Reverse Phase Control Dimmer

[Tony]

Hi

Just curious about something. We probably all know how a standard light dimmer
(or dc motor controller) works. A thyristor is triggered at some point during a
half line cycle and remains on until the the end of the cycle when the current
is zero and the thyristor turns itself off. The earlier in the cycle that it is
triggered the more power delivered to the load.

With reverse phase control, a non-latching switching device (IGBT etc) is
turned on at the start of the line cycle (or zero crossing) and turned off at
some point during the cycle. The longer into the cycle it remains on the more
power delivered to the load.

It would seem that with reverse phase control, high surge currents/EMI would be
eliminated at turn on. So what are the other advantages/disadvantages of this
method? Would reverse phase control be suitable for DC motor control?

Tony

 

[Ralph & Diane Barone]

Hi

Just curious about something. We probably all know how a standard light dimmer
(or dc motor controller) works. A thyristor is triggered at some point during a
half line cycle and remains on until the the end of the cycle when the current
is zero and the thyristor turns itself off. The earlier in the cycle that it is
triggered the more power delivered to the load.

With reverse phase control, a non-latching switching device (IGBT etc) is
turned on at the start of the line cycle (or zero crossing) and turned off at
some point during the cycle. The longer into the cycle it remains on the more
power delivered to the load.

It would seem that with reverse phase control, high surge currents/EMI would be
eliminated at turn on. So what are the other advantages/disadvantages of this
method? Would reverse phase control be suitable for DC motor control?

Tony

Well, for lighting applications, triacs are (were) cheaper than IGBTs. For
other apps, I would consider the possibility of high voltages at turn off.
Perhaps also, once you commit to using a device with controlled turn off,
the possibility of using PWM becomes feasible and you skip right past
reverse phase control directly to PWM.

 

[Spehro Pefhany]

Hi

Just curious about something. We probably all know how a standard light dimmer
(or dc motor controller) works. A thyristor is triggered at some point during a
half line cycle and remains on until the the end of the cycle when the current
is zero and the thyristor turns itself off. The earlier in the cycle that it is
triggered the more power delivered to the load.

With reverse phase control, a non-latching switching device (IGBT etc) is
turned on at the start of the line cycle (or zero crossing) and turned off at
some point during the cycle. The longer into the cycle it remains on the more
power delivered to the load.

It would seem that with reverse phase control, high surge currents/EMI would be
eliminated at turn on.

But would be present at turn-off. You can control the turn off,
deliberately making it more inefficient, to make trade-offs in this
matter.

So what are the other advantages/disadvantages of this
method?

It's more expensive (disadvantage). It's better possible to protect
device that can turn off against overload (electronically, even).
There's some advantage in terms of the power system. It reduces
acoustic noise from the filaments.

Would reverse phase control be suitable for DC motor control?

I don't see a lot of advantage with an inductive load, but sure.

Best regards,
Spehro Pefhany

 

[Adam Aglionby]

Hi

Just curious about something. We probably all know how a standard light dimmer
(or dc motor controller) works. A thyristor is triggered at some point during a
half line cycle and remains on until the the end of the cycle when the current
is zero and the thyristor turns itself off. The earlier in the cycle that it is
triggered the more power delivered to the load.

With reverse phase control, a non-latching switching device (IGBT etc) is
turned on at the start of the line cycle (or zero crossing) and turned off at
some point during the cycle. The longer into the cycle it remains on the more
power delivered to the load.

It would seem that with reverse phase control, high surge currents/EMI would be
eliminated at turn on. So what are the other advantages/disadvantages of this
method? Would reverse phase control be suitable for DC motor control?

Triac/rhyristor/alternistors are a darn sight cheaper than IGBT`s thats for
sure.

Though reverse phase control with stage lighting has the advantage, as
another poster commented, of no filament sing. No big chokes to slow rise
time either so loss of weight and no buzz from dimmers themselves.

This allows lightweight silent dimmers to be mounted near the lights in a
stage application.

Disadvantages are cost, stage dimmers are expensive, stage IGBT dimmers are
very expensive.

Trimming them to always match, at least one early installation was pulled
out and became subject of a court battle, because multiple channels would
not behave identically.

Not entirely sure what the problem is with getting them to behave
identically but there are very few IGBT dimmers on the market ,and some have
been demonstrated as prototypes by major manufacturers, to never appear as a
production item.

Perhaps some people here could outline some of the problems?

Only ones can think of off hand:

http://www.ies.nl/

http://www.rosco-ca.com/products/dimming/IPS_Dimmers.html

Sure some AC motor drives use IGBT to regenerate the sine wave to provide
smooth start up and slow down, again look to others to hopefully fill in
some detail.

Adam