in synchronous motors, these effects and the stator resistance are neglected in the steady-state evaluation. The main flux is thereby simplified and considered as the sum of the contributions from stator magnetizing current and field excitation current. In induction motors the stator constantly supplies a certain amount of magnetizing current to maintain the magnetic flux, resulting in a lagging motor power factor (PF) at all operating conditions. On the other side, synchronous motor offers an extra freedom of motor field current control and hence its operating PF can be adjusted by the drive to leading, unity or lagging as the system operation prefers. Figure 7 demonstrates the motor PF control through variation of field excitation current and magnetizing current while keeping the torque producing current constant for the same load condition. Note that the stator resistance and rotor saliency are neglected in Figure 7 for the simplicity of comparison. Unity PF is achieved when the flux is fully supported by the field current and stator magnetizing current averages around zero (Is_ψs2 = 0) in steady state. Under-excited motor field winding demands positive stator magnetizing current (Is_ψs1 in the same direction of stator flux Ψs) and thus lagging PF. Similarly, over-excited motor field winding requires negative stator magnetizing current to maintain the same flux level and lead to leading PF. Unity PF is often seen in VFD control because it minimizes the motor current under the same load condition. The dynamics of field excitation current control is limited by the design, mainly exciter structure, field winding inductances, and the maximum voltage available for current control. The improved dynamic performance of motor flux control can be obtained through stator magnetizing current during transients.
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Is_ψs3 Is3 (leading) is vs
Xs ef
Is_ψs2 =0
Vs Is2 (unity)
jXsIs1
jXsIs3 jXsIs2
Is_ψs1 Is1 (lagging)
Ef1
Ef2
Ef3
Ψs
(a) Equivalent circuit
(b) Phasor diagram
Figure 7 Leading, unity and lagging motor PF control with stator resistance and rotor saliency neglected. VIII.
TORQUE PERFORMANCE
Depending on the motor constructions and applications, the rotor can be built as salient or cylindrical (nonsalient) poles. Salient-pole motors have non-uniform magnetic flux paths in the air gap, resulting in different directand quadrature-axis inductances, typically with Ld>Lq. In across-line starting, the saliency in the motor boosts the average torque capability in the torque-speed curves. However, due to the same reason, oscillating torque at the frequency of twice the slip frequency is generated throughout the whole startup process [5]. This presents stress to the mechanical system and needs to be carefully evaluated for torsional vibration analysis. The above mentioned is of much less concerns in a non-salient-pole motor because of Ld= Lq. When the salient pole motor fully operates in synchronous mode using VFDs, such oscillating torque component is eliminated for the reason of zero slip frequency. The torque profile of an across-line start method is pretty much determined by motor characteristics and the available line voltage. Similar to the induction motor start, rotor resistance, either in the damping windings or field winding, needs to be tuned to adjust the starting torque capability. The design requires precise knowledge of load torque curves. With VFD control, the electrical torque produced can be anywhere between zero and the maximum possible value (breakdown torque or lower considering the motor and drive design ratings) at any speed. This makes the system suitable for most load and speed profiles. Initial rotor position detection is crucial for a synchronous motor to ensure break-away torque capability under VFD control. Since the magnetic flux is mainly supported from the rotor field current, the electrical torque is produced only when the applied stator current is perpendicular to the main flux. In this sense, sensorless starting of synchronous motor is more difficult than induction motors, since the main flux in an induction motor can be fully defined by the stator voltage and current supplied by the drive. Most sensorless starting methods rely on the motor characteristics such as saliency or transient responses to detect the initial position [6,7]. In addition, in the very low speed region where flux and speed observers are not sufficiently accurate, open-loop operation is often used. The provided stator flux and rotor rotation are not guaranteed to be fully synchronized, which leads to oscillating torque and speed and in the worst case motor stall. Therefore, sensorless operation is not suitable for providing continuous torque in the zero and low speed region. On such occasions, a position sensor is required. With accurate rotor position feedback, the torque ripples during acceleration through the low speed range will be much improved.
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IX.
DRIVE PERFORMANCE
Drives with various types of converter topologies [8,9], such as load commutated inverter (LCI), voltage source inverter (VSI), and current source inverter (CSI), have been applied to synchronous motor control. Theoretically, most motor-side control algorithms can be readily applied to all these drive types. However, the control performance of each type varies due to its uniqueness in power structure and natural characteristics. LCI drives employ SCR thyristors as switching devices. The thyristor does not have self-turn-off capability, but can be naturally commutated by the load voltage with a leading power factor. The LCI features low manufacturing cost and high efficiency mainly due to the use of low-cost SCR devices and lack of pulse-width-modulated (PWM) operation. It is a popular solution for very large drives, where the initial investment and operating efficiency are of great importance. However, the LCI itself is not a grid and motor friendly topology. It normally generates highamplitude low-frequency current harmonics and the grid-side power factor is not controllable. In view of this, active filters or compensators of high power ratings are unavoidable for the LCI to improve the grid connection performance. Other concerns include the difficulty of device commutation at low speed due to the low motor voltage. A method named dc-link pulsing is commonly used to assist device commutation through intentionally created zero-current intervals. A side effect of the method is increased torque pulsation. The dynamic performance of the LCI drive is the poorest among all three due to its naturally commutated operation. Both VSI and CSI utilize active switching devices on the motor side, and thus can adopt suitable PWM schemes together with advanced control strategies to improve motor dynamic performance and reduce torque ripples. Motor PF is controllable with these drives. At the line side, LCI and CSI drives have inherent regenerating capability. In contrast, a large percentage of the industrial VSI drives employ single or multi-pulse diode rectifier at the line side that prevents power to be regenerated back to the line. CSI or VSI drives with an active front end are also capable of line PF correction. For high-power medium-voltage applications, traditional two-level VSI supplies the motor with chopped voltage waveforms and thus generates high dv/dt at the motor terminals. Multi-level VSIs are therefore used to reduce the voltage steps and stress in the motor. CSIs, on the other side, provide the motor load with sinusoidal output voltage and current waveforms with low harmonics because of the filter capacitor at the output. Devices can be connected in series to achieve higher voltage rating, making the structure relative simple and robust for high voltage and power ratings. X.
TORQUE REQUIREMENTS
Ball mills and SAG mills, vertical grinding mills for raw or finishing sections all have a constant torque speed characteristic, however the starting torques, comminution process and material handling is very different. A vertical mill has hydraulics that apply large rollers to a driven rotating milling table. Starting torques depend on the amount and sizing of the material and height of the rollers. A starting torque profile for a SAG or Ball Mill will change with the position of the charge or load. Torques must overcome the gravitational pull on the Mill charge and acceleration of the Mill rotating inertia. With the charge at the bottom of the Mill the breakaway torque is a function of the static friction of the Mill equipment and is usually in the range of 30-50%. Once moving the torque rises quickly with the charge moving up with Mill rotation to a point where the material starts to cascade or tumble and comminution begins. The initial cascading is delayed by a settled charge and momentum and requires a higher torque before settling into a steady cascading and torque. In Figure 8 the starting torque profile is shown for a SAG Mill. Overshoot and peak torques used depends on the acceleration rate of the start. A slower acceleration will reduce the peak torque reached when cascading begins [10]. Should the charge be frozen (more common with the finer charge in Ball Mills) and unable to initially cascade the torques will continue to rise and it is possible the charge may suddenly come loose and plummet to the bottom of the Mill, causing possible damage.
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Figure 8 Mill and motor torques XI.
VFD OPERATION
Under VFD control the synchronous motor can provide torques up to the motor breakdown level from zero speed to above base speed. Full separate excitation must be provided from zero speed for torque to be developed by the VFD. Current draw remains directly proportional to torque under all speeds with a near unity power factor. Fine speed control is possible at any speed from inspection/maintenance to super synchronous levels. In comparison to across-line or LCI operation, modern VFD (including LCI) [11] operation at a higher power factor will reduce motor current and temperature and increase efficiency. In comparison to across-line operation, line side power factor is no longer controlled by the synchronous motor exciter. Although VFD power factor is usually greater than 95%, a leading PF can no longer be achieved. Alternately, several plants have chosen to use a single VFD for starting several Mills, resulting in a variable frequency softstart benefit for multiple Mills [12]. Once brought up to speed by the VFD the motor is synchronized to the line and PF control is returned to the motor exciter to compensate for plant lagging PF. The drive can be used continually on any of the Mills to control speed for different throughput or comminution efficiency. VFD control will allow a synchronous motor to be optimally designed for a specific Mill speed. With the high availability of VFD’s the necessity for a bypass is reduced and so is motor operation at the network frequency. For instance under VFD use a motor base frequency can be reduced since it no longer needs to match line frequency. In this way the number of pole pairs can be reduced (reducing motor complexity and cost) since the VFD can alter its V/F ratio to suit rated voltage and frequency. A 200rpm base speed can be achieved with a standard 60 Hz base frequency on a 36pole motor or a 20 Hz base frequency design on a 12 pole motor or even a 10 Hz base frequency on a 6pole motor. Modern VSI and CSI VFD solutions do not require extra filtering to meet harmonic guidelines and avoid the inefficiencies and capital cost of filtering used by earlier LCI and Cycloconverter solutions. VFD solutions provide high torques down to inching speeds and avoid torque pulsations with sinusoidal current waveforms to the motor. Synchronous motor PF under modern VFD control is usually operating in the high 95-99% while induction motor PF’s range in a 70-90% range. When changing from an older LCI format to a new VFD operating at close to unity PF the current draw and drive size is reduced. As a result the synchronous motor has the lowest operating cost solution. Most users find an efficiency increase of 10% or more with this type of retrofit. Clutches can be left permanently engaged (reducing wear and maintenance) or can be removed altogether since
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torques are now available at low speeds and current inrush is limited to the overloads during starting. This is the case with all synchronous excitation methods except for brushless DC which does not provide excitation (and torque) until rotation generates field current. Inching equipment can also be eliminated. A VFD is sized according to the currents required to produce the worst case starting and running torques. VFD’s are often downsized on retrofit synchronous Mill motors since their operating power factor is increased. It is very important to analyze the motor torque/speed requirements based on loading, acceleration rates, inertias and frozen charge control including indexing, inching, anti-rocking and loosening the charge. Regenerative capability (four quadrant torque speed operation) or at a minimum (two quadrant with) dynamic braking is required for bringing frozen charge back down and anti-rocking. Separate PLC controllers usually provide these auxiliary functions. A worst case duty cycle of high torque operations will dictate the minimum overload requirements for the sizing of rated VFD current. Lower base speed synchronous motors lend themselves to single pinion - ring gear arrangements for Mills and in some instances direct coupling. Loads requiring lower gear-in speeds often share a bearing between the motor and the load. This reduces the complexity of the mechanical drive train. Alignments, vibrations and issues with mechanical natural frequencies are also reduced. Gearboxes and couplings increase the drive train length which lowers the first natural frequency and increases the number of natural frequencies by introducing more elements in the rotating mass. Lower natural frequencies are more active when multiple excitation frequencies exist. The support structure is simplified, more compact and alignment requirements lessened. At a mine site in Colorado, retrofitting a SAG Mill with new control was a major decision with many possible solutions. With a synchronous motor in good condition a modern VFD provided improved operating capabilities with higher low speed torques, reduced current draw and increased efficiencies. A smaller drive was used and doubled for inching duty. Although an encoder proved difficult to install onto existing equipment, starting conditions have so far been within the drive sensorless starting abilities. XII.
CONCLUSIONS
It has been shown that the evolution of the synchronous motor makes it a good fit for Ball and SAG Mill applications especially when combined with a modern VFD. Higher efficiencies, lower currents, improved torque capability and low harmonic content all contribute to lower life time costs as well as improved process control. Improvements are evident on retrofits. Moreover optimized motor designs further reduce Mill drive complexity and increase reliability for these critical processes. Many sites have benefited from a VFD retrofit of existing synchronous motor control with very positive results. XIII.
REFERENCES
[1] James F. Zayechek, LCI’s and Synchronous Motors Applied to Roller Mills, IEEE/PCA Cement Industry Technical conference May 2000. [2] Incze, I.I.; Szabó, C.; Imecs, M., "Voltage-Hertz Strategy for Synchronous Motor with Controlled Exciting Field," Intelligent Engineering Systems, 2007. INES 2007. 11th International Conference on, vol., no., pp.247,252, June 29 2007-July 2 2007 [3] Jain, A.K.; Ranganathan, V. T., "Modeling and Field Oriented Control of Salient Pole Wound Field Synchronous Machine in Stator Flux Coordinates," Industrial Electronics, IEEE Transactions on, vol.58, no.3, pp.960,970, March 2011 [4] Hill, W.A.; Turton, Richard A.; Dungan, Robert J.; Schwalm, C. Louis, "A Vector-Controlled Cycloconverter Drive for an Icebreaker," Industry Applications, IEEE Transactions on , vol.IA-23, no.6, pp.1036,1042, Nov. 1987
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[5] Bredthauer, Jurgen; Wachta, Bernhard; Appleton, Steve, "Synchronous Motors with Cylindrical Rotors for the Petrochemical Industry," Industry Applications, IEEE Transactions on, vol.IA-23, no.3, pp.451,463, May 1987 [6] Feuersanger, S.; Pacas, M., "Initial rotor position identification in medium voltage synchronous machines," IECON 2012 - 38th Annual Conference on IEEE Industrial Electronics Society , vol., no., pp.1852,1857, 25-28 Oct. 2012 [7] J.Holtz, "Sensorless control of induction motor drives," Proceedings of the IEEE Vol. 90, No. 8, Aug. 2002 [8] Kouro, S.; Malinowski, M.; Gopakumar, K.; Pou, J.; Franquelo, L.G.; Bin Wu; Rodriguez, J.; Perez, M.A.; Leon, J.I., "Recent Advances and Industrial Applications of Multilevel Converters," Industrial Electronics, IEEE Transactions on , vol.57, no.8, pp.2553,2580, Aug. 2010 [9] Bin Wu; Pontt, J.; Rodriguez, J.; Bernet, S.; Kouro, S., "Current-Source Converter and Cycloconverter Topologies for Industrial Medium-Voltage Drives," Industrial Electronics, IEEE Transactions on , vol.55, no.7, pp.2786,2797, July 2008 [10] C.D.Danecki, P.F.Thomas,G.A Grandy, S.H.Walters, Selection and Evaluation of Grinding Mill Motors and Drives, July 2003. [11] A.K.Jain, V.T.Ranganathan, Improved Control of Load Commutated Inverter Fed Salient pole Wound Field Synchronous Motor Using Field Oriented Technique, 2012 [12] E.V.Akhmadeev, D.V.Beliaev, A.M.Weinger, S.Weigel, Industrial Implementation of a controlled sensorless synchronous drive with hard start conditions, 2002
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