4 Pole to 12 Pole AC Motor Conversion

Motor

I like what you have done. Thats alot of effort. I suspect you've already considered measuring the stall torque of the 600 RPM motor buy installing a long arm on the shaft and measure its stall torque.

I built a dynomometer for measuring the Torque and RPM of 3 phase motors. If you are anywhere near the Los Angeles area, I'd be interested in sharing my dyno components with you for evaluating your interesting 600 RPM motor re-connect.

Jerry

Jerry,

Yes, the pony brake setup is the next project. Hopefully at the lower rpm and approx 1/3 of the original HP a simple friction setup will work.

Thanks for your dyno offer, but I live in Texas.

I have an old Sanborn strip chart recorder (8 channel with only 4 amps?? I can't remember) set up with 3 current transformers for measuring the phase currents out of my original rotary converter at various loads. Might be a good match for your setup. Worked last time I used it, just looking for a good home for it.

MM

Motor Maniac,

Assume you are talking about an old fashioned Prony Brake, not an inertial dynamometer. The article, "Horsepower and the Prony Brake Dynamometer" In Jan/Feb 2003 issue of Live Steam Magazine offers a good overview and worked out examples using a conventional Prony Brake.

Bob Swinney

Motor

I might be able to save you some time when you do get around to making a dyno. I made about 20 different dynos in my attempt to 'get things right'. Depending on how much you want, you can build a dyno from an automobile disc brake for 10 HP and less. The way you build and instrument your dyno is dependent on how much accuracy you want to get. But, you can get some pretty good information about the 12 pole motor's HP by mounting a car disc brake on the motor. The best dyno I've been able to make uses an aluminum disc and some electromagnets to "load the rotating disc".

You are welcome to contact me 'off line'.

Jerry

Bob,

Thanks for the input. I do not have access to the article, but this is what I therw together over the lunch hour.

I rolled the motor on to its side at the edge of my work bench, the motor free to roll approximately 45 degrees in either direction. I screwed a piece of all-thread into the hoisting hole in the top of the motor. The all-thread now extending out horizontally and past the edge of the bench. I measured and marked the point on the all-thread

I next mounted a Hunter 0-50# spring gauge to a large plate laying on the floor and connected the gauge to the mark on the rod with a wire.

I configured a pair of 1x4's into a tweezer configuration and put the front 1/3 or so about and extending beyond the output shaft.

I started the motor and began squeezing the open end of the boards together. Here are the rather disappointing results:

I did not measure the shaft speed at tthe various loads, but if we use

Me thinks I do not have a motor but a 2.6 KVA heater, possibly a nice hand warmer in the winter.....ARRRG!

MM

Jerry,

I think we were both posting at the same time. I think the disk brake rotor is a great idea, especially if this motor has anywhere near the factory torque of 30 ft-#. Read my post above, the motor puts out only 8 ft-# which I easily soaked up in a couple of 1x4's. A hoaky setup indeed, but I got the info we needed for this expirement.

MM

Motor Maniac, You won't be able to construct a prony brake around torque readings only. You also have to take into account RPM and the length of a torque arm. An inertial dyno may be easier. Check with Jerry re.doing the inertial dyno thing. I think he researched inertial dynos pretty well and may be able to help you.

Bob Swinney

Bob

Not inertia dinos. But I do have a considerable amount of time involved with building a regular dyno where I measure the Torque and RPM while adjusting the load on the motor. I get the best resulkts with the aluminum disc thats attached to the motor shaft, and load it by applying DC thru it. Excel does all the hard work and produces nice graphs of either Torque or HP as a function of RPM for the motor under test. This system sure makes it easy to evaluate the effects of adding idlers and tuning.

Jerry

Pay attention to Jerry..he has devoted a hell of a lot of time and energy developing his data. And the gent is smart as hell too.

Gunner

"A vote for Kerry is a de facto vote for bin Laden." Strider

Guner

Jerry is not so smart, but he does know Don Foreman.

Jerry

Jerry,

Can you elaborate on these statements?

2) ....aluminum disc thats attached to the motor shaft, and load it by applying DC thru it.

#1 I understand that you are inducing eddy currents into the disk, the higher the magnetic field, the higher the currents and resulting torque.

#2 Sounds a bit like a homopolar rotor where, if I remember correctly, current is applied through brushes, one set on the outer diameter, the other set on a smaller diameter or on the shaft.

Thanks,

MM

MM

I made a 14 inch diameter disc from a plate of 1/4 inch aluminum and bolted it to the pulley. I mounted a strong electromagnet, thats free to rotate, out near the disc outer radius. The electromagnet puts its field thru the aluminum. As the current in the electromagnet is increased, the spinning motor is loaded more heavily. I have a load cell on the "free to rotate" electromagnet so the force can be measured with a simple voltmeter and fed directly to Excel, where it the computer does all the 'hard work'.

This 'spinning disc' and 'electromagnet' is dirt simple. No brushes, no super science, all I needed was alot of scrap parts and One Don Foreman.

Jerry

Jerry,

Thanks, sounds like a great setup. I will continue to dig for rewind info to make a 12 pole motor. It can be done, just gotta keep digging. When I get a motor an old shoe can't stop I'll be diggin through my scrap parts (damn, I just love shopping at the dump!!)

Until net time.

MM

Greetings Jerry, It seems that, from your description, the torque measured is equal to the radius of the location of the magnet. So, if a disc of, say, 5 inch diameter with an electromagnet mounted such that the field passes through the 1' radius would give me inch pounds if a pound scale was used. And using the laser tach I just got through Spehro the HP could be figured out by multiplying the rpm by the torque. This sounds like a project that I could do. I could use the digital scale that's used in my kitchen right now. Thanks for posting. ERS

Jerry,

OK, so I visualize your setup something like this: Electromagnet(s) near the edge of the aluminum disk set up "eddy" currents in the disk. The electromagnet is free to rotate in part of an arc around, and with, the aluminum disk. As current is increased in the electromagnets it poses more drag on the disk which rests against a strain gage. The strain gage reads "force" at the distance of the center of the electromagnet from the motor's shaft - this force is torque. The other component of HP is RPM and that is also read from the disk and sent to the computer. I suppose you increase the magnetic drag until the motor just begins to stall, and take that amount of strain and RPM as the point of maximum HP from the motor. It would seem this is a Prony-type brake with a shortened torque arm. Much more compact than the standard Prony Brake with its 5 - 1/4 foot torque arm. Very clever! Is this about right?

Bob Swinney

I' ve always intended to build a small dyno for measurements on small motors and engines. It always seemed to me that a good load might be a small squirrel cage single phase motor, and varying the load using a controlled DC excitation of the stator. The motor provides both the disk (morphed into a cylinder) and the electromagnet. Forced ventilation might be required to keep the rotor cool.

Anyone tried that?

-- Greg

electromagnets

electromagnet

On Fri, 21 May 2004 15:14:54 -0700, "Greg Dermer" wrote: I suppose what you write below will work but a person would need to know how much energy is not being used to slow the motor. I'm sure this is pretty easy for the right people. The dyno using a scale and rpm measurement seems like it would be real simple to get accurate readings. You only need to know the shaft speed and force of the magnet against a scale accurately. The scale and tach are easy and cheap to buy and quite accurate. The power the electromagnet consumes never figures into the calculation. Neither does winding resistance, windage, and heat in the windings. All you care about is how much power is coming out of the motor shaft the way you are operating it. ERS

Bob

You do have it figured right. Or, at least, thats the way I see it. I just conect a couple of multimeters to the RPM detector and the load cell and feed that info right into the computer. Excel draws a plot of HP vs RPM as the motor is loaded. This set up sure shows how the 3 phase motor's ability to deliver power is efected by idler size. Excel draws the series of HP vs RPM for the 3 phase motor when several idlers are used.

Jerry

electromagnets

electromagnet

The basic problem is that your reconnections have produced a field with a 12 pole component but the winding configuration does not distribute the field efficiently.

For simplicity consider a 2 pole 3 phase machine. When any one phase is energised it produces a roughly uniform diametric N-S magnetisation polarisation of the entire rotor. This is because each bunch of phase windings spans as nearly as possible 180 deg - a rotor diameter.

In a 4 pole machine each bunch of phase windings will span 90 deg producing a 4 pole N-S polarisation of the rotor iron i.e 2 N poles and 2 S poles spaced alternately

The same stator wound as a 12 pole 1 phase machine would need 12 sets of windings each spanning 30 deg. Because of the limited number of stator slots a "set" of windings may be no more than a single coil spanning 1 or 2 teeth. The rotor magnetic pattern is then 6 N poles and 6 S poles spaced alternately.

The problem with your 3 phase reconnection is that it is producing 6 overlapping 4 pole fields each spanning 90 deg of the rotor instead the correct 30 deg.

This means that a rewind is needed. With a 12 pole 1 phase machine it is worth remembering that only the wire passing through the stator slot produces useful torque. The wire outside the slots simply connects the ends of the torque producing wires.

The magnetic polarity of the adjacent teeth is determined by the axial direction of the current in the torque producing wires. If the go and return currents on either side of a single tooth produces a N pole the route taken by the external connecting conductors can be routed as convenient.

As an example, a 12 pole winding on a 12 tooth stator could be individual coils on each tooth series connected. A much easier arrangement for experimental purposes is a single prewound large diameter coil which is zigzaged thru alternate stator slots. This produces the same field pattern in the rotor but may be easier for you to manufacture.

For a 24 tooth stator, zigzag thru alternate slots. A second coil zigzagged thru the remaining slots would behave as the second winding of a 2 phase machine. Zigzag winding is rarely used commercially because it's only efficient on multipole machine with small numbers of slots per pole. On most other machines the conventional configuration is more efficient.

You will need to use thinner wire for the rewind because the lower rotor speed means, for a 4 pole to 12 pole conversion, you need

Jim

P.s. Don't forget your rewound 12 pole machine will operate at roughly the same flux density as the parent 4 pole machine. This means that the maximum torque will be about the same and the horse power will be reduced by a factor of 3.