> Just a couple of questions
> - any suggestions on a value for the photo transistor collectors
> - ditto for the R in series with the motor
> - where are the photo-diodes driven from?

I can only provide general pointers. I just don't have the experience
to speak with any authority

The BU508AF has quite a low hfe, typically < 5, so (depending on
the motor wattage ?) the resistor must be capable of passing enough
current to turn the BU508 hard on. The opto must be rated for that
current too, for when it grounds the base

The SCR gate resistors depends on the gate current needed. For
the BT151-800R this is around 10mA

The H-bridge was to be controlled with PIC PWM. Provision was
to be made for the PIC to be on before the HV section so as to
get control of the optos

However, I really hate bangs and smoke and have to admit being
more than a little reticent to actually make this circuit. Although I
was talking with people obviously more learned than myself, there
was some disagreement between them. In future I think I'll stick to
DC motors and just turn AC motors on/off. Variable speed
synchronous drive seems like a hell of a lot of work and just not
worth it for a one-off

This is a quote about the series resistor and other parts

======================================================

You need a series R for sure. Measure the DCR or the motor winding
using a DVM and set R equal to that for starters. You will be able to
reduce it to 1/10 to 1/100 of the motor's DCR later. The filter inductor is
not needed imho. However you could use a capacitor instead of it to
pseudo-resonate the motor at the working frequency. This only works
with very small motors and large capacitors rated for mains ac and high
ac current. So imho short out the choke and remove it. R will dissipate
a lot of heat when large. Plan on 20W ceramic body for start. Even
better, read on:

If you have a drive glitch the bridge will self destruct. You will have
drive glitches. The zeners in the thyristor gates are superfluous. They
will never conduct either way. You are using too many parts imho. The
firing scenario of the thyristors is dubious imho. You are driving a coil
so when the opposite NPN opens, the Cathode voltage will drop, but there
will be no current on the thyristor. Only its gate current will flow
through the coil. Later the coil begins to suck current and the thyristor
will fire suddenly. I don't know what this will do to your waveform, but
it will not help it to be a better sinus, for sure. Meanwhile the gate
resistor on the other thyristor has almost 320V at 30mA across it, which
is 10W or so. This for half the time (so 5W). The resistor will need to be
15W probably. The drive circuit alone will likely burn about 15-20W
(efficiency 65-70% ?). A transformer would outperform it imho. When the
NPN turns off the motor L will generate a terrific kickback and continue
to run current through the thyristor and the opposite thyristor's reverse
diode. This current will continue to flow until the L and the mechanical
motor load and eddy currents and Murphy decide it is time for it to stop.
If this is a little bit later than when you open the NPN under it for the
next half cycle, you lose (everything).

Me, I'd have used two (2 pcs) MOSFETs for this in a scheme like so:

         +320V  +-------------(K D1 A)---+--(R2)--<AC1
                |                        |
              |-                         |
              |<+                        |
         G1---|-+                        |
                |                        |
         GND1---+---(M)---(R1)-------------------<AC2
                |                        |
              |-                         |
              |<+                        |
         G2---|-+                        |
                |                        |
         -320V  +-------------(A D2 K)---+

There are two electrolytic caps missing in the scheme between +320V and
AC2 and AC2 and -300V respectively, each rated about 33 to 47 uF 350Vdc.
I'd suggest BUT11 or other such MOSFETs (abt. 3A 800V I think). Make sure
that you get the kind with reverse diodes inside. Drive circuits are as
you please, but must be push pull, like digital optocouplers with CMOS
compatible outputs (pushpull). They need not supply a lot of current
because switching is in voltage (MOSFET) and slow (coil). I've had
success using two small audio transformers with high insulation rating and
a resistive snubber at the gate for this. Try modem transformers. This
removes the need for resistors in funny places in the gate circuits,
zeners, etc., and drives the beasts rail to rail with very little power.

You want to drive this with a 'magic sine' type of signal. Bear in mind
that the motor will kick back the moment you turn off the drive in either
direction every cycle. Using a rather large capacitor in the snubber
across the motor and reducing its series R will help (try 10uF 400Vac and
100R 5W). Your drive would look like:

       +----+        +----+
       |    |        |    |
G1:----+    +--------+    +---- ...


              +----+        +----+
              |    |        |    |
G2:-----------+    +--------+    +---- ...

Where the total Ton will determine the actual motor voltage. The Vef of a
square wave is 1.0 Vpk and is NOT 0.707 Vpk as in a sine wave. If you will
drive the duty cycle higher than that you will burn the motor. So Ton
(total) should be 0.707 or 2 x 0.350 roughly.

Think hard about fan cooling the motor. Its own fan will not be enough
imho. Serious servos and steppers use forced cooling.

hope this helps,

PS: If the motor is tightly dimensioned and hot then it may hit the Curie
temperature or core saturation or both. If this happens there will be a
temporary dead short as a load to your circuit. You need to make sure that
this does not 'detonate' the thing. Fuses are not enough, but a flameproof
R1 helps a lot imho. You can add another in the ac power lead to lengthen
the life of the rectifiers etc. This is R2 in my schematic.
The optos were to be driven with PIC PWM within a limited frequency
range. Mention was made at the time of design, ISTR, about making
provision for the PIC to turn on before the high voltage section so it
could set the state of the optos

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