Contact Ratings

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Mustwin351

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Didn't quite understand the contact ratings on this time switch. It is a Tork: EWZ101 (unfortunately I could not post a photo but the specs are below)

Resistive: 40A
Inductive: 40A
Ballast: 30A
Tungsten: 15A
Pilot Duty: 720VA
Motor: 1HP

I figured a tungsten load and resistive would be the same...what am I missing?

Also I thought motor loads would be considered inductive loads.
 
Seems to be a wide swing between similar loads, as you have surmised yourself. I'm at a loss to explain... :blink:
 
thanks for the picture.

Yeah the strange ratings make me question its quality....It has some nice features for a cheap price (astromonical timing) and capacitor backup so not even a battery to swap out...but if the contacts or unit is junk then obviously not worth it.
 
jumper said:
This link explains a little bit:

http://www.nkkswitches.com/pdf/electricalratings.pdf

In the link, lamp load is tungsten load.
Click to expand...

Great information! Thanks!

But I still do no understand why there wold be a huge difference between the load ratings of an incandescent lamp and a resistive load like a heater....if anything I would think the heater would be a more problematic load for the contacts as the inrush on startup would be higher than an incandescent lamp. According to the spec sheet the opposite is true...
 
Mustwin351 said:
Great information! Thanks!

But I still do no understand why there wold be a huge difference between the load ratings of an incandescent lamp and a resistive load like a heater....if anything I would think the heater would be a more problematic load for the contacts as the inrush on startup would be higher than an incandescent lamp. According to the spec sheet the opposite is true...
Click to expand...
Very simple. Heaters generally use nichrome (TM) wire which has a relatively low temperature coefficient of resistance.
Tungsten, OTOH, has a very high coefficient. The inrush for tungsten filaments is much higher than for heaters.
In addition, a tungsten filament lamp, especially high wattage, can fail with a vaporization event that causes a very high arc current. This may not affect solid contacts, but can destroy solid state dimmers.

Sent from my XT1585 using Tapatalk
 
GoldDigger said:
Very simple. Heaters generally use nichrome (TM) wire which has a relatively low temperature coefficient of resistance.
Tungsten, OTOH, has a very high coefficient. The inrush for tungsten filaments is much higher than for heaters.
In addition, a tungsten filament lamp, especially high wattage, can fail with a vaporization event that causes a very high arc current. This may not affect solid contacts, but can destroy solid state dimmers.

Sent from my XT1585 using Tapatalk
Click to expand...

thanks for explaining that!
 
170916-1215 EDT

Mustwin351:

What are you missing? An understanding of various components, poor instruction in classes you have had, bad information that is propagated by other electricians, and details on the tests by the vendor.

A typical tungsten filament bulb has a hot resistance about 14 times its room temperature value. A 100 W bulb is about 10 ohms at room temperature. A 120 V 100 W bulb is about 0.83 A at 120 or about 144 ohms.

The higher the wattage of a tungsten bulb the longer is the time to reach it final temperature.

If zero current is flowing in an inductor at time = 0, and if a voltage is applied to the inductor at time = 0, then at time = 0 + a very small time the current in the inductor is still zero. You can not instantaneously change the current in an inductor.

A motor is not an inductor it is a motor. It may have both a varying inductive component and resistive component. Usually a motor is at 0 RPM when started. There is a short time average RMS starting current that is possibly 6 times the average RMS full load running current.

All mechanical contacts tend to have contact bounce on closing, and in turn some arcing when switching on a load.

All circuits have some inductance. Therefore when switching off a load with a mechanical switch there may be some arcing. If switching with a solid state device there my be destruction of that switching device.

Learn more about different devices and their characteristics.

.
 
170917-1404 EDT

Mustwin351:

At my website http://www.beta-a2.com/EE-photos.html see scope plots P1 thru P8 of the turn on current waveforms for a tungsten incandescent lamp, an Ohmite power resistor, and a small transformer.

Note that the Ohmite 10 ohm resistor shows no peak inrush current.

At steady state the 100 W bulb is quite close to a linear resistance on an instantaneous basis for a constant RMS sine wave voltage. For a 60 Hz source sine wave voltage there is virtually no light intensity ripple for a 250 W flood, some ripple from a 100 W standard bulb, and quite a bit of ripple from a 15 W. As the wattage goes down the thermal time constant of the filament becomes shorter, and thus more ripple. As the ripple increases the linearity of the resistance degrades vs time. This is a thermal problem.

When one plots light ripple from an incandescent relative to applied voltage or current, current and voltage are in phase, one sees the light ripple lagging the voltage. This is because beyond the voltage peak energy is still being pumped into the filament at a greater rate than it is being exported from the filament. But then there is a time point where not enough energy is coming in to support outgoing energy.

.
 
170919-1400 EDT

The reference that jumper provided is generally pretty good. However, the discussion in the first part of the discussion under "Inductive Load" is wrong.

Looking into the input terminals of a switching power supply the input impedance is probably not very inductive. More likely it looks like a capacitor. Thus, should not be called an inductive load. If it was a highly inductive load, then there would be no large inrush current.

Why are electricians hung up on saying that inductive loads have a high inrush current? Because they have not studied electrical circuit theory, transient analysis, and do not really know what the load is that they call inductive.

A true linear invariant inductive load with with zero initial current in the inductor at the time a voltage is applied to the inductor will still have zero current in the inductor at the instant of time just after application of the voltage.

If the inductor had a non-zero initial current, then that would be the current flowing in the inductor just after application of the voltage.

What happens to inductor current after t = 0 is a function the other components associated with the inductor, how the components vary with time, and how the source voltage changes with time.

You readers need to run your own experiments to see how various components work. You also need to study circuit theory to avoid making statements that are wrong.

Resistance capacitance circuits with a battery, switch, and an oscilloscope are the easiest to start playing with.

.
 
My guess is that the casual visitor to the subject thinks of motors as at least partially inductive loads (less so when starting!) and knows that motors have a high inrush. So they jump to the unjustified conclusion that inductive loads have high inrush.
 
170919-1728 EDT

Following are two threads I started previously relating to motor loads.

Relative to iwire's comment on my usage or definitions of inrush and starting current. I quoted some comments from a text book on AC Machinery that only used the term "starting current" when describing the current to a motor while starting. Thus, a basis for this usage.

Electricians get very up tight about definitions relative to the NEC, but get outright sloppy when talking about electrical circuits. If two things are quite different it makes no logical sense to call them by the same name.

http://forums.mikeholt.com/showthread.php?t=166485&highlight=motor+starting+current

http://forums.mikeholt.com/showthread.php?t=174880&highlight=motor+starting+current

Transformer, capacitor, and tungsten filament lamps have very different turn on current characteristics than motors.

.
 
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