GB2183048A - Method and apparatus for testing AC generators - Google Patents

Abstract

LED displays (96,98, 100 Fig. 3) are activated by DC voltages derived from the outputs of an AC generator (16 Fig. 1) and a fusion control box (18) to show when the RMS voltage and the frequency of the generator output, and the RMS voltage of the box output, are within acceptable limits. The generator and box are used to energise a heater coil (24) for fusion-joining of thermoplastic pipes. The displays are provided on a small hand-held box 10 containing the solid state circuitry. The apparatus may test only the generator voltage outputs; or only box 18 is not essential. The display the voltage and frequency. A control enables joints to be completed where the generator is only just outside the preferred limits, at the discretion of the supervisor. <IMAGE>

Description

SPECIFICATION Method & apparatus for testing AC generators The invention relates to methods and apparatus for testing AC generators.

The frequency and voltage of the output of an AC generator, even when it is new, cannot be assumed to be exactly equal to desired or rated values nor to be sufficiently close to such values. For some applications, the frequency and voltage of the generator output must be known to be within certain limits. Such an application, for example, is where the output of the generator energises the heating coil forming part of a thermo-fusion fitting made of thermoplastic material such as polyethylene. Such a fitting may be a socket-type fitting used for joining lengths of polyethylene pipe in natural gas distribution systems; or the fitting may be of saddle or tee type.

In another application, for example, the output of the generator energises a heater plate against which the ends of lengths of polyethylene pipe are engaged so as to soften the ends. The plate is subsequently removed and the ends are forced together to form a butt joint in a natural gas distribution system.

In the known method of testing an AC generator useful in such applications, the generator output is fed to a bank of resistors cooled by forced draught from a fan. The resistors provide an imaginary load which is purely resistive and which is not an adequate substitute for a real load, in applications of for example the two kinds mentioned above. Furthermore, the bank of resistors with its case, fan motor and instruments is relatively bulky, heavy and costly.

The object of the invention is to provide a method and apparatus by which those disadvantages are reduced or eliminated.

A method of testing an AC generator, according to the invention, comprises applying the output of the generator directly or indirectly to a real load, deriving from the generator output a first direct current voltage representing the true root mean square of the generator output voltage and deriving from the first voltage an indication of whether the generator output voltage is, or is not, acceptable.

Preferably, in addition to the first direct current voltage a second direct current voltage is derived from the generator output representing the frequency of the output voltage and in which simultaneous indications are derived from the first and second voltages of whether the generator output voltage and its frequency are, or are not, acceptable.

Where the generator output passes indirectly to the load via a control means it is preferred that a further direct current voltage is derived from the output of the control means and a simultaneous indication be derived from the further voltage of whether the output voltage of the control means is, or is not, acceptable.

Preferably, the or each indication is derived using light-emitting devices eg diodes; preferably arrays of the devices are used.

Apparatus for performing the method, according to the invention, comprises a transformer having a primary input winding and a secondary output winding, an rms-to-dc voltage convertor connected to the secondary winding and means to derive from the output of the converter an indication of whether the generator output voltage is, or is not, acceptable.

Preferably, a frequency-to-voltage convertor is connected to the output winding and means connected to that convertor derives from its output an indication of whether the generator output frequency is, or is not, acceptable.

One form of method and one form of apparatus will now be described as examples of the invention with reference to the accompanying drawings in which: Figure 1 is a schematic diagram showing the main connections between the apparatus and a generator, as electrofusion control box, and a fusion fitting; Figure 2 is a scrap view on an enlarged scale of an adaptor used to connect the apparatus to the fitting shown in Fig. 1.

Figure 3 is a block diagram of the apparatus; Figures 4A, 4B and 5 are circuit diagrams of the apparatus; and Figure 6 is a view of part of the apparatus showing the visual indications provided by lightemitting diodes (I.e.d.'s).

Fig. 1 shows the apparatus comprising a box 10 conneced by two conductor leads 12, 14.

The output of the generator 16 is applied indirectly via an electrofusion at 20, 22 of a heater coil (part of which is shown at 23 in Fig. 2) forming part of an electrofusion fitting 24. Typically, for example, the fitting 24 is a tubular item made of polyethylene and having open ends providing sockets to receive the ends of lengths of polyethylene pipe (not shown).

Energisation of the coil softens the polyethylene at the interfaces between the fitting 24 and the pipe ends so that fused joints are formed. The pipes are part of a natural gas distribution system for example and are positioned in a trench in the ground. After all pipes are joined, infull is placed in the trench so that the installation is buried.

The energisation of the coil is accurately controlled by the control box 18 and successful operation requires the generator performance to be known to be within acceptable limits. Once that criterion is met, any fault which may arise in the control box 18 readily becomes apparent so that the correctness of the values of frequency and voltage of the electrical supply can be assured at all times.

Typically, for example, the sockets are sized to receive pipe ends of 180 millimetres outside diameter. The coil requires some 45 amperes at 40 volts, ie 1.8 kilowatts. The generator 16 supplies some 2.5 KVA at full load at 50 herz.

Fig. 2 shows part of the fitting 24 and part of its heater coil 23 having one end connected to the terminal 20 say. The terminal 20 is a pin within a tubular spigot 26 integral with the remainder of the fitting 24. Each terminal pin, such as the pin 20, has a respective conductive adaptor 28. The adaptor 28 is shaped so as to have a cylindrical opening 30 in which the pin 20 is frictionally engageable. The adaptor is also shaped to have a cylindrical pin extension 32 which is frictionally engageable in an opening 34 in a connector 36. The connector 36 is connected to the conductor within a lead 40, say (Fig. 1). The other adaptor engages a similar connector at one end of another lead 42 (Fig. 1). The leads 40, 42 connect the coil 23 to the control box 18.

Each adaptor is made of nickel-silver coated brass, for example, and has a side-arm 44 to which is soldered a conductor 46 of a lead 48, say, (Fig. 1). The other adaptor is correspondingly connected to a lead 50 (Fig. 1). The leads 48, 50 connect the coil 23 to the remainder of the apparatus 10 (Fig. 1).

Each lead 40, 42 has an insulating sheath including a resilient enlarged end cup such as 52.

(Fig. 2) which in the absence of testing of the generator 16 fits tightly over the corresponding spiget 26. For testing however it is acceptable for the adaptor 28 to be interposed without engagement of the cup 52 over the spigot 26.

Fig. 3 shows the block diagram of the circuit of the apparatus 10, and is largely selfexplanatory. The output from the generator 16 is connected across the primary winding 60 of a transformer T1. The secondary winding 64 of the transformer energises a power supply unit 66 providing voltage outputs for the rest of the circuit. The secondary winding 64 has an earthed tapping 68 so tht the voltage across a portion of the winding is fed via connections 70, 72, respectively, to an rms voltage-to-dc voltage convertor 74 and to a frequency-to-voltage convertor 76.

The output from the control box 18 (Fig. 1) is fed to a differential amplifier DA1 and its output is fed to a second rms-to-DC convertor 82.

The outputs from the convertors 74, 76 and 82 are fed to respective conditioning amplifiers 84, 86, 88 respectively. Their outputs are fed to respective l.e.d. driver circuits 90, 92, 94, respectively, connected to respective l.e.d. arrays 96, 98, 100, respectively.

In Figs. 4A and 4B, the points marked "A'' are connected. The values of the components shown in have the values shown below: RESISTORS R1 27 K-ohms R20 1 K-ohms R2 10 K-ohms R21 910 K-ohms R3 4.7 K-ohms R22 910 K-ohms R4 4.7 K-ohms R23 910 K-ohms R5 4.7 K-ohms R24 910 K-ohms R6 24 K-ohms R25 200 K-ohms R7 100 K-ohms R26 200 K-ohms R8 5 K-ohms R27 10 K-ohms R9 100 K-ohms R28 10 K-ohms RiO 47 K-ohms R29 10 K-ohms R11 5 K-ohms R30 10 K-ohms R12 1 K-ohms R31 24 K-ohms R13 68 K-ohms R32 20 K-ohms R14 470 K-ohms R33 20 K-ohms R15 100 K-ohms R34 5 K-ohms R16 100 K-ohms R35 10 K-ohms R17 5 K-ohms R36 5 K-ohms R18 .47 K-ohms R37 1 K-ohms R19 5 K-ohms CAPACITORS C1 2200 micro-farads (electrolytic) C2 0.22 micro-farads C3 2200 micro-farads (electrolytic) C4 0.22 micro-farads C5 0.47 micro-farads C6 0.47 micro-farads C7 0.22 micro-farads C8 1 micro-farad C9 2.2 micro-farads C10 0.1 micro-farad C11 0.1 micro-farad C12 15 micro-farad (electrolytic) C 13 1 micro-farad C14 2.2 micro-farads (electrolytic) C15 0.1 micro-farad POTENTIOMETERS P1 10 K ohms P2 1 Megohm P3 10 K ohms P4 1 Megohm P5 10 K ohms P6 1 Megohm DIODES D1 IN 4148 D2 IN 4148 D3 IN 4148 D4 IN 4148 D5 IN 4148 D6 IN 4148 TRANSFORMER T1 220 volt primary winding, 35 volts either side of centre tap on seconary winding.

RESISTANCE BRIDGE B1 SKB2/02L5A RS Components Ltd.

TACHOMETER TEA 1 LM 2917 National Semiconductor from RS Components Ltd.

VOLTAGE REGULA TORS VR1 LM 7812 VR2 LM 79L12 VR3 78L05 5 volt ANALOGUE DEVICES AD1 UAD536 AJH AD2 UAD536 AJH DIFFERENTIAL AMPLIFIER DA1 LM 324 AMPLIFIERS Al 4 LM 324 A2 4 LM 324 A3 4 LM 324 A4 4 LM 324 BAR DRIVERS BD1 LM 3914 N BD2 LM 3914 N BD3 LM 3914 N LIGHT-EMITTING DIODES LED 1 to LED 25 5 mmx2 mm RS Components Ltd.

The circuits shown in the block schematic diagram in Fig. 3 are completely housed within the hand-held lightweight box 10 (Fig. 1). This facilitates on-site use of the apparatus and represents a major improvement compared with the use of very bulky air-cooled resistance banks.

OPERA TION In the typical use of the apparatus for example, the connections shown in Fig. 1 are made onsite, where the fusion fitting 24 is one of those actually used to join two lengths of pipe in a gas distribution system.

The nominal generator output voltage is 110 volts so the primary winding of the transformer T1, being a 220 volts winding, provides an adequate safety margin to accommodate excessive voltages.

The generator output voltage is monitored by the portion of the secondary winding of the transformer T1, the voltage sample being passed to the rms-dc convertor stage 74 which includes the analogue device AD1 shown in Fig. 4A. The dc output voltage is conditioned by the amplifier A2 (Fig. 4A) to produce a range of voltage applied to the led. bar driver BD1.

This is operated in moving dot mode so that different voltage levels cause different respective l.e.d.'s of the array 96 to be illuminated.

The I.e.d. array is shown in Fig. 6. For generator voltages of 97V or less the red l.e.d. 10 is illuminated, indicating that the voltage is too low. Between 97V and 103V a yellow I.e.d. 9 is illuminated, indicating that the voltage is below that- required by the British Gas Corporation Specification TIN 12. The specification is available from the Corporation to all generator manufacturers. The yellow indication shows that the voltage is just sufficient to operate the control box 18, however.

Between 103 and 120V one or other of three green I.e.d.'s 6, 7 & 8 are illuminated showing that the voltage is within the TIN 12 limits. Voltages above those limits but within the capability of the control box 18 are indicated by a further four l.e.d.'s 2, 3, 4 and 5. Voltages above 145V which may be high enough to cause damage to the control box are indicated by a final red led. 1.

The voltage sample is also fed to a frequency-to-voltage convertor 76 which produces a dc output voltage proportional to the applied voltage frequency. The output is conditioned by the amplifier A3 (Fig. 4B) and fed to a bar driver circuit 92, which operates in moving dot mode.

The red l.e.g. 20 indicates frequencies below 40 Hz. The three yellow I.e.d.'s 19, 18 and 17 indicate frequencies from 40 to 47.5 Hz in steps of 2.5 Hz. The two green I.e.d.'s 16 and 15 indicate 47.5 Hz to 52.5 Hz in two steps of 2.5 Hz, which fall within the TIN 12 limits. The three yellow I.e.d.'s 14. 13 and 12 indicate 52.5 to 60 Hz in steps of 2.5 Hz and the red I.e.d.

11 indicates frequencies above 60 Hz. If a yellow I.e.d. is illuminated the frequency can be handled by the control box 18.

The control box voltage output is monitored at the fusion fitting 24 (Fig. 1) by the connections made by the adaptors 28 described already with reference to Fig. 2. The voltage is not necessarily referenced to earth and is relatively large. Therefore, it is attenuated by a factor of ten and passed through a differential amplifier DA1. The output, referenced to earth, is passed to an rms analogue device AD2 (Fig. 5). The resulting output is conditioned by the amplifier A4 device and passed to the bar driver BD3 in the driver circuit 94. The driver operates the moving dot mode so that an individual l.e.d. is illuminated at each voltage band. However, here there are only five bands.

The red l.e.d. 25 indicates a voltage less than 39V (Fig. 6). The three green l.e.d.'s 24, 23 22 indicate 39.1 to 39.9V in equal steps. The red I.e.d. 21 indicates a voltage above 40V.

In a modification (not shown) an indication of only the output voltage of the generator is derived; or of only the voltage and of the output voltage of the control means, such as the fusion control box for example. In another modification (not shown) the l.e.d.'s are replaced by other light-emitting devices eg lamps; or by other devices eg meters or recording devices eg devices producing a print-out or other indication. Instead of using emissions of different colours from the devices it is possible to rely only on the position of the illuminated device in an array.

Where generators are being compared, a fusion fitting, for example, can be used repeatedly as a load. Although the fitting is not being used to form a joint with a pipe or pipes, it nevertheless acts as a real load.

The invention enables a supervisor on-site to decide that where the generator output is slightly outside acceptable limits, such as the TIN 12 standard referred to above, for example, the joint can nevertheless be completed before the generator is adjusted or replaced.

Also, where as is usual, the generator is driven by an internal combustion engine, the indications derived can assist the supervisor to check the engine petrol/air mixture instead of relying only on excessive throttle setting.

The invention is applicable to generator testing generally. The use of a control such as a fusion control box is not essential. The field of use of the generator is extremely wide and includes for example energisation of heater plates in the butt-fusion joining of thermoplastic pipes in gas distribution systems and other fields; and energisation of hydraulic power packs used to power tools such as road drills and concrete breakers.

Claims (19)

1. A method of testing an AC generator, comprising applying the output of the generator directly or indirectly to a real load, deriving from the generator output a first direct current voltage representing the true root mean square of the generator output voltage and deriving from the first voltage an indication of whether the geneator output voltage is, or is not, acceptable.

2. A method according to Claim 1, in which in addition to the first direct current voltage a second direct current voltage is derived from the generator output representing the frequency of the output voltage and in which simultaneous indications are derived from the first and second voltages of whether the generator output voltage and its frequency are, or are not, acceptable.

3. A method according to Claim 1, or Claim 2, in which the generator output passes indirectly to the load via a control means and in which a further direct current voltage is derived from the output of the control means and in which a simultaneous indication is derived from the further voltage of whether the output voltage of the control means, is or is not, acceptable.

4. A method according to Claim 3, in which the indication relating to the generator output voltage includes an indication of whether that voltage is or is not compatible with operation of the control means.

5. A method according to Claim 3 as dependent on Claim 2, in which the indications relating to the generator output frequency and voltage include indications of whether that voltage and that frequency are, or are not, compatible with operation of the control means.

6. A method according to any preceding claim, in which the or each indication is derived using light-emitting devices.

7. A method according to Claim 6, in which the or each indication is derived using a rectilinear array of devices.

8. A method according to Claim 7, in which indications are derived using rectilinear arrays of devices positioned adjacent each other.

9. A method according to Claim 6, 7 or 8, in which an indication representing an acceptable value is derived by illumination of a device positioned between other devices in the same array the illumination of which indicates less acceptable or acceptable values.

10. A method according to any claim of Claims 6 to 9 are of more than one kind, each kind emitting light of colour different from that emitted by any other kind.

11. A method according to any claim of Claims 6 or 10, in which the devices are diodes.

12. A method according to any preceding claim, in which the load is the heating coil of a fitting designed to be joined by thermo-fusion to thermoplastic pipe.

13. Apparatus for performing the method according to Claim 1, comprising a transformer having a primary input winding and a secondary output winding, an rms-to-dc voltage convertor connected to the secondary winding and means to derive from the output of the convertor an indication of whether the generator output voltage is, or is not, acceptable.

14. Apparatus according to Claim 13, in which a frequency-to-voltage convertor is connected to the output winding and means connected to that convertor derives from its output an indication of whether the generator output frequency is, or is not, acceptable.

15. Apparatus according to Claim 13 or Claim 14, in which there is an input means connectable to the output of a control means via which the generator output is passed to the load, and in which a differential amplifier is connected to the input means and provides an output to further means which derives an indication of whether the output voltage of the control means is, or is not, acceptable.

16. Apparatus according to Claim 15, in which there are two adaptors, each shaped so as to be releasably engaged with a connector on a lead extending to the control means and with a terminal of the ioad, each adaptor being connectable by a respective conductor to said input.

17. Apparatus according to Claim 15, in which said indications are produced using lightemitting diodes substantially as described herein with reference to Fig. 6 of the accompanying drawings.

18. Apparatus according to Claim 13 substantially as described herein with reference to the accompanying drawings.

19. A method according to Claim 1 substantially as herein described with reference to the accompanying drawings.