Immune to Electrocution

[mull982]

Resistance is current limiting.
A 1,000,000 ohm resistor in series with a 73.3 ohm resistor(3 amp appliance @ 220 volts) will equal a 1,000,073.3 ohm resistor, and allow only .00021998 amps which is less then without the appliance in series.

Another words: What rattus said:grin:

Yes but is an apliance with a motor really seen as a constant impedance. Or does the impedance of the device change as the load on the device changes similar to a motor? As the blender is loaded and unloaded wont the impedance change.

Take a motor for example that has a changing impdeance determined by R2/s. A motor is a constant power device which will try to draw rated current to turn the load regardless of what other impedance is in the circuit.

So as an example what happens if we put a resistor in front of a motor? Will the motor not try to draw rated current still? Obviously there will be voltage dropped across the resistor and less avalaibe at the motor so it will in turn then try to draw more current. Is this not ture? So in the case of a motor will a resistor in a circuit really limit the current as it would with a constant impedance device such as a light bulb?

 

[zog]

Don't forget one simple thing we learned in basic electricity current is equal in all parts of a series circuit.

So if we are a 1.5 meg. ohm resistor and in series with a 120 volt light bulb we would receive a 8 milliamp shock.

Your body is nto a series circuit, there are many paraell paths, your nervous and circiltory systems are excellent conductors.

 

[zog]

Mods, I suggest we close this thread, too many dangerous practices and incorrect concepts being thrown around.

We have discussed this topic before, and the real facts are plainly stated in national and international standards, as I have already referenced.

 

[LarryFine]

So as an example what happens if we put a resistor in front of a motor? Will the motor not try to draw rated current still? Obviously there will be voltage dropped across the resistor and less avalaibe at the motor so it will in turn then try to draw more current. Is this not ture?

Yes, that's "ture." :grin: As the motor current increases, so will the voltage drop across the series resistor, and it snowballs. That's why motors abhor low voltage more than most loads.

 

[ronaldrc]

Maybe a better name for the human conductor would be immune to pain.
We know what would happen if he grabbed 1 line and line 2 of a 4160 line
he would cook. Usless he is superman?

 

[rattus]

Yes but is an apliance with a motor really seen as a constant impedance. Or does the impedance of the device change as the load on the device changes similar to a motor? As the blender is loaded and unloaded wont the impedance change.

Take a motor for example that has a changing impdeance determined by R2/s. A motor is a constant power device which will try to draw rated current to turn the load regardless of what other impedance is in the circuit.

So as an example what happens if we put a resistor in front of a motor? Will the motor not try to draw rated current still? Obviously there will be voltage dropped across the resistor and less avalaibe at the motor so it will in turn then try to draw more current. Is this not ture? So in the case of a motor will a resistor in a circuit really limit the current as it would with a constant impedance device such as a light bulb?

Yes, the resistor always limits the current. Try this with a rheostat and a universal motor such as used in power tools and kitchen appliances. You will find that the motor voltage and current fall as the resistance is increased.

Now, if you are driving the motor directly from the AC line, an increase in motor load will cause an increase in current, but if there is series resistance, the voltage drop across R will increase thereby reducing the voltage applied to the motor until a balance is obtained.

 

[mull982]

Yes, the resistor always limits the current. Try this with a rheostat and a universal motor such as used in power tools and kitchen appliances. You will find that the motor voltage and current fall as the resistance is increased.

Now, if you are driving the motor directly from the AC line, an increase in motor load will cause an increase in current, but if there is series resistance, the voltage drop across R will increase thereby reducing the voltage applied to the motor until a balance is obtained.

I'm not sure I understand this. Wont the voltage drop at the motor cause the motor to draw even more current?

I'm not sure I understand this. Can you please help expalin?

 

[rattus]

I'm not sure I understand this. Wont the voltage drop at the motor cause the motor to draw even more current?

I'm not sure I understand this. Can you please help expalin?

Mull, consider a motor in series with a resistor, Rs. Now lock the rotor and treat the motor as a dead short. The current is now V/Rs which is the maximum current that can be supplied.

Short the resistance and the current will increase to its locked rotor value.

Now unlock the rotor and let the motor be modeled as an equivalent resistance, Rm, and let the series resistance be Rs. The motor current is:

Im = V/(Rs + Rm)

Note that this scenario only applies to one set of conditions because Rm varies wildly. A motor is better modeled with a transformer equivalent circuit.

In a nutshell, the motor will draw more current if it is available, but resistance limits that current. Quite sure the old mixers used rheostats. Some of the table fans used inductors to accomplish the same thing.

 

[ronaldrc]

2) With the 3A applicance which can be seen as a constant power load the load is going to pull 3A regardless of the circuit in front of it. The 1Mohm resistance of his body will not do anything to limit the current however as others mentioned all of the voltage in the circuit will drop across his body and there would be nothing left at the load to operate the apliance. The load would pull 3A regardless however with no voltage it should not even operate.

A 20 amp wire is considered a fault path for all the current to travel around it with a dead short. This should be enough current to kick a 20 amp. breaker.Would draw a lot more current than the motor circuit above. An electrical device doesn't try or pull the current through the circuit it limits it according to its resistance or Ac impedance.

Now take 1/4 watt 1 megaohm carbor resistor and apply 120 volts to it, it is in series with this wire line or 120 volts, if your theory above is true why wouldn't the resistor go up in smoke?

 

[mull982]

Mull, consider a motor in series with a resistor, Rs. Now lock the rotor and treat the motor as a dead short. The current is now V/Rs which is the maximum current that can be supplied.

Short the resistance and the current will increase to its locked rotor value.

Now unlock the rotor and let the motor be modeled as an equivalent resistance, Rm, and let the series resistance be Rs. The motor current is:

Im = V/(Rs + Rm)

Note that this scenario only applies to one set of conditions because Rm varies wildly. A motor is better modeled with a transformer equivalent circuit.

In a nutshell, the motor will draw more current if it is available, but resistance limits that current. Quite sure the old mixers used rheostats. Some of the table fans used inductors to accomplish the same thing.

Starting to become a little bit more clear. Thanks for the help.

Lets say that I had a motor that was drawing 100A to turn a particular rated load. Now obviously if I increase the load on the motor the current increases. I guess the motors resistance which you are calling RM then decreases.

Lets say with the motor operating at 100A we thin insert a resistor. Most of the circuit voltage will drop across the resistor and therefore the voltage at the motor will drop. As a result of this voltage drop the motor will then attempt to pull more current. I assume that it does this by lowering its resistance Rm. By doing this wouldn't the motor then cause more current to be pulled in the circuit.

Or does the motor try to pull more current by lowering RM but the current is limited by the resistance in series and therfore max current can only be Vs / Rs + Rm. So although the motor trys to draw more current by lowering its resistance it is limited to the current determined by the combination of the two resistances. Even if the motor dropped its resistance to almost nothing the maximum current would still be limited by the series resistor. Is this correct?

 

[rattus]

Or does the motor try to pull more current by lowering RM but the current is limited by the resistance in series and therfore max current can only be Vs / Rs + Rm. So although the motor trys to draw more current by lowering its resistance it is limited to the current determined by the combination of the two resistances. Even if the motor dropped its resistance to almost nothing the maximum current would still be limited by the series resistor. Is this correct?

Yes, the motor will draw more current to satisfy its load if it is available, but that current is limited by the series resistor and the motor turns slower. Likewise, the resistance of the human body limits the current, but sometimes it doesn't limit enough, and the human suffers a fatal shock.

Consider a motor driven by a stiff source. As the load is increased, the motor draws more current as you say until like Popeye, it cain't take no more, and it stops turning, and we see the locked rotor current.

A running motor generates a "back emf" which opposes the applied voltage. As the rotor speed decreases the back emf falls which causes an increase in current. It is more complicated than that, but maybe you can get the idea. I repeat, we can speak of the equivalent resistance of the motor for a given set of conditions, but that is not a complete model for the motor.

 

[mull982]

Yes, the motor will draw more current to satisfy its load if it is available, but that current is limited by the series resistor and the motor turns slower. Likewise, the resistance of the human body limits the current, but sometimes it doesn't limit enough, and the human suffers a fatal shock.

Consider a motor driven by a stiff source. As the load is increased, the motor draws more current as you say until like Popeye, it cain't take no more, and it stops turning, and we see the locked rotor current.

A running motor generates a "back emf" which opposes the applied voltage. As the rotor speed decreases the back emf falls which causes an increase in current. It is more complicated than that, but maybe you can get the idea. I repeat, we can speak of the equivalent resistance of the motor for a given set of conditions, but that is not a complete model for the motor.

I am familiar with the motor model equivelent circuit and the changing resistance due to slip. But for the sake of this discussion I think we both now agree that as a motor is loaded the overall motor impedance Rm decreases however the current avaliable to it is limited by the series resistance.

 

[rattus]

I am familiar with the motor model equivelent circuit and the changing resistance due to slip. But for the sake of this discussion I think we both now agree that as a motor is loaded the overall motor impedance Rm decreases however the current avaliable to it is limited by the series resistance.

You got it!

In my previous life I made a service call on a print dryer. Complaint was that it wouldn't heat and the motor ran too fast. Well, it didn't take long to see that it was wired to 240V instead of 120V. That blew the strip heaters, but the rheostat protected the universal motor from harm. At 120V, the motor would stop completely. but at 240V, it would keep on turning. The company maintenance guy had assumed it should be wired to 240V without looking at the nameplate.

Had a similar experience with our first house. The so-called electricians had wired a set of outlets in a ring--totally dead, and wired 240V into the washing machine outlet. Plugged in a vacuum cleaner which almost left the floor. Didn't burn it out, but it did cause the brushes to stick.

 

[kid_stevens]

LOL too funny. I showed a telecomm tech why I survived lightning and 100Kva hits. He got knocked across the room by a phone system ring generator (makes telephones ring). The RG puts out 139 Volts AC at miliamps. I touched the same line and told him the RG was okay. Most people have 800K ohms down to 200K ohms resistance. I have over 1.4Mohms. He had 750Kohms. So he gets it and I feel it.

 

[ike5547]

Yeah. He's a semiconductor. :grin:

Yer on a roll.