Magnetic starters

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BigDumbDinosaur
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Re: Magnetic starters

Post by BigDumbDinosaur » Sun Dec 31, 2017 3:21 pm

John Hasler wrote:You got me. It is common to try to use motor starters (with motor overloads) for resistive loads, though. They usually get away with it.
These days, virtually all shop lighting is fluorescent or HID, although LED fixtures are starting to appear. Fluorescent/HID lighting presents a mostly reactive load to the line (even with electronic ballasts, which are a form of switch-mode power supply), especially if the fixtures are el cheapos using magnetic ballasts with inadequate power factor correction. The thermal overloads used with magnetic starters in motor service are too slow to work properly in such applications and often won't trip in time when a ballast starts acting up or is overloaded by a failing lamp. Square-D and others have overloads that respond quickly to gross faults in lighting circuits, while ignoring the inrush transient when the lights are flipped on.

LED fixtures mostly look like resistive loads and should be fused or protected as though they are incandescent lighting.
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tornitore45
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Re: Magnetic starters

Post by tornitore45 » Sun Dec 31, 2017 4:37 pm

LED fixtures mostly look like resistive loads and should be fused or protected as though they are incandescent lighting.
LED and modern fluorescent are far from resistive. Anything electronic has a rectifier with a bic input capacitor after the bridge. High class models have a PFC which makes them near resistive but have a surge current to charge the cap at start up.
Cheaper and practically all consumers products have capacitive input rectifier that load the line for a fraction of cycle at the waveform crest. The line current is composed of narrow pulses. Laws and regulation about the input characteristic differ.
Mauro Gaetano
in Austin TX

spro
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Re: Magnetic starters

Post by spro » Mon Jan 01, 2018 5:43 am

Gosh. You guys are awesome.

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BigDumbDinosaur
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Re: Magnetic starters

Post by BigDumbDinosaur » Mon Jan 01, 2018 1:23 pm

tornitore45 wrote:
LED fixtures mostly look like resistive loads and should be fused or protected as though they are incandescent lighting.
LED and modern fluorescent are far from resistive. Anything electronic has a rectifier with a bic input capacitor after the bridge. High class models have a PFC which makes them near resistive but have a surge current to charge the cap at start up.
Cheaper and practically all consumers products have capacitive input rectifier that load the line for a fraction of cycle at the waveform crest. The line current is composed of narrow pulses. Laws and regulation about the input characteristic differ.
Ahem...any LED or fluorescent fixture sold in North America today *must* have power factor correction that brings it as close to unity as possible. The high power factor requirement is one of several reasons why magnetic ballasts have been supplanted by electronic ones. Any device that has been power factor-corrected to near-unity appears almost entirely resistive after the initial power-on inrush has subsided.

The type of linear DC power supply you describe is seldom used anymore in any consumer products, except in small wall warts. Even there, some power factor correction is applied. Today's switch-mode power supplies, such as those used in computer products, have near-unity power factor and hence almost look resistive to the line.
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John Hasler
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Re: Magnetic starters

Post by John Hasler » Mon Jan 01, 2018 1:44 pm

The presence of a full-wave bridge and filter does not imply that a power supply is linear. The rectified line voltage is used to run the switch-mode supply.

tornitore45
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Re: Magnetic starters

Post by tornitore45 » Mon Jan 01, 2018 3:34 pm

Unless the rule have changed since retiring 4 years ago, PFC, or more precisely harmonic limits are mandatory for power >50W.
It makes sense since the extra cost and added losses negate the minuscule advantage at such low power.
Lighting is allowed PF>0.7 which is just about what peak rectifier let's you squeak by.
The mains switch, no matter what kind, does not care much about PF after the contact close. It does care a lot about the first cycle surge.
Mauro Gaetano
in Austin TX

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