Old machinery, checking motor?

[camoz]

Hi all,

i have discovered the benefits of purchasing old machinery (especially 3 phase), generally now when looking for a new machine, I tend to start looking second hand. Touch wood I have managed to be fairly lucky with no major faults with the machines (or at least none I was not expecting).

I am starting to learn what to look for mechanically, but I am wondering what I could learn electrically (often it is not possible to run machines, and I have had to purchase based on the fact that I am prepared for the motor to be faulty).

I am reasonably comfortable with the different settings on a multimeter, so I was wondering if there is a process I could go through using a multimeter to determine the electrical condition of a motor?

Cheers,

Camo

[BobL]

. . . .I am reasonably comfortable with the different settings on a multimeter, so I was wondering if there is a process I could go through using a multimeter to determine the electrical condition of a motor?

Multimeters are good for identifying shorts and basic coil winding resistances, but that is really about it. What really should be done is to get the machine megger tested. This puts short pulses of voltages higher than 240V onto the machine and this identifies a potential problems like insulator breakdown that may only occur when the motor is running and warm/hot.

I'm fortunate to have access to a megger at work and better still a great techo that also does the testing :D

[Malibu]

G'day Camo,

As Bob says, a multimeter is good for a simple check, but a megger is better.
You can easily get winding resistance on 3 phase with a multi meter and I would expect around 20 to 50 ohms for each winding in the stator (The readings depend on the HP of the motor, smaller ones will have higher resistance) but the important thing is that they should all be near enough to the same readings across the windings. Obviously, open circuit from windings to earth/frame.
For bigger motors with low resistance readings (some will read as a shorted winding... >1 ohm) it helps to leave the leads on the windings while still set to ohms range. SLOWLY spin the motor by hand and watch the meter... the readings will dance all over the place with a good winding, but stay fairly steady if there's a shorted winding.
For single phase motors, it's a little more complicated because you need to find the start and run windings, plus the capacitor, then check each one.

All of the above is almost the simplest test you can do and it's all I ever need in 30+ years of doing machinery breakdowns.
I think I only used a megger a couple of times.

The single simplest test you can do? Stick your nose in the terminal box and have a good long sniff!

[camoz]

Thanks guys for your advice. I will have to have a play in the garage with some motors, and I will be taking my multimeter with me next time (if nothing else it will look like I know what I am doing).
:cool:


cheers,

camo