November 26, 2011
Poulsbo, WA
Making A Meter For A HCCT
By Bob Cascisa
A.K.A.
Be_Zero_Be
There are a number of various homemade and original HCCTs (Hand Cranked Coil
Testers) out there that for one reason or another do not have an original style amp
meter. Original HCCT Weston and Jewell meters are rare and expensive if you can find
one. This article describes how to convert a commonly available AC amp meter to a
HCCT amp meter.
Meter Movement Types
There are three types of meter movements. They are Iron Vane, Electrodynamometer
and D’Arsenval. Here is a link to a brief explanation of the three types on movements :
Meter Movements Introduction
The Weston and Jewell meters used on HCCT’s are Iron Vane type meters with a 0 to
2 Amp scale. We will focus on this type of meter.
Most of these Iron Vane AC Amp Meters have a 5 Amp (full scale) movement. In order
for one of these meters to be useful for a HCCT we must convert it from a 0 to 5 Amp
meter to a 0 to 2 Amp meter. It is a relatively simple thing to do. To do this we must
rewind the coil that surrounds the Iron Vane movement to change the full scale
deflection of the pointer.
Meter Selection
Occasionally at swap meets, or other places, these large (7” to 8” diameter) meters can
be found for cheap. Some manufacturers are GE, Westinghouse, Jewell, Weston and
others. Nice examples of this meter are in Figure 1. Selecting one that can be
converted to a HCCT amp meter might seem to be mysterious, so here are a few things
to look for.
Figure 1. Iron Vane AC Amp Meters (0-2000 & 0-400 Amps)
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November 26, 2011
Poulsbo, WA
1. Find an AC (Alternating Current) Amp Meter (Ammeter).
Some AC Iron Vane voltmeters can be converted but it is more complex to do
so.
2. Look it over.
Make sure the pointer moves freely and it is in generally good condition.
3. Look at the scale.
Iron Vane AC movements are non-linear. Look at
Figure 1. The amount of arc from 0 to 500 on the left meter and 0 to 200 on
the right meter is less than the amount of arc from 500 to 1000 and 200 to
300 respectively. The scale is more compressed at the beginning.
4. Look at the Range values.
The Range values in Figure 1 go from 0 to 2000 Amps and 0 to 400 Amps.
Disregard these numbers. The meter doesn’t really measure 2000 or 400
Amps.
5. Look at the fine print.
Figure 2 shows fine print that says “F.S .= 5 AMP”. This means that the meter
Movement is really 5 Amps “Full Scale”. This is a very common value of full
scale amps for these type meters. This is the meter you want.
Figure 2. Full Scale note.
Figure 3 shows fine print that says “C.T. Ratio 400/1”. This stands for “Current
Transformer” ratio is 400 to 1. A current transformer is a device that steps the
2000 amp line current down to 5 amps to match the full scale Amps of the
meter. (400 times 5 Amps is 2000 Amps).
Figure 3. Current Transformer Ratio note
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November 26, 2011
Poulsbo, WA
Meter Testing
Once you have a meter, verify that it works and verify the accuracy of the meter. I used
a Variac (variable transformer) to provide a 0 to 120 Volt AC source. I used a load
resistor (3 to 5 100 watt light bulbs could be used) to provide a load. See Figure 4 for
the test schematic. Test the meter at 1, 2, 3, 4 and 5 amp loads. Note the scale reading.
Test the meter at the major scale tick marks and note the load current. This will give you
a good idea of the accuracy and linearity of the meter.
SCALE
500
1000
1300
1500
2000
AMPS
1.4
2.7
3.3
4.0
5.2
AMPS
1
2
3
4
5
SCALE
355
740
1115
1500
1925
Table 1. Initial Meter readings
Multimeter
Variac
Meter
To Be
Tested
120 VAC
Load
Figure 4. Meter Test Diagram
Meter Disassembly.
The meters consists of a housing, Scale Plate, Iron Vane movement assembly and a
coil wound around the Iron Vane Assembly.
Housing
The housing can be pressed steel or cast iron. The housing can be removed
from a back plate or a face plate can be removed from the housing.
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November 26, 2011
Poulsbo, WA
Figure 5. Housing removal.
Scale Plate
The scale plate is held on by several small screws. Remove it carefully so as not
to bend the pointer. Watch for the pointer stop bumpers. Sometimes they are part
of the Iron Vane movement and sometimes they are part of the Scale Plate.
Figure 6. Pointer Stop Bumpers.
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November 26, 2011
Poulsbo, WA
Figure 7. Scale Plate Removal
Iron Vane Movement
The Iron Vane assembly is removed next. Take a moment to examine this
assembly. It is a remarkable piece of engineering. The pointer rides on jeweled
pivots. There are one or more adjustable counterweights, a spiral return spring
and a zero adjustment arm.
Store the movement in a safe place. If it is damaged, it could be the demise of
the meter.
Figure 8. Iron Vane Removal
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November 26, 2011
Poulsbo, WA
Dampening
Chambers
(Upper & Lower)
Pointer
Counter
Balance
Weights
Pointer
Return
Spring
Zero adjustment
Figure 9. Iron Vane Movement
Coil Removal
Finally, remove the coil assembly. This is the part to be modified.
Figure 10. Coil Assembly
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November 26, 2011
Poulsbo, WA
Figure 11. Complete Disassembled Meter
Meter Modification
This particular meter was a very lucky find. The scale goes from 0 to 2000. I am going
to modify the meter to read from 0 to 2 Amps. All I have to do is add a decimal point to
the numbers on the scale and I will have it. A reading of 1300 will be 1.3 Amps.
1. Determine and create the new meter face artwork. This will depend on your
creativity and the desired outcome of the modification. If you happen to have a
Weston meter, Fun Projects can provide a high quality reproduction of the
original meter face.
2. Unwind the wire from the coil and count the turns. This particular meter had 36
turns of square copper wire that is approximately equivalent to 12 gauge wire.
Figure 12. Unwinding the coil
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November 26, 2011
Poulsbo, WA
3. Calculate the number of turns needed to increase the sensitivity of the meter
(from 5 Amps full scale to 2 Amps full scale). To do this we must increase the
number of turns in the coil. Use the following formula :
(Old Full Scale Amps / New Full Scale Amps) x Number of Turns in Coil
For this meter, this will be :
(5 Amps / 2 Amps) x 36 Turns = 90 Turns
This is approximate due to the nonlinearities of the basic meter movement.
We must put 90 turns where there once was 36 turns. So, we must use a smaller
gauge wire. I chose 16 gauge magnet wire. It is plenty big enough to carry 2
amps and is small enough to fit 90 turns on the spool. 18 or 20 gauge wire would
also be suitable.
4. Wind the new coil - either by hand or on a lathe or coil winder. The layers do not
have to be perfect, just neat and the correct number of turns. You might consider
putting a few extra turns on the spool for calibration. It is easier to remove turns
then to add them. Put a layer or two of tape over the final product. Leave about
six inch pigtails on the spool. Strip about one inch off of the wire ends and tin
them with solder.
Figure 13. Winding the new coil
5. Install the new coil and reassemble the meter.
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November 26, 2011
Poulsbo, WA
Figure 14. Install the new coil
6. Test the meter for accuracy and linearity. Most analog meters are least accurate
at very low and very high readings. The goal here is to have the meter be most
accurate at 1.3 Amps +/- 0.3 Amps. This meter came out very good. I ended up
with 86 turns on the coil.
Figure 15. Final Calibration Setup
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November 26, 2011
Poulsbo, WA
Before
Modification
Reading
Scale
(Amps)
500
1.4
1000
2.7
1300
3.3
1500
4.0
2000
5.2
After
Modification
Reading
Scale
(Amps)
500
.505
1000
1.008
1300
1.304
1500
1.502
2000
1.955
Table 2. Final Calibration Readings
If the calibration test reveals the readings to be high you must ADD more turns to
the coil and vise-versa. This seems counter-intuitive but remember we are
adding turns to the coil to decrease the full scale value from 5 Amps to 2 Amps.
7. Shorten the meter terminals. Most of these type meters have long (4” to 6”) studs
on the back for the meter electrical connections. On this meter, I machined them
off and drilled and tapped them for easier connections to a HCCT.
Figure 16. Original meter connection studs
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November 26, 2011
Poulsbo, WA
Figure 17. Modified Meter Connection Studs
Conclusion
This particular Westinghouse meter was very easy to modify. It took about half a day.
Although any particular meter may not be as easy as this Westinghouse meter, the
principles and process will be the same.
Figure 18. The final product
The final product does not look any different after the modification. I will decide on the
final artwork at a later date.
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November 26, 2011
Poulsbo, WA
My first HCCT with a modified GE meter and homemade artwork
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