Roof Blind motor faulty?

[zimmi]

I have been asked to repair a faulty roof blind. I have checked mains to the transformer, DC from the transformer to the switch and DC from the switch to the motor. The switch meters correctly when disconnected, ie. when pressed one way the contacts in a line become short and the other way shorts the contacts diagonally. The switch has a non-locked position and a locked position for each direction.

However, when everything is connected and the switch is made in which ever direction nothing happens. When the switch is 'rocked' from the 'off' position to the, lets say, 'up' non-locked position the motor steps very slightly and the blind moves up about 5mm. It is therefore possible, if one had the patience of Job, to get the blind fully open or closed by rocking the switch.

Please can anyone shed some light on why this is happening.

Thanks

 

[Lucien Nunes]

Check the output voltage from the PSU with the switch operated. If the drive is stiff or the motor faulty, it can overload the PSU which will rapidly shut down. Or if the PSU is faulty, it might shut down even with the correct load from the motor. But in either case the voltage might be fine with the switch off.

If the motor does not run in either direction with correct voltage present on its leads, although there could be trouble at the limit switches the chances are it's the motor itself.

 

[Marvo]

The motor might not be run directly from the switch there's likely to be what is basically a stepper motor interface circuit that counts the rotations and stops it in the correct open and closed positions. The control circuit might be a separate item or it could be built into the motor itself. Some photos might help and a make and model number would also be good.

 

[Lucien Nunes]

I would be surprised if it were anything but a DC motor because from his description the switch reverses the supply polarity en route to the blind. It would have two mechanical limit switches each of which puts a diode in the motor feed of such a polarity as to allow the motor only to run away from that limit.

 

[zimmi]

Check the output voltage from the PSU with the switch operated. If the drive is stiff or the motor faulty, it can overload the PSU which will rapidly shut down. Or if the PSU is faulty, it might shut down even with the correct load from the motor. But in either case the voltage might be fine with the switch off.

If the motor does not run in either direction with correct voltage present on its leads, although there could be trouble at the limit switches the chances are it's the motor itself.

Sorry for the delay in respond ing. I couldn't get back to the customer until last week and then my laptop stopped working with a windows problem.

I checked everything through again and this time managed to get the blind to move continuously open and closed a few times and then it started to operate intermittently. Having confirmed voltages, as per your suggestion, the PSU is ok and switch is fine so it has to be the motor. Having further discussion with the customer she said that the blind was about 15 years old and not 3 or 4, as she had previously hinted, and once she left the blind switched on to 'close' for a few days. There are no mechanical limit switches installed, power runs from the user switch directly to the motor.

I can only assume that the entire assembly is suffering from old age and everything is tight and worn, as you suggest. I measured the current while checking the operation and just before the blind stopped the current rose slightly. I know it is not a conclusive check using a multi-meter but I think all the evidence suggests that the motor is no longer up to the task.

Thanks for the suggestions. They got me thinking along the right lines.

Thank you for your suggestions Marvo. I have looked for make and model numbers but they are non-existent. There is a label on the underside of the motor, that I could see when I moved the blind out from the mounting slightly but that will mean more time taking the assembly apart just to see if there is a useful number given the age of the blind. The problem is the amount of money she is willing to spend on the blind. It sort of works at the moment and she will have another think when it stops altogether.

 

[Lucien Nunes]

The limit switches are probably inside the motor housing - there must be some and they could be at fault The momentary rise in current may be caused by the extra mechanical load of their operating cam, or the stiff bit at the end of the travel, so it's probably fairly normal. In any case if it's that old it's not going to be worth delving into. Had the manufacturers wanted it to last longer, they would have used an induction motor.

 

[zimmi]

The blind acts as if there are no limit switches which did surprise me so you are right, they must be faulty. At the end of the day I agree it is not worth any more investigation. It would be nice for everything to last forever but we've all got to eat. Thanks for your help

 

[Marvo]

Did you manage to find the name of the manufacturer. I still think the motor could be encoder controlled but without manufacturer or model number info we'll probably never know.