Hi guys,i got a tachometer from my university lab,and i want to test a bldc motor.I have 4 motors to test,and I just want to know which one is a faulty motor.And I need some help!

Hi,
With just a tach (ie, without voltmeter or wattmeter), you can do this

1. Remove prop. Put a band of white masking tape around the body of the motor. Put two black marks on it with a marker, on diametrically opposite positions to represent a two bladed prop. Run a short burst at full throttle and note rpm. Assuming 8/12v, divide the rpm by 8 or 12. This will tell you the motor's kv fairly accurately. If your tach cannot differentiate between 2,3,4 blade props, then you may need only one marker instead of two. Though the kv value is usually stated, sometimes it is incorrect or unknown.
Will help check that all your motors are same kv or not.

2. Put a prop on. Check rpm at say 1/4, half and full throttle. Note data. Compare motors.

3. Repeat with a few other props. Note and save the data. One day you'll be glad you did.

4. Go to flybrushless.com. You'll find data on hundreds of motors (though the data is not uptodate and does not cover many common modern motors). See if you can locate your motor in the database and compare with your own notings.

5. The site also has thrust vs rpm data for hundreds of standard brand props. Even if your prop is not listed, you get data of other brands of similar dia/pitch. Compare with your notings.

Hope you noted down and saved your own data!
Would be nice if you shared your data on the forum, or with me at least

If you have a wattmeter, you can do some more investigation.
At that stage, you'll have to make something to measure the thrust. But that's another story...

Hope this helps.
Regards

Edit:
I presumed you have an optical tach.
Since it is from a college lab, it could be of contact type.
If so, you'll need a spinner on the prop shaft, with or without prop!

Why not post a pic of your tach.