# Infinite current draw for BLDC/Synchronous AC motor

I have recently created a motor that runs off of outlet power, single phase 120 V AC. My stator is taken from a universal motor from a vacuum cleaner and has two windings on either side, wired to form opposite polarities at every cycle of the AC sine wave. My rotor sits inside the stator and is made of a LEGO body with 4 neodymium magnets arranged in a N, N, S, S configuration each 90 degrees apart from each other. This allows the rotor to rotate either clockwise and counterclockwise depending on the orientation of the permanent magnets and the initial position of the sine wave when it is plugged in. When I plug it into a surge protector, I can only run the motor for around 1-2 seconds at no load before the breaker switch pops. It is worth mentioning that the universal motor from which I took the stator from originally had a brush DC type rotor inside complete with a commutator, brushes, and its own set of 16 windings. Current was being used by both the stator and rotor, but when I replaced the original rotor with my permanent magnet design all the current was being directed only to the stator, which I feel like is what is causing the massive current spike as the current is not being used by multiple components anymore. In the vacuum cleaner I'm pretty sure the current was used by different components such as valves before it got sent to the motor. Is there a way I can design a circuit to limit the current draw from the surge protector? Perhaps I can create a transformer to drop the current, or create a soft start but I do not know how. Maybe I can wire lightbulbs in parallel to act as resistors and drop the current before it gets sent to the motor. My theory is that by adding more resistive components the motor can only receive a limited amount of current and cannot "pull" more current from the power source because of the resistors in the way. However the more components I add the more current is needed for them to operate, so the effect might cancel out. What is the most efficient way?

• Sounds pretty dangerous. Be careful. Running it at a lower voltage (from a step-down transformer) might reduce the current draw. Monitor the transformer to make sure it does not over-heat. Don't leave this thing running when you are not paying attention to it. Fire is a real possibility. Jan 8, 2021 at 7:45
• You would put the lightbulbs in series, not parallel. Jan 8, 2021 at 9:29
• If it was a series wound motor, the field winding would be very low impedance and the rotor would be higher impedance. (Check with a DMM!). In that case, directly connecting the field winding to mains will pop breakers. You need to start again with a proper design.
– user16324
Jan 8, 2021 at 15:25
• Yep. the only reason I went with a BLDC design is because it was far easier to make a rotor out of permanent magnets than to create a motor that needed to have 16 different working electromagnetic windings, not to mention proper tension and placement of the brushes to get optimal drive. In fact, the whole purpose of this project is to create a generator, not a motor so I only care about it generating electricity when I turn the shaft. Jan 8, 2021 at 18:22

You need to first study basic circuit theory more carefully. You also need to understand more about motors.

The rotor of the motor connected in series with the stator is what controlled the current as the motor was used originally. The rotor had resistance, but more importantly, the rotor generated a voltage called "back electromotive force (EMF). that opposed the current.

120 volts is way too much voltage to apply to just the stator. You could connect resistance in series to reduce the current, but that will generate quite a bit of heat and waste energy. It would be better reduce the voltage with a transformer. You need an AC ammeter to measure the current with whatever you can find to connect in series temporarily. You could start with incandescent lightbulbs, but just one will probably limit the current too much. You probably need several lightbulbs connected in a parallel group with the group connected in series with the motor.

Also, with your 4-pole rotor, only two poles will be effectively engaged with the two-pole stator.

If I wanted to do something like this, I would use the stator of a shaded-pole fan motor with a 2-pole rotor. A stator like that would not need anything to reduce the current.

• Yea I wanted to use a 120 VAC to 12 VAC transformer to drop the voltage but I'm worried about the multiplied current coming out of the secondary. I know that current kills so that is why I'm reluctant to use one, unless there is a way I can safely insulate the lines. In the meanwhile I will definitely try the series configuration you mentioned and buy a multimeter to measure the fluctuations in current and voltage. Jan 8, 2021 at 15:57
• 12vac won’t cause enough current to flow through your body! Once the voltage is above 60VAC is when you need to concern yourself with electrocution. Jan 8, 2021 at 16:06
• okay thanks for the clarification Jan 8, 2021 at 16:18
• Most multimeters can not measure high enough AC current for what you described. You may want to consider a clamp-on ammeter,
– user80875
Jan 8, 2021 at 17:40
• thanks for the recommendation Jan 8, 2021 at 20:08