Projector Screen malfuntion

[Farmer Fred]

Myself and another committee member fitted a powered screen in the local village hall but before hanging it we plugged it in and it appeared to work fine.
During the final testing we found it to only start intermittently so we took it down and opened the end covers, one end appears to be a plastic bearing with a large capacitor and a few other electrical components.
The other end houses the Synchronous motor and when you tweak the shaft when the power is on it will start, if you then hold the shaft and apply a load the motor turns the opposite direction.
We found prior to dismantling that when we allowed the screen to wind right in and a load was applied to the motor it immediately started to wind down, we assumed the is the way this type of screen works.
It was purchased from E-bay and had no instructions, Any ideas Please.
Pic is all that was on the box

 

[MJPD29]

I think you might have been ripped off

 

[dmxtothemax]

Have you tried adjusting the end stop switchs ?
there position is quite critical.

 

[gnuuser]

dmxtothemax is correct
an out of position limit switch will cause the stepper drive to do some weird things as it tries to find the home position if it runs at all.
look carefully at where the switches are mounted. you may find some discoloring or location marks on the frame where the switch might have been.
some servo drives need the switches accurately placed within a couple of millimeters tolerance and the drive programmed to match.
an added note the ramp down time or stop time may be just a wee bit late and may have pulled a switch out of alignment.

 

[Lucien Nunes]

Basic electric roller screens are normally driven by a PSC induction motor with two identical windings. One is fed by the 'up' wire via the up limit switch, the other is fed by the 'down' wire via the down limit switch. The capacitor is connected directly between the two windings, so whichever winding is not being made live with mains becomes the capacitor winding. Depending on which of the up or down feeds is made live, so the roles of main and capacitor winding reverse and with them the rotating magnetic field that gives the motor a definite direction.

The screen should never reverse during use - e.g. if while it is down, you energise the up wire, the motor runs in the up direction until the up limit switch opens and it stops. If it reverses when stalled, fails to start until tickled or lacks torque, this indicates that the capacitor is internally open-circuit or disconnected. Without it, the motor cannot generate torque at standstill and has no impetus to run in any particular direction irrespective of whether the up or down wire is live.

A similar behaviour would result if the up and down wires were made live at once, causing the capacitor to be shorted out except when one of the limit switches is open. This should never be allowed to occur but might result from a shorted control switch or wiring, and also if the capacitor is internally short-circuit. Again there will be no starting torque, but in this case the motor may draw excess current, hum loudly and get hot very quickly. When the screen hits one of the limits, normality is restored for a moment as one of the feeds is broken. The motor will reliably restart in the opposite direction and then immediately be subjected to the two-windings-live situation again as the switch recloses.

With the screen in mid-travel (so both limit switches are closed) I would check for shorts between the two inputs, which should not show continuity. The capacitor is the next suspect.

Note that because the motor may at present run in the wrong direction relative to which wire is live, the limit switches are ineffective and it may overrun and wind the screen the wrong way around the roller. The limit switches will then be the wrong way round when normal operation is restored, which could rip or stretch the screen if not spotted.

 

[gnuuser]

without a diagram or pics of the components its hard to diagnose a problem!
being used to syncro drives in industrial equipment my answer focused around the type of equipment i'm used to
anyhow that is a similar issue Ive seen with some early garage door openers as well.

 

[Farmer Fred]

Many thanks gentlemen You have given us food for thought.
We are meeting tomorrow night and we will have another look over the weekend.
Cheers Fred.

 

[static zap]

Have dealt with something ,where limit switches were given memory by a floppy design --Awfull !
(Prepare to be flabbergasted)

 

[Farmer Fred]

Following on from the above comments we took down the screen today Tues 30th Jan and has we had extended the cable between up/down switch and the screen we reconnected is it was originally.
After checking the cable continuity we powered it up and it
went down and after it had travelled about a metre it would stop and start but it would not respond to the up switch.
We let it run right down and as it reached the bottom of the screen it changed direction and wound up to about 4 ins from closed and stopped.
We assumed it had opened a limit switch, so we opened the switch/capacitor end of the screen to discover how the limit switches work.
There are two tubes with a micro switch on one end and a plastic gear on the other end inside a geared plastic ring driven contrary rotating by the roller on which the screen rolls.
Collars inside the tubes operate the micro switches one going up and one going down simultaneously.
Do the limit switches always have to be one open/one closed as we are still getting times when the motor will not respond to the up/down switch, are we missing something??
Thanks in anticipation of more help.

 

[Lucien Nunes]

Anywhere other than at the ends of travel, both limit switches should be closed, as the screen may safely be moved in either direction. When fully up, the up limit should open to prevent it going up further. When fully down, the down limit should open to prevent it going down further.

If the switches are badly maladjusted, e.g. the up limit is set lower than the down limit, then odd things might happen such as neither direction being available. Likewise if the screen has overrun one of the limits (e.g. when capacitor failure allowed the motor to run down via the up wire, the down limit switch would be ineffective, and could be overrun).

From your previous description, where the motor could be reversed manually, there was a problem that was not due to limit switches. Have you fixed that?