Help - how can something need to run on AC and DC at the same time!!!!

[nvirg]

Hi all,
I am a Lab Technician at a High school and today one of our teachers and I were coming up with designs for a motorised toy boat. Were were checking out if the motor was working and how much voltage it took to effectively turn the fan. We hooked the motor up to a portable AC/DC transformer and wired it to the AC connection. NOTHING. We tried it again by wiring it up to DC (which we shouldn't have needed to do). That didn't work either. Then we discovered that the fan would only run if it had one wire connected to the positive AC terminal and the other to the negative DC terminal! We are both Biology majors (little physics background) and were wondering how or why this could occur as it doesn't seem to be logical?
Can anyone help? P.S. We checked the transformer and it was working okay with light globes etc.

 

[Diver300]

Circuits can require AC and DC at the same time to work. DC is electricity at a frequency of 0 Hz, and frequencies can be separated to do different things. An example is the LNB in the middle of a satellite dish. It is powered by DC from the receiver, while the high frequency AC signal to the receiver travels down the same wire, in the opposite direction.

It's quite common to have transformers that have a rectified output, but no smoothing. Often that is full-wave rectified so the voltage dips to zero each half-cycle, so 100 times per second with 50 Hz mains. It can be half-wave rectified where there is no output for half the time.

EIther way, there is an average voltage, the DC component, and an AC component, at either 100 Hz or 50Hz, superimposed on the DC.

It appears that something in the fan requires both an AC component and a DC component to make it work. I can think of a number of ways that could be achieved, but I can't work out why it has ended up like that. If you've got any photos or other information about the fan and the transformer it is meant to work with, that could help. Can you use and oscilloscope to measure the output of the transformer, preferably when the fan is running?

 

[Pommie]

You mention a positive AC terminal which is an oxymoron. AC does not have a positive terminal. I suspect the other AC terminal is open circuit and the DC terminal is providing a return path. To confirm this swap to the other AC terminal and see if it still works - the two AC terminals should be identical so it should still work.

Mike.
Edit, also try the other AC terminal with the DC positive terminal in case it's some weird path through the rectifier.

 

[Reloadron]

OK, not seeing the motor or toy boat. My first guess is that this is a DC motor and any motor with two leads labeled + and - is likely a DC motor. The logic for my thinking here is if a toy boat used an AC motor to turn the propeller it would need an AC source of power which is very unlikely in a toy boat. Where would a toy boat out on water get AC power from? This is why toys having motors have DC motors so they can be battery powered.

Next, how many wires exit the motor?

Ron