Amperage Voltage and Solid State Motor Control

[Mechd07]

Hi,

Please bear with my ignorance of electronics. I realize the answers to my questions may be obvious, just not to me LOL.

I'm trying to reduce the voltage or amperage of a power supply to under 10V or .02 amps from its original 24 volts @ .75 amps. I experimented using two .22 ohm resistors to see if there would be any drop in amperage or volts but saw no change. Can someone give me formulas to reduce the amperage or volts?

Also can a solid state power relay be used as an AC motor speed control (

http://www.futurlec.com/Relays/SSR80A.shtml

) if given PWM on it's input side? I was entertaining using this CVRT (

http://www.futurlec.com/Relays/SSR80A.shtml

) to accomplish this.

Any help would me much appreciated.

 

[waross]

Hello Mechd07
re: SSR70A - SPST 3-32VDC 70A Solid State Relay
The following parameters will prevent this device from working with a PWM input for motor control.
Max turn-on time zero-cross Turn-on: 1/2 cycle+1ms
Maximum Turn-off Time: 1/2 cycle+1ms
This is a "Zero Crossing" device and will only switch on or off when the voltage is crossing the zero point.
It may possibly be used with PWM for heating control.
The PWM frequency for motor control is much more than 60 Hz.
The PWM frequency for heating control is less than 60 Hz. and can be configured as "X" number of cycles on, and "Y" number of cycles off.
yours

 

[waross]

Actually for heating, the control scheme is not actually PWM, but is duty cycle control. At a given setting, there will be only one pulse width. The ratio of the "On" time in cycles to the "Off" time in cycles determines the amount of heating. This type of control is not suitable for motors.
Sorry for any confusion.
yours

 

[jraef]

The answer is still going to be no, but there is some ambiguity in your question. When you say "input side" do you mean the control input or the power input? For instance if you were thinking that you would provide a PWM signal to the 3-32VDC control input side in order to rapidly fire the SSR to try to produce a lower RMS voltage, the SSR cannot react fast enough for that to do any good anyway. If you were thinking of feeding this relay with line power from a PWM derived source, the steep voltage rise of the PWM would ultimately damage the SCR in the SSR.

You also have your terms mixed up in your original statement. You say you want to reduce the amerage to under 10V or .02A from 12V or .75A. Volts and amps, although related, are not the same. If your motor is drawing .75A at 12V, reducing the voltage is going to INCREASE the amperage. Motor current draw is generally load dependant and relates to power, which is formulaic of voltage and amperage.

Please describe your issue in better detail. What do you have, what are you trying to accomplish.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Mechd07]

Sorry for the confusion. I wanted to reduce the voltage or amperage. I knew that amperage and voltage are related but two different things. From what I understood the relay controller can work with either, so I would reduce one. I was looking for an easier and less expensive way to control a powerful motor using a low amout of voltage

I would like to use variable voltage (0-24v) to control a R/C hobby type motor controller that needs pulses between 1-2 ms. I had an idea to use a ‘dc to pulse width modulator

http://www.vellemanusa.com/Downloads/0/illustrated/illustrated_assembly_manual_k8004.pdf

and bypass or eliminate the motor control circuit which is of no use to me because it wouldn't be able to handle a 340 amp motor.

 

[Mechd07]

Thank you for the information waross. It was helpful.

 

[jraef]

I guess I still don't get it. You have a 340A "hobby" motor? What kind of hobby is this?

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Mechd07]

jraef

'Hobby' applies to the radio control model hobby, airplanes, cars, helicopters ect.

The motor I don't have yet, but it's rated at 340 amps. One of the components is a radio controlled (R/C) speed controller. This particular one is used in robotics competitions like Battle Bots used in robotics that needs PWM signals to work. I'm trying to use variable voltage (up to 24v) to control but not create PWM signals of 1-2 ms needed to activate a servo or in my case a speed controller.

I may have to try this method since I learned the solid-state relay and its controller that I mentioned in my first post won't work with a motor.