Dynamic Braking of AC Traction Motors

Trains Magazine Locomotives
1

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

2

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

3

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?

4

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
5

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?

6

The dynamic braking resistor(s) is(are) not directly connected across the DC bus, but connected only when needed for braking. On a passenger locomotive using an inverter for HEP, some of the dynamic braking power can be used for HEP during braking.

7

Yes, it is understood that the dynamic braking resistors are connected across the DC part of circuit ONLY when in dynamic braking mode. This wasn’t made clear in my earlier posts.

While in dynamic braking mode, power still has to come from the alternator to provide the electromagnetic fields to the AC traction motors as part of the braking process. I don’t think there is another source of power going to the traction motors(?)

As the dynamic braking resistors are now connected to the DC bus while in dynamic braking mode, surely, much (most?) of the power from the alternator would be dissipated in the dynamic braking resistors instead of going to the AC traction motors? This doesn’t seem logical, as the traction motors need as much power as possible to provide braking.

Presumably, the AC traction motors while in dynamic braking mode, resist the braking effort by producing a back emf? This back emf, presumably, tends to generate a reverse current which goes back into the DC bus? But how can it do that when power from the alternator is still being provided to the traction motors? There, surely, there would be a conflict of current flow (power from alternator to tarction motors versus the current due to back emf)? And how would the back emf current reach the dynamic braking resistors for dissipation?

Again, if someone would explain the mechanics of how the dynamic braking system actually works in detail would be appreciated.

8

As an aside, the TA doesn’t supply tm excitation during braking except during changeover and low levels of braking.

The crux of the matter is as follows:

The power that flows into the AC motors is in alternating directions at all times. In power mode, power flows into the motor most of the time, but out of the motor some of the time. In braking mode, power flows out of the motor most of the time and into the motor for very brief intervals.

Power is defined by voltage and current, which can be shown as sine waves on a common axis. The direction of power flow depends on the relationship between voltage and current with respect to time, otherwise known as ‘phase relationship.’ The ‘phase relationship’ depends on the speed of the rotating field with to the rotating speed of the rotor. If rotor speed lags field speed, voltage and current are nearly in phase and current flows into the motor most of the time. If rotor speed exceeds field speed, voltage and current are out of phase by nearly 180 degrees,power flows out of the motor most of the time, as in dynamic braking.

I note that that the DC Link caps. excite dynamic braking except as described in the first sentence.

CPM500

9

What gave you the impression that the traction motors are three-phase? They could be called infinite-phase, because of the varying frequency and the multiplicitiy of armature coils, but they are definitely not three-phase motors in the classic sense of what a three-phase motor might be.

That does not prevent them from acting as generators under the conditions specified in a previous pose on this thread. Note also, that on electrified lines, mus and electric locos can feed the catenary, with the power from the dc buss converted to ac at exactly the right frequency, voltage, and phase by computer-controlled choppers with smoothing reactors.

If there are any three-phase induction or hysterises non-synchronous motors, I’d like to know who builds them and where they are used. Their use would certainly complicate the computer control even further for varying speed operation.

10

NO!!

The Power allways flows into the motor when operating as a motor and out from the motor when operating as an induction generator (as in dynamic breaking).

When operating as a motor, the inverter is supplies both motive power and excitation (reactive) current as well as reference frequency, with the mechanical speed a little slower than the “electrical” speed.

When operating as in the dynamic breaking mote the motor becomes an induction generator, the inverter is supplies excitation (reactive) current as well as reference frequency, with the mechanical speed a little faster than the “electrical” speed, and absorbs motive power. This energy is either dissipated in the DC dynamic breaking resistors, or could be “fed back” through an active fornt end, if equipped, to other AC loads.

11

The traction motors on the new ACS-64s ARE 3 phase AC induction motors!

12

CP,

The crux of the matter is how the inverters provide reactive power to the induction motors when in dynamic braking mode.

First point, the inverters can also act as rectifiers (i.e. power flows from AC (motor) side to DC side). The inverters on GE locomotives act like voltage sources, so the switching between inverter and rectifier happens automatically if for no other reason to handle the second point below.

Second point, due to the induction motor being a reactive load, the power on each phase feeding the motor will alternate direction of flow twice for each cycle of the output waveform. On a single phase inverter, this power reversal will be handled by the de-coupling capacitors in the inverter. On a three phase inverter, the net reactive power flow is zero across all three phases - i.e. the reactive power flowing out on one phase will be equal to the power flowing in on the other two phases.

When the power generated by dynamic braking is greater than the losses in the motor and inverter, the inverter will happily run off the power generated by the motors, with the resistors sinking the current needed to keep the DC voltage under control. At very low speeds, the isn’t enough power generated to keep the inverter running, so the main traction alternator will then supply the power. In this case, the dymanic braking power is being dissipated in the motor.

  • Erik
13

JC

Your last sentence-exactly what occurs on the EMD DE/DM passenger locos, with the other load(s) being HEP.

CPM500

14

Please describe in what way are they three phase? As far as I know they are the state-of-the art non-synchronous induction motors with slanted rotating aluminum or copper bars and a series of field coils with their appropriate magnet structure, and computer-timed rotating exitatation, with current reverswal alternating. Current in a coil has only two directions. Induced current in the rotating bars has only two directions.

The classic three-pharse motors had three or six slip rings feeding a rotating arrmature that had, as a minimum, three coils, instead of a minimum of two, but could have a much larger number, again multiples of three instead of multiples of two. Is this the type of motor used?

If not, how does your three-phase non-synchronous induction motor differ from a standard non-synchronous induction motor, the kind that have been standard on ac-motor diesel-electrics, electrics, mu cars, light-rail cars, rapid-transit cars for some time?

The advantages of this motor over any ac motor (or dc motor) with slip rings (or commutator) are far better tolerance of heat overload and lack of maintenance requirements for the slip rings (or commutator).

Possibly, given improved technology and new products for insulation, the old armuture three-phase motor with slip rings is making a comeback. If so, it is news to me, and I would be happy to learn about it. Theoretically, it should have a slight advantage in efficiency.

Again, I wish to remind that neither motor powered the classic single-phase 25Hz electrics like the GG1. Those were commutator motors, basically a dc motor with specific supplementary coils to counter some of the effects of the current reversal and interruption, and they do also operate fine on dx, but would just shake and not rotate, even with no load, at 50 or 60 Hz.

Back to the main argument. Possibly if specifications say “3-phase induction motor” they mean how the comput

15

APOLOGIES!!! PROBABLY ALL RAIL AND TRANSIT INDUCTION BASED MOTORS ARE THREE-PHASE, ANYTHING ELSE WOULD BE WASTEFUL!

By 3-phase, the number of rotating bars and field coils, the same number, is always a multiple of three, not two. 9, 12, 15, 18, 21, 24, 27, all are practical numbers, although the even ones could be operated as single-phase motors if the controls were so programmed. At a moment when coils 2, 5, 8, 11, are at “full power” at “positive polarity”, 1, 4, 7, 10, etc are at approximately one-third-power at negative polarity and heading toward full power at negative polarity, while 3, 6, 9, 23, are at one-third power at negatifve polarity heading towards zero and positive polarity.

These motors thus develop almnost constant torque, whereas a single-phase motor would peak and drop 120 times a second on a 60-cycle feed, Like your typical constant-speed fan motor.

Again, apologies.

16

This text is from a Siemens brochure describing the AR15 locomotive used in Vietnam:

The DC link is supplied with power by means of the brushless three-phase synchronous generator flanged directly to the diesel engine and a B6 rectifier bridge. Each of the 3 three-phase asynchronous motors of a bogie is fed from one of the two partial DC link circuits (which are interconnected in normal operation) by means of a pulse-width modulated inverter; these 3 motors being controlled as a group. Water-cooled IGBT-type power electronics are used. This state-of-the-art propulsion technology enables maximum utilization of tractive effort.

If the field coils, which certainly are part of the motor, are supplied with three phase power, how is this not a three phase motor. As it is an induction motor, no current is supplied to the rotor directly, only to the field coils, and if these are three phase, the motor is three phase.

17

You people are making this way too complicated. Most three phase motors used in industry, and as traction motors in locomotives and mining trucks, are simple induction motors. Their rotating speed is proportional to the incoming frequency. Three phase is used because it is easy to reverse direction by either reversing the phase sequence or simply switching two of the leads around.

This type of motor has no commutator, slip rings, or anything like that.

There are no electrical connections to the rotor, only static coils in the stator. The rotating magnetic field in the stator coils drags the rotor around making the motor turn. I don’t know about locomotives, but most of these rotors are just laminated “star” shaped steel sheets attached to the motor shaft. There is usually one fewer star point than field coils so that the magnetic forces are unbalanced and the motor will start.

Now for the magic. If there is an external load on the motor it will try to turn at an RPM proportional to the incoming frequency. This is why these motors will not “birds nest” or speed up uncontrollably if the load is lost. It’s also why the adhesion factor is so high for these locomotives.

The higher the load, the more power it will draw until it consumes all the power available. If the motor is driven externally and tries to go faster than the incoming frequency would allow, like in dynamic braking, it will start generating power. This flows back through the inverter and is dissipated as DC in the dynamic brakes.

The motor needs to be at least “excited” to start generating. The larger the difference between the actual RPM and the excitation frequency RPM, the more power it will generate.

Forget everything you knew about AC locomotives produced before the 1990’s. Inverter te

18

YOU ARE CORRECT, AND ALL INDUJSTRIAL AND RAILROAD INDUCTION MOTORS ARE ESSENTIALLY THREE PHASE. IF THEY HAD AN EVEN NUMBER OF FIELD COILS AND ROTATING BARS, SAY SIX, OR TWELVE, OR EIGHTEEN, THEY COULD ALSO BE OPERATED IN SINGLE PHASE WAY, LIKE YOUR TYPICAL FAN MOTOR, BUT THIS WOULD BE AN INEFFICIENT USE OF THE WEIGHT AND SIZE OF THE MOTOR. FO RTHREE PHASE OPERATION, THE NUMBER MUST BE A MULTIPLE OF THREE, FOR SINGLE PHASE A MULTIPLE OF TWO.

ALSTOM HAS AN INSIDE OUT MOTOR, WHERE THE ROTATING BARS ARE OUTSIDE, AND THE FIELD COILS INSIDE, AN AC WHEEL MOTOR, THAT CAN BE THE HUB OF THE WHEEL, WITH THE FIELD COILS ON A RIGID AXLE AND THE ROTATING BARS ON THE INSIDE OF THE WHEEL RIM. MAGNET MOTOR AND STORED ENERGY SYSTEMS HAVE ROTATING PERMANENT MAGNETS, A SORT OF INSIDE-OUT DC MOTOR IN PRINCIPLE, BUT AGAIN THREE PHASE EXCITATION OF THE INSIDE FIELD COILS, MULTIPLES OF THE THREE IN NUMBER. THESE MOTORS ARE WIDELY USED IN EUROPE ON LOW FLOOR BUSES, ESPECIALLY THOSE IN USE IN AIRPORTS FOR TERMINAL TO PLANE TRANSPORTATION

SORRY ABOUT A COMPUTER ISSUE FORCING USE OF CAPS

19

Computer problem solved. Siemans and othe alternators connected directly to the diesel do not, of course, have commutators, but they do have slip rings, three being the minimum nummber. Rotating bars, as in induction motors, do not, of course, need slip rings. Alternators as generators built on the induction motor principle are possible, but are less efficient than armature and slip-ring alternators.

There are motors with one less bar than the number of field coils, and the bars are parallel to the shaft. There are motors with the same number of bars as field coils and the bars are slanted. The first type is more efficient but less tolerant of overload. If overloaded it will stall quickly if it cannot keep up very close to programmed speed. The second type always runs slightly under the speed of the signal rotating around the field coils and simply looses more and more speed, running slower and slower, until it stalls. I believe it is this second type in use in most modern locomotives. In either case, stalling does NOT mean exstreme heat and a flashover, as it usually would with a dc motor. Of course in real locomotives, a cicuit breaker woulod open and interrupt the current before flashover, most of the time.

20

And most non-transportation uses, most industrial uses, are constant speed and constant load.