propjobbill

I am trying to test a motor for moto calc to get the imputs for rpm/v, amps, and resistance For the load test I can put a propeller on and use my tach. But how do I get the rpm for no load. Without any thing on the shaft to spin how will I get the rpm reading?

Thanks in advance

DaveFlynn

Put the motor shaft in the drill press or connect it to another motor with a shaft coupling and then measure the rpm and voltage.

Dave

propjobbill

Dave, Thanks I tried what you said, maybe I'm not understand or maybe I am not stating my question correct. With MotoCalc it will let you find the values you need for any motor rpm/k, amps, and resistance. I know that if you chuck the motor up in a drill press and measure the voltage across the terminals you can determine the rpm/v. For example if the drill press is spinning at 1000 rpm and voltage is .5 volt then rpm/k is 2000.

I put the motor in the drill press, I measured the voltage, and then I hooked in an amp meter to find out what amperage the motor was producing. After making those readings I chucked a prop adapter into the drill press and used my tachometer to determine the speed of the drill press by fastening a very thin pop cycle stick to the adapter and measured the rpm. The stick should present very little if any drag on the drill. But when I put those numbers into MotoCalc along with the load numbers it gave me an error.

I tried it with a pop cycle stick cut down to about 3” and cut down to about 1/8” wide. With the stick ground down to a knife edge front and back. When I used these numbers for my entry rpm, volt at terminals, and amps using a power supply, I then used a prop on the adapter which ram at about ½ of the rpm of free load it gave me what I needed. I am sure that my no load numbers were not true no load, but I tested it against a motor that had known values and it was within less than 10% from those known values.

I have been slow to respond because I wanted to make sure before answering that I did the test on several motors. Something is not working right for me using the drill press. I may be wrong, I am not even close to an electrical engineer, but my assumption was if you can use the motor as a generator to find rpm/k that you could also measure the current it produced at the same rpm as an amp reading. Can you tell me what I am doing wrong?

Thanks, propjob

PS I reread my question and I didn't ask it right , Your answer was correct. But along with rpm/k I also need the voltage, and amps at the same time

DaveFlynn

The frequency the motor puts out is too high for most meters to read accurately. You would need a fast 3 phase bridge rectifier to convert it to DC and then filter and read the current.

I don't understand what useful information is in the no load current, or how, or with what it is measured. What you need to predict a motor's performance is Kv, Kt, R and L. Based on data from a Hacker A50-14L, and two no-name motors motor calculators are close to useless. To see it, put into motocalc this: A50-14L, HV45A ESC, 12S1P 2100mAh, 12x8 APC prop. The plane flies great, 8 to 12 min flight times depending on throttle setting. MotoCalc doesn't like this combo at all.

ron_van_sommeren

You could also try 'Drive Calculator'
www.yahoogroups.com/group/D-calc
or MM_calc (English and French)
http://electrofly.free.fr/
-> téléchargements (icon at top of page)
-> moteurs
-> MM_calc (English)

Or Rod Badcock calculators:
http://www.badcock.net/

Or
www.peakeff.com

Vriendelijke groeten Ron van Sommeren
• brushless motor building tips & tricks
• diy brushless motor discussion group
• Drive Calculator download & discussion group
• int. electric fly-in, Nijmegen, the Netherlands

svorkoetter

Propjob:

First, a question: Is this a brushed motor or a brushless one?

You can't measure the no-load current by connecting an ammeter to the motor while it's being run by the drill press. All that tells you is how much current the motor can generate when short-circuited by your ammeter.

To measure the no load current, just run the motor at a low voltage (maybe half the voltage you'd normally use with that motor) with no load on it, with the ammeter in _series_ with one of the battery leads.

Full details on how to measure all the motor constants are in the MotoCalc help: http://www.motocalc.com/motocalc.htm#topic_2131

If your drill press has a synchronous AC motor (most do), then you don't need to measure its RPM. It should be written on the side of the drill press (or inside the top if it's one with speeds selectable by moving the belt to different pairs of pulleys).

[hr]
DaveFlynn:

The frequency only matters if you're measuring a brushless motor. In that case, you do need a rectifier, as described in the MotoCalc help page linked to above.

Kv times Kt is a constant, so knowing one, you automatically know the other. The three parameters of the mathematical model that describes a DC motor are Kv (or Kt), Ra, and Io, the latter being the no-load current. In effect, Io is a measure of the amount of current that "goes to waste", not contributing to actually turning the propeller. Likewise, Ra indirectly determines the amount of voltage that goes to waste. You can't compute motor performance without knowing Ra and Io.

In what way has MotoCalc expressed its dislike for this combination? I'd go and try it for myself, but you haven't told me anything about the plane you're flying it in.

Stefan Vorkoetter
www.motocalc.com
www.stefanv.com/rcstuff

propjobbill

svorkoetter, Thanks for taking the time to explain. I have read the moto-calc help some of the equipment mentioned there I don't have. Some of what they are saying I'm not sure about. I think I will just stick to motors that are listed or ones that the manufacture has already done the test work on. Thanks for the help