Rectify high current low voltage, or low current high voltage via transformer? - Page 1
Forum section: electronics / projects

author: sonnytiger
author_rank: Frequent Contributor
timestamp: August 13, 2013, 10:45:33 pm
content: I got a brushless DC motor from a newer car which powers the steering and figured it could work well as an alternator. I plan on powering an electric motor from it, either a 120V universal motor or a 12V motor. Would I be better off rectifying a large current directly from the alternator or rectifying a smaller current after it has gone through a transformer? Keep in mind the transformer will be operating at a much higher frequency than normal (up to 200Hz) but this doesn't seem like it will be much of a problem to me as a transformer of a given size can handle more power at a higher frequency. Any ideas?Here is a video i made on the project, watch it if things aren't perfectly clear, or just watch it so I can get some exposure for my videoshttp://youtu.be/4VQkAor6P4Q

author: ejeffrey
author_rank: Super Contributor
timestamp: August 14, 2013, 12:05:40 am
content: It is easier to rectify high voltage / low current, at least up to a point. You lose less power to diode drop, so you need less hefty diodes and heat sinks, and you don't have to worry about active rectifiers.The voltage, current, and frequency you are rectifying will determine whether you use SCRs, ordinary silicon diodes, fast recovery diodes, shottky diodes, or shottky diodes plus MOSFETs for active rectification.

author: IanB
author_rank: Super Contributor
timestamp: August 14, 2013, 01:01:40 am
content: Quote from: sonnytiger on August 13, 2013, 10:45:33 pmI presume your video makes clear what seems odd from your first sentence, since on the face of it, "direct current motor" seems incompatible with "alternating current generator".Do I guess that you are bypassing the electronic circuitry in the motor and extracting polyphase AC directly from the motor coils?

author: Kremmen
author_rank: Super Contributor
timestamp: August 14, 2013, 05:23:20 am
content: Let's keep in mind that "brushless DC" is a contradiction in terms. All motors of that family are permanent magnet AC motors, where the term BLDC is usually reserved for those species where the excitation voltage is optimized trapezoidal by virtue of the magnetic path geometry. Thus they are suitable for driving with a DC supplied bridge configuration.As this is an automotive component the supply voltage of the motor is most likely battery voltage directly, i.e. no more than 14+ V. In that context the low output voltage in the video makes sense. If the drill rotates some 1500 RPM as OP states, then the motor nominal voltage could be around 6000 rpm which would make sense. A single pole stator coil fed with 100 Hz would do that.The fact that this is a BLDC motor may be one reason for the significant heating the OP notes on the video. Transformers are optimal at sinusoidal excitation but this motor, used as a generator, will output more like a square wave. There will be a lot of odd harmonic overtones in the output wave, i.e. 3rd, 5th, 7th etc etc. Those will definitely induce hysteresis losses in the iron and even some additional copper losses.Also, if i saw correctly from the video one of the output legs is unconnected. This makes sense seeing how the transformer is single phase. Nevertheless, it will make the coil loading asymmetric thus degrading the output of the generator. A 3 phase generator needs a 3 phase transformer to work optimally.The need for a transformer is mostly dictated by the voltage requirement of the load and the distance to transmit the power. A 12 V universal motor right next to the generator may just need a rectifier bridge and a bit of heavy wiring, whereas a 120V 3 phase induction motor is definitely a candidate for transforming.But please bear in mind that in the latter case the polyphase AC produced by the generator has the original frequency and is in all likelihood trapezoidal in waveform. Thus not a good candidate for supplying a regular induction motor.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: August 14, 2013, 03:10:26 pm
content: Quote from: IanB on August 14, 2013, 01:01:40 amYes this exactly, the common terminology doesn't make sense but it just seems like what is most used.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: August 14, 2013, 03:23:57 pm
content: Quote from: Kremmen on August 14, 2013, 05:23:20 amI realize the terminology and understand what the motor really is for the most part, looking at the waveform on my scope it seems quite close to a sine wave but is a little off, it changes slope at a point on the rising and falling edge, but is otherwise close. The heating I was noting in the video was only in the thick wire from the motor, the core and output windings were all cold even after a minute of short circuit operation. It was no more than 14V, right off the battery as you said. The motor runs quite rough, sounds like a hammer drill when one phase is loaded down like you said, maybe causing the coils to enter at a higher speed then when they leave? I was planning on rectifying the transformed power using a three phase full wave rectifier, definitely not planning on running an induction motor, though that is kind of a cool idea, though from what you say it won't work very well. Did you watch the whole video? Sorry for the garbled speak.Thanks!

author: Kremmen
author_rank: Super Contributor
timestamp: August 14, 2013, 05:51:03 pm
content: OK fair enough, it is perfectly possible that the voltage waveforms are close to sinusoidal. That's good news for a transformer.You say the motor runs rough when the load is unbalanced. Is it smooth when unloaded (excluding a mechanical problem here) or when evenly loaded with say 3 resistors in a star or delta connection? The strong cogging you mention on the video is an indication that the magnetic paths are not strictly sinusoidal since in a 3 phase true sinusoid the cogging torques should cancel almost completely. The apparent roughness can be caused just by that when the load is unbalanced.3 phase rectifier sounds like a good idea. I noticed only after posting the previous that you had actually not mentioned an induction motor at all, that is just me reading carelessly.Yes i did watch the whole video.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: August 14, 2013, 06:59:04 pm
content: ok cool, it just sort of sounded like you didn't all good. it runs nice and smooth if its an even load and when it is unloaded yes, I made another video that I am uploading now where I look at the waveforms of the motor and the transformer output. My scope wouldn't see my stick for some reason.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: August 14, 2013, 09:04:13 pm
content: http://youtu.be/ZorE9rp6Rc4The video