Braking 350w+ motor - limiting brake current with a resistor?

I am building a 7.5" gauge model locomotive (ride-on size) and am interested in having the ability to use a dynamic brake function to control speed while going downhill.

I'm planning to use a pre-made H-bridge module based on the BTS7960 MOSFETs. This is rated at 43A. As is, I have a 24vdc 350 watt electric scooter motor (which works out to around 15A) but may upgrade some if needed.

For braking, I have read that simply turning "on" both lower MOSFETs (or both upper) will provide braking and stop a motor. However given the fact that there will be considerable weight/momentum continuing to turn the motor through such braking, I think dead shorting the motor is a very bad idea for both motor windings and the MOSFETs. I understand a braking resistor can be used, but trying to find the right size is my main question.

Will a 24vdc motor will generate more than 24vdc while coasting at full speed, less than 24v, or the same? And in this case, would I simply apply ohm's law to find a massive resistor that will not exceed the normal current/power rating of the motor?

A DC motor's speed is proportional to the drive voltage. A generator's output is proportional to its speed. Hence a motor operating on over-run cannot generate more voltage than that which has driven it.

However an inductive device (which includes DC motors) will generate extremely high back emf (voltage) if it is suddenly disconnected from its drive source. That's because inductive devices endeavour to maintain current flow and since you have suddenly 'stopped' the current flow the device raises the back emf in an attempt to maintain current.

You will also find that the braking effect drops off rapidly as the speed drops so you really only need to consider a short (excuse the pun) braking resistor requirement. In fact you might find it quite disappointing. You do appreciate there is NO braking effect at zero speed.

Something like a 1ohm resistor rated at 50watts should suffice

Remember too that you can PWM the brake too ! You don't have to give it one hundred percent braking force. Maybe have a seperate control or pedal for proportional brakes.

jackrae:
A DC motor’s speed is proportional to the drive voltage. A generator’s output is proportional to its speed. Hence a motor operating on over-run cannot generate more voltage than that which has driven it.

However an inductive device (which includes DC motors) will generate extremely high back emf (voltage) if it is suddenly disconnected from its drive source. That’s because inductive devices endeavour to maintain current flow and since you have suddenly ‘stopped’ the current flow the device raises the back emf in an attempt to maintain current.

Ok, good to know about the back EMF causing a higher voltage… I guess this was rolling around in the back of my mind knowing a motor was a sort of inductive device and hence the question.

jackrae:
You will also find that the braking effect drops off rapidly as the speed drops so you really only need to consider a short (excuse the pun) braking resistor requirement. In fact you might find it quite disappointing. You do appreciate there is NO braking effect at zero speed.

Something like a 1ohm resistor rated at 50watts should suffice

My concern is the speed may not drop off rapidly… there will be several hundred pounds pushing this thing down the hill. It’s a very slight grade, but enough that something “freewheeling” will soon pick up more speed than I’d like. I would expect it to be at 80-100% of the motor’s rated speed while using dynamic braking for up to a couple minutes at a time. Granted, using PWM to modulate braking effort may result in less total power to dissipate in the resistor.

alka:
Remember too that you can PWM the brake too ! You don’t have to give it one hundred percent braking force. Maybe have a seperate control or pedal for proportional brakes.

That’s the plan. The way I’m planning it, the direction switch is a 3-position switch… forward, dynamic brake, reverse. Then just use the throttle knob to adjust braking force. Same knob, same PWM input on the H-bridge module.

You can't expect more than 350 watts of braking from the motor, without burning it out.

Don't forget that if you rapidly reverse the voltage on the motor (while it is rotating), the resulting current is TWICE the stall current and can rapidly burn out a motor.

This is because in this situation, the back EMF adds to the EMF that you are supplying, while the coil resistance remains of course the same.

JuniusBugg:
Granted, using PWM to modulate braking effort may result in less total power to dissipate in the resistor.
That's the plan. The way I'm planning it, the direction switch is a 3-position switch... forward, dynamic brake, reverse. Then just use the throttle knob to adjust braking force. Same knob, same PWM input on the H-bridge module.

No resistor required. Just PWM the brake function of the H-bridge. The power is dissipated in the resistance of the motor windings, heating up the motor. Hence the power limitation on braking is related to the maximum input power.

Motor armature and commutator/brush resistance determines stall current, which may be many times higher than 'rated power' running current. Power output under stalled conditions is obviously zero, but that doesn't mean there is zero current.
So applying a 'short' to act as a brake could produce extremely high winding currents, which, if sustained, could be sufficient to burn out either windings or commutator. Obviously, when acting as a generator the motor must be rotating (otherwise it wouldn't generate) so you won't get 'stall' current but you may well exceed the rated current specification if the brake resistor value is low enough.
It is rare, but also possible to demagnetise certain types of field magnets if extreme currents flow through the armature. Modern rare-earth magnets are less prone to this effect, but it can happen.

You’ll find lots of useful information on building electric locos at 4qdtec.com and on this forum http://modeleng.proboards.com/

alka:
Remember too that you can PWM the brake too ! You don't have to give it one hundred percent braking force. Maybe have a seperate control or pedal for proportional brakes.

Yes this is the correct way to do things - if you use synchronous rectification mode to drive your motor
braking is automatically handled (ie its a 4-quadrant control method).

Where you will need a resistor is to dump the energy if the DC rail voltage goes too high (due to
recovered energy from braking).

This is again normally done by PWMing the rail into a big dump resistor, you can just use a comparator
to generate the PWM - voltage too high, turn on the dump MOSFET, voltage recovers, turn it off. A small
amount of hysteresis is all you need for this.

I'd suggest researching 4-quadrant motor control.