Dynamic Braking of AC Traction Motors

[CastroPuss]

So the dynamic braking resistor dissipates (excess) power. But as the resistor appears to be connected across the DC part of the circuit, it seems to be dissipating power from the alternator (via rectifier diodes) which is not good (I don't think). This power should instead be going to the traction motors to provide the required excitation/electromagnetic field for dynamic braking. The resistor should ONLY be disspating power from the traction motors while in dynamic braking mode? It seems wasteful to have the dynamic braking resistor dissipate power from the alternator!?

The mechanism of how the alternator provides power to the traction motors and how power from the traction motors is generated and sent back at the same time and then power dissipated in the dynamic braking resistor is not entirely clear.

Can someone explain in detail how the system works while in dynamic braking mode please?

[erikem]

When the inverter is set to run slower than the synchronous speed of the induction motors, the inverters supply power to the DC bus as opposed to consuming power from the DC bus. The traction alternator is not capable of absorbing power from the DC bus, so the power as to go somewhere, hence the resistors.

 - Erik

[CastroPuss]

Thanks for the link! There was some useful information in that topic.

However, unless I missed something, it is still not clear how the dynamic braking resistors work/are connected while the main traction alternator is still supplying power to the AC traction motors while in dynamic braking mode. As mentioned in my first post, it appears the dynamic braking resistors are connected between the rectifier diodes and the inverters on the DC part of the circuit that still supplies power to the traction motors.

Put another way, the AC traction motors in dynamic braking mode still require power to their stator windings, and that power can only come from the traction alternator via the DC part of the circuit and then via the inverters. The dynamic barking resistors being connected at the DC part of the circuit would, surely, just tend to dissipate the power being supplied by the traction alternator via the rectifier diodes, leaving very little (if any) remaining power being delivered to the AC traction motors via the inverters??

This doesn't make sense!?

So how do the dynamic braking resistors actually work in conjunction with power being supplied to the AC traction motors at the same time, and on the same circuit (DC part), while in dynamic braking mode?

[rdamon]

not sure if we can post links to other threads  ...

http://cs.trains.com/trn/f/741/t/107987.aspx

 If not just cut and paste that into your address bar.

 

Or try this ...

 

http://tinyurl.com/o3xg66n

[CastroPuss]

Having difficulty in understanding how dynamic braking works while using 3 phase AC traction motors.

First, it is understood that three phase power is generated by the main traction alternator. This three phase power is then rectified (converted) to direct current (DC) by using diodes. The DC is then passed onto the inverter (consisting of either Gate Turn Off (GTO) thyristors or consisting of Insulated Gate Biplor Transistors (IGBT's)). By adjusting the firing/switching on sequence of the inverter, the DC is then converted back to three phase power for the AC traction motors.

The AC traction motors are usually three phase asynchronous type with "squirrel cage" rotor which has large conductive metal bars instead of windings. The rotor is electrically isolated from the AC traction motor stator, and the stator consists of heavy gauge wire windings. Three phase power from the inverter is applied to the stator windings only.

Now for dynamic braking, it is understood that dynamic braking resistors are still used to dissipate heat/energy in a similar fashion to that used for DC traction motor dynamic braking.

From some diagrams seen so far on the internet, it seems the AC traction motor dynamic braking resistors are connected on the DC portion of the traction circuits between the rectifier diodes and the inverter.

It is understood that to slow the AC traction motor while in dynamic braking mode, the frequency of pulses supplied by the inverter to the AC traction motor is slower than the "mechanical frequency" of rotor rotation due to train momentum turning the wheels and then turning the traction motor rotor. The rotor tries to follow the slower electromagnetic pulse from the inverter,  and this causes the rotor to resist the momentum and then creates braking.

But what is the purpose of the dynamic resistor then? With consideration of where the resistors are placed (between diodes and inverter), it seems that all they do is dissipate the DC power supplied by the rectifier diodes, and it appears to have little or no apparent interaction with the AC traction motors? So it is not clear what effect these resistors have on dynamic braking??