This may be off topic but, I don't think it should be considered that. Many have asked for more insight about controllers so I am liking it.
In answer: A MOSFET is a solid state switch. It is used in a controller to give a modulated varying pulse wave form(PWM).
When talking bridge, most of the time the controller is designed for forward or reverse and is designed with 4 MOSFETs for DC motor control (6 FETs for 3phs control). These (FETs) for short, are arranged to provide two active FETs for each pulse the PWM generator produces. By changing what two of the FETs are energized the output is reverse of the previous arrangement. This arrangement is known as an H bridge where the bar between the legs of the H is the motor winding. The MOSFETs are the building blocks for the bridge and are not the bridge, so to speak.
To increase efficiency in a Buck or Boost circuit ICs are available to drive a MOSFET to handle the back EMF. Because MOSFETs have a lower voltage drop the losses are cut in controllers using them, rather than any other form of diode to control back EMF. This clamping diode as it is known, directs the current held in the coil of the motor, or other inductive load, back into the coil when power is disconnected from the coil. You could say this is free power because the coil needed to be charged in order for the motor, or other inductive load, to become active in the first place, and you are saving the energy that would otherwise destroy the FET used to drive the motor.
Summary:
Controllers that use FETs to control back EMF are more energy efficient. The savings are typically more than a few percentage points, so the money, or effort to make such a controller will save more energy than re-designing a motor.
Fact: PWM motor controls do in fact use the motor as a buck boost DC to DC converter uses a coil. So the same design efforts are used in both systems.
May your charge see you to port.
Kevin Pemberton
On 01/08/2013 08:26 AM, Chris Morriss wrote:
In answer: A MOSFET is a solid state switch. It is used in a controller to give a modulated varying pulse wave form(PWM).
When talking bridge, most of the time the controller is designed for forward or reverse and is designed with 4 MOSFETs for DC motor control (6 FETs for 3phs control). These (FETs) for short, are arranged to provide two active FETs for each pulse the PWM generator produces. By changing what two of the FETs are energized the output is reverse of the previous arrangement. This arrangement is known as an H bridge where the bar between the legs of the H is the motor winding. The MOSFETs are the building blocks for the bridge and are not the bridge, so to speak.
To increase efficiency in a Buck or Boost circuit ICs are available to drive a MOSFET to handle the back EMF. Because MOSFETs have a lower voltage drop the losses are cut in controllers using them, rather than any other form of diode to control back EMF. This clamping diode as it is known, directs the current held in the coil of the motor, or other inductive load, back into the coil when power is disconnected from the coil. You could say this is free power because the coil needed to be charged in order for the motor, or other inductive load, to become active in the first place, and you are saving the energy that would otherwise destroy the FET used to drive the motor.
Summary:
Controllers that use FETs to control back EMF are more energy efficient. The savings are typically more than a few percentage points, so the money, or effort to make such a controller will save more energy than re-designing a motor.
Fact: PWM motor controls do in fact use the motor as a buck boost DC to DC converter uses a coil. So the same design efforts are used in both systems.
May your charge see you to port.
Kevin Pemberton
On 01/08/2013 08:26 AM, Chris Morriss wrote:
A quick reply as I'm sending this from work.
A lot of motor control is related to switch-mode PSU buck-regulator design, so it is useful to get a feel for that first. In fact PWM speed control of a brushed PM dc motor uses circuitry that is almost indistinguishable from a buck voltage regulator. There's a fair amount on the web to lead you into this aspect of it. A PM Brushless dc motor has a more complicated driving arrangement, but it also follows on from this in many ways.
As this is all getting increasingly off-topic for this group, I'll have a look at the motor controller mailing list that Craig mentioned to see if that might be more appropriate for this more detailed techy stuff.
regards,
Chris.
From: electricboats@yahoogroups.com [electricboats@yahoogroups.com] on behalf of Roger L [rogerlov@ix.netcom.com]
Sent: 08 January 2013 13:15
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Motor Controller Modes
Great conversation! I'm enjoying what little I understand about these motor controllers. I've never designed a controller; in fact I've never designed anything more complicated on the electrical side than an electrical/mechanical relay. So I only have a vague idea of the way that things are incorporated into a controller. Right now I'm at the stage of trying to develop an appreciation of what needs to be controlled. And Why & How.I doubt that I'll ever want to design a controller - well, maybe a very simple one - ...., but would like to know enough to discriminate between various designs.Craig, I went to that URL that you posted ....the one at --- MotorControllerManual.html ---- and it is a great help to a guy like me who is just learning about the electrical design side. Although even the definitions of the terms that you electrical guys throw around are a stumbling block at my current level.What the heck is a "MOSFET", ....and why is often compared to a bridge? It would be a great help if you could direct me to some oscilloscope pictures of the driving waveform.Chris, is there some similar explanatory article on your version of a control circuit? Sometimes the differences themselves are illustrative.To both....if you don't mind, I have lots of those kinds of basic questions. Just getting started, I'm still chewing over the implications that there are different ways of controlling current. I didn't even know some of them were possible. To give you an idea of my current level of sophistication. I still think of a motor controller as being a rheostat basically built like the carbon block type that controls a sewing machine, but somehow pulsed so that it drives at the right RPM. I had assumed that ramp times were so close to instantaneous compared with a motor's rotational frequency that they could be safely disregarded. In a purely rotational system, what causes spikes? As for back EMF, my mental version of a sewing machine type controller didn't do anything other than tolerate it, though I can see how back EMF would define a limit on RPM.Thanks,Roger L..........................----- Original Message -----From: Craig CarmichaelSent: Tuesday, January 08, 2013 1:33 AMSubject: [Electric Boats] Motor Controller ModesMax-min current control sounds good. But would require a second
voltage comparator that can handle the spikes and noise. I just
copied the 'fixed off period' mode from another chip. (Allegro
A3938/A3932 IIRC. But that was intended for <1HP and couldn't take
the power/switching spikes from a larger motor.)
As it is, I managed to make the IR2133 into a single chip BLDC motor
controller except for the quad XOR gate to reverse direction.
I believe you but I'm not sure why external diodes would be better...
to share the heat loading?
...or if they were Schottky diodes with a lower forward voltage drop.
Say, I rather like that idea. It might improve regenerative braking a
little?
(I'd more expect this sort of subject would have come up on a motor
controller list like osmc@yahoogroups.com or ?? You just never know
where a conversation will lead!)
Craig
>Thanks Craig, I'll have a look at your controller design.
>
>It's an interesting idea to use a self-oscillating current-mode drive
>for the motor. I hadn't thought of doing the power control this way,
>although the technique is widely used in dc-dc buck regulators. I might
>have used a hysteretic current mode control though, turning the current
>flowing in a stator winding off when it has reached the desired
>set-point, then turning it back on again when it has fallen by a certain
>amount. The inherent source-drain diodes in the MOSFETs provide a
>current path when all four of the MOSFETs are turned off, although I
>have found huge improvements in reliability at full power in class-D
>amplifiers if you use a suitable diode bypass route to prevent the
>internal MOSFET diodes from conducting.
>As you say, the switching frequency will be variable as the di/dt will
>be a function of the motor speed (when the current is increasing
>anyway).
>
>Regards,
>Chris.
>
>In message , Craig Carmichael
>craig=saers=com> writes
> >Um, er...
>>
>>I wasn't thinking of the PWM frequency, which is of course a good
>>point. But I'll surprise you and say it's around 300 Hz. Now I have
>>to explain this...
>>
>>My controllers actually use a dual system. First there's "direct
>>torque control", or as I call it "Current Ramp Modulation", "CRM".
>>(Torque being directly proportional to current.) When you apply
>>voltage to the motor coils, as inductors, the current starts at zero
>>and rises. If the motor is stopped with no back EMF, it rises very
>>quickly, but much more slowly with the motor at full speed. The CRM
>>turns off the coils when the current has ramped up to the maximum
>>level (or alternatively to the control setting below maximum). It
>>turns it off for a fixed period like 50uS, then turns it back on to
>>begin again. So the modulation will be fast at low motor speeds with
>>high torque, and slow at high motor speeds.
>>
>>This minimizes switching losses. It variably reduces switching
>>frequency to no more than necessary for conditions. The problem with
>>it as the main control is that without external feedback, unless
>>torque load rises and falls with RPM (as with, eg, a boat prop), the
>>motor will keep speeding up until it hits max or slowing until it
>>stops for small control adjustments.
>>
>>So in an irregular load like an EV car, PWM (external in present
>>version V2) modulates the CRM (set to max). But since the CRM is
>>controlling the current, the PWM doesn't worry about it, and can
>>switch at any low frequency above where the on-off pulses would
>>become noticeable.
>>
> >So it's actually my controllers that keep the switching frequencies low.
> >http://www.TurquoiseEnergy.com/hybridize/MotorControllerManual/MotorCont
> >rollerManual.html
>>
>>(BTW If the circuit is of interest - There's some component value
>>changes to the schematic C10 is double+ for longer 'fixed off time',
>>and on the layout a wire touches pin 10 of the header strip instead
>>of going around it.)
>>
>>Craig
>>
>
>
------------------------------------
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/electricboats/
<*> Your email settings:
Individual Email | Traditional
<*> To change settings online go to:
http://groups.yahoo.com/group/electricboats/join
(Yahoo! ID required)
<*> To change settings via email:
electricboats-digest@yahoogroups.com
electricboats-fullfeatured@yahoogroups.com
<*> To unsubscribe from this group, send an email to:
electricboats-unsubscribe@yahoogroups.com
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
-- Committing murder in exchange for lifestyle makes you a "thug" not a "Rights Activist"
__._,_.___
| Reply via web post | Reply to sender | Reply to group | Start a New Topic | Messages in this topic (50) |
.
__,_._,___
No comments:
Post a Comment