CA1254612A - Induction motor control system - Google Patents

Abstract

ABSTRACT

An improved induction motor control system. Single-phase and three-phase AC induction motors are controlled in accordance with the equation y = mx + b, where y is the phase angle, m is the slope, x is the firing delay and b is the offset.
Subsequently, the firing delay is increased by predetermined amounts, the behavior of the phase angle is evaluated for several cycles, and the motor is controlled based on the results of the evaluation, thereby minimizing energy consumption.

Description

125~
,~pT IC~T~ON
Or R~Y r DAv I S ~ J~ IIcF~FL J . r~EST'~Q~VE~, ~RT~ J T ~OT-~, ~IC'~3 H JO~SO~ and RO~LD ~ P~ R
FOR
I~PROVE~ INDUC,ION MOTOR CONTROL SYSTE~

3~CXGROUN~ Or T~_ INVENTION

The present invent~on relates to an proved control sys~~r~ -or AC inc~ction motors and, lr. i.s ?refer_ed embodi~ent, o to an ir?roved digital induc~ion motor control sys,em A ?rior art digital induc~ion mo~or control system is shown in U S Patent ~o 4,361,792 lssued November 30, 1982 and assigned to the assl~nee o~ ,he ins,ar~t ap?licG~ion Lhe syste~ disclosed in tha, ?r_or ar. ?atert derives a calibr2ted ?hase angle and then con~rols the opera,ion of the motor ~ased on that phase angle, irrespective of varia,ions in motor loading In this way energy consum?tion is reduced Re~e---ng to h g 1 o~ U S Pa~ent ~o 4,361,792, ar~e- che ~otor has go~ten u? to speed the count in inc-emen~al co~rter 38 and dela~-counter 36 is zero The nu~ber ir pn2se angle counter 34cor~es?onds to the phase zngle between ~o~o~ voltage and ~oto-current zero crossings The count in the incremental counter is incremented every other cycle, which causes the ccunt in the delay coun~er to be similarly increased This increased delay i firing the triac reduces the phase angle and, therefore, ;he count in the phase angle counter Event-ally, ,he nu~be- in ~he ~Z5~
216-"85 ll phase angle counter will 'oe approximately equal to the n~nber

2 in ehe delay counter. The number in the phase angle counter

3 can then be scored in the phase angle register as the calibrated

4 phase angle and used to operate the system.

6 Deriving the calibrated phase angle in this manner, 7, however, means that the number stored in the phase angle 8 ,, register as the calibrated phase angle depends, in part, on 9 1I the inltial motor loading. Greater power savings can be 10 obtained if the motor happens to be heavily loaded when the 11 I calibrated phase angle was derived. The prior art also 12 l, discloses how to modify the initial calibrated phase angle 13, so as to yield a refined calibrated phase angle. See column 5, 14 ,l line 3 through colu~ 6, line 3 of U. S. Patent No. 4,361,792.
15 'I
16 I~ i i ! ,, 18 i!

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26 ' 2~3 216~ i Z S ~ ~ ~ Z

1 1 BRIEF DESCRIPTION OF_THE ~RESENT INVENTION

3 ¦¦ The present invention represents an improvement over 4 1I the invention of U~ S. Pa~ent No. 4,361,792. As noted in that

5 '¦patent, the application was based on limited experimentation.

6 lIBased on extensive experimental work applicants have developed

7 a new and improved method for controlling AC induction motors.

8 ,Applicants have discovered that changes in both the phase angle

9 ,and the firing delay result from changes in motor load and that i0 these changes occur in a characteristic manner which is common 11 to AC induction motors. This characteristlc may be approximated 12 by an equation of the form y mx + b, where y is the phase 13 angle, x is the firing delay, m is the slope and b is the 14 offset. Applicants have also discovered how this characteristic 15 'may be used to control AC induction motors with substantial 16 ,energy savings.

18 In addition, applicants have discovered a way to l9 optimize energy savings by means of a "perturbation" technique 20 ~ in which the firing delay is increased by a predetermined 21 'amount and the behavior of the phase angle is evaluated for 22 several cycles. If the result of the evaluation indicates 23 ~that additional energy savings are possible, the firing 24 ~, delay is again increased by a predetermined amount and the behavior of the phase angle is again monitored for several 26 cycles. This process is repeated until energy savings 27 are maximized.

3~

216-185 Il.
5~61~:

1 BRIEF DESCRIPTION OF ThE; DRAWING

3 Fig. LA is a flow diagram depicting the operation 4 of a motor control system in accordance with the present linvention;
6l!
7il Fig. lB comprises graphs of typical motor responses 8¦¦when controlled in accordance with the present invention;

lol Fig. 2 is a schematic diagram of a preferred motor ~ control system which may be used to implement the present l' ~invention; and 13l, ~ Fig. 3 is a schematic diagram of another motor control 15 ! system which may be used to implement the present invention.

21li 716- 1~5 Ij ~ZS~

DETAILED DESCRIPTION OF T~E PREFERRED E~D3ODI~ENT

3 As already noted, the present invention is based on 4 applicants' discovery that AC induction motors share a common S ¦¦characteristic, namely, that changes in phase angle and firing 6 ¦I delay with variations in motor load can be approximated by the 7 , formula y - mx + b, where y is the phase angle, m is the slope, 8 x is the firing delay and b is the offset. A preferred equacion 9 !I which has been found applicable to all single-phase AC induction lo 'motors tested is PA = - 0.12 DLY + 70. Implementation of this 11 ! equation permits control of the motor with substantial energy 12 1I savings. In the equation, PA and DLY are expressed in units 13 li Of count. In the illustrative embodiment of the invention 14 ~ shown herein, 1 count is equal to 30 microseconds or 0.648 electrical degrees at 60 Hertz.

17 Figure lA discloses generally how the motor is control- j 18 ,,led in accordance with the equation PA = - 0.12 DLY + 70. At 19 1l event N, the motor is energized using a firing delay Dn. The ,phase angle PAn is then measured. The sum of Sn of the delay Dn 21 11 and the phase angls PAn is then used as the address for a computer 22 ! generated look-up table. The contents of that address PAt (phase 23 angle table) is compared with the previous phase angle PAn and 24 1 the result used either to increase or decrease the delay.
1 Initially, PAn will be greater than PAt and the delay will be in-26 creased in accordance with the formula Dn+l = (PAn - PAt~/2 + Dn.
27 This increased delay in energizing the motor results in energy 28 savings. The process is repeated until PAt and PAn are 29 approximately equal. If the motor load increases, PAt will exceed PAn~ Now the delay wlll be reduccd in accordance with ~Z~i4~12 l the formula Dn+l - Dn - 4(PAt - PAn)~ This reduced delay in 2 energiæing the motor reaults in more power being delivered to 3 the motor to handle the increased load and ensures that the 4 motor continues to operate at its designed, constant speed.
S ~ I
6 ¦ Table I, set forth on the next page, is a computer 7 ¦ generated look-up table based on the equation PA = - 0.12 DLY + 70.
8 1¦ The sum of the phase angle PA and the delay DLY is used as the 3 ¦¦ table entry point for initial motor control. The entries in the !
Ilj table are phase angles, i.e., PAt. Thus, if the sum of PA
l~l and DLY is 100 (100 + 0), the phase angle from the table is 65.
1-, If PA + DLY = 106 (100 + 6), PAt = 64. Similarly, if PA + DLY =
13~ 193, PAt - 53; whereas if PA + DLY = 194, PAt - 52. The table 4 ,I provides for 256 sum entry points and these 256 entry points 151l are divided into 16 groups or levels, with 16 entry points in 16ll each level. Thus, sum 0 through 15 fo D Level 1, sums 16 17¦, through 32 form Level 2, etc. It should be noted that small 13l; entry sums indicate high motor loads whereas large sums 19 'I indicate low motor loads. It should also be noted that the 7o ¦ firing delay is never less than 1. Thus, in Levels 1 through 21 ¦ 4 the 100 indicates that a firing delay of 1 is to be 22l, maintained.

25 1!

, :1~S4~
~IE 1 (PA ~ - .12 DLY + 70~
LEVEL Su~ +0 +1 +2 +3 +4 +5 +6 +7 +8 +9 _ _ __ _ _ ._~ . _ _ , , ,. ____ 1! 3 30 100 100100 100 100 100 100 100 i 5 60 100100 100100100 100 j 70 69 69 69 69 69 69 69 68 68 68 ¦ 6 80 68 68 68 68 67 67 67 67 67 67 i 7 90 66 66 66 65 ! loo 65 65 65 65 65 65 64 64 64 64 i 1 8 110 64 6463 63 63 63 63 63 i 1 120 63 62 62 62 62 62 62 62 ' 9 120 . 61 61 , 130 61 61 61 61 61 61 60 60 60 60 ;, 10 140 59 59 59 59 59 59 !l 11 160 57 57 57 57 57 56 56 56 56 56 ! 1 170 56 56 55 55 55 55 . 12 170 55 55 55 55 : 180 54 54 54 54 54 54 54 53 53 53 ~ i 190 53 53 !! I
~ 13 190 53 5352 52 52 52 52 52 ',: 200 52 52 51 51 51 51 51 51 210 .50 50 50 50 50 50 49 ~9 49 49 2~0 49 49 49 49 i~

1 It will be appreciated that a different table can be 2 generated by varying the slope from - 0.12 to some other value 3 such as - 0.18 or by varying the offset from 70 to some other 4 value such as 6B. A different table could also be generated by 5 varying both the slope and the offset. For example, applicants 6 ¦ have controlled single-phase AC induction motors based on a -7 1 table generated in accordance with the formula PA = - 0.18 DLY
8 1 + 72. The slope may vary between about - 0.2 and about - 0.1, 9 ¦ while the offset may vary between about 65 and about 72. Appli-

10 ' cants will now show, by means of a specific example, how the

11, motor is controlled using this type of equation.

12 i1

13 ~¦ Initially, the computer determines the existing phase

14 I~angle PA, adds the start-up firing delay of 1 to PA, and uses

15 llthis sum to enter the table. The computer then reads the phase

16 ¦ angle in the table, which corresponds to this table entry sum

17 and calculates a new firing delay. If PAl = 99, then PAl + DL

18 li= 99 + 1 = 100. Referring to Table 1, address 100 contains a l9 j PAt of 65. Since PAl is greater than PAt, the new firing delay 20 j is calculated as follows: new delay - (PAl - PAt)/2 + old delay 21 1 = (99 - 65)/2 + 1 = 18. The new firing delay of 18 is now 22 ,!implemented and the resulting phase angle is measured, in this 23 llcase 95. The new entry sum is 95 + 18 = 113, which gives a 24 ,¦PAt of 64- The new delay is (95 - 64)/2 + 18 = 33. This 25 Iprocess continues and eventually, assuming that the motor load 26 ~does not change, a point in the table will be reached where the 27 sum remains fairly constant, i.e., PAt approximately equals 28 PAn- If, however, PAt exceeds PAn~ this indicates an increase 2g,in motor loading. The firing delay is then reduced by ~he 30 amount 4(PAt - PAn). See Fig. lA.

1 ¦ The foregoing motor control technique provides a smooth 2 ¦ response to clutched loads without risk of motor stall and without 3 ¦ having to respond with full power, which is an inefficient response 4 1 unless absolutely necessary. The motor automatically responds to 5 1 load changes, in either direction, finding its proper po9ition 6 ¦ in the table for increased energy efficiency. Whlle this basic 7 approach has been proven effective in increasing the energy 8 1 efficiency of all motors tested, many motors possess the potential 9 ! for even greater energy savings. This is due to the fact that 10 1, such motors deviate from the empirical formula PA - - 0.12 DLY
11 1 + 70: ~1) having a slope other than - 0.12; (2) having an 12 lloffset other than 70; (3) having the phase angle and/or delay 13 l¦vary non-linearly with load: or (4) a combination of the foregoing.¦
14 1¦ In order to optimize the energy efficiency of all motors, a 15 i¦further motor control technique has been developed. This 16 linvolves "perturbing" the motor operation by introducing an 17 1 additional predetermined delay in energizing the motor, observing 18 1 the behavior of the phase angle for several cycles in response

19 1 to the delay, and then controlling the motor based on the nature

20 ~ of the phase angle response.

21 1

22 ~¦ After initial control of the motor has been accomplished'

23 .11 in accordance with Fig. lA and Table 1, an additional firing

24 ,Idelay of 20 counts is added to the existing delay. The effect of this additional delay on the phase angle is then determined.
26 ,IThis is accomplished by measuring and storing the value of PA at 27 To (when the 20 count firing delay is added) and at Tl, T2, T3, 28 T4 and Ts (the next 5 positive-going zero crossings of motor 29 current). PAo is the phase angle at To, PAl the phase angle at Tl, etc.

216-185 ~ i ~2S~lZ

1 As shown in Fig. lB, there are three basic types of 2 responses to increasing the firing delay by a predetermined 3 I amount. In a Type I response, the phase angle is reduced from 4 PAo to PAl and ther. remains between PAl and (PAo - PAl)/3, i.e., it recovers to less than 1/3 of the diiference between PAo and PAl-6 j In a Type II response, the phase angle is reduced from PAo to PAl 7 and then recovers to more than 1/3 of the difference between 8 PAo and PAl. In a Type III response, the phase angle is 9 initially reduced from PAo to PAl and then further decreases 10 ¦to a Ievel of PAl - 2. In the event of a Type III response, 11 Ithe calibration procedure is aborted and the computer controls 12 ¦¦the motor based on the conditions in effect at time To. In the 13 llevent of a Type II response, the computer temporarily classifies 14 ,',the delay at Tc (the delay in effect before introducing the 15 ¦ additional Z0 count delay) as the optimum delay for maximum 16 ! energy savings at that load and at those operating conditions 17 11 (e.g., temperature, supply voltage, etc.). The computer will, 18 I however, repeat the process several times to verify that a Type II
19 response continues to be obtained. A Type I response indicates that it may be possible to obtain additional energy savings.
21 In that event the computer increments the firing delay and 22 Il, evaluates the response of the phase angle until a Type Il or 23 ,,Type III response is obtained, i.e., until operating energy 24 l¦efficiency has been maximized.

25 '

26 " Referring again to Table 1, the calibration process

27 for optimizing energy efficiency is generally conduc~ed at

28 Level5 6 - 13. For convenience Table 1, which has an of~set

29 of 70, is referred to as a "70 Table." Experimentation to date has shown that all single-phase AC intuction motor~s tested will - ;O -216-l~S l~æ~lj~

1 ~¦ operate satisfactorily, at all loads, u9ing the 70 Table. Motors 2 ¦ that produce a Type I responss, which indicates that additional 3 ! energy savings may be possible, are assigned smaller phase angles 4 1 corresponding to their look-up table entry sums. This is done, S ¦ each time there is a Type I response, by decrementing the offset.
6 1 In other words, when a Type I response is obtained the equation 7 1 is changed from PA = - 0~12 DLY + 70 to PA = ~.12 DLY + 69.

9 1 For example, assuming that the motor is operating at a 10 ', look-up table entry sum of ll~0, this corresponds to a PAt of 60.
11 If a Type I response is obtained, a new calculation is made based 12 j¦on the equation PA = - 0.12 DLY + 69. This yields a PAt of 59 13 llfor a look-up table entry sum of 140. If another Type I response I
14 lis obtained, a new calculation is made based on the equa~ion 15 ,'PA = - 0.12 l~LY + 68. This yields a PAt of 58 for a 1OO~-UD
16 Itable entry sum of lllO. The effect of this procedure, for a 17 l¦constant look-up table entry sum, is to increase the firing delay 18 jland reduce the power delivered to the motor, thereby increasing 19 llenergy savings. This procedure is repeated until a Type II
20 l¦response is obtained, indicating that energy savings have been 21 llmaximized. In the illustrative embodiment of the invention dis-22 ~lclosed herein, the minimum allowable offset (maximum eDergy 23 !Isavings point> has been limited to 60. A Type III response ¦
24 indicates either that the motor load is increasing or that 25 maximum energy savings have been obtained. In either case, 26 further calibration is halted until stable operation resumes.

28 ~eferring to Table I, the calibration process for 29 maximizing operating energy efficiency is limited to Levels

30 6 - 13. It is possible that some motors, in certain applications, 216-18~ z5~6~z l will operate at one constant load point, thus limiting calibration 2 to a look-up table entry sum corresponding to that load point.
3 Even with a constant load, however, fluctuations in supply 4 voltage, operating temperature, motor friction, etc. will cause the table entry su~s to vary some.what, thus creating the likeli-6 hood a motor varying from one level to another. When this 7 occurs experimentation has shown that when the offsets between 8 adjacent levels differ by more than 5, and the motor is 9 oscillating between such levels, a discernable "bump" is produced which could be disconcerting to the user. Accordingly, 11 1 applicants have devised the following technique for adjusting 12 1! the offsets of adjacent levels so that they never differ by 13 1 more than 3.

15 1 Returning to the preceding example, the table entry 16 1 sum was 140. This corresponds to Level 9 in Table 1. iWhen 17 ¦Ithe calibration procedure is commenced and a Type I response is 18 1 obtained, the computer changes the offset for Level 9 to 69.
19 l None of the other levels is affected. If another Type I
response is now obtained, the Level 9 offset is dropped to 68.
21 This continues until one half the difference between the 22 ll offsets of adjacent levels is equal to or exceeds 3. When 23 ¦¦ that occurs, one half the difference is applied to the adjacent 24 '¦ level, differences being rounded up. In the foregoing example, ¦
25 'I the first adjustment of adjacent levels occurs when the offset 26 I for Level 9 reaches 65 . Then 70 - 65 = 2 . 5, which is rounded up 27 to 3. At this point, Levels 8 and 10 are assigned offsets of 67.
28 When the offset for Level 9 reaches 62, the offsets for Levels 29 8 and 10 are adjusted to 64. Since Levels 7 and 11 now differ from Levels 8 and lO by more than the permlttecl ~moullt, the ~ 6 ~ ~
o~~seos for Le~els 7 and 11 are adjus.ed to 67. The o__set _or Level 9 can now go down to 60 (t..e lowest pe~ ed -n his illus.~ative e~bod;~en. o ~he -nvention), wi-hou. necess~.-t~ng 2ny ~u-,he_ ad,us~ents of ,he 0~-~52.S 0- ,he -djac~n_ levels.

_f ,he motor ccntinues to ooerate at Level 9, no othe callbration at-empt will occ~. I-, however, the motor load changes, causing entry to another level, the computer will comp2~2 ,he new o se- with ~0 (.~.e ~--vlous o se_) a.d w-ll permit a suf icient n~m~e~ o. calibratior. s,e?s to occ~lr so that an of~set OL 60 could be reached. For example, i.- the motor shifts to Level 7, which has an offset o 67, the computer will permit 7 calibra~ion attempts to be e~ecuted.
Levels 1 through 4 never deviate from a maximum firing delay or 1 while Levels 5, 14, 15 and 16 are never cali~rated. They are, however, allowed ~o have their of~sets ad,usted based on the calibration of an adjacent level.

Fig. 2 discloses one motor control system which may be used to implement the present invention. Fig. 2 of the instant application is the same as Fig. 2 of U. S. Patent No. 4,361,792.
The ?resent invention has been emoloyed to control single-phase AC induction motors us ng the syste~
shown in Fig. 2. A suitable computer program ror practicing the presen~ invention with the system shown in Fig. 2 is included at the end of the instant specification.

While the present invention has been described in terms of i~s application ~o single-pnase AC induction mo~ors, ~ 2S4~i1Z
i_ may 21so be ap?l ed ,o 3-?has2 induc ~on mo.o-s. For a Cesc-~ ?~_On 0 `- 2 3-phase c;~ ~al mo.or con,~ol sysea-~ Sea Col--n 6, lines 2O-38 o~ U. S. ~aten- ~o. 4,361,792.
3aced on ~ore ~ ~ed ex?e-i~entatior, applican.s have dlscovered tha. 3-phase AC ~
inauction mo~ors also expe~ience changes in phase angle and _iring delay as a f~nct-on or changes in motor load ~hlch vary in 2 c.~c-_C_e-- s_lc ma-.?.e- --d ,a,~ be a?proxi~ated bv ~',a exp-esslor y = ~x b. ~galn y is the phase angle, ~ is ~he slope, x is _he f_-ing delay and b is the of~se~. unlike the case of slngle-phase AC lnduc-lon motors, with a 3-phase AC
induc~ion motor m is posltive. A prefer-ed equation for controlling 3-phase AC induc~ion motors is PA = + 0.62 DLY , 24.
With 3-phase AC induction mo~ors the slope may vary f-om 0.5 to 0.7, while .he of_set may vary from 10 to 30.

It will be a?pr2ciated by ~hose skilled in the art that the present lnventlon is not liml,ed to the speciflc lllust-at_~e embodimen~ disclosed herein. ~hus, whlle appllcants have disclosed that for slngle-phase AC inductlon motors value fo_ m can be selected ber~een - 0.1 and - 0.2 and values for b can be selected between 6~ and 72, values can also be selec-ed outslde these ranges and stlll success~ully cont_ol many motors:
Similarly, while applicants have disclosed that for 3 phase ~C
induction mo,ors values for m can be selec~ed between 0.5 and 0.7 and values for b can be selected between 10 and 30, values can also be selected outside these ranges and still successfully control many motors. Other modi~ications and lmprovemen~s within the scope of the present invention will also be ap?arent ~o persons sk-lled in ~he art. For example, the present 1 invention msy be implemented without u9ing a progr~med digital 21 computer.

4il Fig. 3 of the present application discloses such a Sj¦motor control system. In Fig. 3 Ql through Q15 are FET
6 transistors which conduct when a positive voltage is applied 7 ¦¦ to their gates, while Q16 is a triac such as a Teccor Q4025V5 and 8 !¦ Q17 is a diode bridge. The components designated Ul - U6, 9i~U8 - U12, U14 - U16, U20, U21, U26, U27, U28 - U30, U38 - U39, lO,IU41 - U44, U60, U64, U72, U73 and U78 are operational amplifiers.
11 U13, U31 - U33, U48, U53, U65 and U79 are inverters, while U56 12 1l is a non-inverting amplifier. U17, U22, U24, U25, U35, U36, 13'!U46, U47, U49, U66, U68, U71, U74 and U80 are AND gates, while '4 U50, U52, U61 and U69 are OR gates. U45 is a NOR gate. U54, i5l¦U55, U57 - U59, U75 and U81 are CMOS Schmidt trigger amplifiers.
16 ¦U18, U51, U67 and U70 are latches, with U67 configured to 17 divide by two. U19, U23, U37 and U62 are counters, U62 being 18 an up/down binary counter. U63 is a four-line decoder which 19 lperforms a D/A conversion based on the output from U62. U76 is 201¦a buffer amplifier, such as a TI 7406~ and U77 may be a Monsanto 21~¦6200 IC chip comprising a pair of optically coupled SCRs connected¦
22 as a triac. A brief description of the operation of the motor 23 Icontrol system of Fig. 3 is set forth below.
24 il 25l! U64 is connected to the line voltage and develops a 26 "squared-up voltage waveform. Q17 and U78 develop a square 27 waveform which is positive when Q16 is off. U66 produces a 28 positive pulse which is synchronized with the line voltage.
29 The output from U66 is used to derive two system ~iming signals. The first is produced by U53. The second is produced 2] 6-185 ~S~6~;~

l by U54 And U55 and follows closely in time the output of US3.
2 Two additional timing signals are derived from the output from 311 U64. One is produced by U56, U57 and U81 while the other is 4 ~I produced by U53 and U59 and follows closely in time the output 511 of U57. The timing for the sample-and-hold circuits QllIU26 6 !~ and Ql2/U27 is provided by Ul3 and U22 - U25. Finally, U35, 7 ~¦ U36 and U37 control the gating of the various types of responses 8 (Type I, Type II or Type III) when the motor operation is 9 ' perturbed to maximize operating efficiency.
10 il ~
11 Converting the phase angle pulse width to a voltage 12 jj is performed by Ql, Q9 and Ul. The output of Ul is PAn which 13 !! is supplied, inter alia, to the sample-and-hold circuits. The l-; "delay" pulse width is converted to a voltage by Q2, Ql0 and U2.
lS ¦¦ U3 and U4 tAke the output of U2 and multiply it by "m" and add 16 il "b," respectively. The output of U4 is therefore PAt. U5 17 compares PAn with PAt. The output of U5 iS positive when PAn 18 ~ is grea~er than PAt and negative when PAt is greater than PAn-1.9 1l U5 controls whether the firing delay is lncreased or decreased.
20 il When the output of U5 is positive, Q3 ls enabled and Q13 is 21 ~¦ disabled and U6 and U8 perform the operation (PAn - PAt)/2 + Dn.
22 !! When the output of U5 is negative, Q3 is disabled and Q13 is 23 l enabled and U28, U29 and U30 perform the operation 24 1l Dn - 4(PAt - PAn).

26 The combination of Ul4, Ul5 and Ul6 function as a 27 zero error detector, i.e., ~hey determine when PAn equals PAt-2a When the output of both U16 and U15 are posi~ive, the 29 perturbation sequence is initiated. The perturbation sequencing circuitry is for~ed by U17, Ul8, Ul9, U50, USI, U20, U21, U32, '16-185 1 ~
~5~i1Z

1 Q4 and Q5. U18 functions as a perturbation in progress" latch.
2 Ul9 keeps track of the number of perturbations that have been 3 initiated. U38 and U39 perform the operation (PAo ~ PAl)/3 ~ PAl-4 1 U41 compares the output of U40 with PAn. If PAn is greater than 51¦ PAl plus (PAo - PAl)/3, then the output of U41 is positive and 6 ! indicates a Type II response. U43 and U42 perform the operation ¦
7 ¦¦ PAl - 2 and U44 compares this with PAn~ If PAn is greater than 8 ~¦ PAl - 2, then the output of U44 is positive and indicates a 9 li Type III response. The outputs of U41 and U44 are supplied to 10 NOR gate U45 which produces a positive output indicative of a Type I response in the absence of both a Type II and a Type III
12 ~¦ response.
13, 14 The gating of the various responses is controlled by 15 ¦¦ U46, U47, U49 and U68. Type I and Type II responses are gated 1611 by U49 and U68 respeceively. A Type III response which occurs 17 during the first perturbation is gated by U47 while one that 18 occurs in a subsequent perturbation is gated by U46. An output lg 1I from either U46 or U47 will produce an output from U52 which j 20 ¦¦ will reset the "perturbation in progress" latch U18 via OR
21 ¦¦ gate U69. When U49 indicates a Type I response, this causes 22`! counter U62 to decrement the offset.

24 1l The delay timing for controlling the delay in firing 25 il the triac comprises U9, U10, Ull, U12, U33, Q6, Q7, Q8, Q14 26 ~ and Q15. U10 and U12 are ramp generators which produce the 27 ~ same fixed ramp starting when the triac current goes to zero 28 during each half cycle of the line voltage. Ull compares these 29 ramps with the new delay in U9. The output of Ull goes 30 positive at the end of the new delay. The output from Ull is '16-185 1 ~ ?
lZS9~6~;~

1 buffered by U76 and fires triac Q16 via trigger U77. U72, U73 2 !1 and U75 determine when a change in line voltage or motor loading 3¦¦is great enough to merit a new perturbation sequence, and initiate 4,¦same.
sjl - .
6 As already noted, this and other circuitry for ~¦¦implementing applicants' invention will be apparent to persons gllskilled in ~he art. Applicants' invention is defined by the g'lclaims ~hich follow.
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Claims (24)

1. A method for operating an induction motor control system comprising the steps of:
(a) energizing an induction motor using a firing delay;
(b) measuring the phase angle between motor voltage and current zero crossings;
(c) determining a phase angle in accordance with the equation y = mx + b, where y is the phase angle, m is the slope, x is the firing delay and b is the offset;
(d) comparing the measured phase angle with the determined phase angle; and (e) altering the firing delay based on said comparison.

2. A method according to Claim 1 wherein the amount by which said firing delay is altered is proportional to the difference between the measured and determined phase angles.

3. A method according to Claim 2 wherein said firing delay is increased by an amount equal to one half the difference between the measured and determined phase angles.

4. A method according to Claim 2 wherein said firing delay is decreased by an amount equal to four times the difference between the measured and determined phase angles.

5. A method for operating an induction motor control system comprising the steps of:
(a) energizing an induction motor using a firing delay;
(b) measuring the phase angle between motor voltage and current zero crossings:
(c) determining a phase angle in accordance with the equation y = mx + b, where y is the phase angle, m is the slope, x is the firing delay and b is the offset;
(d) comparing the measured phase angle with the determined phase angle;
(e) increasing the firing delay; and (f) repeating steps (a) through (e) until the determined phase angle and the measured phase angle are approximately equal.

6. A method according to any of claims 1 or 5 wherein m is negative.

7. A method according to claim 1 or 5 wherein m is between about - 0.1 and about - 0.2.

8. A method according to any of claims 1 or 5 wherein m is positive.

9. A method according to claims 1 or 5 wherein m is between about 0.5 and about 0.7.

10. A method according to any of claims 1 or 5 wherein m is negative and b is between about 65 and about 72.

11. A method according to any of claims 1 or 5 wherein m is positive and b is between about 10 and about 30.

12. A digital method for operating an induction motor control system comprising the steps of:

(a) energizing an induction motor using a firing delay;

(b) measuring the phase angle between motor voltage and current zero crossings;

(c) computing the sum of the measured phase angle and the firing delay;

(d) using said sum to select a phase angle generated in accordance with the equation y = mx + b, where y is the phase angle, m is the slope, x is the firing delay and b is the offset;

(e) comparing the measured phase angle with the selected phase angle; and (f) altering the firing delay based on the comparison.

13. A digital method for operating an AC
induction motor control system comprising the steps of:
(a) energizing an induction motor using a firing delay;
(b) measuring the phase angle between motor voltage and current zero crossings;
(c) computing the sum of the measured phase angle and the firing delay;
(d) using the sum to select a phase angle generated in accordance with the equation y = mx + b, where y is the phase angle, m is the slope, x is the firing delay and b is the offset:
(e) comparing the measured phase angle with the selected phase angle;
(f) increasing the firing delay; and (g) repeating steps (a) through (f) until the measured phase angle and the selected phase angle are approximately equal.

14. A method according to Claim 13 comprising the additional steps of:
(a) measuring and storing an initial phase angle at time To;
(b) increasing the firing delay by a predetermined amount;
(c) measuring and storing the resulting phase angle at times T1, T2 ... Tn;
(d) comparing said initial phase angle with the resulting phase angles; and (e) altering said firing delay based on said comparisons.

15. A method according to Claim 14 wherein the phase angle at Tn is less than the phase angle at T1 and said firing delay is reduced.

16. A method according to Claim 14 wherein the difference between the phase angle at Tn and the phase angle at T1 is less than one third of the difference between the phase angle at To and the phase angle at T1 and said firing delay is reduced.

17. A method according to Claim 14 wherein the difference between the phase angle at Tn and the phase angle at T1 is greater than one third of the difference between the phase angle at To and the phase angle at T1 and the firing delay is increased.

18. A method according to claim 17 wherein said firing delay is increased by decrementing the offset.

19. A method according to claim 12 or 13 wherein m is negative.

20. A method according to claim 12 or 13 wherein m is between about -0.1 and about -0.2.

21. A method according to claim 12 or 13 wherein m is positive.

22. A method according to claim 12 or 13 wherein m is between about 0.5 and about 0.7.

23. A method according to claim 12 or 13 wherein m is negative and b is between about 65 and about 72.

24. A method according to claim 12 or 17 wherein m is positive and b is between about 10 and about 30.