Brushed DC motor control

Thread Starter

gianko

Joined Aug 19, 2006
6
Hi
I started the development of a new project for the control of a Brushed DC motor 100W 24V.
I was thinking of using the Texas DRV8701 to control the bridge (the motor must turn in both directions)
Since I powered the circuit with a swtiching when I want to brake the motor I can't make a regenerative braking but I think to dissipate energy in a resistor. I wanted to insert a resistor in parallel to the motor during braking but I don't understand how to do it.

The procedure I thought to follow was to open the bridge mosfets at this point, while the motor is spinning due to inertia, the voltage must necessarily rise over VM (bridge supply) in order to maintain the current circulation. At this point I close the braking resistor path and I thought to put a diode to protect my switching power supply. But as soon as motor decelerate and voltage drops below the supply voltage it is no longer able to circulate current and no longer brakes but continue turns freely.

I don't understand how to set the braking to keep until the engine stops.
 

AlbertHall

Joined Jun 4, 2014
8,860
If you turn on both, say, both bottom MOSFETs this will short the motor and dissipate the energy in the motor resistance.
[EDIT] See line 2 of this table from the datasheet.
1576257780344.png
 

Thread Starter

gianko

Joined Aug 19, 2006
6
Thanks for fast reply AlberHall

I know the table and I use it but I made tests without mechanical load.

When the motor have a big inertia due to mechanical load I think that shorting the motor must dissipate a lot of energy inside the motor winding. I plan to use the DRV8701P that permit also the "coast" mode and in this mode I thought of applying the external resistor fro braking.

We plan to use this board for different types of BDC motors and systems and I would like to have maximum flexibility in the configuration of the control
 

MrAl

Joined Jun 17, 2014
6,837
What current are you driving the motor at, and what is the measurement of the current when you short out the motor after it has been running for some time?
 

Thread Starter

gianko

Joined Aug 19, 2006
6
We have made some test with a small motor without mechanical load and the DRV8701EVM board
Power supply 24V 1,7A
At motor the measured current is 8A rise until 10A (but depend on current limit set)
We have also tested a rapid change on speed PWM signal from 100% to 0% and the Voltage VM(=24V) on the bridge supply has invcreased until 45/47V for about 100ms (without mechanical load)
Our idea is using a power resistor with his control mosfet connect in parallel to the mosfet bridge. Whne the supply voltage rise above the nominal value we control the resistor for dissipate the energy
 

MrAl

Joined Jun 17, 2014
6,837
Would be nice to see the circuit or at least part of it.

It sounds like the motor is driving the power supply rail higher. That could happen when the power supply is not able to handle reverse currents. A power supply that can handle reverse currents would look more like an amplifier circuit. But yes maybe a resistor will work, but as you increase load you may increase inertia too and that will cause more energy dump into the power supply, so you need a good size resistor probably.
So in that case, what is the problem? Just switch the resistor across the power supply when you are to shut down the motor. An active dump would look like another transistor anyway.

If you can post a circuit or part of the circuit that would help i think too.
 

Thread Starter

gianko

Joined Aug 19, 2006
6
thanks for help MrAl
The mechanical load can be up to 100Kg and I think I hav e a lot of inertia.
No problem for posting my schematic but is work in progress. I made these test for understand how create my circuit.
Test are made with DRV8701EVM board that don't have any circuit for disspiating energy. Unfortunately I dont use the Texas MSP mcirocontroller. I must connect the EVM board brdige to my own micro board with my firmware. This require some time.....

I think (I hope...) using PWM cycle also on the braking resistor can help to reduce his size.
 

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MrAl

Joined Jun 17, 2014
6,837
Just wondering why do you want to use PWM on the resistor why not just turn it on with a single MOSFET?

What kind of mechanical load is it (the 100kg load) is it rotational or lifting or sliding or something else?
I ask because a rotational load will have rotational inertia that will tend to keep the motor running longer or at least with more energy to dissipate. Maybe if you can describe your physical load too.

As far as the circuit goes, it still sounds like you just have to turn a resistor on via a MOSFET. The control would be in the form of monitoring the speed of the motor (or motor current) to see when it stops so you can remove the resistor (via turn off the MOSFET). So basically sometimes the motor will stop faster than other times depending on the load or the original speed of the motor etc. Is that a problem?
 

Thread Starter

gianko

Joined Aug 19, 2006
6
MrAl sorry for so late reply I don't receive the notification mail.
The motor move a rotary table for welding, something like this
the table can be tilted and I don't know how the load is distributed
As you say the load, the speed ecc. change the stop time but I think is not a problem.
I think if I control the "braking resistor" with PWM I can reduce its size
 

MrAl

Joined Jun 17, 2014
6,837
MrAl sorry for so late reply I don't receive the notification mail.
The motor move a rotary table for welding, something like this
the table can be tilted and I don't know how the load is distributed
As you say the load, the speed ecc. change the stop time but I think is not a problem.
I think if I control the "braking resistor" with PWM I can reduce its size
PWM does not change the power dissipation capability as you allude to.
All it does is allow adjustment of the power dissipation but only up to the limit of the resistor power rating.
It does not increase the ability to handle power.
 

MaxHeadRoom

Joined Jul 18, 2013
19,485
If this is for a welding type table, they only require Very low rotation speed usually, on the ones I have built, if fast stop is required a worm and pinion G.B. is used so no over run when back fed.
Max.
 
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