I'll have a go, simply because no-one else has attempted it. I expect to be shot down in flames.

Right, starting from first principles, the motor is being pulsed with maximum voltage pulses of varying width, and during the "off" period the motor "free-wheels" and due to its inherent inductance produces the so-called back-EMF. In a rather simplistic explanation, this was described to me many, many years ago as the motor, or rather the inductor therein, attempting to maintain the status quo. Which of course, it can't.

Now for some funny stuff. For back EMF to develop, there has to be a resistance to develop it across, and as we all know, I=V/R, simplistically that is. But the energy stored in the inductor is, shall we say fixed and was dependant on the current in the first place, therefore we can say that if R is large, then V must also be large to maintain I. And vice versa. So, it is perfectly possible for a inductor, having been fed from say 12v, to generate a back-EMF of many hundreds of volts.

Slight diversion here. In a previous life, used to work for BT as a technician and was once asked to somehow devise a circuit which would determine if a specific spark quench circuit was working correctly. As part of my investigations, I managed to see what looked like a 900v spike from an electro-magnet energised from 50V. The method of monitoring was a bit crude so it might not have been 900v, but nevertheless it darned high, and I didn't fancy my fingers anywhere near it.

Back to business. Assuming then that we can get this high voltage spike, then perhaps all we need to do, and I admit this now pure theory for me, is to connect the back-EMF via a suitably arranged diode to the battery. Hey presto, excess voltage should end up being shunted back into the battery and at the same time clamping the back-EMF to a suitable safe level.

I look forward to being told I'm talking a right load of old cobblers.

Actually, it is possible to use this back-EMF for speed control to ensure constant speed at a particular setting. I have a pair of books by Roger Amos which goes into this in respect of OO gauge model railways. I've had a 0-6-0 tank take about 15 minutes to travel 1 metre. I also used to have a mains operated circuit comprising a thyristor, diac, diode, capacitor & variable resistor for use with my mains drill. It was quite interesting feeling the drill skip-cycling on no-load, and then turning to a deep growl whilst still turning under full load.

Hope this helps,

Peter G. Shaw

Hi Duncan. In simple terms when volts are applied to a permenant magnet motor sufficient current flows into the motor such that the back emf generated equals the volts applied. When no volts are applied to ta permenant magnet motor and it is driven on overrun it generates a voltage at it's terminals which can be well in excess of the battery volts. If the controller allows it, this voltage will produce a current flow into the battery which will brake the motor and hence the loco. To protect the controller electronics, some controllers limit the voltage to say 60 volts on a nominal 24 volt system. If you want more detailed information I suggest you look at the 4QD web site which has an excellent article on this subject. Regards Bob Lilley.

The class 76 locos used on the Woodhead route between Manchester – Sheffield were 1500v DC with regenerative braking.

The idea was that because of the summit in the middle of the route locos would draw current whilst climbing but once over the top the regenerative braking of the loco put power back into the overhead wires.

It used to be said that a loaded coal train braking downhill generated enough current to power an empty train climbing uphill in the opposite direction. Personally I find that hard to believe but I've heard a few people say it was true.

Peter.

Basically one aspect – there is current running about in a the motor when it's driving something. When it's turned off the current is still there and must be given some where to go. A resistor might be used to do that and the time taken to get rid of the current that is stored is proportional to the voltage across it. Often it's simply a reversed diode but if a resistance is added to that the voltage across the motor when it's turned off will be added to the diode voltage so it will decay more quickly.

2nd aspect if a DC motor is being driven by something it will generate power and in simple terms that has to go some where

That's ever so basics but back emf and regenative braking are one and the same thing really. It's just a case of where the power goes. The average voltage on field windings on the motor have an effect on the average voltage the armature develops when it's being driven by some force and also the power the motor can generate. The power it generates that is then absorbed by something determines the braking force that is developed. On electric vehicles it's generally merged in with the normal brakes so that the user is unaware of it. I used the term average because it's usually actually varied mark space ratio pulses of the supply voltage.

John

–

That's a new one on me, I've only been using MOSFETs to switch power in one direction.

On electric/hybrid vehicles the regen energy is ideally put back into the battery. The amount of energy that can be absorbed by the battery without over-voltaging is heavily dependent on the SoC of the battery. Controlling the SoC is thus an important function of the battery management system. When testing batteries the parameter of interest that can be measured is charge acceptance.

Andrew

I was in a bit of a rush with the other post. In some ways it's easier to explain from motor speed and torque control using dc motors and the sep ex I mentioned. The usual way of controlling motor speed is some form of pulse width modulation with the field windings connected to the supply. I hope I get this the right way round – fairly sure I will but it's been a long time.

As the speed of a motor increases the back emf it's armature generates increases and at some point it gets close to the field voltage and at that point it can't go any faster. These are average voltages when things are driven with pulses even when it's the field. They are actually setting mean currents, usually across a diode when the power is off so that the current decay rate is slow. So say the field normally has half of the supply voltage on it. That will limit the speed it can produce and also limit the voltage that it can generate when the armature is driven by say braking but the field can also be changed so the motor can be made to go faster or slower purely on that. This is how the motor is made to produce more volts than the battery that is driving it for regen braking. In practice the whole thing is regulated by armature current both ways – providing power or regenerative braking. Most of the regen will be applied when the accelerator isn't depressed but it can also be blended in with the brakes.

Sep ex is not exactly a safe thing to play around with. Do the wrong thing with the field and the motor may rev so fast that it explodes due to centrifugal force. The other aspect is that field currents have an effect on torque.

I aught to remember which way round things go as I replaced a fair bit of a controller with a micro processor. The states were interrupt driven and noise caused some problems. If I hadn't fitted a switch to make 100% sure thing shut down properly a rather large motor would have exploded showering bits all over the place. That had happened in the past while the original controller was being worked on but the guy working on it ran out of the room.

The armature emf thing I mentioned is a model Motorola have used to describe speed control circuits. I aught to get the book out and check that but it's time for bed. They don't consider sep ex at all. Few do. In this case the motor was designed to be driven like that from a 200 odd voltage battery pack.

John

–

Andrew, check out:

http://www.quora.com/Is-MOSFET-a-bidirectional-device-or-unidirectional-device-What-is-a-clear-explanation-about-this

Neil

Nearly there Duncan but it is even simpler. When regen braking the PWM controller is not in operation and in effect is replaced by a diode which allows current to pass from the motor/generator into the battery, but prevents the battery feeding the motor. Therefore during regen you have a simple dc generator feeding the battery.

You are also correct that the battery would normally be partially discharged thus allowing further charging except in the case of Michigan, described in this month's ME magazine, which has an alternator continuously charging the battery. In this case the battery is fully charged and a high voltage can be generated during regenerative braking.

Trust this answers your query.

Regards Bob Lilley

Edited By Bob Lilley on 21/09/2015 14:37:34

Neil: Thanks for the link, although I would admit to being slightly disappointed. I thought there was some clever way of getting 'reverse power' through a MOSFET that I wasn't aware of. There is an arrangement of two MOSFETs that allows bi-directional current to flow, while paralleling the gates so that only one drive circuit is needed but still blocking current when the gate drive is off. I used the circuit as a connector/disconnector on parallel battery strings for a Formula1 KERS system.

Andrew

If it wasn't possible, then the little chips full of 'analogue switches' wouldn't work…

But my bad MOSFETs have a body diode which means they aren't reversible (at least above Vf for the body diode)…

However most jFETs don't give a (significant) hoot which way round source and drain are. This datasheet clearly states 'note drain and source are interchangeable'.

What I can't see working is a diode alone – that will only work with the motor running ~faster than if it was unloaded and connected to the battery i.e. over-running, which would happen anyway with or without the diode. All the diode would do is stop the battery running the motor at lower speeds..

Neil

Ah yes, the 2N3819…

As Andrew says, you have to put a pair of MOSFETs back to back if you want bidirectional blocking. If you want to keep the losses the same as you would have had with a single unidirectional device, you will need to quadruple the number of devices and hence the cost.

But for a motor drive, you don't need to worry about that. You invariably design the motor so that the maximum back EMF is less than the DC bus. You can control the motor in field weakened operation above this point but this requires clever software and you need to have a plan for when you lose control that doesn't involve popping the switches.

For best operation, you use a half bridge on each phase of the motor, whether brushed, brushless, 3-phase or whatever. This allows you complete control in all 4 quadrants ie speeds and torques in either direction and in any combination. 

For easiest and most versatile control of a brushed DC motor, the best solution is a full "H-bridge" controller. This is 2 half bridges and gives 4-quadrant control, which is what the OP was after. You can buy integrated H-bridge driver power devices with all sorts of in-built protection features or doubtless you can get a ropey equivalent from an auction site.

Murray

Edited By Muzzer on 22/09/2015 13:56:29

In the old days, cars had dynamos which would act as motors. Connect to a battery as normal (for them) – one side to earth on the casing, and the other side to both Field and Armature connections. The dynamo will then turn at a modest and steady speed, and can be used as a motor. Now drive the 'motor' faster in the same direction and it starts to generate, rather than consume. No fancy switching, no change in connections, as long as the rpm is rather more than motoring speed it generates, and in doing so absorbs power from the driver and charges the battery – ie regenerative braking. All that stops the dynamo acting as a motor at low rpm in 'real life' is the cut-out, which disconnects the output at (about) the cross-over point when output changes to input.

Perhaps this will help to understand what is going on?

Cheers, Tim