There is much hype on the quality of new motors used in expensive locomotives, but very little data on their true performance or ratings. Over the years, there have been claims and arguments about the superiority of one type of motor over another. But to my knowledge, no one has proven any of these to be factual. Many have been mislead by so-called experts, who have expounded on second hand statements; resulting in disappointment. With the development of DCC, there has been too much stress on low current draw to meet the limitations of modules. But what is the current at desired operating torque and power in road operation? A motor drawing .2 amp @ 12 volt and a very high 80% efficiency would produce 1.92 watt output. This is barely enough to handle a small switcher with a few cars, excluding gear train losses. The final decision should rest on the marriage of the proper gear train and a well fitting motor to supply the speed and drawbar pull at the desired voltage. The current will fall where it may; hopefully within the maximum motor rating.
One of the main instigations for this development is to compare some of expected improvements in motors with new NdFeB magnet replacements against the original and others on the market. Others may be enticed to fabricate a dynamometer to extend testing data. Although this can not be considered a high precision, calibrated device, the relationships among motors should be accurate enough for comparisons and selection. To this end, this device will permit comparisons to be made.
As seen in the GENERAL DISCUSSION OF MOTORS and the GRAPHICAL ANALYSIS , all parameters are referenced to the torque, due to methods used. But measuring torque presents a problem. Today there are many very expensive, fancy methods with extremely accurate results, beyond the requirements for the motors used in modelling. Most found examples are for large motors, particularly automotive.
Many years ago, Robert Higgins collected data on Sagami motors for NWSL, but the actual set-up or calibration was not found. After considering and trying many possibilities over the years, none proved totally practical. Thanks to some recent research on the Web and elsewhere, a simple inexpensive idea was developed.
Used to measure the output of a prime mover (motor or engine), a dynamometer consists of two parts: a brake to measure torque by applying a resisting force to the shaft and a tachometer to measure RPM. The product of the two, with some constant, yields the output power. By using a fixed regulated voltage source and a good ammeter, efficiency and other data, necessary for graphs, can be calculated.
About 1858 Lord Kelvin developed the "rope" brake, based on an earlier design by Prony, by replacing wooden friction blocks with a length of rope coiled around a revolving shaft. Variations of these are still used in many engineering school laboratory exercises. Designing a small, inexpensive unit presented problems.
First, motors vary in size and shape, but a Micro-Mark vise with rubber jaw pads could clamp them for easy alignment. With varying shaft sizes, NWSL universal joints accommodate them and ease the alignment. Since this vise places the shaft center at a height of about 7", a simple 8" "L" bracket could position the dynamometer vertically and horizontally by clamping it under the vise base. The remaining problem is to make the dynamometer.
Since most new motor specs are rated in the metric system with TORQUE in gram (force)-centimeter (gmf-cm) and power in watts, this will be used. Fortunately 1 ounce-inch equals 72.00775 gmf-cm or 72 for easy conversion. Also 1 mousepower (.001 horsepower) = .746 watt. A set of slotted, gram weights measuring up to 610 was purchased to measure running torque along with two spring scales of 250 and 500 gm to measure stall torque.
A prime item required is a well regulated 12 V DC power supply that will handle at least 4 amps. Reasonable computer switching types can be found at Jameco, All-electronics and others.
There are two distinct types of measurements required to fully graph a motor, both of which include current measurements. The first is a single, short time length, test to determine stall torque and current. A string, affixed to and wrapped around a 1 cm radius drum, will pull against a spring balance and read directly in gmf-cm. Assuming a locomotive has sufficient weight on the drivers to prevent slipping, this would indicate the starting tractive effort, when converted by the gear train and driver diameter calculations to force at the rails, using the methods described in GEAR FUNCTIONS . The drawbar pull in ounces can be found by dividing gmf by 28.35.
In the second type, current and RPM are measured at known torque values based on the stall torque. Theoretically the maximum power is at one half the stall torque and efficiency is somewhat less; but in practice they may vary. Frequently they are beyond the maximum continuous operating current rating.
Since both efficiency and power require drum RPM and working with a laser hand-held proved to be a pain, the next problem involved rigging a convenient tachometer. An inexpensive version ($33 US) used by RC fans measured in 100 RPM, which should be close enough. But it used ambient light bounced from propeller blades. When used under AC lamps, it only registered 3600 RPM for the 120 pps of the US 60 Hz power. Not wanting to work in the dark, an alternative had to be found. The photo cell with extended leads and a 12 bulb were mounted in a piece of 3/8" bar stock with a slit to clear a two holed disk mounted on the shaft.
After several unsatisfactory , CAD drawings with locally found metal components, a 40 mile trip to a commercial metal supplier turned up some very usable remnant aluminum pieces at $2 per pound.
The image shows the first quasi successful assembly with the too large running drum on shaft at right.
Note: Adjust brightness and contrast for optimum viewing.
DYNAMOMETER / VISE, MOTOR, SCALE & TACHOMETER.
Running weights on hanger hooked to box.
8" L bracket clamped under vice base, supporting dyna box at top.
The latest drawings are below.
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FULL LAYOUT W/ VISE & MOTOR.
Vise and motor on left.
8" L bracket clamped under vice base, supporting dyna box at top.
The lower section is broken to show the height may vary with the vise chosen. The unit may be clamped at a bench corner to permit weight pan and scale to drop below bench top level.
Note: Adjust brightness and contrast for optimum viewing.
BOX DETAILS.
Drum is yellow.
Tachometer disk is blue.
String locations are solid line in end view.
The right string makes one or two turns around drum for running measurements, while the left is hooked on the pin only for stall torque measurement and must be removed during running tests.
Stall torque and tachometer tests have been successful, but running portion needs work. For those who might be interested, full fabrication and operation details will be added when finalised.
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