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| How Do AC Motors Provide DC Power During Regenerative Braking? (1647972) | |
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How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by heypaul on Sun Apr 26 23:21:31 2026 I've long wondered about how much energy is actually retrieved by other trains as a result of regenerative braking?Now I have further muddied the situation. The new technology trains have ac motors, which require inverters to convert the 3rd rail DC power to AC. My question is when the train brakes, how does the AC motor/generator provide DC voltage to the 3rd rail? A separate rectifier? Or can you invert the inverter ? |
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Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by Olog-hai on Mon Apr 27 02:34:37 2026, in response to How Do AC Motors Provide DC Power During Regenerative Braking?, posted by heypaul on Sun Apr 26 23:21:31 2026. Apparently the newer LIRR trains have bidirectional converters, which act as both rectifiers and inverters. |
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| (1647975) | |
Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by heypaul on Mon Apr 27 08:55:26 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by Olog-hai on Mon Apr 27 02:34:37 2026. . Apparently the newer LIRR trains have bidirectional converters, which act as both rectifiers and inverters.Okay, that's interesting, so one component does 2 jobs. So next question does each car have its own bidirectional inverter? And if such an inverter fails, how does that affect the car's operation? |
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| (1647978) | |
Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by Jersey Mike on Mon Apr 27 14:05:34 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by heypaul on Mon Apr 27 08:55:26 2026. One inverter unit per car, one inverter unit per truck and one inverter unit per axle are all available options. Not sure what the LIRR chose, but likely truck or car since the M7's are over 20 years old at this point.The rule of thumb for AC propulsion systems is that energy can be sent from anywhere to anywhere with little added complexity in the hardware. This means that much of the regen braking is provided by the vehicle's own HEP load, which is generally 600-100hp per train. It's not uncommon for modern electric locomotives and MUs to not be equipped with resistance braking. |
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| (1647979) | |
Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by AlM on Mon Apr 27 14:42:49 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by Jersey Mike on Mon Apr 27 14:05:34 2026. It's not uncommon for modern electric locomotives and MUs to not be equipped with resistance braking.Huh? Sure, if you want to slow down you go into generator mode and make the rolling motion of the cars generate electric power that is accepted by some nearby load. So far, so good. But what if electrical connectivity is broken? Then there is no load to receive that generated voltage, and the train doesn't slow down. |
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Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by Bill West on Mon Apr 27 16:23:46 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by AlM on Mon Apr 27 14:42:49 2026. You go from Plan B back to Plan A, air brakes.Bill |
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| (1647989) | |
Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by AlM on Tue Apr 28 09:43:41 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by Bill West on Mon Apr 27 16:23:46 2026. Aren't air brakes a form of resistance braking? They cause friction between the wheels and the tracks.Or does "resistance braking" not mean what I think it means? |
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| (1647996) | |
Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by Spider-Pig on Tue Apr 28 15:05:41 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by AlM on Tue Apr 28 09:43:41 2026. No.In other words, "resistance braking" is not Plan B, it's plan Ω. |
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| (1648009) | |
Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by AlM on Wed Apr 29 13:29:28 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by Spider-Pig on Tue Apr 28 15:05:41 2026. If you've answered my question, which was, how can it be that new MUs don't have resistance braking, then I don't understand the answer. |
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| (1648012) | |
Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by Bill West on Wed Apr 29 16:26:27 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by AlM on Wed Apr 29 13:29:28 2026. It’s a language weakness.In the 1840s railroads had hand brakes that created mechanical friction between the shoe and the wheel. Although never called resistance it was indeed friction “resistance” braking. In the 1880s? Westinghouse used air to wind up the brakes, it was called air brakes. In the 1920s? catenary powered locomotives reconnected the motors to back feed into the catenary. It was called regenerative braking. In the 1940s? diesel electric locomotives reconnected the motors to an electric resistance (resistors) arranged in a grid. It was called dynamic braking. In the present era, power electronics allows several combinations of these ideas to be applied on a loco or MU car. Given that flexibility it has now become common to distinguish whether an MU car uses regenerative or dynamic braking by referring to the “feeding the motors to the resistor grids” mode as having the electrical resistance as the destination for the energy. IE when in dynamic braking, the car uses “resistance braking” equipment. This is electrical “resistance”. I think there are many places now where a new person, not knowing enough history, invents new word uses when perfectly good phrases already exist. Bill |
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| (1648013) | |
Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by AlM on Wed Apr 29 17:22:05 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by Bill West on Wed Apr 29 16:26:27 2026. Ah, thank you. But I think you raise even more questions.If electric voltage is applied to a motor, it can make wheels move forward and accelerate a train. But if you now divert that power to a grid of resistors, it stops accelerating the train. But it doesn't actively slow the train down, other than by letting air resistance and rolling friction with the tracks do their thing. So how is that any kind of braking? The automobile equivalent would be taking your foot off the gas pedal, but not stepping on the brake. |
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| (1648014) | |
Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by Fisk Ave Jim on Wed Apr 29 18:10:52 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by Bill West on Wed Apr 29 16:26:27 2026. At Metro North, there were instances where trains (ACMU's) would be sent out on the road using "straight air" as the engineers used to refer to braking w/o dynamic brakes |
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| (1648015) | |
Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by Train Dude on Wed Apr 29 19:29:52 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by Bill West on Wed Apr 29 16:26:27 2026. I recall many instances where a contactor on a Westinghouse car would hang up in braking and while the rest of the cars were in power mode, that car would be generating braking current until the grids began to melt. |
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| (1648016) | |
Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by Stephen Bauman on Wed Apr 29 20:47:35 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by AlM on Wed Apr 29 17:22:05 2026. If electric voltage is applied to a motor, it can make wheels move forward and accelerate a train.Other than a permanent magnet DC motor, there are 2 windings. One is stationary, the other is on the rotor. The stationary winding produces a magnetic field that is equivalent to the permanent magnet motor. When a voltage is also applied to the rotor winding, then it becomes a motor. If voltage is applied only to the non-moving winding and the rotor is rotated, it becomes a generator. If current is drawn from the rotor winding, then the rotor will encounter a torque that will oppose the rotor's continued rotation. The magnitude of the torque is proportional to the rotor's rotating speed. If the rotor winding is not connected to anything (open circuit), there will be no opposing torque within the motor other than bearing friction. The automobile equivalent would be taking your foot off the gas pedal, but not stepping on the brake. There is still engine braking, if the car is in gear. The equivalent would be for the driver to place the car in neutral or depress the clutch. Keeping the car in gear and releasing the accelerator is equivalent to either dynamic or regenerative braking. The difference between dynamic and regenerative braking is where the generator's power is dissipated. If it's used to store energy energy that can be converted back into electrical energy, or directly powers another motive device, then it's regenerative braking. If it's dissipated as heat into atmosphere then it's dynamic braking. |
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| (1648027) | |
Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by Spider-Pig on Thu Apr 30 11:36:51 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by AlM on Wed Apr 29 17:22:05 2026. But if you now divert that power to a grid of resistors, it stops accelerating the train. But it doesn't actively slow the train down, other than by letting air resistance and rolling friction with the tracks do their thing. So how is that any kind of braking?This is not true. As Mr. Bauman explained in a more detailed response, as a "motor" and "generator" is the same contraption, the motor generates electricity, and because in this house we obey the laws of thermodynamics, that electrical energy is either dissipated as heat, charges a battery, or used to drive another motor elsewhere. When I take my foot off the gas* pedal, that in fact causes my car to actively slow down, with the battery recharging. There's an onscreen bar that shows this and the slowing down is very noticeable. *Yes, just like I still "dial" a telephone, and then "hang up." |
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| (1648031) | |
Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by trains61 on Thu Apr 30 14:05:00 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by Spider-Pig on Thu Apr 30 11:36:51 2026. To sum up what Bill West and Mr. Bauman posted, motors convert electrical energy into mechanical movement, generators convert mechanical movement into electrical energy. Traction motors generally perform both functions.The Lurkers'Guild Hope this helps |
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| (1648043) | |
Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by AlM on Fri May 1 07:20:57 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by Spider-Pig on Thu Apr 30 11:36:51 2026. This is not true.Actually, it is true, but irrelevant. I was referring to the electrical network's voltage being diverted to a grid of resistors. But that isn't what's happening, which was my misunderstanding. The power contained in the forward motion of the vehicle is being used to generate electricity which goes into the grid of resistors (or in other circumstances, back into the network or into recharging a battery). And that slows down the train or the Tesla (where the vehicle's power goes into recharging the battery). |
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| (1648047) | |
Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by TRAIN DUDE on Fri May 1 13:24:14 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by AlM on Fri May 1 07:20:57 2026. LOL |
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Re: How Do AC Motors Provide DC Power During Regenerative Braking? |
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Posted by Jersey Mike on Tue May 5 07:00:26 2026, in response to Re: How Do AC Motors Provide DC Power During Regenerative Braking?, posted by AlM on Mon Apr 27 14:42:49 2026. Then dynamic brakes aren't available, however with year-round HEP loads its more efficient to remove the weight and complexity of the dynamic brake resistance grids at the cost of somewhat more friction braking. |
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