I know back in the DC traction motor days the nominal operating voltage of locomotives was nominally 600 volts. Or at least all the electrical cabinets had 600 volt stickers on them.
Not having been on a locomotive since AC traction motors became ‘the way’ I am wondering what is the nominal operating voltage of the 21st Century AC traction locomotives? Do EMD (Progress Rail) and GE use the same operating voltages?
Don’t know about diesel electrics, but the AC traction motors used in transit trains are typically 480v three phase driven by VFVV circuitry.
AC locomotives are weird in how they actually make their AC voltage. The diesel engine spins the alternator which produces AC which is rectified into DC then it’s chopped via inverters back into AC as they need it at specific frequencies to power the traction motors.
My understanding (after being corrected on the old forum, where there are threads about this) is that the DC-Link on AC locomotives is typically 1200 to 1500VDC (reasonably smoothed DC, rather than rippled rectified full-wave AC).
A modern version of dual-mode-lite would handle all the power connections between locomotives at this reference voltage, with conversion to variable three-phase AC taking place via inverters on the individual locomotives, for individual axles.
I seem to recall that the DC link on the original EMD AC locomotives could go higher than 2KVDC. At higher roads speeds the inverter switched from PWM mode (switching frequency higher than effective AC frequency, to switching at the AC frequency. This required DC link voltage to be increased as road speed increased.
I sure hope bogie_engineer comes back to the ‘new and improved’.
I think M636C and at least two of the Australian engineers would have distinctive competence on a range of AC locomotives.
My imperfect recollection is that the DC link voltage is varied on IGBT equipped inverters as well. At low speeds and high current, the reduction in DC link voltages lowers the switching losses from charging discharging Coss. IGBT’s switch faster than GTO Thyristors, which means the inverter can operate in PWM mode at higher track speeds.
Would be very interested to hear what Dave or Peter has to say about the subject.
The EMD DC link voltage can go as high as 2800V, I believe GE limits theirs to around 1600V. The EMD voltage was based on the requirements of Siemens inverters when EMD first got into AC traction. It does result in smaller cable diameters but greater insulation capability.
Dave
Dave, thanks for chiming in!
One reason for the lower DC link voltages on the GE units is that GTO thyristors as used in the Siemens inverters can be made to handle higher voltages than IGBTs as used in the GE inverters.
Side note about voltage rating for power semiconductors. For silicon FETs and IGBTs (which are really a variant of a FET), the recommended maximum DC link voltage is half the breakdown voltage. Breakdown voltage is defined as the voltage in which a device in the “off” state starts conducting a specified current, typically on the order of a milli-ampere. The reason for the factor of two safety factor is to prevent device failure when struck by a cosmic ray neutron. Silicon Carbide devices can be operated at 2/3rds of their breakdown voltage.
For the most part, the terminal voltage of a variable speed motor increases with motor speed, whether the motor is a DC series, AC induction or AC synchronous.
Walt, I guess the 600 volt danger label on the electrical cabinet has been changed to 25kv.
I understand the chain of events: Diesel/Alternator/Rectifer/Inverter but as Walt asked what is the voltage AT the TRACTION motor?
Of course the DC motors overheated at low rpm and high torque demand. I would guess the variable frequency alternating current feed to the traction motor is the secret. And thus a normal 60 cycle AC voltmeter would not be effective to measure traction motor voltage.
Then backwards in the transitions of electrical power the high voltages need heavy insulation. Am I correct that traction motors with wire windings so close would need insulation for high voltage protection and would not be practical? True/False?
Again: what voltage does the AC traction motor have albiet a special AC volt meter that takes into account frequency NOT 60 cycle?
Excellent read on details above in this thread. Question still remains: voltage AND amperage at electrical terminals of traction motor? endmrw0223250951
I suspect that the danger label says 2.5kV, not 25kV for an AC Diesel electric.
Unless there is a filter between the inverter’s active devices (GTO Thyristors, IGBT’s and soon to be SiCFET’s) and the motor, the peak voltage will be the DC link voltage, but the rms voltage (effective) will be less. For an induction motor the terminal rms voltage will be roughly proportional to the square root of the tractive effort times the road speed. If it exists,the datasheet for the motor would have the numbers for current and voltage.
One reason that induction motors are more rugged than DC motors is that the rotor conductors are bars as opposed to wires on a DC and can carry a lot more current, which then results in more torque.
Motor windings will be subject to spike voltages exceeding the link voltage value due to fast switching transients of IGBTs. Snubber circuits doesn’t completely eliminate these spikes. When these motors are brought in for overhaul, they are tested for punch through between stator and frame, rotor bar separation, motor bearing asperity. Additionally, some facility’s have stator signature analysis to detect stator winding to winding punch through.
My bad, KV jump. Thanks and I understand the AC induction motor principle. Then 3 ph AC is more efficient than single phase. Do the AC locos have 3 phase current for traction motors?
Of course an induction locked rotor motor does not have back emf and will be a problem. With slow rotation at slow speeds does the back emf lessen since there is this variable frequency AC?
endmrw0223251309
A bit of simple E&M physics: The voltage induced in a winding is due to the change flux with time multiplied by the number of turns, and where flux (Webers) is the product of area and flux density (Tesla). With a motor running at constant current, the back emf increases/decreases with increasing/decreasing speed. The back emf represents the electrical power being converted to mechanical power.
The AC traction motors do use three phase as the is the simplest way to produce the rotating field needed to make an induction motor work efficiently. When motoring, the rotor of the induction motor revolves slower than the field, which then induces a current to flow in the bars of the squirrel cage rotor that generates torque (sort of like a fluid clutch). The difference between the speed of the rotating field and the rotor is called slip, and up to a point, torque increases with increasing slip. When the rotor is revolving faster than the field, the induced current is reversed and the motor now acts as an induction generator.
There are a couple of features about 3 phase induction motors that are nice to have in a traction motor. One is that the torque is constant, and the other is that the torque will drop rapidly if the wheels start slipping.
There are semester long college courses on motor theory. But yes, the locomotive motor control circuitry converts the DC link to 3 phase AC, which of course is Variable Voltage and Variable Frequency .
Keep in mind that the ‘secret’ of using induction motors on locomotives and railroad cars is not in varying “voltage”, as with a DC traction motor, but in controlling the phase via syntiesis. The field strength is arranged so it’ ‘rotates’ through the windings in sequence, both inducing a magnetic field in the rotor and then attracting that induced magnetic field to produce armature rotation. If you power the armature, or equip it with permanent magnets, you don’t need to allow ‘slip’ for the field to produce rotation, and you can alter the phase electrically so that the magnetic attraction just ‘leads’ the armature to produce peak torque even if the armature isn’t rotating at all (“locked-rotor”). Naturally there is resistance heat produced, which needs good traction-motor blowing, but it is not high-current through stalled armature windings hot.
Induction motors present a different impedance to the applied source as the frequency is varied. To compensate for this, the voltage has to be varied.
UCB was on the quarter system when I was there, so the course I took on electric machinery was only a quarter versus a semester. I also had the lab that was associated with the course. Turns out that the course was one of the more useful for my career, which at times involded designing magnets for NMR applications.
As for the impedance of induction motors increasing with frequency (2pif*L), that’s only part of the story as the contributors to the terminal voltage of an induction motor are a combination of inductive reactance and the back emf of the induced currents in the rotor.
W_H, the torque of a synchronous motor (permanent magnet or electromagnet) is proportional to the sine of angle difference between the rotor and the rotating field. The induction motor torque depends on the slip frequency. One advantage of a synchronous motor is an improvement in efficiency at a cost of a more complicated inverter control algorithm. Another advantage is an even finer control of wheelslip.
Synchronous slip, as controlled by varying motor frequency and voltage is certainly an inverter control function. Further intensifying this is wheel spin/slide control as well as dynamic/regenerative braking control. Most of this is accomplished by proprietary “chipsets” where the motor magic happens. I’m not sure if Siemens holds most of the patents on these chipsets. What theory describes these algorithms goes beyond the typical information presented in a motor theory course. For those that work in a Railway electronics back shop , or maintenance engineering/training environment know this well.
I suspect that the software uses some variant of Field Oriented Control (FOC), with some additional features geared to RR traction motors. FOC only requires current and voltage sensors, but will give accurate information on motor speed and torque.
For induction motors, “synchronous” refers to synchronous speed. If the rotor is turning at exactly synchronous speed, the rotor windings will not be cutting through the magnetic field “lines” and this no voltage or current will be induced.
The variable frequency variable voltage inverters allow use of high efficiency induction motors as the frequency can be adjusted for maximum starting torque versus having to modify the rotor to produce good starting torque with a constant frequency.