# Why would DC motors be preferred over brushless AC induction motors for cable cars propulsion?

According to this article San Francisco cable cars system uses a set of traction cables driven at constant speed and each car can connect to the cable when its operator want the car to move and disconnect from the cable when the car needs to stop.

The article further says the cable drums are being driven by 510HP (380 kW) DC electric motors that were installed during restoration in 1982. So this is not a case of just continuing to run some well built old motors.

Now why would DC motors be preferred?

An AC induction motor with a squirrel cage rotor has no brushes and runs continuously just fine. Speed variation which is hard for AC motors is not needed for this system, so that's not a problem. 1982 is long after the War of Currents ended and AC became preferable for distribution. Driving a DC motor requires a powerful inverter and DC motor brushes will need servicing and there're no obvious benefits.

So it looks like an AC induction motor would be preferable for this application.

Why would DC motors be preferred?

• As pure guesswork, it might be that they have a DC supply and so an AC motor would require an inverter in the Car. Aug 15 '13 at 9:08
• @medivh: There's no traction power equipment in the car - the car just grips the moving cable. The system originally used steam engines for driving the cable. Aug 15 '13 at 9:11
• As pure guesswork, it might be that they have a DC supply near the motor, and so an AC motor would require an inverter near the motor. Aug 15 '13 at 10:06
• It is highly unlikely that these DC motors have brushes.
– user3624
Aug 15 '13 at 17:29
• Why is that unlikely David?
– Eric
Aug 15 '13 at 18:03

This touches on basically two things: AC current and electrical machines. Let's start with the first.

The war of the currents was very, very long ago. In a time where there was only theoretical understanding of switching power supplies, but no means to actually implement this kind of device. In order to distribute power over long distances, you want to reduce the current as much as possible because current is what causes losses. So you want to have very high voltages, in the 400kV-4MV range. However, as you distribute out towards smaller and smaller units, the required physical size of such conductors as well as safety issues require the use of lower voltages. In homes, you want to use safe low voltages, i.e. <600V. For medium scale distribution, something in between is preferred, nowadays standardized to 10-40kV. So for grid-scale distribution, you require at least two big conversion steps.

By far the easiest and most reliable way to do this is with transformers. They are solid-state, very well understood, absolute lowest complexity. Of course, transformers only work well with AC current of some kind, and in order to minimize overall transformer size but avoid the worst parts of skin and proximity effects, we chose about 50-60Hz as the AC frequency. This is why things are the way they are now.

However, this does not mean that AC is 'better' than DC. At the time, using AC was the better choice. Nowadays, DC is. At least for grid purposes.

AC is a much less efficient way of transporting large amounts of power. It is required for transformers, but otherwise it is horrible. It is very easy to do a first-order calculation to see how bad this is.

Take a distribution system that has a 1000V conductor insulation rating, i.e. I can safely transport energy at 1000V. Let's say I want to transport 1000W of power over here. This means that I can either do 1000V, 1A or 707VAC, 1.4A. It is immediately clear that DC resistance losses in this transmission line will be $I^2=(\sqrt{2})^2=2$ times as high for AC compared to DC. Or alternatively, a transmission line with equal current rating will be able to carry $\sqrt{2}$ times the power when operated with DC instead of AC.

However, there are more distribution advantages to DC. The most prominent is skin effect, although the proximity effect is something you might also want to look up. Skin effect causes changing current to bunch up near the surface ('skin') of a conductor. This means that the current density is not uniform over the conductor cross-section, and this causes the effective resistance of the wire to increase as frequency goes up. Especially when dealing with very high currents like in transmission lines, this can cause tens or hundreds of percents of effective resistance increase. A mitigation is to use hollow wires, wound plate or Litz wire, but these are fairly expensive methods.

Now, with DC transmission being very clearly the best solution of power transmission, why don't we do that everywhere? The reason is simply of cost and complexity. Even though we have very efficient and reliable DC-to-DC grid converters nowadays, they are inherently much more complex and still measurably less reliable than old fashioned transformers. Until very recently (post-2000!) the increased cost of maintenance and increase in downtime weren't worth the 40+% reduction in effective grid losses.

So that is why ideally, you want to use DC everywhere where otherwise AC would be used. Now onto the machine discussion.

This also boils down to an efficiency/performance vs. cost discussion. Very analogously to the transmission line argument, DC motors have performance that is very favourable for use in transportation equipment. AC motors generally sport piss-poor stall torque, whereas torque at stall is maximum for DC motors. Also, power density is much better with DC motors, reducing physical size, weight, etc. All very nice things for transportation. Also, it is very easy to do regenerative braking on DC motors, whereas AC motors require some more work in order to regenerate into a battery.

The downside is obvious: they need brushes, and those require maintenance. AC induction motors are basically safe life, so they don't ever need maintenance. Even though they are technically inferior in most ways, lately they have been increasingly employed in public transportation. The biggest downsides of yore - very pronounced cogging despite using skewed squirrel cages, lack of torque, complexity of frequency drives (yes, most public transportation runs from DC transmission lines, so you need to do conversion to AC with a machine drive of sorts) have basically been sorted out. Also, AC induction motors scale better with higher power - a 1MW induction motor is actually smaller than a DC brushed variant. And apparently - I don't have the exact stats but this is what I have been told - the slightly lower efficiency and drive concerns are easily offset by lower maintenance.

• The DC motors advantages would only be important for motors installed on cars, but in this system the motors are installed on real property and run at constant speed - so the stall torque, power density and regenerative braking are almost irrelevant. Aug 15 '13 at 9:25
• Then it's probably an efficiency/driving complexity design decision. (if I'd have to guess) Aug 15 '13 at 9:34
• Stall torque may be important when you restart the cable car system in the morning and have several tons of cable to accelerate from rest up to normal running speed? Aug 15 '13 at 11:05
• @RedGrittyBrick: Well, maybe, but I guess they could employ some kind of assistance motor for that operation. This takes like 5 seconds per day but brushes wear out all the time. Aug 15 '13 at 12:44

The key is that although the DC motor will slow down when the cable is grabbed, the DC motor can take shock loads far better than an AC motor. That shock of trying to accelerate a stopped car is huge, but as the grip is established slowing the motor down, the current will rapidly rise increasing the torque far more effectively. The same principles apply to DC traction motors which much produce massive torque at zero speed to accelerate a mass. In the case of the cable car, no sophisticated control is required, it is very simple and the speed of the car is not critical. As the torque requirement tapers off, the speed will naturally come back up and current decreases and counter EMF rises. The DC motor is a natural brute producing high torques und high load at low speeds. All this happens based on it natural design and no sophisticated control is required to boost the torque.

I don't know if this question is able to be answered by anybody here (unless somebody here happens to be on the team that designed this system). It could be a legacy issue where the old system used DC motors which made it easier to stick with DC motors. It could be a performance issue ... the motor/gear reducer system needs a certain output speed and torque and maybe the DC motor and reducer system can give them what they need better (more efficiently, less costly, etc.) than an AC motor and reducer system can. There are certain maintenance issues that can arise with AC motors (e.g., AC motors on VFD's can see premature bearing failure due electrical discharge machining (EDM)). But again, without knowing a lot more about the application and what systems were considered, it is difficult to say why they chose this one over any alternatives.

According to this brochure from GE (pages 10-11), the S.F. cable car system uses GE's Kinematic CD6000 line of brushed DC motors.

• Makes sense, but not the best efficiency. They stayed with what they had because 3-phase brush-less motors are very expensive, and the electronic 3-phase controller is very expensive. The total system cost to change to 3-phase would be in the millions of dollars. If it works, don't fix it.
– user105652
Apr 25 '16 at 4:02