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Document Type and Number:
WIPO Patent Application WO/2008/027874
Kind Code:
A current limiting DC motor starter (FIG 7a) employing a closed loop current measurement (58) to provide precise current control that adapts to changing motor conditions, integrated (104, 106) with a solid state reversing motor starter The motor starter further includes a calibrated over current indication (44, 46, 48) and a control system interface that reduces the likelihood that cable faults and other single failures will cause spurious actuation of the motor.

Application Number:
Publication Date:
July 24, 2008
Filing Date:
August 28, 2007
Export Citation:
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WESTINGHOUSE ELECTRIC CO. LLC (4350 Northern Pike, Monroeville, PA, 15146-2866, US)
International Classes:
Attorney, Agent or Firm:
VALENTINE, James, C. et al. (Westinghouse Electric Co. LLC, 4350 Northern PikeMonroeville, PA, 15146-2866, US)
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Wh . at . is .. ciainied . js:

1. A current limiting DC Motor Starter comprising: a switching mode current regulator; and a solid stale reversing motor starter placed in a series circuit with the switching mode current regulator.

2. The current limiting DC Motor Starter of Claim 1 wherein the switching mode current regulator comprises: a source input terminal for connecting a source of DC current; a switch in series with the input terminal, the switch having a switch control circuit operable to open the switch when the current through the switch exceeds a first predetermined value and to close the switch when the current downstream of the switch falls below a second predetermined value; a motor output terminal for connecting a motor winding in series with the switch; a diode placed across the motor winding between the motor winding and the switch, the diode being oriented to block current from the source of DC current connected through the source input terminal, from circulating through the diode, and a current sensor for monitoring the current through the motor winding and providing an output representative thereof to the switch control circuit.

3. The current limiting DC Motor Starter of Claim 2 wherein the switch comprises an Insulated Gate Bipolar Transistor.

4. The current limiting DC Motor Starter of Claim 2 wherein the second predetermined value takes into account the hysteresis in the series circuit.

5 ' 1 he current limiting DC Victor Starter of Clasra 2 including three control input terminals Io DC Motor Startøi comprising a first contiol input lerminal for acln ating or deacln aling the switching mode cυi rcnl regulator. a second control input terminal which connects or disconnects a polaπh υf an armature of the motoi so that when connected will cause {he armatuie to turn in a first direction and a thsrd control input terminal which connects oi disconnects * the poianf) of the armature of the motor so that when connected W JII cause the armature to turn m a second direction

6 The current hunting DC Motor Starter of Claim 5 wherein when the motor ϊS not running no \ oitage JS present at the three control input terminals

7 rhe cunem limiting DC Vlotoi StaUer of C laim 2 including an ON ercurrent protection de\ ice that monitors a state of the s\\ i tch and pro\ ides an output to a motoϊ control centeϊ when the switch remains in an open state for greater than a first pieseiected time inters a!

S The current limiting DC Motor Starter of Claim 7 the ox ercurrent protection de\ ice computes a fust timing circuit that JS retπggered e\ er> time the switch is opened to prox ide a first gπ en output (or at le-a&t a second pteseleeted time mien al substantia!^ equal to the time the switch will remam open under a normal startup condition, wherein the first preselected tune inten al JS substantial^ Iongei than the second preselected tune inten al and the second pi ejected time inten al t& reset e\er> time the first timing circuit is retπggered, a second timing cnciiit that is tπggeied λλhen the sw itch is opened to pro\ ide a second gn en output foi the first preselected tune inten al and JS not retπggerijd b\ subsequent openings of the sw itch until the first preselected time inten al has timed out, arid

a comparison circuit that monitors the first gn en output and the second giλ en output and pio\ ittes a thud output io the motor conuoϊ center when the comparison eucuil senses ihat the second gn en output is not pjesent when the first gn en output is detected



CROSS REFERENCE TO RELATED APPLICATIONS This application JS related to eopending applications Sena! Nos.

(Attorney Docket Nos NSD 2005-01 S and NSD 2005-0 J 9} filed concurrently herewith.


1. Field of the Invention

(0001 { This invention relates in genera! to DC motor stalling circuits and more parti cuiarh to a DC motor starting circuit with a current limiter

2, Backgrmmd lnforniation

(00O2J When DC motors are used for \ ah e operators, the high starting current can place undue burden on the electrical cables and a batten- power source. The current in a DC motor is limited by {he self generated electromotive force (EMF) voltage that is proportional to the machine speed. When the motor JS first started, the speed and hence the EMF is zero (OK thus allowing the current to rise to the rated value known as the "locked rotor current", which can be up to ten times the name plate current rating of the motor. As the machine accelerates, the EMF increases and the current decreases until a stead\ equilibrium is achieved depending on the load. FIG 1 shove s the current and speed as a function of time for a t\ pical 15 horsepower motor. The current curx e is represented by reference character 10 and the speed cuπ e is represented by reference character 12 jθOO3[ Com eotional reduce \ oltage motor starters limit the current but do not adapt to changing conditions of motor temperature or supply \ oltage. In a com entional reduced \ oHage motor starter, additional resistance is included in the circuit to limit the current and is then switched out as the motor accelerates Tins may be done as a single step or in multiple steps. The switching mas be based on fixed times or on a measurement of the motor speed (actually the EMF developed across the motor) A circuit for implementing this approach is illustrated m Figure 2A R and L

respectively stand for the resistance and inductance of the motor windings. EMF is the electro-molive force developed within the motor. Resistance Rl is greater than R2. In operation the Switch S is sequenced from OFF to Rl . Then, after a preselected time interval has passed or molor speed has been achieved the Switch S is sequenced to R2 and then when the motor readies full speed the Switch S connects directly to the power supply. This approach has several drawbacks. In the switched resistance approach, {he current limiting is only prov ided at initial start up. If the current increases later due to overload conditions, then the current will increase until it is interrupted by an over-temperature protection device, if one is included in the circuit. Furthermore, the resistors must be sized to dissipate a large amount of heat. For example, in the 15 horsepower model used to produce the graphs shown in the figures, the RMS power dissipation of the first stage resister is 6.9 KW, Additionally, once the starting resistances have been chosen, it is difficult to make changes in the field to accommodate the as installed conditions experienced for each specific motor operated valve.

[00041 The advent of high power semiconductors allows a switch mode voltage regulator approach employed by the prior art to control the voltage of a DC motor as a means to control the motor ' s speed, to be adapted to overcome the issues identified above for the conventional reduced current motor starter This is particularly important for some critical applications such as, advance passive nuclear power plants, where DC motors will be employed for safety grade valve operators because no class 1 E AC power source is available. Because these motors must operate from the 125 VDC power station batteries, it is not possible to increase motor voltage for the larger size motors to decrease the operating current. The advanced reactor Utilities Requirements Document for these passive nuclear power plants requires that the cables connecting the valve operators be sized to earn ' the full locked rotor current on a continuing basis. This is due, iti part to a United States Nuclear Regulatory Commission Regulatory Guide (RGl .106) that requires overcurrenl protective devices to be bypassed in the event of a safety actuation of the valves. As mentioned above, conventional relay based motor starters have used reduced voltage means to limit starting current by sw itching resistors into the circuit upon initial energization This

com entional approach has the di aw backs thai \%eιe mentioned abov e thai need to be ox acorae

}0005| λccordmgh it is an object oi this im ention to pi en ids a current lmiilmg DC moloi staiter that emplo\ s a switching mode curient regulator in a feedback contiol loop to liitiit the motoi cuπent to a pieset \ alue

|0006| It is a fuithei object υf this m\ ention to pio\ ide an impiov ed cuπent limiting

DC motor startei that intei-tau:s a snitching mode cunent iegulaloi with a solid state rex ersmg motoi stastei

|0007| It is an additional object of this im ention to pπn ide a current limiting DC motor starter that empio> s a SXλ itching mode cυiient regυiatoi emplo> ing closed loop cuiient mea^uremcnl to pi ox ide precise cunent contiol that adapts to chaiigiiig conditions

|0008J It is the further object of this im ention to pio\ iUe an iniprm ed cuiient b nil ting DC motor itartei tliat pτox ides an ox crctuicnt indication to a contiol &} item as ueil as features to enable us calibration

|Q009| Further, it iv the object of thιt» in\ en Ii on to pioude an nxiprtn ed cuirenl hmitmg DC motoi statter ha^ ing an mteiface xMth a control s\ stern that is arranged m such a manner that cabie faults and other single failuies \\ι\\ not cause spuiious actuation of the motor

JOOIO] It is an additional object of this inv ention to prov ide such an improv ed current limiting DC motes staitet that is specif icalh suited fot ^afeh gtade applications in nucleai pouei plants

Sl MM '\RY OF ϊH£ TM fcN UON

[θOϊ ϊ j This invention accomplishes tlw forijgooig objects bs integiaiing a sv\ ιtchmg mode current leguJatυr vx itli a solid state ie^ ersing motυi staiter I he switching mode cunent regulator of this inv ention uses closed loop current measuiemeiits to pi ox ide piccise cuirenl control that adapts to changing conditions The cunent limiting DC motoi stai tei pan ided foi hei em also pro\ ides ox crcui i ent mdicati on to a control s\ stem and features to enable its calihation The interface to the control <λ stem is

arranged in such a manner that cable faults and other single failures w ill not cause spun cαis actuation of the motoi

}OOϊ2| The switching mode cuiienl regulates of thi^ inv ention includes* a source input terminal for connecting a source of DC current and a switch in series with the input terminal Hie switch has a switch control circuit operable to open the switch when the current tυ the sw itch exceeds a predetermined \ alue and to close the switch w hen the current dow rntteam of the sw itch falls below a second predetermined λ alue A motor output terminal is provided fos connecting a motor wsruling in scries w ith the sw itch 4 diode is placed across the motor winding betw een the motor winding and the switch, the diode being onented to block current from the source of DC current connected through the source input terminal, fiom cncuiating thiough the diode A current sensor monitors the current through the motor w Hiding and pro\ ides an output representaln e thereof to the switch contiol circuit In the preferred embodiment the switch is an Insulated Gate Bipolar Transistor and the second predetermined v alue takes into account the b\ steresis m the series circuit

[QO I3| Preferabh , there are three control input terminals to the OC motor starter circuit The Inst control input terminal ts for acln ating or deacm ating the sw itching mode current tegυlatυr The second control input teimmaJ connects or disconnects a first polaπt\ of an at mature of the motor so that when connected v\ ι)l cause the armature to turn in a first direction The third control input terminal connects or disconnects a second polaπt> of the at mat me of the motor so that when connected v>ι!l cause the armatuie to turn m a second direction The cueun is aπangcd so that when the motor ι.s not running no \ oStage is present at the Lh tee control input terminals so that spurious signals do not cause inadv ertent actuation of the motor JOO Mf Destiahh the current limiting DC motor starts t of this im ention further includes an o^ei current pi election de\ ice that monitois the state of the switch and prov ides an output to a motor control center when the switch remains in an open state for greater than a first preselected time inten a! The o\ ercurrent protection de\ ice includes a first timing ciicuit. a second tuning circuit and a comparison cucuit The first timing circuit ts ttiggcred e\ en. ttme the switch ts opened to prm ide a first smen output for at least a second preselected time mterv al substantial!) equal to the time the

switch will remain open under a normal startup condition. The first preselected lime inten al is substantial K longer than the second preselected time interval and the second pieslected lime snten al is reset ex ery time the first timing circuit is relriggered The second liming circuit is triggered when the switch is opened Io proMde a second gn en output for the first preselected time inter * al and ss not retnggered b> the subsequent openings of the switch until the first preselected time intenal has tuned out. The comparison circuit monitors the first gn en output and the second gh ert output and prox ides a third output to the motor control center when the comparison circuit senses that the second gn en output is nol present when the first giλ en output is detected.


(0θ15J A further understanding of the im ention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying draw ings m which

}OOI6j FlG. 1 is a graphical representation of a plot of current and speed v ersus time of a fifteen (15) horsepower DC motor upon startup;

|0017] FIG. 2a is a simple schematic circuit diagram of a com entiona! reduced x oltage motor starter Io illustrate the concept of adding additional resistance to the motor circuit upon startup until the motor comes up to operational speed,

(00ϊ8{ FlG 2b is a simple schematic circuit diagram of a current regulator to illustrate the sw itching mode concept of this im erttioii

}0019| FIGS 3a and 3b are graphical representations of motor current and speed versus time for both a conv entional and a switching mode current regulator, with all design parameters at thesr nominal \ alues.

|0020| FIGS 4a and 4t> illustrate the comparison shown m Figures3a and 3b with an increase of motor resistance,

(00211 FIGS 5a and 5b show the comparison of Figures 3a and 3b as affected h> in inctease in load.

|βϋ22| FJGS 6a and 6b show the comparison illustrated by Figures 3a and 3b after a battery \ oϊtage drain;

}OO23| FfCiS. 7a, 7b and 7c is a full circuitry schematic of the preferred embodiment of a current limiting motor starter contemplated by this invention;

|0024| FIGS. Ha, 8b and 8c is the circuit diagram illustrated in FlG 7 showing component values.


}0025{ This invention integrates the concept of a switching mode λ oltage regulator with a solid state re\ ersing motor starter. TIi e ιmpro\ ed switching mode current regulator of this im ention employs closed loop current measurement to provide precise current control that adapts to changing conditions. This invention also provides an o\ ercurτenl indication to the control s> stem as well as features to enable its calibration Additionally, the interface to the control system \s arranged in such a matter {hat cable faults and other single failures will not cause spurious actuation of the motor.

(0026 { The concept of a switching mode current regulator emplos ed by this inv ention shown in FIG. 2b To start the motor, the switch 14 is dosed The inductance 18 of the motor initially resists the change in current; howex er {he current does increase w Uh an L/R tone constant When the current reaches the desired limiting \ aiue the switch 14 is opened. Current continues Io flow through the diode 22 but now decays at the L R time constant When the current falls below the set limit b> some h\ steresis \ alue the switch is again closed. This continues as the motor accelerates which causes the EMF \ oitage to increase proportionally to the speed. Increasing EMF causes the current to be reduced until it reaches the point where the set limit is not exceeded After this time the switch remains closed. The result of the current switching mode concept o\ er that illustrated in FIG. 2a can be appreciated from the graphical representation illustrated in Figures 3a - 6b which plot current and speed of the motor x ersus time under v arious motor operating conditions

}002?| FJGS 3a and 3b show a comparison between the tw o different control schemes with all design parameters at their nominal \ allies The response of the S»o schemes is similar although the graph of 3b. representaih e of the sw itching mode, can be seen to reach full speed a Ii Ui e quicker. This is due to the fact that the current is kept closer to the limiting \ a!ue during starting. Thus the average starting torque of the motor is higher.

J0028J FIGS 4a and 4b show the effect of increased motor (or circuit) resistance by fifty percent {50%). This increase in resistance may result from higher motor temperature (either from higher ambient temperature or from pre\ ious running) or from changes to the circuit such as corrosion of the connections. The nominal resistance of a large DC motor is low. typically a fraction of an ohm. It does not take much of a change to impact the response Also, the increase in the resistance could be from a more permanent source such as not sufficiently accounting for the resistance of the cable leads. From Figure 3a it can he seen that the com eotional reduced \ oitage starter initially produces less than the intended current, hence less torque under these conditions resulting in a slower acceleration of the motor. The current limit is exceeded in the later stages of starting because the motor speed, and hence the BMF. is less than planned at the time the resistors are switched out

|0029] FIGS 5a and 5b show the effects of another parameter change, that being an increase in load by 50 % Such a load increase max" occur in a motor operated vaKe if the stem packing oilers more resistance from over tightening by maintenance to reduce leaks, etc. As can be seen, under these conditions, the conventional motor starter design does not keep the current below the set limit, whereas the one employ ing feedback control adapts to this situation jθO3O[ Additionally, in critical safety related applications, such as ate found in nuclear power plants, the motor operated \ ah es must be operable e\ en if the battery x oHage is reduced because they have been drained from long duration usage. Figures 6& and 6b show a comparison of the O control schemes under conditions of 70% supply \ oitage. It can be seen that the coin enϋonai approach does not produce the allow ed current during starting thus leading to a tow er torque and a siower

acceleration of the motor. The control provided by this invention again adapts to the changing parameters providing the best possible torque under ali conditions. }0031| A full circuitry schematic of the preferred embodiment of this invention is illustrated in Figure 7. The main switching component in the current limiting DC motor starter circuit illustrated in Figure 7 is an Insulated Gate Bipolar Transistor (IGBT) device such as is available from Powerex, Inc. of Youngwood. Pennsylvania. The product family called Intelligent Power Modules combines high speed, low loss IGBTs vvith optimized gate drive and over-current and short-circuit protection circuitry. The device with a part number PM800HSA060 is capable of switching 800 Amps at 600 Volts.

(0032 { The IGBT 24 in FIG. 7 serves as the current switch 14 previously shown in FlG. 2. Pairs of IGBT devices 26 and 28 or 30 and 32. are energized to route the armature current in one direction or {he other to determine the direction of rotation of the motor (M). The control interfaces to these devices is done with HCPl, 4504 optocouplers 36, 38, 40 and 42, per the recommendation of the IGB T manufacturer. When IGBT 24 switches off, motor current continues to circulate through the free wheeling diode 54.

(0033 { Motor current is detected using a hall effect current sensor 56. such as the Honeywell CSLAl EL, which is capable of measuring up to 625 Amps. At zero current, the output of this device is offset to Vcc/2. As the current increases so does the output voltage at about 7 raV per Amp. This voltage is compared to a fixed setpoint by comparator 58. The setpoint voltage established by the adjustable resistor voltage divided network of resistors 76, 78 and 80. The fixed resistors 76 and SO of this network establish the range of adjustability. Resistor 82 provides a small positive feedback current to give hysteresis to the comparison.

}0034| When the measured current is less than the setpoint value, the output of the comparator 58 is high turning on transistor 74 and hence, through optocoupler 34, turning on the ICJBT switch 24 allowing motor current to How. Tins switching is interlocked by optocoupler 44 which is a control system input 62 to turn the motor starter on. When the current rises above the setpoint (plus hysteresis), the comparator 58 output goes low and subsequently switch 24 is turned off The current will decay

at the motor L/R lime constant until it again falls below the selpoint value at which lime switch 24 is once again turned on. This switching will continue, at a frequency determined by the size of the hysteresis band and the L/R time constant of the motor until such time as the EMF generated by the motor reduces the motor current to the point that the setpoint is not reached. In the simulation mode? used to compare this invention to conventional control approaches which produced the graphical representations identified in Figures 3 - G, the switching frequency was found to be on the order of 30 Hz.

|QO3S| The control inputs to the motor starter come from a host control system as three separate signals, RUN, 62, OPEN, 64, and CLOSE, 66. The OPEN and CLOSE signals alternately turn on the pairs of armature polarity switches 26/28 or 30/32. Optocoupiers 46 and 48 provide an interlock so that in the fault state where both of the inputs are turned on. all of the current steering IGBFs will be turned off preventing any motor motion. As previously mentioned, the RUN input 62 enables the main switch 24 to be turned on. By providing this signal coincidence, the potential for spurious motor operation due to control failures is reduced. Terminal 68 is the reference or return line for the control inputs. Light emitting diodes 84. 86 and 88 are provided for indication of device operation.

}0036] The control signals 62, 64 and 66 are low voltage When the motor is not running, no voltage is present on these control lines. This provides protection against spuπous motor operation that could otherwise result from cable faults {hot shorts) that may result from cable fires or other physical damage. The input voltage is set for 48 VDC by the values chosen for resistors SH), 92 and S>4 that limit the current that passes through the optocoυpers 44, 46 and 48. respectively. This input voltage is the voltage employed for generating the signals originating from the Components Interface Module that will be used on the APl OOO advanced nuclear power plant designed by Westinghouse Electric Company LLC, Pittsburgh Pennsylvania, However, other input voltages could be easily accommodated by selecting different values for the resistors 90, 92 and 94.

[0037] Because the switching of the armature polarity IGBTs 5 26/28 arid 30/32, is slow and is done at currents considerable less than their rating, external suppression,

be\ OJKI that proN ided mtemalh to the dei ice. is not needed For s\\ itch 24, the Uansienls that could result from switching highK inductn e loads &αch as the motoi ate a\ oided due to the fiee wheeling diode 54 1 io\\e\ ej. because of the iesidual inductance in the lead from the pow er bus to the switch, there would be transients induced as a result of the switching Therefore, resistor ioo and capacitor 102 are pro\ ided as a snubber circuit to suppress these tiansients The manufacturer ' s jtecommendation is that the tune constant of the snubbei be appio\irriateh three lmies the switching pes sod, so Jt mas be dependent on the specific \ ah e motor on a ease b> case basss For the fifteen horsepower sample case used in this embodiment, \ allies of Si ) UiI and 2 Oμf ma\ be appropriate tυ gn e a time constant of 1 OO msec {0038 j In addition to the shoit circuit protection, which could be a magnetic cucυn breaker u (> as shown m the upper left hand comer of the schematic of Flu 7, a com enfiønai motor starter will ha\ e an o\ eicurrent pi election de\ ice w hich is sized based on ITS appummating motor tempeiatuie in a cuiient sensun e de\ ice The concept of such a deuce is to tap the cuciut when a sustained cui rent caused b> an ON erload condition that could heat the motor windings to a pomt that exceeds the insulation rating The si/ing of such de^ ices has pro\ en to be ptobSematic and the pυteiitiai for fahe tups is sigiiificant Foi safeh ieiated motors, the United States Nuclear Reguiaton Commission ha.s required that the action of such dex icjs be b> passed in the e\ ent that the demand for the \ aS\ e motion results from an actual safelv need (RG 1 H)(>) The αse of cuttenl iunUtng b\ this jmentioti makes such a device moot Ne\ ertlielcss» n is dcsuable to detect ON erload conditions to inform the control s\ stem so that appropriate actions can be taken, including shutting off the motor if the demand is not critical ' 1 his detection is pro\ ided b\ a pair of 555 timer mlegiated circuits K)4 and 106 When the comparator 58 output goes low. indicating that the cuπeot is being limited, both timer cucuit^ U4 and 106 are triggered at inputs 1 OX and l lo causing their outputs 1 i 2 and 114 to go h tgh 1 i mer circui f 104 ϊS tetiiggβied e\ en time the comparator goes low b> the discharging of the capacitor J 1 7 thiough the tramislor 1 1 (< The timei cucuit S (>4 has a ielatn its short time out period of approximator 2^0 msec The effect is that its output 1 12 vull remain high for the duration of the time that the current is besng limited h\ the opening of switch

24 Timer circuit 106 ss not relngge-r b> the suecessπ e switch pulses, mstead it times out after an adjustable period on the order of a few seconds If the timer ciicuil 10ft output becomes low at a time when the timer cucust S (»4 output is still high, indicating that the current limiting has pessisted foi longer than the pieset time period, then the optocouplers 50 and 52 conduct, causing timer circuit 106 to reset pres enting it from responding to further triggering pulses, so long as the optυcouplers 50 and 52 are conducting, and at the same time signaling the o\ eicurrem condition to the contiol ss stem Ih * o ugh optocoupler outputs 124 The tinier circuit 106 resets when the current limiting stops b> allowing timer circuit HH to time out and Hs output M 2 go low

|0039j The sensitπ ch of the hall effect cunent sensor 56 is dependent to an extent on the geometn of the w ire earn ing the motor current passing through it and the coupling of the magnetic field pioduced b> the cunent to the core of the toroid of the sensoi Fot this reason, calibtation featuies aie pio\ ιιicd to allow the cunent limiting Io be preciseK set in situ A multiple turn w inding 122 through the apertuse of the current sensor 5<> is brought out to test points I Pl and FP2 B\ injecting a known current source through this winding, the output λ ullage of the sensυr 56 can be measured at test point (TP4) The iatio of tuιns> of the calibration winding to the single turn of the motor circuit prov ides a multiplier fiom which a calibration cur\ e can be determined The calibration cuπ e thus determined is used to determine the setpoHit. which is then dialed m while measuring its \ alue on TP 3 Since the comparator 58 will he on during this proccduie (smcc the motor current is zero) the voltage at TP3 will include the h\ stetesis x alue and will thus be a ltue mea&ute o( the point at which the current will be switched off Hie function of the comparator 5H and the \ alue of the h\ steresis caα then be detetnaned b> \ an ing the calibration cuuent (multiplied h> the winding iatio factoi ) abo\ e and belθλ\ the action \ alues and obscn mg the resulting \ oltage changes on 1 P5

J0040J Accordingl y . this im ention pro\ ides a cunent himtiαg DC 1 motoi stailer λ\ ith pjecise current control that adapts to changing conditions and pio\ ιdes o\ eicurrent indication to the control Sλ stem as well as features to enable its calibtatton

I t

Additionally, the interface to the control system is arranged m such a manner that cable faults and other single failures wiii not cause spurious actuation of the motor. }0041| While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breath of the appended claims and any all equivalents thereof.