US3892104A

US3892104A - Cryogenic freezer with variable speed gas control system

Info

Publication number: US3892104A
Application number: US399769A
Authority: US
Inventors: David J Klee; Roger A Howells
Original assignee: Individual
Current assignee: Individual
Priority date: 1973-09-20
Filing date: 1973-09-20
Publication date: 1975-07-01
Anticipated expiration: 1992-07-01
Also published as: DE2441757B2; NL7411500A; JPS5548233B2; GB1481452A; CA998538A; JPS5056646A; NL178720B; DE2441757A1; BE818982A; FR2244973B1; NL178720C; FR2244973A1; DE2441757C3

Abstract

A cryogenic tunnel-type freezer is disclosed comprising a cryogenic liquid spray zone, a plurality of gaseous precooling zones having non-enclosed recirculation fans, and at least one enclosed, variable speed gas control blower for producing and varying the net forward flow of cold gas from the liquid spray zone, in which the gas is generated, toward the inlet end of the freezer through each of the precooling zones in countercurrent heat exchange with the product to be cooled and/or frozen; the speed of the gas control blower being directly responsive to changes in the amount of cryogenic liquid being supplied to the spray zone.

Description

United States Patent Klee et al.

CRYOGENIC FREEZER WITH VARIABLE SPEED GAS CONTROL SYSTEM Inventors: David J. Klee, Emmaus; Roger A.

Howells, Orefield, both of Pa.

Filed: Sept. 20, 1973 Appl. No.: 399,769

US. Cl. 62/186; 62/216; 62/374;

62/380 Int. Cl. FZSd 17/04 Field 01 Search 62/177, 178, 179, 180, 62/186, 216, 222, 373, 374, 375, 376, 380, 63

Relerences Cited UNITED STATES PATENTS 9/1959 Bahnson et al .x 62/178 X July 1, 1975 Primary ExaminerWilliam F. ODea Assistant Examiner-Ronald C. Capossela Attorney, Agent, or Firm-Ronald B. Sherer; Barry Moyerrnan {57] ABSTRACT A cryogenic tunnel-type freezer is disclosed comprising a cryogenic liquid spray zone, a plurality of gaseous precooling zones having non-enclosed recirculation fans, and at least one enclosed, variable speed gas control blower for producing and varying the net forward flow of cold gas from the liquid spray zone, in which the gas is generated, toward the inlet end of the freezer through each of the precooling zones in countercurrent heat exchange with the product to be cooled and/or frozen; the speed of the gas control blower being directly responsive to changes in the amount of cryogenic liquid being supplied to the spray zone.

13 Claims, 4 Drawing Figures 1 CRYOGENIC FREEZER WITH VARIABLE SPEED GAS CONTROL SYSTEM BACKGROUND OF THE INVENTION May previous cryogenic freezers have been designed for cooling and/or freezing articles such as food products with liquid nitrogen (LIN), or with other cryogenic liquids such as liquid CO liquid air, and various inert halogenated hydrocarbons. In such freezers, the cryogenic liquid is contacted with the product in a liquid bath or spray zone wherein the liquid is vaporized into its gaseous state, and the resulting gas is then passed through a plurality of precooling zones in countercurrent heat exchange with the product to cool and/or partially freeze the product prior to the product being contacted with the cryogenic liquid. However, there has long been a substantial problem in accurately controlling the amount and velocity of the net gas flow through the tunnel toward the product inlet end while, at the same time, not permitting ambient air to enter the open ends of the freezer, nor permitting excessive amounts of the very cold gas to escape from the product discharge end of the freezer; all of which conditions are critical in order to achieve high thermal efficiency of the freezer. This critical problem of controlling the gas flow so as to continuously maintain the above-indicated conditions is further complicated by the fact that the amount ofgas generated per unit time in the spray zone varies over an extremely wide range. That is, the pounds of gas generated per hour in the spray zone varies widely as a result of many factors such as, for example, changes in the type of product, changes in the initial temperature of the product, changes in the desired final temperature of the product, and frequent variations in the production rate including such wide variations as between a maximum production rate and a zero production rate whenever the freezer is suddenly put into a temporary idling condition for any one of many reasons.

Many prior attempts have been made to solve this gas control problem such as by the use of gas control baffles or dampers as disclosed in US. Pat. Nos. 3,345,828; 3,403,527; 3,600,901 and in co-pending application Ser. No. 292,869 now US. Pat. No. 3,813,895. However, all of these damper and baffle type systems for controlling the gas flow have required relatively complex electro-mechanical systems, and they have not been highly accurate primarily because the amount of gas flow through a variable position baffle or damper is not a linear function of the change in the baffle or damper position throughout the range of movement thereof.

SUMMARY OF THE INVENTION The present invention provides a simplified and substantially more accurate gas control system for precisely controlling the forward net gas flow toward the product inlet end of the freezer so as to automatically vary the net gas flow in a linear manner in direct response to any change in the amount of cryogenic liquid being introduced into the spray zone which, of course, is directly related to any changes in initial or final temperature of the product, the type of product, and the production rate. Briefly summarized, the present gas control system includes a gas control blower driven by a variable speed DC. motor the speed of which is varied by an electrical control circuit which is directly responsive to the pressure of the cryogenic liquid in the spray header system; the pressure in the spray header system being a direct and linear function of the amount of cryogenic liquid being supplied as will be more fully described hereinafter.

It is therefore a principal object of the present invention to provide a cryogenic freezer having a simplified and substantially improved control system for more accurately controlling the net gas flow through the precooling zones of the freezer in heat exchange with the product.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view having portions cut away to more clearly illustrate some of the internal components;

FIG. 2 is an enlarged sectional view taken along view lines 22 of FIG. 1, including an illustration in phantom line showing the movable top and bottom sections of the tunnel in their open position;

FIG. 3 is a fragmentary perspective view of the interior of one of the stationary tunnel sections illustrating the details of the conveyor belt supports which are mounted solely on the stationary sections;

FIG. 4 is a simplified cross-sectional view of several zones of the freezer including the variable speed gas control blower and the associated electrical circuitry whereby the speed of the gas control blower is varied in direct and linear response to changes in the pressure of the cryogenic liquid in the spray header system.

DETAILED DESCRIPTION While the gas control system of the present invention may be employed in freezers of widely varying design, the gas control system will be described in connection with the particular freezer design comprising the subject matter of copending application Ser. No. 292,869 now US. Pat. No. 3,8l3,895. Referring to FIGS. 1 and 2, the overall freezer is generally designated by numeral l0 and includes a frame 12 having legs 13, longitudinally extending box beams l4, l5 and lateral supports 16 extending between the box beams l4, 15.

The frame 12 supports an elongated tunnel 17 defined by a series of alternate movable sections 18 and fixed stationary sections 19 the latter of which are of short length to facilitate cleaning. The stationary sections 19 are each supported by a pair of

vertical support posts

20, 21 which extend upwardly from the box beams l4, 15, respectively. The bottom portions 49 of the movable sections 18 are supported by angle frame assemblies 22 which extend beneath the movable bottom portions 49 and are, in turn, supported by lifting screws 23 of conventional jacks 24 which are mounted on the lateral supports 16. Each angle frame assembly 22 may extend beneath from two to five movable bottom portions and cooperates with four of the lifting screws 23 and jacks 24 to vertically move the bottom portions of the movable sections 18 so as to enable thorough cleaning and inspection.

The freezer 10 also includes a mesh conveyor belt 25 which extends through the tunnel l7 and has upper and

lower reaches

26, 27, respectively. The belt 25 extends over

pulleys

28, 29, 30 at the inlet end of the tunnel 17 (left side as viewed in FIG. 1), and over a pulley 31 at the outlet end of the tunnel 17 (right side as viewed in FIG. 1). The belt 25 is driven by means of a motor 32 through a drive chain and sprocket arrangement (not shown). Food or like articles intended to be conveyed through the tunnel 17 for processing are placed on the conveyor belt at a loading table portion thereof 33.

A cryogenic fluid such as liquid nitrogen is admitted to the interior of the tunnel 17 through a supply line 34 which communicates with a spray header 35 mounted in a stationary section 19 disposed near the exit end of the tunnel 17. The liquid nitrogen may be emitted through spray nozzles 36 directly on the articles moving through the tunnel l7 whereupon it will vaporize and become a gas. The gas is forced to move in a plurality of recirculating paths by the effect of radial fans 37 which are mounted in the stationary sections 19. The structure and unique operation of the radial flow fans 37 are more fully explained in the above-noted US. Pat. No. 3,403,527 the complete specification of which is hereby incorporated by reference. However, the present invention may be used in conjunction with any type of recirculation fans including, for example, conventional axial flow fans. The fans 37 are driven by fixed speed AC. motors 38 also mounted on the stationary sections 19. Transverse zone baffles 39 depend downwardly from the upper portions of the movable sections 18 on each side of the fans 37 to define cooling zones therebetween while permitting some of the gas to pass therebelow and flow into the adjacent upstream zone in cascading fashion. Anti-swirl vanes 40 may be provided for inhibiting swirling action of the gas around the fans 37. Rather than having the gas spill into the room the gas is preferably removed from the tunnel 17 by means of an exhaust duct 41, mounted in the first stationary section 19, and a remote blower 42 which has little or no effect on the actual movement of the gas through the tunnel 17. Thus, the blower 41 is primarily used for safety reasons, e.g., to preclude the build-up of non-life supporting nitrogen gas in the room in which the freezer and operating personnel are located.

Referring now particularly to FIG. 2, there are illustrated details of a movable section 18. Each movable section 18 includes a pair of hinged

top covers

43, 44 having upper portions 45 and depending side portions 46. The side portions 46 terminate in inclined surfaces 47 which sealingly mesh with correspondingly inclined surfaces 48 of a bottom portion 49 in the closed condition of the movable section 18. The bottom portion 49 is supported by and fastened to the angle frame assembly 22 whereby it may be moved downwardly away from the

top covers

43, 44 by means of the jacks 24 as seen in phantom in FIG. 2. A vertically disposed slide pin 50 extends downwardly from the angle frame assembly 22 and slidingly cooperates with a slide bushing 51 carried in one of the lateral supports 16 for preventing lateral movement of the bottom portion 49 during vertical movement thereof.

The jacks 24 are of conventional design and may be manually, electrically or pneumatically operated. A typical model jack may be the Duff-Norton Miniature Jactuator No. M-2555 or M-2625.

The top covers 43,44 are each carried by sleeves 52 which are keyed to torsion bars 53 which are, in turn, mounted in flanged support blocks 54 mounted on the stationary sections 19. The

covers

43, 44 may be swung open approximately 80 degrees and counterbalanced in the open position by torsion bars 53. The top covers 43, 44 further include handles 55 which facilitate manual opening and closing thereof. Toggle latch means (not shown) may be provided for securing the top covers 43, 44 in the closed position.

As seen most clearly in FIGS. 2 and 3, the upper reach 26 of the conveyor belt 25 is supported on plastic wear strips 57 mounted on lateral support bars 58 which, in turn, are carried by longitudinally extending side rails 59 supported by clips or brackets 60 mounted in the interior side walls of the stationary sections 19. The wear strips 57 can be formed of any suitable high molecular weight material capable of withstanding the low temperatures and providing the necessary antifriction function. For example, polytetrafluoroethylene (e.g., Teflon) can be satisfactorily employed. The lower reach 27 of the conveyor belt 25 is, on the other hand, supported on stainless steel bars 61 mounted on vertical support pins 62 extending upwardly through inclined sloping surfaces 63 of the bottom of the stationary member 19.

From the foregoing description it will be apparent that the structure thus far described permits the movable sections 18 to be fully opened so as to gain complete access to all of the internal portions of the tunnel, including the interior of the short stationary sections 19, so as to enable thorough cleaning and full visual inspection of the entire tunnel.

As further illustrated in FIG. I, the gas control system more fully described in co-pending application Ser. No. 292,869 utilized a movable, arcuate baffle 64 to variably obstruct the flow of gas from radial flow fan 37 toward the spray zone, and thereby, created a net forward flow of the gas toward the inlet end of the freezer; the forward net flow being variable by changing the position of the pivoted baffle 64. While this type of control system has been proven to be very effective in certain sizes of freezers such as, for example, a freezer having a 4 foot wide conveyor belt and having an overall length in the order of 36 feet, difficulties have been encountered in using this type of movable baffle system for freezers of substantially greater length and/or having a substantially narrower cross section such as, for example, a freezer having a 2 foot wide conveyor belt and having a length of 50 or more feet. Thus, the present invention provides an improved gas control system which is very effective in all sizes of freezers, and which is not subject to the previously mentioned nonlinear characteristics of pivoted baffles and dampers. In addition, the present gas control system responds very rapidly to any change in the amount of cryogenic liquid supplied to the spray zone, whereas, prior systems using temperature sensing devices inherently result in a significant time lag between the change in the amount of cryogenic liquid being supplied and the required gas control correction.

As shown in FIG. 4, the present invention provides a separate variable speed gas control blower in addition to the radial flow fans 37. This gas control blower 70 includes a scroll casing 71 which is secured to the internal surface of the upper portion of one of the fixed sections 19. The scroll casing 71 encloses a conventional centrifugal fan 72 driven by a shaft 73 extending through the upper portion of the fixed tunnel section 19 and connected to a variable speed DC. motor 74 which is also mounted on the same fixed tunnel section 19. It will also be apparent that instead of shaft 73 being directly connected to motor 74, an intermediate belt or gear drive unit could be provided such as to operate the fan 72 at a different speed than that of the motor 74. For example, the intermediate drive unit could provide either a fixed or variable speed ratio by using a belt, chain or gear drive unit.

In order to direct the cold nitrogen vapor forwardly toward the product inlet end of the freezer, one of a pair of relatively short horizontal partitions 75 are secured to the sides of each of the cover portions 43-44 and, similarly, one of a pair of flow deflectors 76 are mounted to the underneath sides of each of the top portions 43-44. Thus, when the cover portions are in their closed position, the pairs of partitions 75 and deflectors 76 combine to define a short duct extending the full width of the freezer through which the cold gas is forced by centrifugal fan 72 so as to be discharged in a downwardly and forwardly directed manner.

In the embodiment illustrated in FIG. 4, it will be noted that the variable speed blower 70 is located between two of the radial flow fan zones such that a portion of the gas recirculating in the radial fan zone on the right side of the gas control blower 70 is drawn into the center inlet of the gas control blower and discharged therefrom toward the radial flow gas zone located on the left side of the gas control blower so as to create the net forward flow of gas through the nondirectional radial flow fans zones which recirculate the gas. Alternatively, it will be understood that the gas control blower 70 may be located immediately adjacent the spray zone, or in any one or more of the fixed tunnel sections 19 between the spray zone and the product inlet end of the freezer. However, it has been determined that the optimum location of the enclosed gas control blower or blowers should be at one or more locations in the tunnel at which the temperature of the gas is between minus 50 degrees and minus 70 degrees F. within which the temperature range the gas will be cold enough so as not to create a frost accumulation problem within the scroll casing or discharge duct, while at the same time, not being so cold and therefore so dense as to require substantially greater amounts of horsepower to operate the blower within the preferred speed ranges. Of course, in addition to the use of a plurality of gas control blowers 70 spaced along the length of long tunnels, pairs of gas control blowers 70 may be utilized in the same fixed tunnel section. That is, two or more gas control blowers 70 may be provided side by side as, for example, in the case of a freezer having a conveyor belt width of 4 to 8 feet.

As previously stated, the speed of the gas control blower 70 is precisely controlled as a linear function of the amount of liquid nitrogen being supplied to the freezer through spray header system 75. In the preferred embodiment, this is accomplished by connecting a small pressure sensing line 77 to the spray header system 35 downstream of the main liquid control valve 78 which is regulated by an actuator 80. Valve actuator 80 is preferably controlled in a completely automatic manner by the liquid nitrogen control system as described in US. Pat. No. 3,6l3,386 the complete specification of which is hereby incorporated by reference. However, regardless of how liquid nitrogen valve 78 is controlled, including manual operation or any automatic system, the pressure of the liquid nitrogen in the spray header system downstream of liquid control valve 78 within the range of 0 to 10 p.s.i.g. is a direct and linear function of the amount of cryogenic liquid being supplied to the freezer. Thus, whenever the flowrate of liquid nitrogen is varied for any reason such as, for example, whenever the production rate and/or the initial or final product temperature is changed, the pressure of the liquid between fixed nozzles 36 and valve 78 accurately reflects any such change.

Pressure sensing line 77 is connected through a small, restricted orifice device 82 which functions to smooth out and eliminate spurious fluctuations in the pressure of the liquid nitrogen in the spray header system. The pressure signal is then transmitted through line 84 to a conventional transducer 86 which, solely for purposes of example, is shown as comprising a pressure responsive bellows 87 which actuates the movable contactor of a potentiometer 88. Thus, the sensed changes in the pressure of the liquid in the spray header system vary the output voltage signal of transducer 86 which is connected by lines 99 to a conventional, DC. power supply 100 preferably of the silicon controlled rectifier type. DC. power supply 100 receives an A.C. input voltage through lines 103 and supplies a constant DC. voltage through lines 101 to the motor field, while supplying a variable DC. voltage through lines 102 to the motor armature so as to vary the speed of motor 74. As a result, the speed of gas control blower 70 is continuously and automatically varied so that its pumping capacity always matches the precise amount of gas generated from the amount of liquid supplied to the liquid nitrogen spray zone. Thus, the net forward flow of gas through the freezer is always maintained equal to the rate of gas being generated in the spray zone so that all of the critical gas flow conditions previously described are always maintained regardless of any of the aboveindicated changes in the operation of the freezer.

In addition to the completely automated control system just described, a back-up manual control is provided in the form of amanually variable potentiometer 104 which is connected by lines 106 having switches 108 to lines 99 having switches 110.

Switches

108 and 110 are preferably of the relay type actuated by a solenoid 111 which is connected to a power source 112 through a mode selector switch 113, and this relay circuit preferably includes an indicator light 114. Thus, closure of mode selector switch 1 l3 energizes the solenoid 111 which closes switches 110 and opens switches [08 so that the automatic control system is in 0peration, and this automatic mode of operation is indicated by indicator light 114. However, if mode selector switch 113 is opened, or if any malfunction should occur in the relay circuit such that solenoid 111 becomes deenergized, switches 110 drop to their failsafe open position, and switches 108 drop to their failsafe closed position, whereby gas control blower may continue to be varied by manual operation of potentiometer 104 which then controls the speed of motor 74, and hence the speed of gas control blower 70 through the DC. power supply 100.

The present invention also provides a particularly sensitive maximum speed potentiometer 116 which adjustably limits the maximum speed of the gas control blower 70. This potentiometer has at least 3, and preferably, 5 to 10 turns so that it can accurately adjust the maximum operating speed of the gas control blower 70 which establishes the relationship between the liquid nitrogen pressure in the spray header system and the proper RPM of the gas control blower corresponding to that pressure.

While the foregoing description has illustrated the liquid nitrogen spray system 75 as producing direct contact of the product with liquid nitrogen, the present invention is equally applicable where the liquid nitrogen is introduced and vaporized in any one or more of the gas zones. That is, either in place of, or in addition to, the previously described spray system 75, liquid nitrogen may be introduced through a spray system such as that shown in phantom line, and generally designated by numeral 75A, which includes a supply line 34A, a liquid nitrogen control valve 78A, and one or more nozzles 36A which spray or otherwise inject the liquid nitrogen directly into the relatively warmer gaseous nitrogen wherein it is vaporized without direct contact with the product. The principal advantage of this type of liquid nitrogen injection system is that it can be used with any type of product including certain types of delicate products which may be damaged by a direct spray. When this type of cryogenic liquid injection system is used, the pressure of the liquid being injected is sensed by a sensing line 77A, which is identical to sensing line 77, and all of the other components of the gas control system remain structurally and functionally identical to that previously described.

From the foregoing description of one preferred embodiment of the invention, it will be apparent that the present invention provides a mechanically simple and reliable variable speed gas control blower which can be accurately and automatically regulated by a simple and highly reliable electrical control system so as to always maintain all of the necessary and critical gas flow conditions regardless of wide changes in the various operating conditions of the freezer. In addition, the pressure in the storage tank (not shown) often varies during filling of the tank, and also varies as the hydrostatic head of the liquid in the storage tank drops during operation of the freezer. These pressure changes in the storage tank change the rate of flow of liquid through control valves 78 and/or 78A. However, the pressure differences producing these changes of flowrate are also sensed by lines 77 and/or 77A, and the control system automatically adjusts the speed of gas control blower 70 in accordance with any such change in the amount of cryogenic liquid being injected.

Since numerous modifications and changes will be readily apparent to those skilled in the art, it is to be understood that the foregoing description is merely illustrative of one preferred embodiment of the invention, and that the true scope of the invention is not to be limited other than as expressly defined in the following claims including all equivalents thereof.

What is claimed is:

l. A cryogenic freezer for freezing a product comprising an elongated insulated tunnel including a con veyor belt, a cryogenic liquid injection system and a plurality of gaseous cooling zones through which the product is passed in heat exchange relationship with the gas resulting from vaporization of the injected cyrogenic liquid, each of said plurality of cooling zones including recirculation fan means for recirculating said gas in contact with the product, gas control fan means for controlling the net forward flow of gas through said cooling zones, variable speed motor means for driving said gas control fan means at various speeds, and a control system operatively connected to said variable speed motor means and to said cryogenic liquid injection system for automatically varying the speed of said gas control fan means as a function of the amount of cryogenic liquid being injected into said freezer.

2. The cryogenic freezer as claimed in claim 1 wherein said gas control fan means is located along the length of the tunnel at a position at which the temperature of the gas in the tunnel is colder than minus 50F.

3. The cryogenic freezer as claimed in claim 1 further including manual electrical control means for manually varying the speed of said gas control fan means, and mode selector switch means for operating said control system in either an automatic or manual mode.

4. The cryogenic freezer as claimed in claim 1 wherein said cryogenic liquid injection system includes injection means for injecting cryogenic liquid into at least one of said gaseous cooling zones for vaporization in contact with the gas recirculating therein.

5. A cryogenic freezer as claimed in claim 1 including an intermediate drive unit connected between said variable speed motor means and said gas control fan means.

6. The cryogenic freezer as claimed in claim 1 wherein said control system includes pressure sensing means connected to said cryogenic liquid injection system, and transducer means for converting the sensed pressure of the cryogenic liquid into an electrical control signal.

7. The cryogenic feeezer as claimed in claim 6 further including pressure smoothing means connected between said cryogenic liquid injection system and said transducer means for suppressing spurious pressure fluctuations.

8. The cryogenic freezer as claimed in claim 1 wherein said gas control fan means comprise a blower mounted inside said insulated tunnel including an outlet positioned so as to discharge the gas from said blower in a direction toward the product inlet end of the freezer.

9. The cryogenic freezer as claimed in claim 8 further including a discharge duct connected to said blower outlet for directing the gas toward the product inlet end of the freezer.

10. The cryogenic freezer as claimed in claim 1 wherein said variable speed motor comprises a DC. motor, and said control system includes a DC. power source having a variable DC. output connected to said variable speed DC. motor.

1 l. The cryogenic freezer as claimed in claim 10 including a maximum speed limiting potentiometer con nected to said DC. power supply and having at least three turns for accurately setting the maximum RPM of the variable speed motor.

12. The cryogenic freezer as claimed in claim 1 wherein said cryogenic liquid injection system includes spray header means for directly contacting the product with cryogenic liquid and thereby vaporizing the liquid to a gas, and wherein said plurality of gaseous cooling zones comprise precooling zones positioned between the product inlet end of the tunnel and said spray header means.

13. The cryogenic freezer as claimed in claim 12 wherein said gas control fan means is located between two of said gaseous precooling zones.

t i l

Claims

1. A cryogenic freezer for freezing a product comprising an elongated insulated tunnel including a conveyor belt, a cryogenic liquid injection system and a plurality of gaseous cooling zones through which the product is passed in heat exchange relationship with the gas resulting from vaporization of the injected cyrogenic liquid, each of said plurality of cooling zones including recirculation fan means for recirculating said gas in contact with the product, gas control fan means for controlling the net forward flow of gas through said cooling zones, variable speed motor means for driving said gas control fan means at various speeds, and a control system operatively connected to said variable speed motor means and to said cryogenic liquid injection system for automatically varying the speed of said gas control fan means as a function of the amount of cryogenic liquid being injected into said freezer.

2. The cryogenic freezer as claimed in claim 1 wherein said gas control fan means is located along the length of the tunnel at a position at which the temperature of the gas in the tunnel is colder than minus 50*F.

10. The cryogenic freezer as claimed in claim 1 wherein said variable speed motor comprises a D.C. motor, and said control system includes a D.C. power source having a variable D.C. output connected to said variable speed D.C. motor.

11. The cryogenic freezer as claimed in claim 10 including a maximum speed limiting potentiometer connected to said D.C. power supply and having at least three turns for accurately setting the maximum RPM of the variAble speed motor.