Heater Element Impedance

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weressl said:
You mean duration of the inrush of the incandescent bulb is much smaller than the compressor. Otherwise the incandescent maximum inrush is 1:54 compared to the compressors 1:5. So incandescent inrush is much higher than motoric loads.
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Yes, duration. So short in duration that I'm not sure if it can be captured without a scope or PQA!
 
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philly said:
Dont you have to protect the cable from overload, as you would in any other application?
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Yes, and the MCP won't do that. The code doesn't say "overload", it says "shall be protected against overcurrent in accordance with their ampacities..." which is about the same thing you are talking about. 240.4 (2005).

cf
 
Cold Fusion said:
Yes, and the MCP won't do that. The code doesn't say "overload", it says "shall be protected against overcurrent in accordance with their ampacities..." which is about the same thing you are talking about. 240.4 (2005).

cf
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Right I agree. But does the NEC specify what will provide the overload protection, either a fuse, CB, or overload relay?
 
philly said:
Right I agree. But does the NEC specify what will provide the overload protection, either a fuse, CB, or overload relay?
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Look at post 22. That will give you some ideas of the discussions I have had with designers on this subject. Again I suggest you look at 240.9 about thermal devices used for overcurrent. Note the reference in 240.9 to using thermal devices in motor circuits.

As for the requirement for a fuse, CB, or overload unit, look at 240.20.A I don't think an overload unit qualifies as an overcurrent device for non-motor loads. As I said earlier, others have disagreed with me - and these would be knowledgable "others"

cf
 
weressl said:
Could you do a favor and run the same trace on the switch off bounce/interruption? Would be much appreciated!:smile:
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Both were done with a Fluke 43B PQA in its inrush function, only options are the current divisions and max duration of one screen. Besides, I've never heard of bounce/interruption!:confused: Maybe there is another scope term for what your refering to? I own a few scopes also.

I was able to capture inrush on the same 100W buld using a AEMC F05 but it will capture down to a 1/2 cycle.
 
090205-1908 EST

weressl:

The clutch control system was designed in 1971. Solid state devices at that time with the required peak voltage capability were SCRs. Possible, but the relay was easier and less expensive. Drop out time of the clutch was the most important requirement. This meant large turn off voltage peaks.

Also use of the relay provided something for the electricians or jobsetters to do.

Something we should have done was develop an automatic polarity reverser. The clutch was a nominal 105 V DC and 1 A device, and we used parallel silver cadmium oxide contacts with a 10 A rating.


The peak current to a 100 W lamp turned on at the peak of the sine wave is about 170/10 = 17 A. If turned on at a zero crossing it is substantially lower. The steady state RMS current is 0.83 A and the steady state peak current is 1.18 A.

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090205-1934 EST

weressl:

What load, resistive or inductive, and for what switch conditions are you interested. I assume for a small relay or contactor with mechanical contacts. A KUP relay has a higher mechanical resonance frequency for the contact mechanism than a larger relay. I believe the KUP falls in the millisecond range, maybe less than 1/2 millisecond.

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gar said:
I use a P&B KUP type relay for DC switching of a highly inductive clutch. These relays last about 3 to 6 months before the metal conical mound on one contact gets to be too large for reliable switching. This mound corresponds with a cavity in the other contact.
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gar said:
090205-1908 EST
The clutch control system was designed in 1971. Solid state devices at that time with the required peak voltage capability were SCRs. Possible, but the relay was easier and less expensive.
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Gar -
Did you happen to look into a free-wheeling diode across the clutch coil?

Remembering back to 1971, silicone diodes were available, but I can't recall if anyone was putting them across inductive loads yet.

cf
 
gar said:
I use a P&B KUP type relay for DC switching of a highly inductive clutch. These relays last about 3 to 6 months before the metal conical mound on one contact gets to be too large for reliable switching. This mound corresponds with a cavity in the other contact.
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So, swap the polarity, and get more time from the same contact set.
 
090205-2143 EST

Cold Fusion:

What happens when you put a back biased diode across an inductor, apply a current, and then abruptly open the current supply (a switch)?

The stored energy in the inductor tries to maintain the same current flowing before and after the switch is opened.

This causes the reverse biased diode to become forward biased and thus present a short circuit across the inductor. The discharge time constant is L/R. Where L is the coil inductance and R is the internal resistance of the coil. Because the coil has a relatively low resistance the discharge time is relatively long. The low resistance is where the stored energy is dissipated.

If you can make this discharge resistance large, then the clutch drop out time is reduced. Our circuit used an external capacitor and series resistor to shorten the drop out time and limit the peak voltage to 1000 to 2000 V across the coil.

If you leave this snubber off the time will be shorter and the voltage higher with most of the energy being dissipated in the spark or arc across the switch contacts.

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gar said:
The clutch was a nominal 105 V DC and 1 A device, and we used parallel silver cadmium oxide contacts with a 10 A rating.
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I wonder if it would be viable to add a diode-resistor snubber to the system.

You would have a trade off between resistance, voltage developed across the contacts as they are opening, and current decay rate. But it seems to me that the arc is maintaining the current flow for a period of time, and that with a suitable resistor you might get faster clutch operation.

-Jon

Edit to add: whoops, I see that you've answered this already!
 
090205-2257 EST

winnie:

The capacitor resistor snubber was optimized by the clutch manufacturer for minimum drop out time.

A resistor diode will to some extent do the same as the capacitor resistor, but will have an instantaneous voltage rise to whatever the external resistance and the inductor current determine. This will still be an exponential decay with no zero crossing.

The capacitor resistor uses a lower resistance than would be used with a diode and thus the initial voltage jump would be smaller. The capacitor in combination with the inductance creates a resonant circuit and this causes a voltage zero crossing that may extinguishes any contact arc. I believe the circuit was close to a critically damped oscillation.


Larry:

As I indicated in an earlier post we probably should have developed and added to the circuit a function to alternate polarity. This would have greatly extended the contact life. Essentially this is what happens in an AC circuit.

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090208-1542 EST

bill:

The top trace is the output of a comparator. The comparator switches between high and low with a fractional part of a millivolt differential input voltage. At the input is a source voltage of about 8 V RMS to a 1K resistor, a shunt back-to-back pair of 1N4148 diodes for amplitude clipping, and last a 1 K resistor to one of the differential inputs. The other differential input is to a 2.5 V DC bias which was also the other lead from the 8 V AC signal.

Each transition of the top trace occurs at a voltage zero crossing. So high is 8.3 MS follower by low of 8.3 MS.

Where I am shifted about 90 deg for turn on is about 4 MS from the left side of the scope scale. See photo P1.

So the top trace is combined with a random trigger in a NAND gate to produce the trigger to the first 555. The first 555 controls the turn on phase angle, and the second 555 the on duration. Since the switch is a Triac the turn off is always at a zero crossing.

.
 
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