Mosfet & Motors

[Bizzy]

Good afternoon
I am working on making a pulse motor with a flywheel. I want to use a mosfet as the component to pulse the power into the motor.
When I attach my multimeter to the circuit I show I am getting 12.5 volts when i trigger the mosfet, which is enough to run the 12 volt motor.
However the motor doesnt turn.
When I put The motor directly to the battery it runs so I know that those two are working as well.
I am speculating that maybe the mosfet isn't allowing enough amperage. Or perhaps it is not staying on for a long enough time.
Any help anyone could give me would be a great help.
thanks
Bizzy

[citfta]

Need to see the circuit

Hi Bizzy ,

To the forum

If you can post a schematic of your circuit someone may be able to help you. Otherwise we can only guess what your problem is. I have a circuit I designed that is pulsing a motor using a Mosfet. It is not hard to do.

Carroll

[Bizzy]

Hi Bizzy ,

To the forum

If you can post a schematic of your circuit someone may be able to help you. Otherwise we can only guess what your problem is. I have a circuit I designed that is pulsing a motor using a Mosfet. It is not hard to do.

Carroll

hI Carroll
Thanks for welcoming me.
I am at work and dont have the specific numbers for each component but can get that for you if you need it.
The attached diagram shows basically what I am working with.
The coils on the left are in a three phase configuration. the idea is that when magnets pass over the coils it produces a charge which feeds into the three phase rectifiers. The capacitor after the diodes of the rectifier stabilizes the voltage.
Everything else looks like a normal mosfet circuit.
And when I place my multi meter where the motor is I read 12.50 volts. So I know that the mosfet is opening to allow voltage from the battery to flow.
However like I said the motor simply doesnt turn.
I am wondering if the mosfet is not open long enough or it is choking off the current, or am i missing something.
Any help you can give would be greatly appreciated.
thanks
Bizzy

[citfta]

Hi again,

If you have one lead of your meter connected to one side of the motor and the other lead of the meter connected to the other side of the motor and you are getting 12.5 volts then there must be a bad connection somewhere on the motor itself. If the mosfet was not turned on all the way then you would not get the full 12 volts to the motor because limiting the current would cause the voltage to drop. If you have a small 12 volt light bulb like a tail light or similar bulb, connect it in place of the motor and see if you can get it to turn on.

Did you simplify the circuit you posted because I don't see anything in the circuit to pulse the mosfet? Once the cap is charged the mosfet will just stay on.

When you get home from work try checking everything again and try the bulb idea if you have one and post what you find and I will try to help again. I am in the Eastern time zone but I am usually up rather late so I will look for your posts later.

Good luck, Carroll


PS Can you repost your circuit showing exactly where you had your meter leads? I have worked in electronics for over 50 years and I have still put my leads in the wrong place sometimes when I wanted to check something.

[Bizzy]

Hi again,

If you have one lead of your meter connected to one side of the motor and the other lead of the meter connected to the other side of the motor and you are getting 12.5 volts then there must be a bad connection somewhere on the motor itself. If the mosfet was not turned on all the way then you would not get the full 12 volts to the motor because limiting the current would cause the voltage to drop. If you have a small 12 volt light bulb like a tail light or similar bulb, connect it in place of the motor and see if you can get it to turn on.

Did you simplify the circuit you posted because I don't see anything in the circuit to pulse the mosfet? Once the cap is charged the mosfet will just stay on.

When you get home from work try checking everything again and try the bulb idea if you have one and post what you find and I will try to help again. I am in the Eastern time zone but I am usually up rather late so I will look for your posts later.

Good luck, Carroll


PS Can you repost your circuit showing exactly where you had your meter leads? I have worked in electronics for over 50 years and I have still put my leads in the wrong place sometimes when I wanted to check something.

Hi Caroll
I simply didnt descibe myself clear enough, which I do often.

When the motor wouldnt turn I replaced the motor with the multi meter. So basically in my diagram replaced the motor with the multi meter.
The timing mechanism are magnets at the end of the flywheel that pass over the coils. Because there is only two sets of magnets on the fly wheel the motor should only pulse twice during a revolution. Let me know what you think.
I will however try the bulb idea tonight.
Thanks
Bizzy

[citfta]

Now I understand

Ok Bizzy now I understand. You need to test the circuit with both the meter and the motor connected at the same time. Then you can tell what is really going on. A meter will only need a tiny fraction of an amp to show voltage. A motor may need several amps to turn if it is a medium size motor or is driving a load.

Do you mean the six coils are going to be energized by two magnets? And the voltage is going to turn on the mosfet? I guess I need to see a picture of the coil and magnet arrangement to understand how that will work. I would think a single set of two coils and two magnets would work just as well, but I probably don't fully understand yet what you are doing. If you don't get tired of all my questions we'll get it working yet.

[Bizzy]

Ok Bizzy now I understand. You need to test the circuit with both the meter and the motor connected at the same time. Then you can tell what is really going on. A meter will only need a tiny fraction of an amp to show voltage. A motor may need several amps to turn if it is a medium size motor or is driving a load.

Do you mean the six coils are going to be energized by two magnets? And the voltage is going to turn on the mosfet? I guess I need to see a picture of the coil and magnet arrangement to understand how that will work. I would think a single set of two coils and two magnets would work just as well, but I probably don't fully understand yet what you are doing. If you don't get tired of all my questions we'll get it working yet.

Hi Carroll
Good evening
My goal is to develop a trigger for a pulse motor which is not so fragile as a reed switch and last longer.
As far as the coil numbers...I thought of that too while I was driving home tonight. I went from six coils to four coils. In a backward coil configuration( I cant scan at home so I will from work on Tuesday)
Basically instead of going from a half phase to another half phase with the magnets, you go from a whole phase to a whole phase, which are in a series. If you kept it in a two phase you would get not enough voltage. The only differance from your disciption is i use five magnets for four coils. In my previous alternator experiment I learned that if the number of coils is even the number of magnets must be 1 number higher.
Anyway I got my motor to start !!!
But It takes to long to power down. It ran for almosy 10 seconds. A revolution would be about 2 secounds. So if I could trouble you again... Once you start the pulse motor, How can you make sure the motor shuts off right away and does keep running and over heat the mosfet.
Thanks again for letting me bounce ideas off of you.
Bizzy

[citfta]

Congratulations!

OK, very glad you got it to run. To try and answer your question will involve a little bit of learning about mosfet circuits. A mosfet is a voltage controlled device as opposed to a regular bipolar transistor which is a current controlled device. What that means in your application is that as soon as your cap starts to build a charge the mosfet is going to start turning on. But it will only be partially turned on until your cap reaches a high enough level to turn it all the way on. This means it will not be letting all the current through at first and will get warm and also not let the motor accelerate to full speed until the cap is charged to a high enough level. And when you turn it off it will not go all the way off until the cap is almost fully discharged. The way around all of these problems is to use a mosfet driver chip. The ones I like to use are the FOD3180 chips. These are like opto isolators that have special circuitry built into them that make sure the mosfet is fully on or fully off. The nice thing about using these is the input current to turn them on is very small. You are only turning on an LED inside the driver chip. So you can use a reed switch, an optical switch or a hall switch to control the driver chip. The other advantage is there is complete isolation between the control circuitry and the power circuitry. DoubleD who used to post on this forum some told me about them about a year ago and they have made my circuits much easier to build and control. If I have time tomorrow I will try to draw up the motor controller circuit I am using and post it for you. I am controlling my motor using a Picaxe programmable controller to put pulses to the mosfet driver chip. This way I can control exactly what kind of pulses I am sending to the motor. I hope this info helps some.

What did you find that was causing your motor to not work before?

See ya, Carroll

[Bizzy]

OK, very glad you got it to run. To try and answer your question will involve a little bit of learning about mosfet circuits. A mosfet is a voltage controlled device as opposed to a regular bipolar transistor which is a current controlled device. What that means in your application is that as soon as your cap starts to build a charge the mosfet is going to start turning on. But it will only be partially turned on until your cap reaches a high enough level to turn it all the way on. This means it will not be letting all the current through at first and will get warm and also not let the motor accelerate to full speed until the cap is charged to a high enough level. And when you turn it off it will not go all the way off until the cap is almost fully discharged. The way around all of these problems is to use a mosfet driver chip. The ones I like to use are the FOD3180 chips. These are like opto isolators that have special circuitry built into them that make sure the mosfet is fully on or fully off. The nice thing about using these is the input current to turn them on is very small. You are only turning on an LED inside the driver chip. So you can use a reed switch, an optical switch or a hall switch to control the driver chip. The other advantage is there is complete isolation between the control circuitry and the power circuitry. DoubleD who used to post on this forum some told me about them about a year ago and they have made my circuits much easier to build and control. If I have time tomorrow I will try to draw up the motor controller circuit I am using and post it for you. I am controlling my motor using a Picaxe programmable controller to put pulses to the mosfet driver chip. This way I can control exactly what kind of pulses I am sending to the motor. I hope this info helps some.

What did you find that was causing your motor to not work before?

See ya, Carroll

Hi Carroll,
I was putting too much voltage into the gate from my trigger coils. Once I turned the three phase into a two phase it was only producing 2.45 volts. I knew my mosfet had a thresh hold voltage of 2 volts, but I didnt realize until last night it also has a maximum voltage of 4 which means when I was throwing 6 volts at it it was not turning on. But when I was only using 2.45 it was...
I will be very interesting is seeing your circuit and trying that out.
I am also wondering if the capacitor coming off the rectifier diodes is too big. I am thinking that it may be feeding too much power to the gate. I may be able to switch that with a smaller one so that power to the gate drops off sooner.
Thanks again
Bizzy

[Bizzy]

Trigger coils

Hi Carroll
I tried to down load pictures of the trigger coils but our home isp didnt want to cooperate. I will try again later when I get home.
In the mean time I am attaching a diagram of the trigger coils.
The standard 3 phase is what I used in my original alternators. Although If this works right I will use a five phase coil. Any way this was sending too much voltage to the mosfet and wouldnt open the gate.
So I went to the 2 full phase.
In the diagram the magnets move over the coils from left to right. In the 3 phase coils the magnets develop a charge in the "secondary" coils and follows the current into the "primary" coils and the AC power then flows into the rectifiers.
In the 2 full phase coils the magents flow left to right over the first set of coils producing a charge which flows into the rectifiers. the magnet then moves over the second coil set and sends an additional charge to the rectifier. This gives me a controled and constant volatge which can open the gate.
I tinkered alittle bit this morning before work and realized that the rectifier capcitor is keeping the gate open. I know this becasue when I disconnect it it the motor stops.
Thanks
Bizzy

[citfta]

Simple motor control circuit

OK Bizzy,

It sounds like you are making progress. Attached is the simple motor control circuit I am using. All of the components are fairly cheap and I think I got all of them from Mouser or Jameco or maybe Digikey. The mosfet is good for up to 55 volts and 80 amps. So it should handle a decent size motor. You will need a heat sink on it though if you start pushing it toward the limits. If I can be of any more help just ask.

Carroll

[Bizzy]

OK Bizzy,

It sounds like you are making progress. Attached is the simple motor control circuit I am using. All of the components are fairly cheap and I think I got all of them from Mouser or Jameco or maybe Digikey. The mosfet is good for up to 55 volts and 80 amps. So it should handle a decent size motor. You will need a heat sink on it though if you start pushing it toward the limits. If I can be of any more help just ask.

Carroll

Hi Carroll
I dont see the attachement any where. could you repost it?
Thanks
Bizzy

[citfta]

Sorry about that

I was correcting it because it was too small to read after I posted it. Now it is too large but at least you can read it. I am still learning how to take a drawing I have made and get it the right size to post. Let me know if you can read it ok now.

Carroll

[Bizzy]

I was correcting it because it was too small to read after I posted it. Now it is too large but at least you can read it. I am still learning how to take a drawing I have made and get it the right size to post. Let me know if you can read it ok now.

Carroll

Hi Carroll
I still dont see the attachment yet. Is it on your reply?
Thanks
Bizzy

[Bizzy]

I was correcting it because it was too small to read after I posted it. Now it is too large but at least you can read it. I am still learning how to take a drawing I have made and get it the right size to post. Let me know if you can read it ok now.

Carroll

Hi Caroll
Ok never mind I found it. I must be getting old
Thanks!
Bizzy