Why Does Regeneration Raise the Bus Voltage? ── Regen Resistors Explained with Live Circuit Diagrams

Part 3 of the series “Control-Circuit Protection and Troubles, Explained with Live Circuit Diagrams”

Last time, comparing four ways of stopping a motor, the one that looked the most elegant was regeneration: instead of burning the energy off as heat, hand it back to the supply as electricity. But the article ended with a caveat ── most real power supplies are not built to “accept” electricity flowing back in. So where does energy with no one willing to take it actually go?

The answer is this article’s main character: the rising voltage of the DC bus. Energy with nowhere to go piles up in the bus capacitors, and the voltage swells and swells. The “overvoltage trip” of an inverter or servo amplifier, and the part called a regen resistor (brake resistor) in the catalogs, are both tied to exactly this phenomenon. In this article we watch the voltage balloon ── and watch a resistor pull it back down ── using interactive live circuit diagrams and real-time charts.

What this article answers

Common questions, and the sections that answer them.

Question	Section
Where does regenerated energy finally end up?	§1
If the receiving end were an “ideal supply”, would anything happen?	§2 (live figure)
Why does the bus voltage rise in real equipment?	§3 (live figure)
How does a regen resistor (bus-side brake resistor) actually work?	§4 (live figure)
Is the brake resistor from Part 2 the same thing as a regen resistor?	§5 (live figure)
How does real equipment deal with all this?	§6 / FAQ

1. Today’s stage ── stressing the bus with an injection instead of a motor

In Part 2’s Fig. 4 we followed the regenerated current up to the point where it “heads back toward the supply”. This time we turn the camera around and make the receiving end ── the DC bus ── the main character. The DC bus is the shared DC backbone inside an inverter or servo amplifier, and large capacitors (the bus capacitors) hang on it to keep the voltage steady.

That is why no motor appears in today’s figures. The regenerated current that a spinning motor would push back is simplified into an injection from a “48 V source + 100 Ω” (the simplest possible teaching model of a coasting motor acting as a generator; effects like the generated voltage sagging with speed are left out). With that simplification, the story reduces to an “injecting side” and a “receiving side”, and what happens on the bus is laid bare.

The circuit skeleton is shared across the series: left of the vertical boundary is the regen side (the source of the injection); right of it is the DC bus side. The “Logic Y0” button on the panel switches the injection on and off ── in other words, it stands for “regeneration is coming back right now”. The only thing that changes from figure to figure is how the bus receives it.

V1 Vbus (bus voltage) ── watch whether it rises or gets clamped I1 Inject current ── flows while Logic Y0 is ON

A few ground rules for the controls, up front. Each figure also has its own step-by-step guide right above it.

• The buttons become active once the panel shows “Connected” (wait while it says “Loading simulator…”).
• The charts scroll like an oscilloscope, showing roughly the last 0.05 s. The numeric value also appears on the panel as “Vbus = … V”.
• The simulation only runs while the figure is on screen. If a chart looks frozen, scroll back to the figure.
• In these figures, a bus voltage that has risen may stay up even after you switch the button OFF ── that is the point of the article, not a glitch. To start over, use the “Reset circuit” button at the top right of the panel to reload the figure to its initial state.

2. With an ideal sink, nothing happens (Fig. 1)

The first figure re-establishes “the world of Part 2”. If the bus were an ideal 24 V voltage source ── a perfect sink that swallows any current you feed it ── regeneration coming back would cause no drama at all. Every other figure in this article is read against this baseline.

The ideal source absorbs the returning current as-is, so the bus voltage does not budge. In Part 2’s Fig. 4, 【V1】 leveled off near the supply voltage precisely because the receiving end was this ideal source. ── An enviable world that exists only inside a simulator, really. In the next figure, we swap this sink for something much closer to real hardware.

3. A bus that can’t absorb ── the voltage swells (Fig. 2)

Most real DC supplies make their DC by rectifying AC through diodes, and diodes are one-way streets: a supply built that way can push current out but cannot take current in. This figure boils that property down to its minimum: 24 V supply → diode → bus capacitor (470 µF). The supply can hold the bus at 24 V through the diode, but nothing on the bus can flow back into the supply.

This is why “we can regenerate” does not mean “problem solved”. The returning energy is turned away by the supply, so it has no choice but to accumulate in the bus capacitor ── and a capacitor is a part whose voltage rises the more you store in it. As long as the injection continues, Vbus keeps swelling.

	Fig. 1 (ideal sink)	Fig. 2 (realistic bus)
【V1】 during injection	Stays at about 24 V	Creeps steadily upward
Does current return to the supply?	The ideal source absorbs it	The diode blocks it
Where the energy goes	Recovered on the supply side	Accumulates in the bus capacitor
After the injection stops	Unchanged	The risen voltage stays

4. Add the regen resistor and see what changes (Fig. 3)

If the energy can only pile up, give it a place to be dumped. Onto Fig. 2’s bus we add a switch + 80 Ω resistor between the bus and 0 V. This is the skeleton of the part sold as a regen resistor or brake resistor (bus-side) in the catalogs: the surplus energy is converted to heat in this resistor and disposed of. This figure is the highlight of the article ── same injection, and the fate of the bus voltage flips with one button.

	Bus brake R = Off	Bus brake R = On
【V1】 during injection	Creeps upward (same as Fig. 2)	Clamps near 24 V
Where the energy goes	Accumulates in the capacitor	Mostly heat in the resistor
Practical meaning	Straight road to an overvoltage trip	Disposed of by the regen resistor; operation continues

One important difference from real hardware needs stating. In this figure a human toggled the resistor with a button; in real equipment a circuit called a regen chopper (brake chopper) watches the bus voltage and connects the resistor automatically the moment a threshold is crossed (for a 24 V system, some agreed value around 30 V, say), disconnecting it once the voltage falls. In other words, the thing you just did with your finger ── “connect when it rises, release when it falls” ── automated at high speed, is the real product. The skeleton of what it does is exactly this figure’s button.

5. Is Part 2’s brake resistor the same as a regen resistor? (Fig. 4)

In Part 2’s FAQ, the question “Is a brake resistor the same thing as a regen resistor?” got the answer: “the part looks the same, but there are two places to connect it”. This figure puts both in one circuit so you can compare them directly. There are two resistor paths: Terminal R (150 Ω) sits between the injection node (the spot standing in for the motor terminals) and 0 V ── the same hookup as Part 2’s Fig. 3 “brake resistor”. Bus R (80 Ω) sits between the bus and 0 V ── the same hookup as this article’s Fig. 3 “regen resistor”.

	Terminal R (Part 2’s brake resistor)	Bus R (this article’s regen resistor)
Where it connects	Across the motor terminals (before the bus)	Between the DC bus and 0 V
Its job	Keep the energy from reaching the bus	Dispose of what has reached the bus, as heat
How it looks on the chart	Never rises far in the first place	Pulls the risen voltage back down
Catalog name	Brake resistor (across the terminals)	Regen resistor / brake resistor (bus-side)

So here is the full answer to “are they the same?”: as parts, both are power resistors ── but they hold different posts in the circuit, and when a catalog says “regen resistor” it usually means the bus side. What Part 2 answered in words, this figure lets you confirm with your own eyes.

6. What real equipment does ── including options beyond burning it off

Today’s figures are teaching models trimmed down to “diode + capacitor + resistor”, but real countermeasures share the same skeleton. In the order you meet them in the field:

• Let the capacitors take it ── for short, small bursts of regeneration, the bus capacitance alone can absorb the energy. The reason Fig. 2 “doesn’t fail instantly” is this buffer.
• Burn it off with a regen resistor + regen chopper ── the automated version of Fig. 3, and the most common countermeasure. When a servo or inverter catalog lists a “regen resistor (optional)”, this is it.
• Let another axis use it ── multiple servo amplifiers can share one DC bus, so energy returned by a decelerating axis feeds an accelerating one. Redistribute before you throw away.
• Actually return it to the mains (power regeneration) ── a regenerative converter feeds the bus energy back into the AC supply. Chosen where large energy comes back repeatedly ── elevators, cranes ── it is, in effect, Fig. 1’s “ideal sink” purchased with money.

Which one to pick depends on how much energy comes back, how often, and what the equipment may cost. But whichever option is on the table, the starting question is the same one the four figures asked: “where does the returning energy go?”

Summary ── designing regeneration means designing the receiving end

• Regeneration is not finished at “return it to the supply”. If the receiving end cannot accept it, the energy piles up in the bus capacitors and the bus voltage rises (§2, §3).
• With an ideal sink the bus voltage never moves; what makes reality different is the supply’s rectifier diodes blocking the way back (§2, §3).
• Left alone, the climb runs into component voltage ratings ── hence the overvoltage trip (§3).
• The regen resistor (bus-side brake resistor) converts the excess into heat and clamps the voltage; in real equipment a regen chopper automates the on/off (§4).
• Part 2’s brake resistor (across the terminals) and the regen resistor (on the bus) look alike but hold different posts: the terminal side “keeps it from rising”, the bus side “knocks down the rise” (§5).
• Beyond burning it off: let the capacitors take it, share it between axes, or return it to the mains with power regeneration (§6).

In this series, Part 1, “Coil Back-EMF Explained with Live Circuit Diagrams ── What the Flyback Diode Really Does”, covered the momentary surge, and Part 2, “Stopping a DC Motor, Explained with Live Circuit Diagrams ── Coast vs. Brake vs. Regeneration”, covered where the kinetic energy goes. This article picked up where they left off: how the returning energy is received.

FAQ

Q1. What exactly is the “DC bus”?

A. The shared DC backbone inside an inverter or servo amplifier that distributes the rectified DC to the rest of the circuitry. Large capacitors (bus capacitors) hang on it to steady the voltage. It is also the first place regenerated energy arrives ── 【V1】 in this article’s figures measures exactly this bus voltage.

Q2. Why can’t an ordinary DC supply accept regenerated current?

A. Because most supplies make their DC by rectifying AC through diodes, and a diode is a one-way street. Current can only flow outward, so energy pushed back from the bus cannot enter the supply. The single diode in Fig. 2 is that property reduced to its minimum.

Q3. If the bus voltage keeps rising, what actually happens in real equipment?

A. First it approaches the rated voltages of the bus capacitors and switching devices, risking degradation or failure. Real equipment trips on overvoltage (an alarm stop) before that point, based on its bus-voltage monitoring. The classic symptom ── shorten the deceleration time and overvoltage alarms suddenly appear ── is this exact mechanism. The fixes are to decelerate more gently, or to provide a sink such as a regen resistor.

Q4. Who switches the regen resistor on and off in real hardware?

A. A switching circuit called a regen chopper (brake chopper). It connects the resistor when the bus voltage crosses a threshold and disconnects it when the voltage falls, repeating at high speed ── the automated version of what you did with the button in Fig. 3.

Q5. So can I treat “brake resistor” and “regen resistor” as the same thing?

A. As parts they are the same family (power resistors), but there are two hookups: across the motor terminals (Part 2’s Fig. 4, this article’s Terminal R) and on the DC bus (this article’s Fig. 3 and Bus R). When a catalog says “regen resistor” it usually means the bus side. Fig. 4 lets you compare the two roles directly.

Q6. Burning the energy off feels wasteful ── can’t it be recovered?

A. It can. Options include sharing one DC bus across several axes so the energy feeds another axis, and “power regeneration”, where a regenerative converter returns the energy to the AC mains. Both raise the equipment cost, so they are weighed against how much energy comes back and how often. For small amounts, burning it off in a resistor remains the cheapest and most dependable answer ── that is the practical market consensus.

Q7. How do these figures differ from real hardware?

A. Regeneration is replaced by a “48 V + 100 Ω” injection, so the real motor’s inertia and its sagging generated voltage are not reproduced, and the component values are teaching values chosen for visible motion. A real regen chopper also switches automatically and fast, where the figures use a manual button to expose the skeleton. Use the figures for the principle; use the equipment datasheets for real design numbers.

Q8. Logic Y0 is OFF but the bus voltage won’t come down ── is that a bug?

A. Not a bug ── it is Fig. 2’s main point. The stored charge has nowhere to go, so the voltage remains after the injection stops. (Real bus capacitors do the same: high voltage lingers after power-off, which is why equipment manuals specify a discharge waiting time.) To start over, press “Reset circuit” to reload the figure to its initial state.