Newsgroups: sci.engr.electrical.compliance
Subject: Re: Measuring Load across an electrical motor
References: 
[email protected] (Gerard) writes:

> I am interested in buying a pan type concrete mixer and would like to
> know if there is any way to constantly measure how hard the electrical
> motor is working.  

Sounds like an application for a meter that measures how much current 
that motor takes. The harder the motor works, more current it takes 
from the mains supply. 

Easiest trick for testing is to put a clamp multimeter over the 
current supplying conductor (one phase on three phase power, 
in sungle phase system live or neutral conductor) 
to see how much AC current flows. 

> In order to cast a good concrete product, I need my
> concrete mix to be of a certain consistency.  If its too hard, it will
> have many holes.  If it is too liquid, it will be low strength.  So, I
> thought that if I could measure how hard the motor is working, I could
> add water to my mix until a certain load is achieved.  If the paddles
> are in the mixer are finding it hard to pull through the mix, then the
> amount of energy used by the motor should be higher than the amount of
> energy used when the mixture is more liquid.  Can this be measured and
> displayed on some kind of digital readout with good accuracy?  

The digital multimeter as I described is one option. 
What would be the actual accuracy and how well would it measure 
what you want is a question. 
Current meters are available easily with 3 digit displays 
with better accuracy better than 1% on directly connected models 
and few percent (in)accuracy on clamp on models.  

> Also,
> would this measurement be easier to do with a single phase or 3 phase
> electric motor?  Both options are available to me. 

With single phase power this measurement is pretty straghtforward. 
Just measure the current on the live or neutral conductor. 
A clamp type AC current meter is the instrument used by 
electricians on repair/maintenance work to do current measurements. 
Maybe there is some "screw-in" instrument you can put there 
as well. Some kind of panel meter with suitable AC current range 
sounds as suitable instrument. There mighnt be some wirign related 
regulations on installing this kind of instruments. And there 
might be some other instumenrs maybe specifically designed for this 
in the market.

In three phase system to get the most accurate results you 
would need to measure the current on all three phase conductors 
and calculate a sum of those. In practice in the real world 
applications where you can expect that the power feed is OK 
(all phases present, voltages on them do not differ much etc.) 
and the motor is not broken, those phase currents have pretty 
much the same value, so you can usually go on with measuring just 
one phase. This is done in the same way as with single phase 
live wire current measurement. 

This the measuring technology and theory part. 
How to actually wire this kind of measurement device to 
the motor circuit safely, somebody else can tell it. 
I don't know all the electrical installion rules on this. 


-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/