OT - Capacitor Needed Power Supply running Motor?

sbaum

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• Nov 20, 2014

• #1

No PLC's involved here but since this is where I frequent I'll give it a try.

The task...
Need to test run 1/2HP 12VDC 39FLA Motors and we want to create a power supply for the purpose.
The idea is to use a 120V to 12V transformer and a full wave bridge rectifier.

Do I need filtering capacitors to eliminate the ripple or will the motor run fine without? This will only be used for testing a motor for 5 min or so.

 

bernie_carlton

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• Nov 20, 2014

• #2

The rectified DC voltage will be almost 17 VDC peak (12 * 1.414). Check if that will cause problems.

 

dlweber

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• Nov 20, 2014

• #3

If I remember right RMS and the dc you get out of a rectifier (average) are not the same RMS uses sqroot 2 (1.414), dc uses p-pvac/pi for full wave rectification

EDIT: On the filtering it tends to depend on how sensitive the motor is, the motor may have ripple tolerance specs associated with it. I have seen it before as a percentage I believe

 

Last edited: Nov 20, 2014

James Mcquade

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• Nov 20, 2014

• #4

a full wave bridge vill not give you 12 volts dc. it will be .646*12= 7.75. its been 20+ years since i did electronics so my memory is a little fuzzy.

regards,
james

 

Eric Nelson

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• Nov 20, 2014

• #5

This PDF should be helpful.

http://www.hammondmfg.com/pdf/5c007.pdf



-Eric

 

dlweber

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• Nov 21, 2014

• #6

The .9 that the .pdf says makes sense, 2*sqroot2 is about .9 pi. That is a much easier way to figure it out, since no components are "ideal" the error is probably negligible.

 

jrwb4gbm

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• Nov 21, 2014

• #7

sbaum said:

No PLC's involved here but since this is where I frequent I'll give it a try.

The task...
Need to test run 1/2HP 12VDC 39FLA Motors and we want to create a power supply for the purpose.
The idea is to use a 120V to 12V transformer and a full wave bridge rectifier.

Do I need filtering capacitors to eliminate the ripple or will the motor run fine without? This will only be used for testing a motor for 5 min or so.

Click to expand...

You won't need any filtering capacitors. We ran machines (with more than 140 motors ranging from 1/2 to 2 hp) on DC SCR phase control power supplies for many years without any filtering or problems. The RMS (average) DC voltage from the unfiltered full wave bridge will be approx. .9 of the supply voltage.. Here's a suggestion. If you are just making a test for 5 minutes or so, get a 12 volt car battery and jumper cables to power the motor.

 

kallileo

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• Nov 21, 2014

• #8

I would also suggest using a 12V car battery for the test. There is no need to complicate things for a 5 minutes test.
Just be carefull because you will have a voltage drop on the battery when the motor will be on. You can have a charger always connected to the battery.