# Slow Decay Mode Brakes Motors Harder? Really?

#### K-Sci

Joined Feb 12, 2021
4
I've encountered two articles, one here and one by TI, saying slow (current) decay mode (circulating currents in the top or bottom of the bridge) stops motors faster than fast (current) decay mode (using the bridge diodes or transistors to connect the supply voltage so it opposes the established current). A motor will stop faster if the current (torque) is reversed more quickly. By definition, a fast decay mode reduces the current (torque) faster than a slow decay mode. How is it possible that a motor would stop faster when the current (torque) driving the rotation in the direction of the inertia is higher?

Am I missing something?

#### shortbus

Joined Sep 30, 2009
8,591
Why are you equating torque and current, when talking about braking?

#### K-Sci

Joined Feb 12, 2021
4
Why are you equating torque and current, when talking about braking?
Current through a coil produces the same magnetic fields and magnetic fields induce the same currents in a coil regardless of why there is current flowing or why the rotor is turning. If mechanical friction were zero, the only way to stop a motor more quickly (or at all) would be to generate more current and torque opposing the established direction of rotation.

I'm not clear on the reason for your question.

Joined Jul 18, 2013
23,123
The method sometimes used in 3ph motors is 'Plugging', This entails reversing the 3ph power immediately while the motor operating, the down side it produces a great deal of heat in the rotor and can cause problems if not allowed to dissipate between braking stops.
For the DC bridge braking, the conduction of the bottom two switching devices is preferred if the fastest method is desired.
Max..

#### shortbus

Joined Sep 30, 2009
8,591
I'm not clear on the reason for your question.
Because torque is not current. You are mixing different terms and that is probably why you can't find an answer to your problem.

#### K-Sci

Joined Feb 12, 2021
4
The method sometimes used in 3ph motors is 'Plugging', This entails reversing the 3ph power immediately while the motor operating, the down side it produces a great deal of heat in the rotor and can cause problems if not allowed to dissipate between braking stops.
For the DC bridge braking, the conduction of the bottom two switching devices is preferred if the fastest method is desired.
Max..
The device I'm designing is a power assisted propulsion for a 200Lb push-cart that uses a 12V 50Ah Li Polymer battery to supply electronic equipment. The power assist is used to prevent back strain for users that push the cart from room to room every 5-10 minutes over an 8 hour shift.

My plan for the design is to use a 4-quadrant FOC controller to drive a BLDC Motor. The motor has distributed windings that produce sinusoidal back-EMF (Technically this is an AC PMSM). To the extent that I can keep the DC bus voltage from rising above the maximum charging voltage for the battery, the braking can be regenerative. Once it hits that limit I will have to use the low-side transistors.

When braking using the low-side of the bridge doesn't the motor's back-EMF drive current in a loop through the low-side lof the bridge. The only elements capable of for power dissipation in that loop are the motor resistance, FETs, wires, and other parasitic resistances? If I understand this right (not guaranteed) doesn't that mean that the cart's inertial energy ends up as I^2*R losses in the stator windings?

What's bothering me is that I'm reasoning this through on my own and I'm not sure if I'm missing some key element of understanding.

#### K-Sci

Joined Feb 12, 2021
4
Because torque is not current. You are mixing different terms and that is probably why you can't find an answer to your problem.
Don't the torque and current vary according to the motor's torque constant even when the motor is operating as a generator or shorted through the low-side windings?

Joined Jul 18, 2013
23,123
Then there is electrical torque and mechanical torque.!!

Electrical motor torque is proportional to the product of magnetic flux and the armature current.
Mechanical or load torque is proportional to the product of force and distance. Motor current varies in relation to the amount of load torque applied.
When a motor is running in steady state, the armature current is constant, and the electrical torque is equal and opposite of the mechanical torque.
When a motor is decelerating, the motor torque is less than the load torque.
Conversely, when a motor is accelerating, the motor torque is higher than the load torque.

#### kaindub

Joined Oct 28, 2019
70
I hope you understand what four quadrant means
A four quadrant controller can go forward and backwards , driving the load, as well as forward and backwards braking , in a controlled manner, the load.
Since you are using a battery, the braking power will be fed back into the motor. You dont need to do anything for this to happen (except reduce the rotating field frequency to the motor)
DC braking is sometimes used at the end of the deceleration to 'lock" the rotor since the breaking effect is not continuous to zero speed.
DC braking is a somewhat crude method of braking and the effect of DC braking is dependent on the speed of the rotor as well as the applied current. And its not linear as the speed changes.

#### du00000001

Joined Nov 10, 2020
63
What most answers are focusing upon seems to be the inductive component (aka "coils") of a motor. Which is more or less negligible. And "current decay" - whether fast or slow - is merely a symptom, not a good characteristic for motor braking.

What counts with respect to motor braking is the motor's momentum once it's rotating.
To brake the motor you somehow have to reduce the rotational energy of the motor to zero. Beyond powering it "the reverse way", the second-best option is to simply short its windings, thus shorting the EMF generated. IOW: shorting a generator is the best way to stop it as fast as possible. Might even be you want to make it braking somewhat "less hard" - the shorting the windings is really efficient when it comes to DC motors. (But I expect AC motors to brake not much slower.)

Joined Jul 18, 2013
23,123
. (But I expect AC motors to brake not much slower.)
AC induction motors require DC injection or some other kind of dynamic braking.
(or plugging as per #4).
Max.

#### du00000001

Joined Nov 10, 2020
63
AC induction motors require DC injection or some other kind of dynamic braking.
(or plugging as per #4).
Max.
Might be ACIMs (cage rotor) cannot be braked to standstill. But you can slow them down significantly by simply shorting the stator windings.
As there is quite a number of AC motor types: whether you can brake them down to standstill by shorting the windings depends on the exact model.