Hi

In Nerds CD1, Part 6, at the start of time 1:00, the instructor speaks about the amps being 28, but the scope is not showing amps rather 2.8v, do I take it what the resistance of the motor is 0.1 and I=VR? but if so, why was not it indicated? am I missing something?

thx

thanks for that.

why would not the scope run its own functions and just display the amp according to the setting of the clamp?

Carl Grotti wrote on Feb 1^st, 2009 at 8:45am:


The current clamp has two calibration settings, set by a slider switch on the handle of the probe.

   1. 1 mV/10 mA (100 mV = 1 A)
     — use this for testing currents up to 20A.
   2. 1 mV/100 mA (10 mV = 1 A)
     — use this for testing current up to 60A.

screen                   setting 1                       setting 2

2.8v                       28A                              280A

BTW
which clamp is supplied in the Quadkit, The PP218 or the PP264?

Quote:


PP264 Current Clamp (60A AC/DC) with BNC and 3 metre screened lead

PP266 Current Clamp (600A AC/DC) with BNC and 3 metre screened lead

Hi Tom,

And you have done a mighty fine job

Second to none in support and dedication

THANKS Tom !

dave

Carl Grotti wrote on Feb 1^st, 2009 at 12:49pm:


I am not an EE but have always known that V=IR is a non breakable law.
why it can not be applied here?
I am guessing the reason being that there could not possibly be a good explanation for the relation. if so, my question be if R is dropping due to motor getting faster would not that satisfy the change in relation on the graph?
why R changes? maybe R of a slow running motor is affected be EMF and when motor speed increases R starts dropping due to less EMF.... I do not know. just a guess. but if you know why Mr. Ohms fail here, let us know.

BTW, R is not just the static ohm, but rather the resistance, reactance, and impedance.

Quote:


Sam, you are correct.

The better answer is that you have an active component involved and so the math becomes more complex. You don't throw the law out.

Any circuit with an active component can be broken down into its different stages and ohms law can be applied to figure out the various values.

CEMF is just another voltage potential in the circuit and the math is exactly the same. You just need to know the given CEMF for that given instant.

The specs needed for motors and such will be hard to come by though. I don't know what Tom covered, but there will be differences between permanent magnet motors and series wound motors (which are mostly a thing of the past in our biz).

A little experimentation and you can come up with some amperage curves. PM motors tend to want to run at a specific rpm range or not run at all.

I would recommend anyone involved with heavy scope useage to take at the minimum a semester class at their local JC. A class that is a "survey of electronics". I opted for a 32 unit program which I have never regretted. It's been a while, I forget things, but it just takes a peek at the old books to bring things back to remembrance.

The only down side is that most all of the classes are biased towards AC circuits. Motors especially. One of the very best I remember is a semester of passive circuit analysis. Simpler by far than that covering active circuits, but all active circuits can be broken down into passive circuits.

Quote:


I think this might add some clarity, maybe not.  We don't act as if the induced voltage can be sustained at any serious amperage.

So yes, I have a different take on what controls motor speed. Given a properly applied potential (varies on the type of motor), its mostly its torque spec and the applied load. Up to a point, of course.

PM motors once again, tend to want to either run within a given range, or not run at all. That is why starter failures seem so different today than they used to be.