A couple more motor control questions - Braking

[Echinos]

I've been doing some research about braking a DC motor. I am building
an electric model train, and I will definitely need some sort of brake
or speed limiter (or both .

I've read that shorting the motor, or opening both of the high or low
sides of an h-bridge driving the motor, will cause the motor to "work
against itself" and slow it down. I understand that, but in my
situation, the motor will be going downhill with anywhere from 200-600
lbs of weight on a certain section of track, for up to 20 seconds. It
seems to me that shorting the motor in such a case will likely result
in a burnt motor or h-bridge (or both). I am assuming that the
downhill situation will likely generate a lot more current than the
motor draws under normal operation.

So, I looked into dynamic braking - I like the idea, and it doesn't
seem too expensive to buy a pre-designed dynamic brake resistor pack,
but I would do without it if I could.

There's also regenerative braking, but I am assuming that it might be
bad to put a high current back into the battery. It's a car battery,
so it might be OK, but I haven't looked that up yet, but, would it
supply enough braking action anyway?

Am I correct in saying that I will not get any braking action by
simply reducing the PWM frequency to the h-bridge, other than friction
in the motor and elsewhere? ie, I will definitely need some sort of
"active" brake?

I would like to somehow set a maximum speed for the motor, but to have
that speed adjustable, so I can have different settings for kids and
adults. I would like to tie the brake to this speed, so that it kicks
in automatically, rather than having a manual brake.

Would it be possible to connect a second motor that I could use as a
"braking" motor? Just make it oppose the direction of movement?

There's also the option of disconnecting the motor from the circuit if
there's too much current, and have a mechanical brake.

Thanks in advance.

 

[BobG]

Whats more important? Saving/recycling the energy using regen, or just
having a modulatable brake for the downhill run? Going downhill the
motor will be generating, but I think you need a big contactor to
change the top of the h-bridge from the battery to the load
resistor... then the throttle (pwm duty) is the brake.

 

[John Popelish]

I've been doing some research about braking a DC motor. I am building
an electric model train, and I will definitely need some sort of brake
or speed limiter (or both .

I've read that shorting the motor, or opening both of the high or low
sides of an h-bridge driving the motor, will cause the motor to "work
against itself" and slow it down. I understand that, but in my
situation, the motor will be going downhill with anywhere from 200-600
lbs of weight on a certain section of track, for up to 20 seconds. It
seems to me that shorting the motor in such a case will likely result
in a burnt motor or h-bridge (or both). I am assuming that the
downhill situation will likely generate a lot more current than the
motor draws under normal operation.

So, I looked into dynamic braking - I like the idea, and it doesn't
seem too expensive to buy a pre-designed dynamic brake resistor pack,
but I would do without it if I could.

There's also regenerative braking, but I am assuming that it might be
bad to put a high current back into the battery. It's a car battery,
so it might be OK, but I haven't looked that up yet, but, would it
supply enough braking action anyway?

Am I correct in saying that I will not get any braking action by
simply reducing the PWM frequency to the h-bridge, other than friction
in the motor and elsewhere? ie, I will definitely need some sort of
"active" brake?

I would like to somehow set a maximum speed for the motor, but to have
that speed adjustable, so I can have different settings for kids and
adults. I would like to tie the brake to this speed, so that it kicks
in automatically, rather than having a manual brake.

Would it be possible to connect a second motor that I could use as a
"braking" motor? Just make it oppose the direction of movement?

There's also the option of disconnecting the motor from the circuit if
there's too much current, and have a mechanical brake.

First of all, dynamic braking (based on the power generated
by the motor) works only when the motor is turning, so you
probably still need some sort of mechanical brake to hold
the train when it is stopped or nearly stopped, instead of
using battery power to hold the motor still.

Controllable dynamic braking with an H bridge involves
briefly shorting the motor with the bridge, so that the
generated voltage ramps the motor current up to some desired
value (the braking torque is proportional to the motor
current) with a ramp rate dependent on the generated voltage
and the motor inductance. Then open the short circuit and
let the motor inductance pump that current into the battery,
with a decay rate dependent on the difference between the
battery voltage and the generated voltage and the motor
inductance. You repeat this cycle fast enough that the
current is controlled between a maximum and minimum limit,
to produce the desired average braking torque, specified by
the position of the brake pedal. To perform this process
accurately and smoothly, the motor current needs to be
measured during the cycle, so that switching decisions can
be made based on its rising and falling through the setpoint
limits.

If you add an electrical magnetic mechanical brake, you can
bring this into play once the motor is turning so slowly
that the desired braking torque (motor current) is not
achievable during the shorted part of the cycle in a
reasonable time, or if the current falls instead of rising
during the short.

 

[BobG]

Good explanation John! I have never been able to get a clear
explanation of how a motor generating somewhat less than the battery
voltage could be used for regen until now. Sounds like they use the
motor windings and the h-bridge switches like a boost converter!

 

[John Popelish]

Good explanation John! I have never been able to get a clear
explanation of how a motor generating somewhat less than the battery
voltage could be used for regen until now. Sounds like they use the
motor windings and the h-bridge switches like a boost converter!

Exactly so. This method prevents the motor from actually
being driven in reverse, but just extracts energy from the
generation process. There is nothing technical preventing
you from augmenting the current ramp-up part of that cycle
by applying battery voltage across the motor, adding to the
generated voltage with the bridge, effectively driving it in
reverse. But then you have to have controls that treat the
motor as a servo, with speed feedback, to prevent the brake
pedal from sending the train backwards after it stops.

 

[Echinos]

Whats more important? Saving/recycling the energy using regen, or just
having a modulatable brake for the downhill run? Going downhill the
motor will be generating, but I think you need a big contactor to
change the top of the h-bridge from the battery to the load
resistor... then the throttle (pwm duty) is the brake.

I'm not really worried about saving energy too much, but if I can
manage to implement it, I will. It is more important to have a
modulatable brake or "top speed".

 

[Echinos]

First of all, dynamic braking (based on the power generated
by the motor) works only when the motor is turning, so you
probably still need some sort of mechanical brake to hold
the train when it is stopped or nearly stopped, instead of
using battery power to hold the motor still.

Yes, that is of course true. I would be happy with a speed limit for
keeping the thing from zooming down the hill, and use a mechanical
brake for bringing the train to a stop. I should be able to bring the
train to a very slow speed with the throttle, and then I could have an
emergency brake" for stopping, and also for obstacles on the track
that might require a quick stop. The emergency brake could disconnect
the motor when it is engaged.

Controllable dynamic braking with an H bridge involves
briefly shorting the motor with the bridge, so that the
generated voltage ramps the motor current up to some desired
value (the braking torque is proportional to the motor
current) with a ramp rate dependent on the generated voltage
and the motor inductance. Then open the short circuit and
let the motor inductance pump that current into the battery,
with a decay rate dependent on the difference between the
battery voltage and the generated voltage and the motor
inductance. You repeat this cycle fast enough that the
current is controlled between a maximum and minimum limit,
to produce the desired average braking torque, specified by
the position of the brake pedal. To perform this process
accurately and smoothly, the motor current needs to be
measured during the cycle, so that switching decisions can
be made based on its rising and falling through the setpoint
limits.

Ok, so it is sort of like a charge pump? More complicated, of course,
but sounds similar. I have been looking for regen braking circuits
etc; is there a chip or an example schematic you might know of that I
might be able to put to use?

Also, if I were to do it as you've explained, is it likely that I can
get significant braking action over a prolonged (30 s) period of time
with no danger to the battery? I'm using a 12V car battery; if I were
to slam on the pedal with 500lbs on the train going downhill, might I
put too much current through the battery?
I should go re-read some stuff on SLA batteries (and charging them) to
see how much current they can handle. I will have to do quite a bit of
testing, I imagine. I could always put in another battery to help out.

If you add an electrical magnetic mechanical brake, you can
bring this into play once the motor is turning so slowly
that the desired braking torque (motor current) is not
achievable during the shorted part of the cycle in a
reasonable time, or if the current falls instead of rising
during the short.

I hadn't thought of that sort of brake. I like it! I was thinking only
purely mechanical. You're thinking of something like a disc brake with
a solenoid or something controlling the pads?

 

[Echinos]

Exactly so. This method prevents the motor from actually
being driven in reverse, but just extracts energy from the
generation process. There is nothing technical preventing
you from augmenting the current ramp-up part of that cycle
by applying battery voltage across the motor, adding to the
generated voltage with the bridge, effectively driving it in
reverse. But then you have to have controls that treat the
motor as a servo, with speed feedback, to prevent the brake
pedal from sending the train backwards after it stops.

Ok, so I'm going to go look up boost converters, for one.

This method of essentially driving the motor backwards is interesting
- If I had a controller that had a pot where halfway was stop (or,
neutral), and one way was forward and one way was backward, I could
simply turn the pot the other way to brake the motor. I would still be
worried that the fight between the battery and the motor under a high
load situation might create fireworks.

 

[John Popelish]

Yes, that is of course true. I would be happy with a speed limit for
keeping the thing from zooming down the hill, and use a mechanical
brake for bringing the train to a stop.

That speed control should probably be in effect during
powered motion, as well as during braking. This involves
measuring the speed (usually just by averaging the voltage
across the motor) and having an override circuit that takes
over and lowers the throttle if the speed is too high, or
activates a regenerative dynamic braking once the overridden
throttle goes to zero. This takes some sort of control loop
to be a smooth and stable operation.

I should be able to bring the
train to a very slow speed with the throttle, and then I could have an
emergency brake" for stopping, and also for obstacles on the track
that might require a quick stop. The emergency brake could disconnect
the motor when it is engaged.

Or add to dynamic braking.

Ok, so it is sort of like a charge pump? More complicated, of course,
but sounds similar. I have been looking for regen braking circuits
etc; is there a chip or an example schematic you might know of that I
might be able to put to use?

I haven't designed this, lately, and would have to look around.

Also, if I were to do it as you've explained, is it likely that I can
get significant braking action over a prolonged (30 s) period of time
with no danger to the battery?

You could safely produce as much braking torque this way ans
you could safely produce driving torque. Since all
mechanical and electrical losses add to the braking
deceleration capability, but subtract from the driving
acceleration capability, this method could slow the train
more aggressively than the same motor and battery could
drive it. Also, there is no limit to how long you can ask
the battery to keep the train in motion, but there is a time
limit on how long you can decelerate it, so you can break in
an overload condition that doesn't last long enough to
overheat anything. Also, you can switch in a dump resistor
to absorb some of the energy, so the motor and H bridge
capacities are really the braking limit, not the battery.

I'm using a 12V car battery; if I were
to slam on the pedal with 500lbs on the train going downhill, might I
put too much current through the battery?

I think the length of the down hill run is a lot bigger
limit than the initial stop. Car batteries can deliver and
absorb rather large pulses of power, briefly. If the
battery can drive the train up the hill, it can almost
certainly control its speed going down the same hill.

I should go re-read some stuff on SLA batteries (and charging them) to
see how much current they can handle. I will have to do quite a bit of
testing, I imagine. I could always put in another battery to help out.

Reducing the peak stresses on the batteries will definitely
extend their life.

I hadn't thought of that sort of brake. I like it! I was thinking only
purely mechanical. You're thinking of something like a disc brake with
a solenoid or something controlling the pads?

Magnetic safety brakes are generally activated with a
permanent magnet or spring to break by default, and released
with a small electromagnet that cancels the permanent magnet
field or compresses the spring. That way, when you turn
the power off, the brakes hold the thing still. This is a
pain, however, if the battery goes dead and you have to
push. So a normally released brake that is activated by an
electromagnet might be better, with a mechanical parking
type brake in addition.

http://www.mikipulley.co.jp/en/product/category.php?id=pcb

 

[John Popelish]

Ok, so I'm going to go look up boost converters, for one.

This method of essentially driving the motor backwards is interesting
- If I had a controller that had a pot where halfway was stop (or,
neutral), and one way was forward and one way was backward, I could
simply turn the pot the other way to brake the motor. I would still be
worried that the fight between the battery and the motor under a high
load situation might create fireworks.

As I said, the biggest problem with reverse drive to brake
is that things don't stop when you reach zero speed, but
change direction.

 

[Jasen Betts]

I've been doing some research about braking a DC motor. I am building
an electric model train, and I will definitely need some sort of brake
or speed limiter (or both .

look into the controllers used for electric scooters, they do most of
what you want.

Bye.
Jasen

 

[Echinos]

(snip)

That speed control should probably be in effect during
powered motion, as well as during braking.

Yes, that was my intention. A likely use for the train is to let
younger kids operate it, so I would want to be able to set a
reasonably slow speed limit, but also have it adjustable for hauling
cars for track work etc.

You could safely produce as much braking torque this way ans
you could safely produce driving torque. Since all
mechanical and electrical losses add to the braking
deceleration capability, but subtract from the driving
acceleration capability, this method could slow the train
more aggressively than the same motor and battery could
drive it. Also, there is no limit to how long you can ask
the battery to keep the train in motion, but there is a time
limit on how long you can decelerate it, so you can break in
an overload condition that doesn't last long enough to
overheat anything. Also, you can switch in a dump resistor
to absorb some of the energy, so the motor and H bridge
capacities are really the braking limit, not the battery.

Yes, it seems that a combo dump resistor/regen braking would be the
most robust.

I think the length of the down hill run is a lot bigger
limit than the initial stop. Car batteries can deliver and
absorb rather large pulses of power, briefly. If the
battery can drive the train up the hill, it can almost
certainly control its speed going down the same hill.

Well, on this track, the uphill portion is much more gradual; the same
elevation is done in 2 or 3 times the length of track. So, the
downhill will be more current. When you say that car batteries can
handle large brief pulses of current, do you mean the pulsing during
regen braking, or the average pulse over the entire braking period?


Thank you very much for your advice. It has been quite helpful.

 

[John Popelish]

On Jul 26, 11:06 am, John Popelish <[email protected]> wrote:

Yes, it seems that a combo dump resistor/regen braking would be the
most robust.

All that may be necessary to control the dump resistor is an
over voltage sensor across the battery. Any time the
battery exceeds, say 15 volts, switch the dump resistor in
until the voltage falls to below, say, 14 volts. Most of
the time this would result in brief , repetitive periods of
having the dump connected.

Well, on this track, the uphill portion is much more gradual; the same
elevation is done in 2 or 3 times the length of track. So, the
downhill will be more current. When you say that car batteries can
handle large brief pulses of current, do you mean the pulsing during
regen braking, or the average pulse over the entire braking period?

Both. Once the battery has been used a bit and is no longer
fully charged, you can blast rather large pulses of current
into it, as long as it doesn't get anywhere close to being
full charged, again. There is some internal resistance in
the battery that will produce heat, regardless of the
direction of the current through it, but as long as the
temperature of the electrolyte doesn't overheat, you should
not worry too much about braking current if the battery
voltage does not get excessive. The H bridge should have
considerable low internal series resistance capacitor across
it, so that the high frequency part of the chopped wave does
not need to go all the way back to the battery. Adding a
little inductance between this local energy store and the
battery would greatly increase the filter effect, lowering
the RMS current through the battery caused by the pulsing.
This will lower the battery temperature rise caused by the
high amplitude, short duration (millisecond) pulses, whether
during driving or breaking.

A battery temperature interlock might be a good idea,
though. It wouldn't have to disable the motor, just lower
the maximum speed to a very low value, so you can still limp
home.