Trains with AC drive

[Perry Babin]

Do all locomotives with AC driven traction motors use Variable Frequency Drive to determine the motor speed?

[Overmod]

All modern ones do.

A number of famous locomotives used direct AC motors at the beginning of the 20th Century, notably the PRR FF1 'Big Liz' (counterpart to the HB1 simple articulated).  These used giant synchronous motors that locked into slightly slower phase with a rotating field, producing enormous torque... at one or two speeds.  Uphill and on the level, it was all the same.  This was so bad that the 'replacement technology' was actually a motor-generator locomotive: the synchronous motor drove a DC generator with variable field excitation to control conventional DC motors,  And all the subsequent action was with rectifiers -- ignitrons, solid-state devices, all running 'diesel-style' DC traction motors 

In a sense this practice continues in modern AC-drive units, which rectify the essentially-variable-frequency AC from the traction alternator (admittedly the 'variable' frequency is fixed in steps corresponding to the governed engine rpm in each run notch) to what is called the DC-Link.  The AC to the motors is synthesized from this.

[Perry Babin]

If I read right (various places), AC is better than DC on tough starts because there is essentially no slip due to the variable frequency drive. 

If the speed is locked to the 8 notch speeds, how is this true?

I could see it being better if there was an infinitely variable frequency control but it would seem that DC would be better on starts (if the wheels weren't allowed to slip). 

[SD70Dude]

The 8 throttle notches govern engine RPM and/or potential power output, they do not have a fixed relationship with the locomotive's speed or traction motor RPM. 

Contrary to what you may have read, AC traction units can and do slip, sometimes quite violently.  They are a lot better than DC units in the vast majority of situations but they are not magic, and given the opportunity in pouring rain or deep snow an ES44AC can chew up rail just like an SD40.

In my opinion the biggest benefit of AC traction motors is their durability, I don't think I've ever had to cut out a defective AC motor whereas this is a regular occurrence on DC units.  And even if you do have to cut out an AC motor you will still have working dynamic braking, on a DC unit you lose DB completely as soon as you cut out a motor.  

[Pneudyne]

Referring to the original question, if one looks back beyond the “modern” era, then the answer is “no”.

 

Single phase commutator motors, used in new production electric locomotives until the early 1990s, operated on a single fixed supply frequency.  There were one or two in Europe that could operate on either 16⅔ or 50 Hz, but that was simply to facilitate the use of locomotives on different catenary systems.  These motors behaved somewhat like DC series wound motors , and so their operating speeds were essentially frequency-independent.  The PRR GG1 had this kind of motor, operating on 25 Hz.  Regeneration was possible with this kind of motor, and later Swiss examples could regenerate to a standstill, stopping any train that they could start on the same grade.

 

Although rectifier locomotives, initially with ignitrons and excitrons, and later with solid state devices, fairly quickly displaced types with 25 and 50 Hz single-phase motors, that was not the case with 16⅔ Hz in Europe.  Some European operators, such as SBB Switzerland, stayed with single-phase motors until the “modern AC” era.

 

Historically, three phase induction motors were used with either fixed frequency three phase supplies or with fixed frequency single phase supplies via a rotary phase converter.  Speed control was through combinations of pole changing, phase changing and cascade-parallel connection.  Four speeds was about the maximum typically used,  Transition between speeds was aided by the use of bulk liquid rheostats to which the induction motor rotors were connected.  Regeneration was automatic.  The biggest three-phase installation was in Northern Italy, and it survived for a surprisingly long time, notwithstanding the apparent drawbacks.

 

There was some development of the phase converter technique post-WWII, represented by the Hungarian V55 locomotive, with five stepped speeds, and the SNCF CC14000, with continuously variable frequency and voltage obtained entirely by electromechanical means.

 

One could say that the modern AC technique is a realization, through the availability of suitable electronics, of both the small-signal and power types, of ideas that go back to the early days of electric traction/transmission but were difficult, if not impossible to implement electromechanically.

 

 

Cheers,

[Erik_Mag]

Perry Babin

Do all locomotives with AC driven traction motors use Variable Frequency Drive to determine the motor speed?

A more rigorous answer to the question is that all modern AC drives using either polyphase induction motors or polyphase synchronous motors use a variable frequency drive. The GN and Italian three phase electrifications, the N&W electrification and first generation VGN electric locomotives used wound rotor three phase motors with large 3 phase rheostats for starting, but ran at constant speed. These locomotives also used cascade connections for half speed or quarter speed operation, akin to the series versus parallel connection of DC traction motors.

Speed control for AC series motors is done by varying the supply voltage.

Advances in power electronics have made variable frequency drives much easier to implement. The variable frequency "drives" are often termed converters as opposed to rectifier/inverters as power flow between the AC side and DC side can be set to flow either way depending on the timing of the switches (induction machines will at as motors if the AC frequency is greater than the shaft speed times the number of pole pairs in the winding, and act as generators if the AC frequency is less).

The advanatges of modern AC drives over DC motors include:
AC motors tend to be much more rugged than DC motors
AC induction motors have a much steeper torque versus speed than DC series motors, any slip will cause a drop in motor torque that will limit the amount of slipping - effect is even stronger with AC synchronous motors.

[Perry Babin]

On some trains, a mid DPU is running much harder than the lead locomotives. Does operating a DPU like this mean that it has to be either DC drive or asynchronous AC drive (not possible synchronous AC motors with VFD).

[BaltACD]

Perry Babin

On some trains, a mid DPU is running much harder than the lead locomotives. Does operating a DPU like this mean that it has to be either DC drive or asynchronous AC drive (not possible synchronous AC motors with VFD).

I most likely means that the Engineer is operating his train with the DPU 'Fence' up wherein he can have the lead engine consist and the DPU engine consist doing different things at the same time - depends upon the geography that the train is experiencing.

Engine consists control all engines in the consist with the same control inputs.  The engine consist can have either/both AC or DC engines.  DPU consists have ONE engine in that particular consist that responds to the radio controls initiated by the Engineer controlling the train.  DPU consists can be more than a single unit.  DPU is a matter of configuring specific engines in the various engine consist(s) to acknowledge DPU status - master/slave(s).

Most territory is far from grade stable, where the gradient within the trains length is the same.  Even in the best graded rights of way there are undulations; undulations that your eyes can't perceive, but thousands of tons of freight highlight with exclamation.  The Engineers biggest job in moving their trains over any territory is to control the slack so that the in train forces don't tear the train into two or more pieces.

[timz]

Pneudyne

PRR GG1 had this kind of motor, operating on 25 Hz.  Regeneration was possible with this kind of motor ...

The GG1 could have had regenerative braking if PRR had wanted? Or would it need stuff that hadn't been invented by then? How does a commutator AC motor produce constant-frequency AC current that's in phase with the catenary AC, whatever the locomotive speed?

[timz]

Perry Babin

On some trains, a mid DPU is running much harder than the lead locomotives. Does operating a DPU like this mean that it has to be either DC drive or asynchronous AC drive (not possible synchronous AC motors with VFD).

Best quit supposing that anything has to be anything. No reason the DPU couldn't be an ABBA set of FTs.

By the way: some engines do have synchronous AC motors. Dunno how many -- doesn't SNCF have some?

[JayBee]

timz

By the way: some engines do have synchronous AC motors. Dunno how many -- doesn't SNCF have some?

 

 

All the powercars for newer TGV trainsets use Permanent Magnet Synchronous AC motors. These can be even lighter for the same power output compared to earlier powercars. TGV powercars went from Rectifier with DC motors in the first generation sets, to Synchronous AC motors in the second generation, to 3-phase Asynchronous AC motors in the third generation to Permanent Magnet Synchronous Motors in the fourth generation. The first two generations have been retired.

 

Also the AGV trainsets used by Italo in Italy are produced by Alstom and use the same motors as the latest TGV trainsets.

[Perry Babin]

With the type of synchronous AC motors used in the US locomotives, if the drive is telling it to run at X RPMs and the load is causing it to run significantly slower (0.75X RPM, for example), does the motor draw significantly more current than it would if it was running, under a lighter load, at X RPMs (or X RPMs - a small amount of slip)?

[JayBee]

Freight locomotives using AC drives in the US use 3-phase Asynchronous motors. As load on the motor causes rotational speed to be less than the frequency supplied by the convertor, the slip angle increases. The slip Angle is the difference between the energized coil on the stator and the shorting bar on the rotor carrying the same phase. If the slip angle is positive (the rotor bar is lagging behind its counterpart on the stator) the motor will produce torque trying to accelerate the rotational speed and the train. If the slip angle is negative (which happens in dynamic braking) there will be resistance to the motor turning, hence slowing the train. These things happen without any action by the Engineer or Locomotive control computer. Modern locomotives measure the speed that they are travelling along the track very accurately, this allows the Engineer through his controls to signal to the Engine Control Computer whether he wants the train to be in Power or Dynamic Braking. The ECC in turn will signal to the Power Converters to either increase or decrease the Engine throttle and frequency of the Power Convertor output to the traction motors.

[Erik_Mag]

Re: slip and asynchronous (induction) motors

Slip is the difference in the rotational speed of the rotor versus the rotational speed of the magnetic field generated by the current in the field windings. The advantage of a polyphase motor is that the appropriately placed field windings produce a rotating field when fed by polyphase AC. This relative motion induces currents in the rotor, hence induction motor, which then interacts with the rotating field to generate torque. Up to a certain point, a larger slip will result in more current being induced and thus generating more torque.

"Angle" and synchronous motors:

Synchronous motors have some means besides current induced by sli to generate a magnetic field in the rotor, be it permanent magnets or electromagnets powered through slip rings or other methods. This magnet wants to line up with the rotating field unless pulled away by an applied torque. If the torque is the direction of rotation, the "motor" will act as a generator (alternator), and will act as a motor if the torque is in the opposite direction. The amount of torque will vary with the sine of the angle between the rotating field and the field poduced by the rotor - going past 90º will cause the  motor to "pull-out" - generally a bad thing.

 

[Perry Babin]

Do any US freight train locomotives use synchronous AC traction motors with VFD drives?

[Overmod]

Perry Babin

Do any US freight train locomotives use synchronous AC traction motors with VFD drives?

It was my understanding that all AC traction motors used in 'current' North American production locomotives use field induction of current in rotor bars, and have neither permanent magnets in the rotor structure or any form of slip-ring or brushed connection to generate rotor field electrically.

A reason for the 'slip' in these motors is that the field coils have to induce current flow and hence a magnetic field in the rotor bars before the rotating field can act on them to produce torque.  Otherwise the fields could rotate indefinitely with 'nothing magnetic to act on' as in a synchronous motor...

A major argument against using PMs in rotors of this size of motor is that if you begin to approach the Curie point, the magnetic field starts to die (and this is usually from the 'outside in', which further reduces the torque for a given field strength).  I don't remember which of the current high-strength magnetic materials have an elevated Curie point, but it helps greatly not to have a material subject to degradation in the first place.

[Erik_Mag]

Overmod

A major argument against using PMs in rotors of this size of motor is that if you begin to approach the Curie point, the magnetic field starts to die (and this is usually from the 'outside in', which further reduces the torque for a given field strength).  I don't remember which of the current high-strength magnetic materials have an elevated Curie point, but it helps greatly not to have a material subject to degradation in the first place.

Samarium Cobalt. Needless to say, the stuff ain't cheap.

One problem with PM synchronous motors is that the terminal voltage scales directly with speed.