2:1 ratio step down transformer question from newbie

[wozzer]

After looking through my spares I found I actually had a 24v-0-24v transformer, so I hooked it up and tried the voltage, it was 24vac so I then hooked up my my motor and it worked ok, am just a bit worried now that it is going to be dangerous in any way regarding the primary, I have properly insulated this and wondered if it would be worth me putting an inline fuse to the motor just in case the tranformer fails. I have got to modify the mountings to hold the motor and don't want to do this until I am happy that this is a safe way of converting my 50v - 24vac.

 

[duke37]

The value of a resistor can only be found by experiment. At 125mA you need 200 ohm to drop half the voltage but the motor will not be a simple resistance so there may need to be some deviation from this.
The voltage across the motor would need to be measured off and on load and the value adjusted accordingly.
The power dissipated would be about 3W so two 2W 100 ohm resistors in series should do. You could try lower powered resistors for a few seconds to get the right voltage.

A fuse in the motor supply will not help unless the motor fails.
A fuse in the transformer supply may need to be much larger than you expect since there is a need to provide the energising curent for the transformer, this will depend on the size of the transformer.

 

[wozzer]

Thanks, I tried the 24-0-24v transformer but it has a job to start running on initial motor power up, do you think the resistor way is safer and will work better, will the resistors run hot or should I try a 30v-0-30v transformer?

 

[duke37]

If the motor does not run up properly, either the motor is not powerful enough or the voltage is not sufficient.
Measure the voltage at switch on.

A resistor may make the problem worse since the motor will take more current at switch so the resistor will drop more voltage.

The resistor will be safer than a transformer if you do not insulate the primary properly.

Changing from a 24-0-24V to a 30-0-30V transformer will not make a significant difference but transformer resistance will be important.

The resistors will run warm, that is what resistors do. The bigger they are, the cooler and the more reliable they will be.

 

[gorgon]

Thanks, I tried the 24-0-24v transformer but it has a job to start running on initial motor power up, do you think the resistor way is safer and will work better, will the resistors run hot or should I try a 30v-0-30v transformer?

Just a thought, why did the old motor die, could something in the mechanics load it too much and make it burrn out?

TOK