12V DC vs 12V AC Transformer for Homemade Electric Motor Kit Drawing 4A at 200-300 RPM

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#1 21674063 23 May 2015 10:55

Glen Boehm Glen Boehm

Anonymous

Post #1
21674063 23 May 2015 10:55

I'm a mechanical engineer and I'm building an electric motor kit for kids to learn about electrical/mechanical design. I've made the parts for the motor but my electrical knowledge is low. I want to use a 12V transformer to power the motor. Does it make a difference if I use a 12V DC or 12V AC transformer? The motor runs at about 200 to 300 RPM and draws about 4 amps. I was using a 12V DC battery from my garage opener to test the motor. I then tried using a 12V AC transformer for a halogen light fixture and it wouldn't run the motor. Does anyone have some ideas for an inexpensive power source that would best suit this application? Thanks for your help.
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#2 21674064 11 Jun 2015 02:13

Benjamin Michael Tecson Benjamin Michael Te...

Anonymous

Post #2
21674064 11 Jun 2015 02:13

It seems like your design is a DC motor. Therefore, using an AC power supply will not create a single unidirectional magnetic field. The forces that are suppose to make the motor turn are cancelled out due to the Alternating Current.

I suggest you buy a Universal Power Supply which has a 12V DC output.
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#3 21674065 17 Jun 2015 12:45

Chuck Sydlo Chuck Sydlo

Anonymous

Post #3
21674065 17 Jun 2015 12:45

If it is a brush type motor it will run on AC or DC. I presume it is a brushless type as it isn't working on AC. Just look for a 12VDC "wall wart" type of transformer that hangs on the receptacle. I have a couple in my junk box. You should be able to get one for a phone or radio, just make sure it says DC. Some of the ones for LEDs don't rectify to DC so avoid those.
#4 21674066 17 Jun 2015 15:07

Glen Boehm Glen Boehm

Anonymous

Post #4
21674066 17 Jun 2015 15:07

Thanks for the information. Would it make a difference using AC or DC power source if I used straight copper wire or carbon brushes? Also, many of the inexpensive 12VDC power supplies like the wall wart are only rated below 2 amps. This motor draws 4 amps. When I look for other power DC power sources, it then steps up to a bigger box unit costing over $100. I can't find a cheap power source that supplies the higher amperage.
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#5 21674067 18 Jun 2015 09:24

Chuck Sydlo Chuck Sydlo

Anonymous

Post #5
21674067 18 Jun 2015 09:24

Look for one that has the transformer in the middle of the wire, like this example: http://www.lightinthebox.com/60w-12v-5a-ac-po...nce-security-camera-ac-110-220v_p1774720.html

They tend to be a higher output. I have a spare one from and old printer
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#6 21674068 18 Jun 2015 19:26

richard gabric richard gabric

Anonymous

Post #6
21674068 18 Jun 2015 19:26

It might draw that current when running. What is the current when the rotor is locked, at startup there is no back enf, and the current requirement is very much larger. You have to make sure the power supply doesn't sag too much when starting. Brush type won't make a difference, aside from the contact area, on a motor like this,
cheers,
Richard
#7 21674069 19 Jun 2015 07:43

Glen Boehm Glen Boehm

Anonymous

Post #7
21674069 19 Jun 2015 07:43

Thanks Richard. I've been using a 12v DC battery from my garage door opener. I
assume the start up current required is higher than 4 amps. I'm trying to find an inexpensive power supply that will run the motor. Batteries need to always be recharged so i was looking for a 110V plugin source. What about a power source like this:
http://www.alibaba.com/product-detail/Constant-Voltage-Single-Output-12V-10A_1496625416.html
#8 21674070 19 Jun 2015 12:40

richard gabric richard gabric

Anonymous

Post #8
21674070 19 Jun 2015 12:40

Firstly, measure the current with the motor stalled. That will tell you what the peak current is. A good SMPS power supply will have fast acting over current protection, which will normally allow automatic startup once the offending load is removed. So you need to sort out a power supply with that in mind.
It could be that you could pick up a cheap transformer with a suitable rating, put a rectifier on the output, and try that, a grunty old battery charger with a suitable rating might do it, that was all they normally contained. Good quality ones will have a charger controller, but that could be bypassed, in either case, you might find a cheap second hand one locally, just a thought,
cheers,
Richard
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Topic summary

The discussion addresses the suitability of 12V DC versus 12V AC transformers for powering a homemade electric motor kit drawing approximately 4 amps at 200-300 RPM. The motor is likely a DC motor, as AC power fails to produce a unidirectional magnetic field necessary for rotation, causing the motor not to run on 12V AC transformers. Brush-type motors may run on AC or DC, but brushless motors require DC power. Common 12V DC "wall wart" power supplies often provide less than 2 amps, insufficient for this motor's 4-amp draw. Higher current DC power supplies, such as those with integrated transformers mid-cable or repurposed printer power supplies, are recommended. Startup (stall) current can exceed running current, so power supplies must handle peak loads without voltage sag. Suggestions include measuring stall current, using switched-mode power supplies (SMPS) with overcurrent protection, or employing a rectified AC transformer or a secondhand battery charger with appropriate current ratings. An example of a 12V 5A AC power supply suitable for LED lighting and security cameras was referenced, as well as a 12V 10A constant voltage power supply from Alibaba for higher current needs.
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12V DC vs 12V AC Transformer for Homemade Electric Motor Kit Drawing 4A at 200-300 RPM

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Topic summary