Stopping a DC Motor, Explained with Live Circuit Diagrams ── Coast vs. Brake vs. Regeneration

Part 2 of the series “Control-Circuit Protection and Troubles, Explained with Live Circuit Diagrams”

As you know, a motor does not stop the moment you cut the switch. The rotation carries momentum (inertia), and current is still flowing in the windings. So where do that momentum and that current go? The different answers to that question are exactly what the terms coast, short-circuit brake, brake resistor, and regeneration distinguish ── terms you need to handle with safety in mind. ── If all you want is definitions, any glossary has them. But which loop does the current circulate in, where does the heat come out, does anything return to the supply or not ── a static schematic rarely makes those distinctions stick.

In this article we watch what happens right after a DC motor’s drive is switched off, using live circuit diagrams and real-time voltage and current charts. All four ways of stopping share the same circuit skeleton and the same controls, so the differences in current path and where the energy ends up become plainly visible. Every figure is interactive: toggle the drive with the button and watch the chart with your own eyes as you read.

What this article covers

Common questions and the sections that answer them.

Question	Section
What happens to the motor’s current and rotation after the drive is cut?	§1
What happens if you just cut the drive and do nothing (coast)?	§2 (live figure)
Why does the short-circuit brake work? Which loop does the current take?	§3 (live figure)
How is a brake resistor different from a plain short?	§4 (live figure)
What exactly is “regeneration”? What happens when energy returns to the supply?	§5 (live figure)
How do you choose among the four stopping methods?	§6 (comparison table) / FAQ
How does this relate to Part 1’s coil back-EMF story?	§1 · FAQ

1. A motor is a spinning generator ── stopping means choosing where the energy goes

Inside a DC motor is a coil (the armature) spinning between magnets. That means a motor inherits the coil property we met in Part 1 ── it hates having its current changed abruptly. But a motor adds a second property that a relay coil never had: as long as it spins, it generates a voltage of its own. Motors and generators share the same construction; spin one from outside and it generates. The moment you cut the drive, a motor still turning on its inertia is, quite literally, a generator.

Let’s take stock of the energy involved. In Part 1, the troublemaker was the energy stored in the coil’s magnetic field ── it shows up as a single brief surge. A motor has that too, plus something much bigger: the kinetic energy of rotation. It dwarfs the magnetic energy and takes far longer to disappear. So “stopping a motor” is really a design decision: where, and into what, do you dump (or recover) that kinetic energy? The options are coast, the short-circuit brake, the brake resistor, and regeneration.

All the figures below share one skeleton. Left of the vertical boundary is the external side (the motor); right of it is the drive-switch side. The path is: 24 V supply → 50 Ω series resistor → drive switch → DC motor → 0 V. The “Logic Y0” button on the panel plays the role of the drive command. The only thing that changes from figure to figure is the stop path added around the motor’s terminals.

The instrumentation is the same as in Part 1: the real-time charts above each figure trace 【V1】 (the voltage at the motor’s + terminal) and 【I1】 (the motor current) over a sliding window of roughly the last 0.05 seconds of simulated time. If you want a fresh start, the “Reset circuit” button at the top right of each panel reloads the figure to its initial state.

V1 Voltage at the motor’s + terminal ── watch the spike, or the ceiling, right after OFF I1 Motor current ── watch how fast it decays and which path it takes

2. Coast ── cutting the drive with no path prepared (Fig. 1)

The first figure has no stop path at all. Cut the drive switch and the motor’s circuit is simply left open. That is coasting (free-run stop), and every other stopping method in this article is read against this baseline. Turn “Logic Y0” ON, watch the current flow and the speed build, then switch OFF before the speed tops out ── and keep your eyes on the 【V1】 chart right after OFF.

At the instant of OFF, the current left in the windings loses its path, and the same momentary surge we saw in Part 1 appears on 【V1】 (a motor winding is a coil, so this much is review). What follows is this article’s real subject. Even after the current dies, the motor keeps spinning on inertia, still generating voltage at its terminals as a generator ── but the circuit is open, so no current flows. No electrical braking happens at all, and the speed drifts down slowly on nothing but bearing and load friction. “I switched it off, and it just keeps turning” ── coasting is the stopping method that leaves the kinetic energy entirely to friction.

Note: Figures 1–3 include a small series RC that stands in for the capacitance and leakage of real wiring, added so the post-OFF ringing does not go on forever. It is not part of the main path.

3. The short-circuit brake ── connect the terminals and the current keeps circulating (Fig. 2)

Now we add a stop switch across the motor’s terminals. Cut the drive, then close this switch, and the motor’s two terminals are shorted together. A spinning motor is a generator ── and a shorted generator drives current. The current keeps circulating around the small loop motor → short switch → motor, and its energy source is the rotation itself. In other words, the rotation gets braked. This is the short-circuit brake (a form of dynamic braking).

In the figure below, that stop path is on a button. First leave “Short-circuit brake: Off”, run a cycle ON → OFF, and confirm the surge from Fig. 1. Then, after OFF, switch the brake to “On” and repeat the same steps. Same circuit, same controls ── the 【V1】 spike is tamed, and you can watch the current circulate through the short loop instead.

One important check while you are here: the short-circuit brake’s current never returns to the supply. It circulates only between the motor and the short switch, and the rotational energy is burned off mainly as heat in the armature (winding) resistance. It is tempting to imagine that “braking = feeding electricity back to the supply”, but that is §5’s story (regeneration). The short-circuit brake simply converts the energy to heat on the spot and throws it away.

	Brake Off (coast)	Brake On
【V1】 right after OFF	Sharp momentary spike	Spike is tamed
Current path after stop	None (open)	Motor ↔ short-switch loop
How the speed falls	Slowly, on friction alone	Fast, by generator braking
Where the energy goes	Surge · friction	Mainly armature heat
Returns to supply?	No	No

4. The brake resistor ── choosing where the heat comes out (Fig. 3)

The short-circuit brake has two weak points ── the heat lands in the armature, and the current is huge. Both can be eased at once by replacing the short with a resistor of a value you choose. That is the brake resistor. In the figure below, the stop path across the motor’s terminals is a 150 Ω resistor in series with a switch. The controls are the same as Fig. 2: run OFF with “Brake resistor: Off” to see the surge, then repeat with it “On”.

Compare with Fig. 2 and you should see 【I1】 decay more gently than with the plain short. A larger resistance means less braking current and a softer brake; smaller brings you back toward the short-circuit brake. And the heat now comes out mainly in the external brake resistor rather than the armature. Letting a properly heat-sinked resistor absorb the kinetic energy, instead of the heat-sensitive motor itself ── that is why “brake resistors” (often sold as “regen resistors”) exist around servo amplifiers and inverters. The designer chooses both the braking strength and the place where the heat appears ── that is the essential difference from a plain short.

5. Regeneration ── building a path back to the supply (Fig. 4)

The last method is different in character from the other three. To the Fig. 1 circuit we add a single diode from the motor’s + terminal to the 24 V rail (anode on the motor side, cathode on the supply side). After the drive is cut, whenever the spinning motor’s terminal voltage tries to rise above the supply, the diode conducts and current flows into the supply side. Instead of burning the kinetic energy as heat, we hand it back as electricity ── this is the doorway to regeneration.

The controls are the usual ON → OFF. This time, right after OFF, watch the current dots head through the diode toward the supply. And instead of Fig. 1’s sharp spike, 【V1】 tops out near the supply voltage ── the current that had nowhere to go now has a road home to the supply.

6. Choosing among the four ── by current path and heat location

Here is everything from the four figures on one sheet. It is not that one method is better than the others: you choose by answering three questions ── which loop does the current take, where do you dispose of the energy, and how fast must the motor stop. That is this article’s conclusion.

Method	Current path after stop	Where the energy goes	Stopping speed	In one line
Coast (Fig. 1)	None (open)	Friction (+ OFF-instant surge)	Slow, uncontrolled	A stop you never really designed
Short-circuit brake (Fig. 2)	Shorted-terminal loop	Mainly armature heat	Fast (current is large)	Powerful, but the motor bears the heat
Brake resistor (Fig. 3)	Terminal loop + resistor	External resistor’s heat	Tunable via resistance	Strength and heat location by design
Regeneration (Fig. 4)	Back into the supply	Supply side (recovered, ideally)	Depends on the receiver	Hinges on having somewhere to return it

One more thing, whichever method you pick: the OFF-instant surge still needs its own protection (Part 1’s diode and friends). In the brief moment before the stop-path switch closes, the winding current has nowhere to go. Design the stop, and protect against the surge ── as two separate jobs, both done. That is the practical thread connecting Part 1 and this article.

In real products you will rarely wire these bare circuits yourself: motor drivers and H-bridges come with the short-circuit brake (brake mode) and regen paths already built in. But when a datasheet says “brake”, “coast”, or “regenerative”, the four figures in this article are exactly the map of what those words mean.

Summary ── “stopping” means deciding the current’s path and the energy’s destination

• Cutting the drive does not erase the motor’s current or rotation. A spinning motor is a generator, and designing a stop means choosing where the kinetic energy goes (§1).
• Coast has no path: no electrical braking, stopping left to friction, and the OFF-instant surge appears for the same reason as in Part 1 (§2).
• The short-circuit brake circulates current in a terminal loop, turning the rotation’s momentum into armature heat for a fast stop. Nothing returns to the supply (§3).
• The brake resistor is “a short with a chosen Ω”: the designer picks both the braking strength and where the heat comes out (§4).
• Regeneration is a path back to the supply ── but most real supplies cannot accept it, which raises the next problem: bus voltage pump-up (§5, sequel planned).
• Whatever the method, OFF-instant surge protection is a separate, additional requirement (§6).

The foundations of this article ── “a coil hates abrupt current changes” and “where the surge lands” ── are laid in Part 1, “Coil Back-EMF Explained with Live Circuit Diagrams ── What the Flyback Diode Really Does”, and the output stage that drives the motor is covered in “Relay, Transistor, or Triac? Understanding PLC Output Types with Live Circuit Diagrams”, both with the same kind of interactive figures.

FAQ

Q1. Coast, short-circuit brake, regeneration ── each in one line?

A. Coast: provide no path and wait for friction. Short-circuit brake: make a loop across the terminals and burn the energy as heat on the spot, stopping fast. Regeneration: make a path back to the supply and return the energy as electricity. The axis that separates them is which loop the current takes after the stop.

Q2. How is this different from Part 1’s coil surge?

A. Part 1 was about the energy stored in a magnetic field ── a single brief surge, countered with a “momentary escape path” such as a flyback diode. This article is about disposing of the kinetic energy of rotation ── far larger, lasting far longer, and countered by designing the stop itself. In a motor both happen at once, so you need surge protection and stop design as two separate items.

Q3. Does the short-circuit brake’s current return to the supply?

A. No. The current circulates only in the loop between the motor and the short switch, and the energy is dissipated mainly in the armature resistance as heat. Energy returns to the supply only when a path toward it exists, as in Fig. 4 (regeneration).

Q4. Are “brake resistor” and “regen resistor” the same thing?

A. As a component, both are power resistors; the name usually just follows the context. But there are two distinct hookups: across the motor terminals as in Fig. 3, or on an inverter/servo DC bus, where it burns off the bus-voltage rise caused by regeneration (sequel planned). When a catalog says “regen resistor”, it usually means the latter.

Q5. Can the short-circuit brake be applied at any time?

A. No. Because the loop resistance is tiny, the braking current at the moment of engagement is large, and from high speed or high inertia it can exceed the ratings of the switch or the motor. Real drivers and motors specify the conditions (speed, duty cycle, and so on) under which the brake may be used. When in doubt, lean toward a brake resistor that limits the current.

Q6. What actually goes wrong when regenerated energy returns to the supply?

A. The receiver is the problem. Fig. 4’s ideal source absorbs the current, but most real DC supplies are not built to sink current. With nowhere to go, the current charges the bus capacitors and the bus voltage climbs. Inverter “overvoltage trips”, bus-side regen brake resistors, and regenerative converters are all responses to this.

Q7. Why do the figures tell me to switch OFF before the speed tops out?

A. Because once an unloaded motor reaches full speed, its generated voltage (back-EMF) nearly equals the supply and the current almost stops flowing. Cut the drive then, and there is little current left to lose, so the surge is small too. Switch OFF during acceleration ── while current remains ── and the behaviour of current losing its path shows up clearly.

Q8. Which of these is the “brake mode” of an H-bridge or motor driver?

A. Usually Fig. 2’s short-circuit brake: both low-side (or both high-side) switches of the bridge turn on together, shorting the motor terminals. In datasheet language, coast (freewheel) corresponds to Fig. 1, brake to Fig. 2, and regenerative braking to the Fig. 4 family of paths.

Q9. Does the same thinking apply to AC (induction) motors?

A. The skeleton ── decide where the kinetic energy goes ── is the same, but the paths are built differently. With an inverter-driven AC motor, deceleration regenerates through the inverter into the DC bus, leading straight into the Fig. 4 sequel world of bus pump-up, brake resistors, and regenerative converters. AC-specific methods also exist, such as DC-injection braking.

Q10. How does the simulation differ from real hardware?

A. It is stripped to the principle’s skeleton. The motor constants (torque constant, inertia, friction) are teaching values chosen to make the behaviour visible, and the 50 Ω series resistor doubles as a way to slow the current dots. The small series RC in Figures 1–3 stands in for real wiring’s capacitance and leakage, purely to damp the ringing ── it is not part of the main path. Fig. 4’s supply is an ideal voltage source, so the bus-voltage rise that troubles real hardware does not occur. Use the figures to understand the principle, and motor/driver datasheets for real design numbers.