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90V DC Motor Controller

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Gerald Schwartz

New Member
Hello
I am in the process of building a VERY simple DC motor controller for a 1/3hp, 4a, 90 volt dc motor...I will be using a variable AC switch on the input side, a 5010 full-wave rectifying diode, and would like to add a smoothing capacitor in the output curcuit. My lack of understanding arises in computing the uf rating of the smoothing capacitor. In my searching I have found various formulae which indicate that, at 120V ac input voltage at 60 cycles, with a four amp current draw of the motor, the uf requirement would be in the neighborhood of 2778uf.... My use of capacitors has been limited to repairing motors and building various static and rotary 3ph converters. I have never seen a run-type capacitor anywhere near the rating required in the formulae...I assume that an oil-filled capacitor would be required. What am I missing? In this simple circuit, what capacitor (uf and voltage) would suffice. Jerry
 

4pyros

Well-Known Member
Most Helpful Member
I have never seen a run-type capacitor anywhere near the rating required in the formulae
No you will not. A run are start cap for an AC motor. Is nothing like a smoothing capacitor in a power supply witch is what you are making.
 

Gerald Schwartz

New Member
smoothing capacitor

Thank you for your response...how would I describe the required capacitor in order to acquire one... eg; 3000uf at 200v? Are the required capacitors readily available in hobby shops or on Ebay? Thank you in advance. Jerry
 

alec_t

Well-Known Member
Most Helpful Member
I will be using a variable AC switch on the input side
Exactly what are you varying? Are you trying to control (ultimately) motor speed?
 

Gerald Schwartz

New Member
dc motor speed controller

Again, thank you for the replys. My ultimate purpose is to build a crude, inexpensive dc motor controller to run one of several 90V dc motors that I have. The motors will be used on metal cutting machinery to either run a spindle or provide for a variable speed rate for table feeds. These are analog machines...no controllers, etc. My level of expertise in matters electronic is just high enough to keep me from sticking a fork into a light socket, hence my obviously amateurish questions. I will begin searching for an aluminum electrolytic capacitor (or several) in the 3000 µf at 200v range. Jerry


"If it ain't broke...fix it until it is...."
 

4pyros

Well-Known Member
Most Helpful Member
I would just buy some. Thay can be had for like $50 to $100. Its not a one off if you need more than one. No sense messing around.
 

Diver300

Well-Known Member
Most Helpful Member
You don't need it to be smoothed. The power produced by a single phase AC motor varies a over the mains cycle, and will probably be negative at times. The inertia of the rotor keeps it going.

You still have the rotor which holds more energy than any capacitor you would want to use.
 

Gerald Schwartz

New Member
aluminum electrolytic capacitors

Thanks for the help and your efforts in responding. I found numerous samples of the type and capacitance on Ebay selling from $10.00 US. Jerry
 
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