AC vs DC?

[GP40-2]

I would think that when push comes to shove and you have the choice of either moving a train because of mismatched units or hold tonnage for the right units the freight should move.  Lets keep the customer happy.

How efficient are these units? 

It appears that Mr. Krug indicates 80 to 85% efficiency.  What can be done to increase that efficiency to a higher level, or is it a figure that is accepted.

ed

From Al Krug's site:

"C44AC

The BNSF has only three of the AC traction versions of the C44. Loco numbers 5600-5602.

The table below shows the Tractive Effort readings taken from the engineer's TE meter at various speeds. If you multiply the TE times the speed (in ft/sec) then divide by 550 (definition of a HP) you can calculate the HP. Since the diesel engine developes 4400 HP and the TE x Spd figure is about 4100 HP this means the overall "transmission" efficiency is about 93%.

The table also shows the Main Alternator Volts and Amperage. By multiplying those two numbers to get the watts and then dividing by 746 (definition of a HP) you can calculate the HP output at the main alternator. This value comes out to about 4240 HP. That means the main alternator is about 96% efficient. Since the overall transmission efficiency is 93% this means the AC invertors, AC traction motors, and the gearing lose about 3% of the available power. "

Take it for what it is worth, but his figures from the BNSF are very close to CSX's data, and GE's "factory specs" for GE's AC locomotives.

http://www.alkrug.vcn.com/rrfacts/dash9.htm

[erikem]

I would think that when push comes to shove and you have the choice of either moving a train because of mismatched units or hold tonnage for the right units the freight should move.  Lets keep the customer happy.

How efficient are these units? 

It appears that Mr. Krug indicates 80 to 85% efficiency.  What can be done to increase that efficiency to a higher level, or is it a figure that is accepted.

ed

From Al Krug's site:

"C44AC

The BNSF has only three of the AC traction versions of the C44. Loco numbers 5600-5602.

The table below shows the Tractive Effort readings taken from the engineer's TE meter at various speeds. If you multiply the TE times the speed (in ft/sec) then divide by 550 (definition of a HP) you can calculate the HP. Since the diesel engine developes 4400 HP and the TE x Spd figure is about 4100 HP this means the overall "transmission" efficiency is about 93%.

The table also shows the Main Alternator Volts and Amperage. By multiplying those two numbers to get the watts and then dividing by 746 (definition of a HP) you can calculate the HP output at the main alternator. This value comes out to about 4240 HP. That means the main alternator is about 96% efficient. Since the overall transmission efficiency is 93% this means the AC invertors, AC traction motors, and the gearing lose about 3% of the available power. "

Take it for what it is worth, but his figures from the BNSF are very close to CSX's data, and GE's "factory specs" for GE's AC locomotives.

http://www.alkrug.vcn.com/rrfacts/dash9.htm

97% efficiency sounds a bit high for the combination of inverters, traction motors and gearing - I would expect the traction motors to be slightly less efficient that the main alternator. I'd also be interested in the speed for which that efficiency number is valid - the minimum possible losses for an induction motor are set by the percentage of slip, that is the difference between synchronous speed at shaft speed. The transmission efficiency of an AC locomotive lugging at say 2-3 mph has got to be a lot less than 90%, mainly due to the slip in the induction motors.

There was a recent article in one of the rags I get at work about the Chinese going to permanent magnet synchronous motors for air conditioners - these are lighter and more efficient than induction motors (the efficiency is due to the lack of slip in the synchronous motor - think locking torque converter versus non-locking torque converter). Thought was that might be a nice idea for a traction motor, especially allowing for better efficiency at low speeds - turns out that GE is working on it. One downside is that the inverter control system will be a bit more complicated - torque in an induction motor is proportional to the slip (difference between synchronous speed and shaft speed) - where torque in a synchronous motor is proportional to the angle between the field generated by the permanent magnets in the rotor and the field generated by the windings in the stator.

[Hugh Jampton]

Synchronous motors have been used in traction applications for coming on 25 years now. Think TGV,, Virgin also use them in their Voyagers.

[JayPotter]

So, what happens when you mix AC and DC?  Lets say you got some AC units in which you are down to 5mph on a grade.  Do the motors in the DC units burn up? 

I believe that DC-traction units oftentimes have protective circuitry that reduces traction motor current when the motors are in danger of overheating.  The only example of this with which I'm the least familiar is on CSXT's ES44DCs.  They have what GE refers to as a "thermal simulator" that, if my understanding is correct, computes traction-motor temperatures and automatically reduces prime-mover horsepower output to whatever level is necessary to protect the motors.  So when an ES44DC is in a consist with AC-traction units in Ed's "5 mph on a grade" scenario, the train will eventually stall unless the AC-traction units alone are capable of moving it.

Jay Potter

 

 

[oltmannd]

So, what happens when you mix AC and DC?  Lets say you got some AC units in which you are down to 5mph on a grade.  Do the motors in the DC units burn up? 

I believe that DC-traction units oftentimes have protective circuitry that reduces traction motor current when the motors are in danger of overheating.  The only example of this with which I'm the least familiar is on CSXT's ES44DCs.  They have what GE refers to as a "thermal simulator" that, if my understanding is correct, computes traction-motor temperatures and automatically reduces prime-mover horsepower output to whatever level is necessary to protect the motors.  So when an ES44DC is in a consist with AC-traction units in Ed's "5 mph on a grade" scenario, the train will eventually stall unless the AC-traction units alone are capable of moving it.

Jay Potter

Interesting!  Enforcing the TMs short time ratings  would be make the Mech. Dept very happy, but would make Transportation kind of squeemish.  I think they'd rather have the flexibility to smoke the motors a bit and avoid a stall - motor maintenance doesn't come out of their budget, after all.

I can see in this era of AC locos that having such a system in place would be a great thing.  Dash 8s, 60 series and newer ought to be capable of something like this through software.  Older locos would be out of luck.

[beaulieu]

So, what happens when you mix AC and DC?  Lets say you got some AC units in which you are down to 5mph on a grade.  Do the motors in the DC units burn up? 

I believe that DC-traction units oftentimes have protective circuitry that reduces traction motor current when the motors are in danger of overheating.  The only example of this with which I'm the least familiar is on CSXT's ES44DCs.  They have what GE refers to as a "thermal simulator" that, if my understanding is correct, computes traction-motor temperatures and automatically reduces prime-mover horsepower output to whatever level is necessary to protect the motors.  So when an ES44DC is in a consist with AC-traction units in Ed's "5 mph on a grade" scenario, the train will eventually stall unless the AC-traction units alone are capable of moving it.

Jay Potter

Interesting!  Enforcing the TMs short time ratings  would be make the Mech. Dept very happy, but would make Transportation kind of squeemish.  I think they'd rather have the flexibility to smoke the motors a bit and avoid a stall - motor maintenance doesn't come out of their budget, after all.

I can see in this era of AC locos that having such a system in place would be a great thing.  Dash 8s, 60 series and newer ought to be capable of something like this through software.  Older locos would be out of luck.

MTP, or Motor Thermal Protection has been standard since the UP's C36-7s on GEs, EMD started with the 60-series. It's not foolproof but it has reduced the incidence of burned up traction motors.

[owlsroost]

Minor point, but as far as I know the Voyager DEMU's use Alstom ONIX 3-phase asynchronous drives (rated at 235 kW per motor). According to the Alstom web site, they only offer synchronous motors at 1150kW and 2800kW ratings (as used in French TGV and locomotives).

Tony

[beaulieu]

Minor point, but as far as I know the Voyager DEMU's use Alstom ONIX 3-phase asynchronous drives (rated at 235 kW per motor). According to the Alstom web site, they only offer synchronous motors at 1150kW and 2800kW ratings (as used in French TGV and locomotives).

Tony

I think that synchronous AC motors are pretty well dead, now that asynchronous motors and variable voltage - variable frequency  inverters have been perfected. Even the latest TGV power cars, those for the new POS sets for the TGV Est, now have asynchronous motors, as do the latest Alstom locomotives, the Prima series.

[owlsroost]

I think that synchronous AC motors are pretty well dead, now that asynchronous motors and variable voltage - variable frequency  inverters have been perfected. Even the latest TGV power cars, those for the new POS sets for the TGV Est, now have asynchronous motors, as do the latest Alstom locomotives, the Prima series.

I agree with you - if I remember correctly (it's a long time ago) the French went for synchronous on the early TGV sets because they couldn't fit powerful enough DC motors within the space and weight constraints they had, and high power async inverter drives (25 years ago) were big, expensive and not very reliable e.g. the DB E120 - and probably not French enough either :-)

I think the Eurostar sets were the first TGV-derived trains to have async drives - and that was partly I suspect because the traction equipment was British designed and built.

So how long do you reckon before EMD and GE stop offering DC drives ? - we've already reached that point with electric locos and EMUs in Europe.

Tony

[beaulieu]

I agree with you - if I remember correctly (it's a long time ago) the French went for synchronous on the early TGV sets because they couldn't fit powerful enough DC motors within the space and weight constraints they had, and high power async inverter drives (25 years ago) were big, expensive and not very reliable e.g. the DB E120 - and probably not French enough either :-)

I think the Eurostar sets were the first TGV-derived trains to have async drives - and that was partly I suspect because the traction equipment was British designed and built.

So how long do you reckon before EMD and GE stop offering DC drives ? - we've already reached that point with electric locos and EMUs in Europe.

Tony

I think the builders will have to get the cost differential down to about $250,000 or so, before the economics sway the purchasing decision. Of course the exact figure depends upon the railroad, they each do their own cost benefit analysis, how they use their locomotives has a big influence.

[erikem]

Minor point, but as far as I know the Voyager DEMU's use Alstom ONIX 3-phase asynchronous drives (rated at 235 kW per motor). According to the Alstom web site, they only offer synchronous motors at 1150kW and 2800kW ratings (as used in French TGV and locomotives).

Tony

I think that synchronous AC motors are pretty well dead, now that asynchronous motors and variable voltage - variable frequency  inverters have been perfected. Even the latest TGV power cars, those for the new POS sets for the TGV Est, now have asynchronous motors, as do the latest Alstom locomotives, the Prima series.

Synchronous motors are aided as much by VV-VF inverters as induction motors. What has changed with the "new generation" of synchronous motors is the use of high energy product permanent magnet material such as NdBFe. For a given size motor frame, replacing the squirrel cage with a permanent magnet rotor will give both an increase in available torque (the permanent magnet generate a higher field than the squirrel cage) and an improvement in efficiency (no induced current losses in the squirrel cage).

[GP40-2]

Minor point, but as far as I know the Voyager DEMU's use Alstom ONIX 3-phase asynchronous drives (rated at 235 kW per motor). According to the Alstom web site, they only offer synchronous motors at 1150kW and 2800kW ratings (as used in French TGV and locomotives).

Tony

I think that synchronous AC motors are pretty well dead, now that asynchronous motors and variable voltage - variable frequency  inverters have been perfected. Even the latest TGV power cars, those for the new POS sets for the TGV Est, now have asynchronous motors, as do the latest Alstom locomotives, the Prima series.

Synchronous motors are aided as much by VV-VF inverters as induction motors. What has changed with the "new generation" of synchronous motors is the use of high energy product permanent magnet material such as NdBFe. For a given size motor frame, replacing the squirrel cage with a permanent magnet rotor will give both an increase in available torque (the permanent magnet generate a higher field than the squirrel cage) and an improvement in efficiency (no induced current losses in the squirrel cage).

My understanding of synchronous AC motors used in RR applications in the past was they were not as rugged in freight use due to having the heavy permanent magnets mounted on the rotors. Induction motors are relatively indestructable-especially in North American operating conditions. Maybe GE's work will solve this problem. You are right about the efficiency of induction motors at low speeds. GE's own data show a drop in overall efficiency at max CTE down to 86%. However, induction motors are highly efficient at higher rotational speeds, which is why I said that people who think AC traction is only for lugging trains up a mountain at 8 mph are missing the total picture of this technology.

[Randy Stahl]

The GG-1 was pretty rugged and they went like hell !!!

[GP40-2]

The GG-1 was pretty rugged and they went like hell !!!

Very true, however, GG1s used universal AC motors, which like traditional DC motors have high maintenance brushes and commutators.

The AC motors we are talking about have permanent magnets, but do not use brushes and commutators. They need an electronic control system similar to asynchronous motors.

[erikem]

My understanding of synchronous AC motors used in RR applications in the past was they were not as rugged in freight use due to having the heavy permanent magnets mounted on the rotors. Induction motors are relatively indestructable-especially in North American operating conditions. Maybe GE's work will solve this problem. You are right about the efficiency of induction motors at low speeds. GE's own data show a drop in overall efficiency at max CTE down to 86%. However, induction motors are highly efficient at higher rotational speeds, which is why I said that people who think AC traction is only for lugging trains up a mountain at 8 mph are missing the total picture of this technology.

As far as I know, there hasn't been any application of synchronous traction motors for US freight service. I would guess that the European synchronous traction motors used rotor windings and not permanent magnets - most likely using the amortisseur windings for starting and low speed operation with excitation applied once the motors came up to speed.

The issue with older permanent magnet materials wasn't a problem with mechanical strength, Alnico is a lot less fragile than NdBFe, but resistance to demagnetization. It is unlikely that the fields in a motor would be strong enough to demagnetize NdBFe, but it would be easy to do the same for Alnico. One issue with NdBFe is that it is difficult magnetize large blocks of the stuff and assembling a large block out of small blocks is -um- a challenge (and there are some safety issues with handling large blocks of magnetized NdBFe).

You're right in that induction motors are quite efficient at higher speeds, but the modeling done by GE's research center in Bangalore still predicts a small improvement at those speeds.