rheostat for electric motor

[bunghole]

hi guys,

i am trying to find a rheostat to control a 12vdc 80w fan motor.
i have measured the current withdrawal using an analog panel meter. i found this information, with fan blades attached:

startup (not rotating to max): 0 - 10amps, dropping to about 5 - 7 amps after approx. 2 seconds

running current: between 5 and 7amps

maximum power applied, holding rotor still: around 15amps

i have tried a 20amp rheostat from a cordless drill, although the controller feels like a sliding variable resistor, the output tends to use 'steps' of output power, similar to an emu.
i am running this fan off a 12vdc, 18Ah SLA.

if anyone has any ideas, please reply, or any advice for that matter. every little bit helps!

ta

 

[Leftyretro]

hi guys,

i am trying to find a rheostat to control a 12vdc 80w fan motor.
i have measured the current withdrawal using an analog panel meter. i found this information, with fan blades attached:

startup (not rotating to max): 0 - 10amps, dropping to about 5 - 7 amps after approx. 2 seconds

running current: between 5 and 7amps

maximum power applied, holding rotor still: around 15amps

i have tried a 20amp rheostat from a cordless drill, although the controller feels like a sliding variable resistor, the output tends to use 'steps' of output power, similar to an emu.
i am running this fan off a 12vdc, 18Ah SLA.

if anyone has any ideas, please reply, or any advice for that matter. every little bit helps!

ta

Well a more modern approach would utilize a PWM speed control to a power mosfet switching transistor. Much more efficient and better control.

Lefty

 

[bunghole]

i'm familiar with those things, but i still would need the circuit. i'll try google and see if i can find anything. i would like to have regenerative braking as well. (or if it can put power back into the battery) it needs to be 12vdc capable of at least 20 amps or more if i find, but does anyone here have a diagram i can use?

ta

 

[Hero999]

What are you talking about?

There's lots of stuff on Google.
https://www.google.com/search?hl=en...result&cd=1&q=motor+speed+control+PWM&spell=1

 

[mramos1]

Look at the third link down G Forest Cook. Pretty basic parts to aquire.

If you are in the States, most Radio Shacks have most of the parts, probably not the FET. You can hit mouser.com and order with priority mail and have it all pretty quick.

 

[bunghole]

one of them http://solorb.com/elect/pwm/pwm1/ on there says to use a IRFZ34N FET for 10A , but are there any FET's that go up to 20A or morethat i can use in this circuit? can i use a SSP60N06 that rates at 60A?

 

[dknguyen]

**broken link removed**

You can use any FET you want as long as the gate drive voltage is high enough and the gate drive can provide enough of a current pulse to turn the gate on and off fast enough.

Don't forget a diode in anti-parallel with the motor to protect the MOSFET from inductive current spikes produced when the transistor interrupts current flow to the motor.

If the gate drive requirements are not met directly by the 555 timer, then stick a driver IC in right before the MOSFET (and after the 10K resistor and 555 timer), like an LM5112.

 

[bunghole]

alright, i'll do it, but will a SSP60N06 work?

 

[Hero999]

Why don't you look up the datasheet using Google and check that the current and voltage ratings meet or exceed your requirements?

 

[Boncuk]

You can parallel as many IRFZ34 as you require. I wouldn't go higher than about 7A per MosFet of that type and mount them on a good heat sink too.

They can easily be controlled by an NE555 for PWM.

Boncuk

 

[Hero999]

Go for the IRF540, it's cheap reliable and well tried and tested.

 

[bunghole]

i found this in another thread, and this would be ideal. yet, i still have questions:

is there anything else i can use to replace q1?
does q2 have any specific value, or what kind it needs to be or a part number?
can i use a 60amp mosfet for q3?

 

[audioguru]

The opamp produced PWM is not very fast so the driver Mosfets are not required.

The driver Mosfets would both be conducting over 1A through themselves when they are halfway switched.

Use the opamp output in series with 22 ohms to the gate of the power Mosfet.

 

[Optikon]

i found this in another thread, and this would be ideal. yet, i still have questions:

is there anything else i can use to replace q1?
does q2 have any specific value, or what kind it needs to be or a part number?
can i use a 60amp mosfet for q3?

Why bother with S2? are you ever going to open circuit the inverting terminal of that opamp? The circuit will behave erratic if you do... (read as: dont do it).. if you want to shut off the system, that's not the place to do it..

 

[bunghole]

could i just drive q3 directly from the output of u4 via r3, or can someone please make a suggestion for q2? i am going to omit s2 (originally added for seperate pot to control speed)

here is the url of the original circuit:
https://www.electro-tech-online.com/threads/pwm-controller.21834/.

and there is a jpeg of the circuit, with my notes added in (scan of printout)

 

[audioguru]

could i just drive q3 directly from the output of u4 via r3?

That is what I said. Then R3 should be reduced to 22 ohms.
Q1 and Q2 were shorting the supply when they switched.

 

[bunghole]

could four MJ2955'S be linked together to equal one 60amp MOSFET?

 

[audioguru]

could four MJ2955'S be linked together to equal one 60amp MOSFET?

An MJ2955 (and a 2N3055) needs a base current of up to 3.3A for it to switch 10A with a voltage loss of up to 3V. That is a lot of loss.
Use a Mosfet instead.

 

[bunghole]

what would be the voltage of the output of u4, cause i was thinking of an SSP60N06,

 

[audioguru]

Your driver Mosfets fight each other when they switch because they both conduct a few Amps through each other.
The frequency is very low so the fighting Mosfets can be replaced by complimentary transistors that provide plenty of current without fighting.

I have never seen Korean Mosfets before. Do they work reliably?

Why is your schematic a fuzzy JPG file type that is the size of my neighbourhood??? A GIF or PNG file type is extremely clear.