Re: Controlling a motor in presence of phase shift


fssg wrote:
I am trying to control speed of a single phase fan motor using triac phase
control, across a optoisolated boundary. The motor could be of shaded-pole
or capacitor-run type, between 30 and 200W.
The 4N25 optocoupler detects the zero crossing, a microcontroller turns on
an optotriac LED after the required delay, and holds it on until about 20deg
before the end of the half cycle.

I have noticed that 100% power (where the voltage across triac is almost
always zero) is at about 25deg for a 173W capacitor-run motor, 60deg for a
50W shaded pole motor. This is giving me two problems:
1. If I turn on the triac eariler than this point, I start to get
'half-waving' (?) but I can't see why this happens? Shouldn't it just
continue to run at 100%?
2. Since I don't know what motor my users will plug into it, how do I
prevent this happening? My zero crossing comes straight from the mains -
could I put the zero crossing detector across the triac to detect when the
triac turns off and have my timing referenced to that point instead?

I also notice that on the shaded-pole motor, the circuit doesn't seem to
turn on the triac properly at high firing angles. Starting at 140deg, I can
see where the triac is being turned on, the voltage across the triac dips to
zero briefly but then rises back up to mains voltage. At about 125 deg it
starts to do the negative half cycles properly, then positive half cycles
start working
from about 92deg. Does anyone know why this might happen?

Thanks for any help.

I cant read the circuit but I can guess what its supposed to look like.
Firstly because the motor is inductive the current crossing point (turn
off) is delayed and not very predictable, it also moves about during
operation. Timing your firing from the point of turn off rather than
mains crossing will improve performance no end. Secondly the triac
turns off at zero current but the voltage is not zero, this causes a
step voltage to be applied to the triac with can cause it to reconduct
again. You need a triac with a very good dv/dt rating and bigger than
normal snubbers. Thirdly at high firing angles the current does not
have time to rise above the triacs latching current so it goes off as
soon as the pulse ends, by using an opto to monitor the triac voltage
you can detect this condition and refire as required, this opto will
also give you your timing signal.

.

Relevant Pages

  • Re: Controlling a motor in presence of phase shift
    ... The 4N25 optocoupler detects the zero crossing, ... What you want is IMHO impossible with your circuit. ... That SCR is actually a TRIAC, ... almost always zero) is at about 25deg for a 173W capacitor-run motor, ...
    (sci.electronics.basics)
  • Re: Controlling a motor in presence of phase shift
    ... The 4N25 optocoupler detects the zero crossing, ... What you want is IMHO impossible with your circuit. ... almost always zero) is at about 25deg for a 173W capacitor-run motor, ... If I turn on the triac eariler than this point, ...
    (sci.electronics.basics)
  • Re: Controlling a motor in presence of phase shift
    ... The 4N25 optocoupler detects the zero crossing, ... I have noticed that 100% power (where the voltage across triac is almost ... 50W shaded pole motor. ...
    (sci.electronics.basics)
  • Re: Triac Switch Inductive Loads
    ... I'm looking to use a moc3032 zero crossing couple and a triac to swich an ac motor on, now my understanding is that because it's a zero crossing it will only turn on at the base of the wave and not in mid wave, now using a microcontroller can i pwm this signal and chop ... The zero crossing firing delay will prevent you from turning the TRIAC on anywhere except at the very beginning of each voltage half cycle. ...
    (sci.electronics.design)
  • Controlling a motor in presence of phase shift
    ... I am trying to control speed of a single phase fan motor using triac phase ... The 4N25 optocoupler detects the zero crossing, ... I also notice that on the shaded-pole motor, ...
    (sci.electronics.basics)