D-C amplifier controls electric furnace

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Research Paper (National Research Council of Canada. Division of Building

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D-C amplifier controls electric furnace

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Research Paper (National Research Council of Canada. Division of Building

Research); no. DBR-RP-159, 1962-06-01

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D-C amplifier controls electric furnace

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ser

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PRICE 1 0 CENTS

NATIONAL RESEARCH COUNCIL

DIVISION OF BUILDING RESEARCH

D-C AMPLIFIER CONTROLS ELECTRIC

FURNACE

by

9. H. McGUlRE

REPRINTED FROM C A N A D I A N CONTROLS A N D INSTRUMENTATION VOL. 1, N O . 2, MARCH-APRIL 1 9 6 2 p. 37-38 RESEARCH PAPER N O . 1 5 9 OF THE

DIVISION OF BUILDING RESEARCH

OTTAWA NRC 6 8 9 2

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This publication is being cli~tributed by the Division o f Building Research o f the National Research Council. It

should not be reproclucecl in whole or in part, witlzollt perrni~.siotz o f the original p~~blisher. The Divi.,ion would be

glacl to be of n~.>i.>tance in obtaining such pern7i.,sior1.

Pllblications of the Division of Builcling Resecrrclz tnuy be obtainecl by mailing the appropriate retnittance, (a

Bank, Express, or Post Office Money Order or a cheque r71acle paycrble (11 prrr it7 Ottawa, to the Receiver General

of Carzacla, credit National R e ~ e a r c h COLIIICI'I) to 111e Natiorzcrl Researcll C O L I I I C ~ ~ , Ottc~wcr. Slatnps are not ncceptnhle.

A coupon systenz hrrs been introclucecl to rnalce paynzents for pllblications relatively sitnple. Coupons are avnil-

able in denorninations of 5 , 2 5 anti 50 cents, and /nay be obt(rirzec1 hy r?~aliing (1 renzittnnce us inclicatecl above. These

coupons rnay be usecl for the p~~rclzase of all National Resenrch Corlrzcil p~lb1iccltion.s incl~lrling specificaliorls of tlze

Canaclicrn Goverrzrnent Specifications Board.

D-C

amplifier controls electric furnace

At NRC's Building Research Division,

a

lab-style d-c amplifier was found to make

an excellent (and economical) automatic controller for the combustibility furnace

By J.

H.

McGuire, Fire Section easiest method of control by means of ture of about 1,500 deg F (say 4 0

Div. of Building Research, N.R.C. a d-c amplifier would be a circuit mv). T h e instrument should not b e

operating o n a n on-off principle, the damaged by an input of t _{4 0 m v}

A n automatic controller f o r a n electric power being disconnected when the even when set to its most sensitive conlbustibilitv furnace at the ~ i ~ tenlperature attains the control value. i ~ i ~ ~ range.

of Building Research was required, to eliminate tedious and time-consuming manual control. T h e controller had to be able to select any temperature

I I I 1

10 2 0 30 4 0 50

T I Y E i N MIH.

Figure I

within the range 200 F to 1,400 F and hold it within 5 F. Initial warmup had to be as rapid as possiblc.

A t the same time, purchase of a d-c amplifier for another purpose was un- dcr consideration. Since both requirements wcre intermittent, the possibility of using the d-c amplifier as a furnace controller was investigated. This was all the more desirable in that a control circuit consisting essentially of a d-c amplifier and relay was found to bc probably thc least costly type which could be procured.

T h e temperature sensing elcmcnt in the combustibility furnace is a chro- mel-alunlel thermocouple^ and the

Thermal lag between a temperature rise of thc furnace heating element and the subsequent response of the sensing elcment should not be exces- sive. T h e system constitutes a relaxation oscillator and is in fact a n almost exact thermal analogue of a resist- ance-capacity tube oscillator. T h e con- stants of the system must therefore be such that the amplitude of the oscillation is very low.

By experiment, the pcriod of the oscillation was f o ~ l n d to be about 5 minutes and the amplitude about 6 dcg F peak-to-peak. This period is suitable for a n "on-off" type controller and the amplitude variation constitutes an accuracy of control of

+

0.5 percent. While this is acceptable, it can be im- proved.

During the warm-up period thc d-c anlplificr is heavily ovcrloaded ncga- tively, and when the furnace temperature overshoots the control level a positive overload may wcll result. T o allow a control differential of the order of 1 or 2 deg F it was ruled that the gain of the amplifier should be thc least sufficient to give a full-scale output with a n input of 100 microvolts, corresponding to about 5 deg F . Higher ranges are also desirable, and the highcst should preferably be suitable for setting thc backing-off voltage (which will be negative) to a valuc corrcsponding to a maximum tempera-

T h e input impedance requirement of the d-c amplifier is governed prin- cipally by the impcdance inherent in the source of backing-off potential and is not stringent. T h e o u t p ~ ~ t of the amplifier must be sufficient to operate a simple relay.

T h e necessary frequency response of the d-c amplifier is determined by the expected duration of the on-off pe- riods. Since this latter is of the order of nlinutes a response time of better than 1 0 seconds is all that need be asked.

A proprietary electronic nlicrovolt- nleter was found to meet the above requirements, except that its output was not sufficient to operate a simple relay. This difficulty w a s easily over- come by interposing a one-stage transistor amplifier between the instrument and the relay. A s regards general ap- plication it was very acceptable. T h e design is such that zero drift is only significant o n the most sensitive range (10 microvolts) and thc input impedance (1 niegohm) is very high for a chopper type amplifier.

Controller circuit functions

T h e circuit of the complete controller is shown in Fig. 2 a n d functions as follows. The microvoltmeter range is first set to allow nleasurenlent of the desired backing-off voltage which is adjusted with the input switch at "set- up". T h e input switch is then returned to "normal", the rnicrovoltn~eter range

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switch sct to 100 niicrovolts (full scale), and the nieter on the microvoltmeter disconnected. This latter action is taken to ensure that the nieter is not damaged as a result of overload- ing.

T h e instrument operates on a niodi- fied "on-off" principle. Instead of thc controller's switching the power to the furnace o n and off, it merely raises or lowers a pre-set voltage by 25 percent. This type of operation gives a lower amplitude of thermal relaxation oscillation than does a pure "on-off" type. F o u r snialler values of voltage variation may be selected by changing the tapping on the transformer shown a t the bottom left of Fig. 2.

T h e voltage which will maintain the furnace at the required temperature must therefore be known correct to say t 15 percent. T h e variac must be set to give this order of voltage.

When this action has been taken the controller will operate and the furnace may be switched on. During the warmup period none of the control relays A , B, o r C is operated and a high voltage is applied to the furnace to reduce this time to a niininium. When the output from the thermocouple equals the sun1 of the backing- off potential and the differential repre- sented by the range setting of the niicrovoltmeter, relay A operates. This is a relatively sensitive relay with only one contact and its only function is to operate relay B. T h e first time relay B operates, relay C also operates and hence-forth remains on because power is applicd to it via contact C,. Contact C, disconnects the furnace from the fast warmup voltage and applies the pre-set voltage t 25 percent, depend- ing 011 the position of contacts B, and B,. When B is operated the low voltage is selected. When the furnace tends to cool, relay B (following A) is de-energized and the voltage applied to the furnace is raised to the pre-set value f _{25 percent.}

A typical control curve is shown in Fig. 1. It will be noticed that although

the initial overshoot indicates a sub- stantial thermal lag in the response of the sensing thermocouple to a n in- crease in the heater temperature, control is still quite accurate.

From this result it is apparent that the value of 25 percent choscn for the variation in the voltage applied to the furnace is sufficiently sniall to give adequate control.

T w o further controls illustrated in Fig. 2. should be described. The fast warm-up reset control is called for when a new and higher control teni- perature is chosen during operation and a fast warni-up to the new temperature is required.

T h e switch disconnecting the fast warni-up facility is included as a safety feature when equipment is completely unattended.

When the equipment was first set up, the action of the B and C rclays in switching the voltage to the furnace was found to interfere with the operation of the n~icrovoltmeter. As a re-

sult, a single operation sometimes be- came three or even five operations. F o r example, instead of B being energized as o n e operation, it would be energized, drop off a fraction of a second later, and then be re-energized. The effect was very s i n ~ p l y eliminated by desensitizing the transistor ampli- ficr by a n output amplitude control already incorporated in the microvoltmeter. Previously relay A had operated at about 10 percent full scale microvoltmeter output. T h e control was adjusted until the relay only operated a t 9 5 percent full scale output.

T h e modified on-off controller described has been found a most satis- factory and economical means of con- trolling a sniall electric furnace and gives an accuracy of control which is better than rcquircd. Its design may readily be adapted to the control of other electric furnaces, and when not serving such a purpose the individual units constitute useful pieces of lab- oratory equipment. 0

A lirt o f all pu0licntions of [Ire Division of Bi~ilcling Re.cearch is nvcril~lble nrzd nzay be obtrrirzerl frotn the Pi~O1icntiorz.y Section, Division o f B~~ilrlirr,q Research, National Re.rearc1z Co~itlcil, Ottawa, Canada.