PWM with low side MOSFET drive for DC motor (coast or brake when off?)

I should probably add that the motor is braked in the opposite direction that it is driven (i.e. resists turning backward).

"Brandon"

** Nonsense.

That diode shorts the back emf after each pulse of current fom the PWM drive - ie it keeps the current flowing in the same direction in the motor.

** Yon can add breaking with a second MOSFET wired to short the motor whenever there is no PWM drive.

Pulls it up real quick.

......... Phil

When you switch off the power to the motor there is no reason the motor should stop unless it has an internal brake that brakes when there is no power. I have a motor that has seperate power for the brake that you power to turn off the brake.

The motor itself has inertia and essentially acts like a generator when there is no power. So if its spinning it will continue to spin unless there is some mechanism to stop it. (it will, of course, gradually slow down due to friction and load)

My guess is that since you are saying it has a built in brake and it works "one way" that you need to change the "polarity" of the motor.

The diode only provides current flow for the back emf to reduce the voltage spike when turning off the mosfet.

I do not believe it is possible to brake a motor without some built in brake system or "reversing" direction. I mean this w.r.t to modifying the PWM.

In fact, its logical to assume it would be a bad idea. With PWM you are turning off the power to the motor and if it braked then you would drastically cut your efficiency. (although a smoothing cap would void that so I suppos it still could work with the right motor)

If the motor itself does not have a built in brake(or one in the right direction, which would simply say that you have it hooked up backwards) then you can use an H-Bridge to run the motor bidirectionally and simply slow the motor down by reversing the polarity. In this case you will need to monitor the motor's back emf to know "how much" to slow it down.

I'm not sure if thats a good idea but it would probably work.. at least for a while.

Control is more important than efficiency for me here, and with with

I was using an H-bridge before (an NJM2670) but it seemed like overkill considering that I only need to run the motor in one direction and for the project I'm working on the less space I take up on my circuit board the better. It did work like I wanted it to, though: it braked when both inputs were the same and so running PWM to one of them gave me the results I wanted (i.e. a 50% PWM duty cycle ran the motor as if half the supply voltage was applied).

I like the idea of using another MOSFET to short the motor during the off part of the duty cycle, but at the moment I don't see how to add it to the circuit so that I can turn it on and off with a logic signal. Perhaps I am missing the obvious as I haven't done much with circuits in awhile.

"Brandon"

** Who's dumb idea was that ??

Not mine.

Do learn to read.

...... Phil

you need a dynamic brake circuit added.. use a high side Mosfet (P channel) to short the motor leads with a series load resistor when you want to do a DB .

--
http://webpages.charter.net/jamie_5"

Brandon, you need to read up on 4 quadrant control so that you can actively brake the motor, that is to say actively take energy out of the motor and return it to your source; this will need an H-bridge. When you switch a resistor into the circuit this will brake the motor effectively but the energy is lost as heat into the resistor. This can cause problems if you have a lot of energy in the motor and load as the resistor will get very hot and can fail without sufficient cooling. On top of this its damn inefficient and the resistor may have to be quite large.

The act of applying the voltage supply in the opposite polarity in order to brake the motor is called plugging and whilst effective results in massive heating in the motor. If it's something you do rarely then maybe it's ok but if you are braking and/or changing motor direction regularly then you will cause the motor windings to heat up and ultimately fail. Also if you reverse the polarity how do you know when to turn off before the motor starts rotating in the opposite direction?

Don't short the motor with a MOSFET, you will get a huge current in the MOSFET and it will fail very quickly and with nice big flames and the room will stink for a week!

When taking energy out of the motor (regenerating) you need somewhere to put it. If the whole system is operating from just a battery then it can go back in there. If it's a d.c. power supply then there will be some capacitance available, but this may not be enough and when the capacitors become fully charged you will start driving up the d.c. rail so what you thought was a 12V supply could end up with much larger voltages and cause failure in the d.c. supply. Adding more capacitance may resolve this, or a battery. Alternatively if it's powered from the a.c. mains then you could put it back there but this is a whole other area of pain, literally if you come into contact with the high voltage!!! HAZARD!!!

I hope this helps a bit.

Dave.

"Dave"

** That is a load of absolute SHIT.

Dave is a moron.

...... Phil

Never heard of DB resistors? Its a standard, We've been doing it for years in the plant. No problems with that method.. Using a 4Q drive is way over kill for the simple task at hand and the stress on the motor to get maximum stall effect most likely is going to be more stressful than just using DB resistors. If the op wants instant stop, he'll need to use a locking clutch when the motor is de energized. In such applications, maybe he would be better off with a stepping motor if the RPM's arn't to much.

"

Well, there might be a heating issue though.

The H-Bridge isn't much larger than than the mosfet(ok, it is but shouldn't matter too much I would imagine).

This would probably work but I guess the only real way to know is to try. You will need another logic pin and its essentially the same as the first mosfet except you put a small resistor(for testing purposes... try different values) between the mosfet and ground.

In your code you simply turn on the other mosfet after the first is turned off(not during each cycle but when you want to brake it). This swithces in the load to the motor and will slow it down.

Hopefully you are using a uC so it will be easy. If your using a PWM IC(unless it has some controll pins for "braking") then its going to be a little harder and not as perfect.

Essentially what you have to do is branch off the digital signal, filter it, invert it, and then to the second mosfet. That way when the PWM is off it will be get inverted to high and then turn on the secondary mosfet. You have to filter it else it will do this once every cycle and be a bad thing. The problem with the filtering is you have to have enough to get the PWM's average value without much ripple(might not have to be all that great actually but as long as it represents a high for your inverter) but also have it drain enough so that it responds quickly when the PWM is shut off. (you can invert then filter too)

Hopefully that makes sense and hopefully your using a uC as it is 100x easier and cheaper.

Phil,

Please don't be so disrespectful as I am trying to help the OP, if you disagree then fine I can accept that but please also supply your argument.

Well DB resistors are exactly what I was talking about when saying they are ineffecient as they release the energy as heat, as I said in my first paragraph. Yes it works fine but it is inefficient and may result in a large device depending on the amount of energy involved.

What stress on the motor?

Yes, but it's an extra component as would an H-bridge and controller be.

Well maybe, but as you say, speed and loads on the motor will determine this.

Why? That is what generally happens. I have done it :)

I suppose with a sufficiently large mosfet and a sufficiently small motor it would work.

--

John Devereux

No, the diode conductive the current flow to prevent an inductive spike. Back EMF opposes the flow of current in the motor. That's why current decays as the motor speed up. The back EMF approaches the drive voltage thus the effective voltage across the motor windings becomes small.

Sure it is. You won't be able to grake to or near zero but you can get quite high braing forces.

Or you can feed the energy back into the power bus. Such regen braking is common on AC and PM motors on EVs.

Robert

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Yes.

The diode simply conducts the current that would otherwise cause an inductive spike.

Back EMF opposes the the applied voltage and is thus blocked by the diode. That's why the motor current decays as the motor reaches speed. You need to short the motor to have braking due to the power control. In the short term what happens in that case is the current decays from the drive current and then reverses as the back emf is able to overcome the inductance and start the motor generating.

Look for the terms 1/2H and regen braking (and maybe breaking I've seen a lot of misspellings of brake recently). Full H's have their place as well. If driven correctly you get regen out of a 1/2H down to slow speeds. If you don't have a supply bus that can tolerate regen you may have to add a braking load to dissipate the energy. Or if you only go from on to stop then you could simply short the motor through a sufficiently large resistor.

Unless you have a full H the final brake to stop will involve friction either in the load or as a separate mechanical brake (maybe both).

Robert

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Conducts

grake -> brake braing -> braking

That was a particularly bad bought of typos.

Robert

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