M7 question

[Frank]

What is the reason why the M7s do well in the snow? Is the M7's mechanical equipment better protected from the elements?

[Nasadowsk]

AC motors don't have any brushes or commutators, which are what goes first on DC motors. The rest of the industrial world figured out years ago what a maintenance bear DC motors are - today, they're used for little beyond appliances or tiny, specialized appliactions (chemical injection pumps come into mind - but they're virtually no HP). In the various industrial stuff I've seen, I know of no large DC installations, even in places where variable speed is needed. I've seen the bizzare wound rotor AC motors, which have slip rings and a weird control system - those are going out of style fast, but were popular for a while - that's how much bad DC motor maintenance issues can be. And I'm talking shop floor type stuff, not hanging under a train where there's always junk to get sucked in.

Going to AC was basically it - the ventilation system on the M-1s was a known weak point (though in all honesty, many other MUs of the era had the same design), not to mention it got rid of that huge cam controller, which was another weak point. The Silverliners do a little better because they're phase angle control, but you can't do that on pure DC without some weird tricks...

[Long Island 7285]

After a Budd trainset passes, does the 3rd rail have to be de enigized then re enigized with AC for an M7 trainset to pass?

[Clemuel]

7285,

The third rail is always supplied with 750 DC. The M-7's have inverters that convert that DC power to variable frequency AC to operate the AC motors.

Clem

[Long Island 7285]

thanks Clem.

[Frank]

I have another question about the M7s: Are they capable of 100mph?

[DutchRailnut]

yes the M7 and even M7 is capable of 100 mph, its their listed max speed.
Max service speed is 80 mph however and ATC is usualy set for that .

[jtr1962]

I have another question about the M7s: Are they capable of 100mph?

Someone here said they took a cab ride in an M7 and hit 115 mph near Babylon. I'm not entirely sure that's possible. I believe the M7's traction control computer tapers off the tractive effort so the train tops out at exactly 100 mph, but it could have been malfunctioning or overridden on said train. After all, it's just software that limits the train speed. The traction motors easily have enough power to propel the train at 115 mph given their horsepower ratings.

[4 Express]

I have another question about the M7s: Are they capable de-railing? I would think not because they are just SOO SUPER!

Any car can derail, the M7's are no different than any of car when it comes to that.

[emfinite]

I highly doubt any M7 went 115mph in revenue service. Whoever witnessed that was probably doped up on smack. I bet the engineer had Mickey Mouse ears too. The set MAS for these cars is 80mph, but sometimes it can be 79, 81 or 82mph depending on it being offset a bit. They WERE ran at or around 100mph for testing I heard, but definitely not in revenue service.

Joe

[Head-end View]

The M-7 would be a good commuter car if:

1- The seats were as wide as those in the M-1/M-3's

2- The number of seats per car was at least the same as the M-3.

3- The door opening/closing mechanism (built in the 21st Century) was as quiet as built 35 years ago on the M-1.

4-The passengers could see out the front, and back of the train.

And Dutch: before you say "I paid for my ticket, so sit down and enjoy the ride," picture yourself riding in the back-seat of a car with both the windshield and rear window covered so you could only see out the side windows as you went down the road. Then you'll know how I feel riding in the M-7. Chuckle !

[Nasadowsk]

I'll take a wild guess that 60Hz at the TM == 100mph +- 1 or two MPH, the inverters hard cut off at 60hz.

This isn't like a DC motor guys - frequency more or less == TM rpm on an AC. Thus, you can set the inverters for whatever max you want, and they won't go above that, regardless of load.

So, no, an M-7 can't go 115 in revenue service. I doubt they're physically capable of it, period.

And, max frequency is a software programmed value, there's nothing to 'break' that could allow it to change. You either change it or it doesn't change.

With the specs of the TMs, the gearing, the wheel diameter, it's trivial to calculate max speed in MPH, or more exactly, speed Vs inverter hz (slip is negligable).

Of course, you could reprogram an M-7's inverters to limit themselves to like 20 or 30 hz, in which case the thing would max out at 50mph or so hehehehe.

[jtr1962]

So, no, an M-7 can't go 115 in revenue service. I doubt they're physically capable of it, period.

And, max frequency is a software programmed value, there's nothing to 'break' that could allow it to change. You either change it or it doesn't change.

Yes, I know how an AC motor works. Very easy to get a hard speed cutoff compared to a DC motor.

I'm just repeating what I read somewhere else on these forums, and agreeing that the person who said it was most likely high on crack. Of course, if the engineer could disable ATC, reprogram the inverter with a higher frequency limit, and the traction motors had enough power to reach 115 then it would be possible. Unlike DC motors which tail off sharply in HP as their speed increases, AC motors maintain a more or less constant HP until they get close to the frequency of the driving waveform. Let's see, the LIRR limited the M7s to something like 700 HP so an eight-car train would have 5600 HP. That's more than enough to do 115, provided all the other "ifs" I mentioned were met. Whether or not the train would be stable at that speed, or the traction motors would fly apart, or the ROW would allow it are yet more questions. As I said, I found the whole scenario rather implausible. I would believe 100 mph, though, but that means either a malfunctioning or overridden ATC.

BTW, I used to occasionally watch the LIRR main near Forest Hills in the mid 1980s. It wasn't that uncommon to hand time the M1s/M3s coming through at better than 90, and 100 or so once in a while. (Yes, I know how to hand time trains with a stop watch with a pretty small margin of error.) I guess with the M7s those days are long gone unless they raise the MAS.

[Frank]

So, no, an M-7 can't go 115 in revenue service. I doubt they're physically capable of it, period.

And, max frequency is a software programmed value, there's nothing to 'break' that could allow it to change. You either change it or it doesn't change.

Yes, I know how an AC motor works. Very easy to get a hard speed cutoff compared to a DC motor.

I'm just repeating what I read somewhere else on these forums, and agreeing that the person who said it was most likely high on crack. Of course, if the engineer could disable ATC, reprogram the inverter with a higher frequency limit, and the traction motors had enough power to reach 115 then it would be possible. Unlike DC motors which tail off sharply in HP as their speed increases, AC motors maintain a more or less constant HP until they get close to the frequency of the driving waveform. Let's see, the LIRR limited the M7s to something like 700 HP so an eight-car train would have 5600 HP. That's more than enough to do 115, provided all the other "ifs" I mentioned were met. Whether or not the train would be stable at that speed, or the traction motors would fly apart, or the ROW would allow it are yet more questions. As I said, I found the whole scenario rather implausible. I would believe 100 mph, though, but that means either a malfunctioning or overridden ATC.

BTW, I used to occasionally watch the LIRR main near Forest Hills in the mid 1980s. It wasn't that uncommon to hand time the M1s/M3s coming through at better than 90, and 100 or so once in a while. (Yes, I know how to hand time trains with a stop watch with a pretty small margin of error.) I guess with the M7s those days are long gone unless they raise the MAS.

I don't think the M1/3s go 90 or 100mph in revenue service. The highest I've recorded on an M1/3 train was 81mph on the Main Line.

[jtr1962]

I don't think the M1/3s go 90 or 100mph in revenue service. The highest I've recorded on an M1/3 train was 81mph on the Main Line.

I'm sure they don't any more thanks to ATC. 20 or so years ago is when I'm talking about. Having had plenty of practice timing model trains when I used to calibrate my HO train speedometers, and real trains at both Forest Hills and Princeton Junction, I have a pretty good idea what 80 mph looks like even without a stopwatch. Yes, the M1s/M3s used to come through Forest Hills at well over 80 quite a bit, although I don't recall ever timing them at much over 100 (maybe 101 once). It was a lot easier to go over the limit 20 years ago than it is today. As a locomotive engineer, safety concerns aside you would have to be nuts to break the limit with all the recording and safety devices in place nowadays as it's a sure way to end your career.

While on the subject of how things in the M7 work, how exactly does the ATC limit the train speed? Do the traction motors simply cut out once you reach the set speed, or does the tractive effort start tapering off when you reach set speed, and drop to zero at set speed plus a few mph? The latter solution would result in a balancing speed a couple a mph over 80, or exactly 80 if you set the traction motors to start tapering off at 78 or 79.

Another thing I'm curious about-since maximum traction motor frequency is easily reprogrammable, why not just set it for 80 mph so ATC will be just a failsafe backup system? You can always reprogram it later should the LIRR ever raise the MAS (which I doubt they'll do any time soon anyway).