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Trying to figure out how to make small dynomometer for electric and piston motors.

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Patrick

Sat Mar 21 2015, 07:14AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

im trying to figure out how to measure shaft output from some of my electric motors and piston engines. Im getting back into the SMPS and high voltage and some of my projects are merging, Theres no way to avoid this required measurement any more.

ive been looking for small ones to buy on ebay, but i only see 400hp ones for 5000$... theres this one meant for my purposes

but i cant figure out how it works.

id like to know torque x rpm, that seems most useful for my purposes.

i realize theres a type used in the auto industry based on a torque arm and rpm meter. a force meter and rpm meter i can handle, but what do i attach the force arm on to thats generates rotational resistance?

id like to use a generator + SMPS to make a remote power generator... to replace car batteries.

Dr. Slack

Sat Mar 21 2015, 09:09AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Patrick wrote ...

i realize theres a type used in the auto industry based on a torque arm and rpm meter. a force meter and rpm meter i can handle, but what do i attach the force arm on to thats generates rotational resistance?

The simplest way to get a small, controllable, load torque is to use a motor as a generator, with a resistive load on it. If you are testing a motor, then an identical motor used as a generator is perfectly sized for the job. Either a PM motor for DC output, or one of these newer three phase very efficient types. Electro-mechanically it is very tame. If you use a fixed resisitive load, then the current drawn increases with the voltage, resulting in torque load proprtional to speed. If you use incandescent lamps, then they draw a more constant current with varying voltage, so give a more constant load. If you use zener diodes, or a bridge rectifier into a car battery for instance, then they only draw current at a certain voltage, so they will try to hold the load at or below a certain speed.

Then you just need to measure the speed, and arrange a torque arm to mount the generator onto.

Patrick

Sun Mar 22 2015, 07:17AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ok so i hold a constant load (via resistor disipation) while the rpm varies and i measure force?

Linas

Sun Mar 22 2015, 08:47AM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

maybe you make load with power point tracking, so load at any rpm will be as close as output of your machine ?
(idea is to match resistance of your system, generator resistance should be equal to load resistance to transfer power, you know what i am on about)
Just idea

Dr. Slack

Sun Mar 22 2015, 09:11AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Patrick wrote ...

ok so i hold a constant load (via resistor disipation) while the rpm varies and i measure force?

I think you've missed the point. Dissipating the power electrically is so flexible that you can do what you darned well like, with constant this or tracking that. What load conditions do you want to put on your prime mover? You can create them!

Using a motor as a generator does most of the difficult mechanical bit, it being built to rotate an' all. Then all you need to do is measure the torque that's needed to stop your generating motor from rotating, do remember to be careful about any stray unmeasured torques in any lead-outs you have. And the speed.

A significant part of a dynamometer, for any power above a watt or so, is shifting the dissipation away from the shaft area. A water brake shifts it to a radiator, a generator shifts it to some suitably-sized resistor somewhere.

The way I'd tend to use this is, instead of trying to recreate specific load and speed conditions that you think you might want to use, is to sweep the entire operating range of the motor, to characterise it, and then interpolate to specific conditions from a 2D graph of its performance.

klugesmith

Sun Mar 22 2015, 04:52PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Yes, what Dr Slack said.
Power is force x speed (torque x RPM). For speed you can use an optical tachometer, among other methods. Microphone?

You need some kind of brake to apply a resisting force, then measure the relative torque between the motor and the brake. Ever see the acronym BHP?

I once demonstrated this to some kids, using an electric motor with 1/4 inch horizontal output shaft. Put a wooden clothespin on the shaft, sticking out sideways, with the other end of clothespin pressing on a scale. Presto: distance x force = torque. Tighten the clothespin (e.g. with rubber bands) to increase torque, until the braking area burns up.

The fun part is making an adjustable brake that won't burn up. Prony used a leather belt around the smooth rim of a flywheel. Stationary exercise "bikes" use fans, or brake pads on a flywheel, or adjustable magnetic dampers. I bet you could use a marine propellor or paint-stirring paddle dipping into a bucket of water. Or electric motor. Or aluminum disk with its rim between the poles of a magnet.

One year I advised some kids who wanted to comparing biofuels in a diesel engine on a go-kart. They ended up using the kart to tow a truck along a level street.

radiotech

Mon Mar 23 2015, 09:09AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

There is a homemade dynamometer discussed here.

Ash Small

Mon Mar 23 2015, 02:54PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

"A water brake is a type of fluid coupling used to absorb mechanical energy and usually consists of a turbine or propeller mounted in an enclosure filled with water.

As the turbine or propeller turns, mechanical energy is transferred to the water due to turbulence and friction. The shock caused by the acceleration of the water as it passes from pockets in the stator to the pockets in the spinning rotor requires energy. That energy heats the water due to the friction as the water moves through the water brake. Almost all of the horsepower of the system turning the rotor (usually an internal combustion engine) is converted into a temperature change of the water. A very small amount of energy is taken by the bearings and seals within the unit. Therefore, water must constantly move through the device at a rate proportional to the horse power that is being absorbed. Water temperature exiting the unit must be kept under 120-160F (50-70 Â°C) to prevent scale formation and cavitation. The water enters in the center of the device and after passing through the pockets in the stator and rotor exits the outside of the housing though a controlled orifice. The amount of loading is dependent on the level of water inside the housing. Some water brakes vary the load by controlling the inlet water volume only and have a set outlet orifice size depending on the desired hp to be absorbed and some control both input and output orifices at the same time which allows greater control over outlet water temperatures. The housing is vented to the outside to allow air to displace the water as the water level in the unit rises and falls.

The amount of torque that can be absorbed is defined by the equation T=kN2D5 where T = torque, N = RPM, D = the diameter of the rotor and k = a constant dependent on the size and shape and angle of the rotor/stator pockets.[1]

Systems which require the torque of the system under test to be measured typically use a strain gauge mounted on a torque arm that is attached to the housing perpendicular to the input shaft. The housing/stator is mounted on roller bearings and the rotor is mounted on roller bearings within the housing/stator so that it can turn independently of the rotor and frame. The strain gauge connects the torque arm to the frame assembly and keeps the housing from spinning as housing tries to turn in the same direction of the turbine. (Newtonâ€™s third law).

The amount of resistance can be varied by changing the amount of water in the enclosure at any one time. This is accomplished though manual or electronically controlled water valves. The higher the water levels within the brake the greater the loading. Water brakes are commonly used on some forms of dynamometer but have also been used on railways vehicles such as the British Advanced Passenger Train."

Electroholic

Tue Mar 24 2015, 03:55PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

Depending on the RPM and Torque range you are looking at, maybe its worth looking into some type of Magnetic brake/clutch system?

ConKbot of Doom

Tue Mar 24 2015, 08:21PM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329

Electroholic wrote ...

Depending on the RPM and Torque range you are looking at, maybe its worth looking into some type of Magnetic brake/clutch system?

Hmm, maybe graft part of a small centrifugal blower into a copper disc for air flow, and use a door lock electromagnet and appropriate controls to load it? Or an external blower for cooling that way the torque on the electromagnet is your full engine torque.

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