Speed control using potentiometer

[dynamo135]

I want to control the speed of my 350 watt electric motor using a potentiometer. The motor draws 22 amps and 24 volts. I'm planning on using a 24 v 17.5 amp battery. What would the resistance of the potentiometer have to be?

 

[carbonzit]

You're not going to be able to control your motor directly with any kind of variable resistance.

You need to look at something in the SCR (silicon controlled rectifier) or PWM (pulse-width modulation) area.

What kind of motor: universal (AC/DC, with brushes)? induction?

 

[dynamo135]

its a DC motor, i dont know whether its with brushes or not but its a scooter motor. hope that helps

 

[Jaguarjoe]

Here ya go. Scroll down to the 350 watt, $35 unit:

https://www.electricscooterparts.com/speedcontrollers24volt.html

There are probably hundreds more like it out there. If you want to build one, Silicon Chip magazine shows how to make a really nice one. Really nice:

https://www.siliconchip.com.au/cms/A_110145/article.html

 

[aljamri]

or use 555 with a suitable replacement to the transistor to handle your current.

**broken link removed**

 

[dynamo135]

thanks so much....but im still confused on what wires i would wire to the 555 timer. like what pins connect to what. and how would i vary the speed, is there some thing i could use as a throttle? and i looked at the 555 timers, they happen to be really small, so would i have to crack it open to replace the transistor?

 

[Jaguarjoe]

You would vary the speed with "R1", the pot, short for potentiometer. Look at the upper left side of the schematic.
The transistor you could replace is labeled "Q1" on the right side of the sch. The one specified on the sch is able to handle the load of your motor so it does not have to be replaced.
You do not crack an IC open for anything unless you are trying to see what's inside of one but that's not too fruitful. Sometimes when things don't go as planned they crack open by themselves
If this is your first attempt at building electronic things, you should start with something much simpler like blinking LED's with a 555. Experimenting with power electronics can be pretty spectacular if it isn't done exactly right.

 

[carbonzit]

You do not crack an IC open for anything unless you are trying to see what's inside of one but that's not too fruitful. Sometimes when things don't go as planned they crack open by themselves

Actually, we super-technical types like to say that sometimes the "magic smoke" leaks out of ICs.

 

[dynamo135]

What would the model number of the transistor needed for this circuit? And how would i connect the transistor to the 555 timer? I took on this project for school without realizing the amount of knowledge needed to complete it and since its due a week from now theres no backing out of it now. I am trying to motorize a bicycle with an electric motor.....I have taken care of the mechanical aspect of this but the electrical part is whats giving me problems.

 

[Jaguarjoe]

You will find the part # right next to the transistor on the schematic. IRFZ something or other. You will find out how to connect it by looking at its "datasheet" which is readily accessible on the net. It will show a drawing of its package with its 3 leads, gate, source, and drain. If you look at its schematic representation it will show the gate, source, and drain schematically which will relate to your circuit schematic.

You will need a big heat sink for the transistor. It will be dissipating 8 watts of power. From the datasheet you will find the thermal characteristics, like its Θj-c, the thermal resistance between the transistor's guts and it's case, probably 1 or 2 degrees C/Watt. There are many heatsink calculators on the net.

Be sure to power the 555 with at least 9 volts and better yet 12 volts to make sure it can turn the transistor on hard. The drawing says 3-18, but it can not work at 3 volts which kinda makes me wonder about the whole circuit.

 

[carbonzit]

Given the O.P.'s obvious lack of electronics background, I see two possible courses of action here:

1. Provide clear instructions on how to put this project together. Don't make him (her?) do research, since a lot of this stuff is over their head at this point. No harm, no foul. Give schematics, part lists and complete instructions.

2. Or suggest that they start with something simpler (as was already mentioned above).

Personally, I'd like to see someone provide #1. Just because they don't understand all the principles involved doesn't mean they can't put together a working speed control. Give them a break here and don't berate them for their lack of knowledge.

 

[ronv]

Speed control

Here is a schematic for what you want, but I doubt you can get the parts in a week.
If the motor turns when you pedal you might get away with a big relay that you turn on once you are moving.

PS you can get the parts at Mouser.com

 

[carbonzit]

Ronv--you're a real trooper!

O.P.: let us know if you need help interpreting the schematic.

 

[dynamo135]

thanks for the schematic ronv. I believe I can interpret the schematic because I do have a very basic knowledge of electronics, but I found a solution to my problem by taking the trigger from an electric drill since the motor inside a drill is a dc motor anyways. There are probably a million reason why this is an unsafe idea but I used the drill trigger as a throttle and since the transistor was somehow damaged, I spliced the emitter and collector wires together and just left the ground wire open. The wires on the throttle get hot after about 10 seconds of running the motor. I was wondering if using lower gauge wire would help this problem or is the problem somewhere inside the trigger of the drill itself that can't be solved with lower gauge wire. Thanks everybody for helping

 

[carbonzit]

If the wires get hot but not the controller, then yes, you should use heavier-gauge wire. If the controller gets hot, though, you're pretty much screwed, as that means it's not able to handle your load current.

Look at the rating for the average electric drill (3/8", is that right?) to see how much power it draws. I'm guessing it's less than your motor.