Traction AC DC

The squirel cage induction motor can be said to be the work horse of industry. It is rugged and there is little that can go wrong with it. However it was used only where speed control is not an issue. So in traction duty the DC motor reigned supreem. However today with vector control power electronics it is possible to give the induction motor full torque from zero to rated speed and there are no brushes to worry about. But then the brushless DC motor is now competing with AC induction motor drives. My gut feeling is that the AC induction motor may be best for load haulers, while the BLDC motor may be better for smaller vehicles.

Upon reviewing the locomotive traction motor I have decided that there are advantages to both, however, the AC motor has these advantages:

• No brushes to inspect and periodically change
• No commutator to periodically inspect and clean, or tear down the motor to
• machine.
• No undercutting the mica insulation
• No brush holders to clean, test tension, and adjust spacing
• Less electrical losses created from heat due to winding resistance values
• Less rotating weight
• Lower cooling air requirements
• Reduced factors of electrical grounds due to carbon dust, moisture, etc.

Where are Tesla and Edison when you need them?

Firstly thanks for making a thread on my pet topic.

Trucks.

Look up GE Transportation Rail and select Off Highway Vehicles.

AC traction does surpass DC by a sufficient margin to warrant the higher cost and complexity. The main advantages operationally are improved safety, better thermal capacity at low speed, no commutation envelope to worry about, stronger retard, higher speed, improved slip and slide control, limp home function, long motor life, no stall damage, less RPM in retard and less maintenance.

The above 300tonne class of electric drive trucks (Komatsu, Caterpillar/Mitsubishi, Hitachi and Liebherr) are AC and AC is also available in selected smaller trucks(Komatsu 240t and Unit Rig 150t).

Loaders.

Le Torneau have traditionally used DC but are increasingly using switched reluctance motors which for low speed applications have advantages over even the squirrel cage especially in low speed thermal capacity and low speed braking (the rotor can be locked electrically, indefinitely stalled if you like).

All of the above manufacturers make fit for purpose electric drive equipment (there will be a bit of a wait for the Cat as it is new) so log on to their sites and enjoy.

If you then have some specific questions add another post to your thread and I'll answer your questions. Brushless DC so far doesn't get a guernsey largely because it can all be done more cost effectively and reliably by the above two motor designs neither of which need permanent magnets.

Thanks for your response Emjay4119 i'll check out the site

From my original thread "ac would have to match the beter feature of dc while surpassing the worst features" is what i am looking at, particualrly on grades, i guess.

DC system can offer huge starting torque but with series motors have runaway problems down hill.

The VFD is complex, but has virtually no moving or wearing parts, and permits the robust simplicity of the AC motor to be used over a wide speed range. This is slightly compromised by the necessity at this time of the speed encoder.

The DC controller is relatively simple, but the DC motor therefore requires a mechanical equivalent of the VFD which takes the form of commutator, brushgear and brushes in the motor. These components are all subject to wear and tear and consequently require attention and maintenance. Modern DC machines can typically run for around 2 years without significant maintenance being necessary.

Other considerations are

- Design maturity.

- Overall energy efficiency.

- Productivity. - Reliability (unplanned maintenance).

- Maintenance requirements (planned maintenance).

- Economic impact (life cycle cost).

Basically none of your second post is applicable. Modern DC trucks don't use series motors, they use seperately excited shunt motors supplied with controlled DC field current (up to 400 amps) from a static exciter (MFSE) which is supplied with single phase AC from a tertiary winding in the traction alternator. The alternator field is also controlled by a static exciter (AFSE) which is supplied from its own tertiary winding. Initial excitation is provided by a battery boost circuit and the suicide circuit kills the alternator field when it is not specifically commanded for Propel or Retard. Prior to the start of propel the alternator is unexcited when the propel logic is true then Battery Dc is fed into the rotating field windings until the alternator field tertiary winding starts to produce power. At this point the batt boost stops and AFSE takes over via the GF contactor. With the alternator sufficiently excited P1 (the propel armature contactor) picks up and MFSE supplies strong motor field via MF contactor. AFSE excites the alternator for the required starting current. Current and voltage are monitored and controlled within the parameters of the motor. Once the motor starts to spin naturally back EMF is generated and the armature voltage rises. To increase speed further MFSE reduces field current. When desired speed is approached the armature current and supply voltage are decreased. In retard engine revs are maintained to supply motor field current. P1 is opened and RP1 is closed which routes generated current to the resistor grids. Motor field current is limited to keep generated current within the commutation envelope. As speed falls the voltage generated by the wheel motors falls causing the retard current and torque to reduce. To maintain torque RP2 closes to short out part of the resistance and maintain torque, then RP3 through 9 do likewise. Retard will slow the truck to almost standstill.

I'm out of time right now. Post again if you need more.

From my original thread "ac would have to match the beter feature of dc while surpassing the worst features" is what i am looking at, particualrly on grades, i guess.

DC trucks are faster on grade than Mechanical trucks. AC trucks are generally faster still.

The VFD is complex, but has virtually no moving or wearing parts, and permits the robust simplicity of the AC motor to be used over a wide speed range. This is slightly compromised by the necessity at this time of the speed encoder.

All electric trucks are fitted with speed sensors on each wheel motor for many reasons, the primary one being safety. The AC trucks use speed sensors on four wheels for the slip and slide control and inverter torque control.

The propulsion system utilises the same type of alternator as the DC truck with much the same three phase full wave rectifier. The alternator voltage is still controlled by AFSE but MFSE and the associated tertiary winding are not required. AFSE controls the DC link volts. The two inverters control the current and frequency.

The grid blowers contain the only brushgear on an AC truck. AC blown grids will replace the DC grid blowers in the future.

The DC controller is relatively simple, but the DC motor therefore requires a mechanical equivalent of the VFD which takes the form of commutator, brushgear and brushes in the motor. These components are all subject to wear and tear and consequently require attention and maintenance. Modern DC machines can typically run for around 2 years without significant maintenance being necessary.

Yes the DC control system is simpler, it only uses one processor while the AC control uses 2.

The AC truck uses less contactors which reduces contact tip maintenance.

Traction DC motor brushes need attention every 250 hours. AC trucks have 500 hour service intervals.

- Design maturity.

Yes DC traction is mature (extremely reliable) technology which has not been applied to the largest trucks (and is unlikely to be). AC traction has only scratched the surface.

-Overall energy efficiency

Both electric technologies are far superior to mechanical drives for efficiency. One mine has proven that to carry 345 tonne loads with an AC electric truck uses 160 litres per hour of diesel (60 litre engine tier 2 emissions) and a mechanical truck carrying 345 tonnes uses 320 litres per hour (107 litre engine tier 0 emissions).

The efficiency advantages of AC come from the even greater ability to back off engine revs at full speed and the lower revs in retard. The speed is maintained by the inverters which only call for sufficents DC link volts and engine revs to overcome rolling losses.

- Productivity. - Reliability (unplanned maintenance).

Both are more productive than Mechanical trucks due to faster on grade speeds (Electric trucks round up their mechanical brothers on the ramp at mines that allow overtaking) and reduced time spent in servicing.

Service on an AC truck can easily be achieved in a shift with minimal manning.

Unplanned maintenance is similarly low, as the mature DC technology is very sorted out.

- Maintenance requirements (planned maintenance).

Minimal Maintenance very few oil compartments in either case and no traction brushes for the AC models. Also no Differential gearing to fail.

- Economic impact (life cycle cost).

Compared with mechanical trucks the waste oil alone is a huge saving. AC trucks are the coming wave but not all agree that they have a lower life cycle cost than DC, mostly because the data doesn't yet exist. Some 30 year old DC trucks are still going strong with high availability, so what is their real life cycle.

Hope that helps. If you want AC operation details just ask.

A small footnote; In the future if diesel engines are no longer allowed for emission reasons (somewhere beyond tier 4) electric trucks can be powered by fuel cells or a trolley wire, try that with a mechanical truck. Of course that explains the pace of AC electric truck development, all trucks will soon need tier 4 compliance and fuel efficiency limits which can only be achieved by AC traction.

Thanks again, Emjay4119 your comments are most helpful.

I got my reported "runaway" of the traction system from the underground coal sector where the haulers (shuttel cars) use a combination of 250 DC series (SCR mains or battery) and AC 1000volt (VFD).

I have heard that heat is an issue with the AC type in the under ground machines.

The Joy 15SC Shuttle Cars that I've worked on use 2 Proprietary packages (Right and Left hand traction). The supply is 1000VAC and a 1000/250v traction transformer is included for isolation. These packages only operate in 2 quadrants, there is no regenerative braking. Brake adjustment is critical on these models. Newer machines run VVVF with 4 quadrant operation, they would be much easier to drive.

With AC drive, heat from braking should not be an issue as the regeneration is into the mains. Heat generated by losses in a hot pit could be an issue. Heat certainly can be a problem with DC units as the only heatsink is the flameproof enclosure (AC would be no different). Where the DC drives are used on the continuous miner the packages are bolted to an integrated water cooled heat exchanger welded into the flameproof enclosure. Shuttle cars on the other hand do not have water connected.

The temperature limitation is due to the ratings of the flameproof enclosure rather than the operation temperature of the packages themselves. The case temperature of enclosure is limited primarily to prevent the ignition of a layer of coal dust.

Check out GE's locomotive division web site. In recent years almost all diesel locomotive produced have been of the ac design, starting I believe with the model 44ac, that uses a 4400hp diesel to power an alternator to drive the traction motors.

Yes, you are right they go up to 6000 HP.