How is dynamic breaking achieved in the AC motors insofar as the polarity is unable to be reversed in an AC motor?
Richard Kates
by changing the ac traction to dc while braking this is done with a very intelegent computer an the altanator does not know its doing it the power is stored in huge grids in locomotive on sides thats what those big fins on the side are there cooling fans for the huge copasators that are being used an when they get to capacity they dischare into the outside rail hope this is what your looking for
henry
WHAT!
This is actually close to correct in some respects.
AC locomotives generate their power as AC but this is converted to DC at about 1000 volts in a rectifier and then provided to inverters which convert it back to AC at a variable frequency to control the speed of the motors, and a variable voltage to control the power in the motors.
When running in dynamic brake, the motors act as alternators and the power is fed back through the same inverters and comes out as DC.
The DC current is fed through resistance grids which are exactly the same as those in a DC locomotive (although sometimes bigger in the AC loco) and the grids are cooled by fans driven by DC motors.
In GE’s new hybrid loco, the DC current from dynamic braking can be used to charge batteries and this power can be re-used when starting or climbing hills.
M636C
For what? To help
?
I think the main job of the copasators is to smooth the pulsing DC coming out of the rectofitters. I’m not sure about using them in braking because you can run AC or DC through the resistance grids.
You can’t run AC back thru a inverter and get DC. You have to run it thru a rectifier.
Oh yes you can!
The IGBT inverters now used in locomotives can do almost anything. The type of current and the output frequency can be controlled from DC to 60 Hz and above, and can be three phase or single phase. The output can be varied as required. On the Amtrak GE Dual Mode locomotives, there are five identical inverters, one for the HEP supply and one each for the motors. Any one of these can be used for HEP power if the unit being used for HEP fails, allowing the loco to continue on three motors.
The Alaska Railroad SD70MACs can use one inverter for HEP while the other inverter supplies four motors, or as a normal six motor unit in freight service.
M636C
It is somewhat more complicated than running the AC traction motors as alternators. Those traction motors cannot be alternators because they are induction and not synchronous AC machines.
The power of any AC device depends on the alternating voltage and current waveforms and especially the relative phase angles of those waveforms. The “active” power is the RMS magnitude of voltage V times the RMS magnitude of current I times the cosine of the phase angle difference between the voltage and current sine waves. The “reactive” power is |V| |I| times the sine of the phase angle difference. The active power is the thing that supplies tractive effort to the motor while the reactive power represents energy that is swapped back and forth between the motor and the inverter at twice the excitation frequency.
If you apply a voltage to the motor at a frequency slightly higher than the rotational speed of the motor, that “rotating” magnetic field will induce currents in the shorting bars of the rotor of that induction motor that will produce tractive force, and the phase angle between voltage and current applied to the motor will be such that cosine will be positive, indicating that power is being delivered to the motor. If you apply a voltage at a frequency slight lower than the rotation of the motor, that will induce currents to produce a retarding force, and the cosine of the voltage-current phase angle will be negative, indicating that the motor is returning power back to the inverter.
The trick is to have a control system that can precisely control the AC frequency applied to the motor to get tractive or retarding force across a range of wheel speeds. The other thing to take care of is that if the inverter is supplying retarding force, the net effect will be for the inverter to back feed power into the DC bus – in that instance, the inverter acts as a kind of synchronous rectifier that “uninverts” the AC back to the DC bus –
[quote user=“Paul Milenkovic”]
It is somewhat more complicated than running the AC traction motors as alternators. Those traction motors cannot be alternators because they are induction and not synchronous AC machines.
The power of any AC device depends on the alternating voltage and current waveforms and especially the relative phase angles of those waveforms. The “active” power is the RMS magnitude of voltage V times the RMS magnitude of current I times the cosine of the phase angle difference between the voltage and current sine waves. The “reactive” power is |V| |I| times the sine of the phase angle difference. The active power is the thing that supplies tractive effort to the motor while the reactive power represents energy that is swapped back and forth between the motor and the inverter at twice the excitation frequency.
If you apply a voltage to the motor at a frequency slightly higher than the rotational speed of the motor, that “rotating” magnetic field will induce currents in the shorting bars of the rotor of that induction motor that will produce tractive force, and the phase angle between voltage and current applied to the motor will be such that cosine will be positive, indicating that power is being delivered to the motor. If you apply a voltage at a frequency slight lower than the rotation of the motor, that will induce currents to produce a retarding force, and the cosine of the voltage-current phase angle will be negative, indicating that the motor is returning power back to the inverter.
The trick is to have a control system that can precisely control the AC frequency applied to the motor to get tractive or retarding force across a range of wheel speeds. The other thing to take care of is that if the inverter is supplying retarding force, the net effect will be for the inverter to back feed power into the DC bus – in that instance, the inverter acts as a kind of synchronous rectifier that "uninverts&quo
Well, there you have it, the inverters are indeed back feeding the DC bus. Maybe regarding the inverters as rectifiers is not quite the right analogy. One of the pre-rectifier means of converting AC to DC was the rotary converter – not exactly a motor-generator set, but a synchronous AC machine that operated some kind of commutator to switch the AC into a rectified waveform. I guess the inverter acts as a solid-state synchronous converter to allow the back flow of AC power to the DC side.
If you are back feeding the DC bus, connecting the DC bus to the dynamic brake resistors could sustain the DC voltage to allow the inverter/traction motor combination to be self exciting, but as the EMD manual says, this is probably a precarious situation from a control standpoint without the alternator supplying voltage to stabilize things.
This is not quite the same thing, OK, it is nothing at all like DC dynamic braking where a transition is made where the traction motor field windings get current from the main alternator on one circuit, and the traction rotor windings supply voltage and current to the dynamic brake resistors on a completely separate circuit. With the AC locomotive, it sounds like the inverter DC bus, the main alternator, and the dynamic brake resistors are all on one circuit, and it all has to be controlled some how. Not an easy job: I talked to the guy from EMD at our Engineering Career Fair who did the design of the AC electronics on their latest locomotive model, and the fellow looked like he hadn’t slept in four months.
Thank you guys, always wondered how this was done.
Frank
I am so glad I wasn’t the only one!
Paul is on the right track (pun not intended) in comparing the ‘inverters’ to the rotary converters - the terminology actually fits. A ‘rectifier’ implemented with thyristors (SCR’s) is often referred to as a converter because power can be sent both ways. With a ‘single’ converter, the terminal voltage has to be reversed to reverse the power flow as the current is limited to flowing in one direction. A ‘dual’ converter can be set up to mimic the actions of a rotary converter - i.e. keeping the voltage polarity constant and reversing the power flow by reversing the direction of the current.
The difference between a converter and an inverter is that commutation in the converter is provided by the AC side, where an inverter provides its own commutation. Commutation is an issue with plain thyristors as once they are turned on (fired) they continue to conduct current until something else stops the current flow.
A common configuration (topology) for a variable voltage variable frequency inverter is an H-bridge with pulse width modulation where the switching frequency is several times higher than the highest expected output frequency. These inverters by designed will allow power to flow both ways - even without regeneration this feature is needed to be able to handle reactive loads such as motors.