Re: [CAD_CAM_EDM_DRO] Re: LARGE DC MOTOR speed control question

STAN MCDONALD wrote:

> Someone stated putting a cap across the output of the rectifiers to
> increase the voltage closer to the 180 that the motor needs
> because when fed with 120 volts it comes to 130 rectified. Since my
> motor is a 3 hp dc 180 volt arm and 90/180 field and i have 180
> arm and 180 field, and the motor draws 17 amps (name plate). What
> value of cap do I need and can I out the cap on the input of the
> rectifiers instead.

Roughly 140,000 uF will give about 1 Volt ripple. You could probably
tolerate
several times that, so probably as little as 5000 uF will work fine.
If you put the capacitors on the input, you will blow breakers or the
caps.
They will try to remove all AC from the line, thereby drawing huge
current.

>
> When I measured 240 AC rectified I got a reading of 280 and clearly
> this is way more then the 180 that the motor asks for and the
> motor will run but she gets warm enough you can fry eggs on it.

Right, you need to rectify 120 V AC to get approx. 170 V DC. Rectifying
240 V AC
will give about 340 V DC. With the large current draw, you might use a
240/120
step-down transformer before the rectifier.

Jon