20 replies to this topic

[SkiBachelor]

Sam Magazine, November 2002:

Quote

Last year, two new Leitner-Poma lifts were installed with AC drives. These new AC drives are more expensive to buy at a cost of $80,000, but a lot cheaper to run and maintain. Unlike DV motors, AC motors are brushless, and in lift applications, sealed against contamination. So, unlike DC motors, they require no carbon brush replacement, and no regular maintenance to dismantle and clean the motor of carbon residue and other contaminants. AC motors don't require an internal flow of air, with associated filters, for cooling, another additional maintenance expense with DC systems. The new, larger AC rives still generate heat and have their own cooling requirements, but an AC motor is cooled externally, a much simpler setup that helps keeps contaminants out of the motor's of the motor's core. The new AC motor is essentially maintenance-free.

[liftmech]

One of those lifts is the Independence SuperChair at Breckenridge. The motor room is quite spacious without the blower attached to the motor, and the motor itself isn't quite as big as a DC motor. BUT... the lift shack is essentially two-storied because the drive is monstrous. I hear that the AC drive is the way to go because it not only requires less maintenance, but is more energy-efficient as well, thus paying for itself in a few years.

[Allan]

Apparently they offer excellent speed control as well... Not like the AC drives of yesterday.... with the drum controller and resistor grids, that get hot enough to melt a plastic fan... oops :)

[Dawson]

This might be a stupid question, but how do you control the speed of an AC motor? And, (again maybe a stupid question) but why is the drive for an AC motor more cumbersone than a DC drive?

[Allan]

Old AC drives control the speed by diverting power through the resistor banks, this leads to hot (and I mean REALLY hot) resistors, these are AC Wound rotor motors. New AC drives achieve speed control by varying the voltage and frequency of the electricity being supplied to the motor.

[Allan]

I don't think AC drives are more cumbersome at all... The new ones require very little maintenance, while the older ones are pretty much the same... all we need to do is lube the bearings monthly, which pretty much keeps them happy!

[liftmech]

Some clarification is in order here, I think. What we refer to as the drive is the electronics which control the speed of the motor. The motor is the physical or 'business end' part of the system. An AC drive, like Allan said, used to consist of banks and banks of resistors (think of pinching a garden hose to reduce the flow of water) which generated heat. An AC drive today generates much less heat, but still takes up space. It is (and I'm not certain how, but bear with me) more energy-efficient- perhaps because of the characteristics of an AC motor versus a DC motor.

[Kelly]

My guess is that if your area is near cheap power (within 300 miles of Columbia River Hydro Dams) and you have low horsepower lifts it might be tough to recover initial costs with the yearly 4-6% savings on power, even if they do throw in the free set of ginsu knifes with each purchase. Notice nothing is said about the annual maintenance costs to the drive itself. Or the cost of the building to house the large drive. I wonder what the drive system cost differential is to a DC?

900HP that’s one big motor!

http://www.controldesign.com/Web_First/CD....Document&Click=

Ryan B

[Allan]

What about within 20 miles of the Columbia River Dams? :)

[Matt]

AC drivers work by converting AC input power into DC. The DC is then converted into 3-phase AC that is variable in both voltage and frequency.

If the voltage to an AC motor is lowered in order the lower the speed, but the frequency is unchanged, the motor will experience exessive heating. A modern AC drive matches the frequency and voltage to deliver optimum efficiency. Most are even capable of 85Hz output, allowing the speed to be increased above what would be capable on 60Hz power.

[Duck]

We've run VFD's here @ 120 Hz, although just in testing. :)

-Iain

[Matt]

What kind of power levels are you testing at?

[Duck]

Power levels? 230V, 3 phase. The VFD wasn't on a lift, it was on a SEW Eurodrive running a gearmotor for a calendar roller tower for part of a huge automation system we did a few years ago. Each set of rollers was driven by its own VFD so we could control and tune the speed at each stage independently.

-Iain

[Mike]

The summit lift that will be built this summer at breck will probaly be AC. Allen Bradley has a liquid cooled AC drive that is very compact. The 400 hp one that breck is considering is the same size as a 800hp DC drive.

[Allan]

Heh, our magic carpet has a SEW eurodrive in it :)

[Aussierob]

I'll be very happy the day there are no more DC drives in lifts. DC motors are nothing but a pain in the . ABB's DTC drives can easily handle the ski lift application to any horsepower rating. You do need more power conditioning equipment which makes the drive larger than a DC one onf the same size. They are also more expensive to buy. The upside is you get the reliable, almost maintenance free AC motor.

[sd40t-2]

The use of AC (induction / non-brush) motors in variable speed industrial applications is not new. Railroad locomotive manufactures were beginning to experiment in variable frequency propulsion in the late 80s and began to market to the railroads in the early 90's.

Not to say that AC drives are without problems. Some computer control systems don't handle the currents and voltages as well in practice as in theory. Obviously, the currents and voltages of most lifts are not nearly as high as railroad motive power but the control systems are/could be very similar.

I'm very surprised that the lift manufacturing industry has not taken to using AC drives on a large scale long before now.

Just my 2¢ worth..

SD
D. Speirs

[Matt]

I've never worked on a lift, just have an interest in high power systems. Therefore, I have no idea how much maintenance is associated with the DC motor(s). How much maintenance goes into dealing with the DC motor?

Has anybody on this forum had any experience with the new AC drives in chair lifts? I'd be interested to know if they're using induction, wound rotor induction, synchronous, how many poles, motor RPM etc.

[Aussierob]

Dc motors are quite maintenance intensive. The brushes and commutator need to be regularly checked and they require a thorough cleaning at least once per year to remove carbon dust. There are also commutaion problems to contend with. Ski lifts operate across the full speed and load range which DC motors don't like too much. There are many different problems and the symptoms don't always point to the problem. If it gets bad enough before you figure it out the commutator turns into a fireball and its all over.

AC drives these days are variable frequency and standard squirrel cage motors can be used. Forced cooling may be necessary depending on the application. As most gearboxes have input speed from 1200-1800 RPM, a 4 or 6 pole is used.

[liftmech]

Aussierob, on Apr 5 2005, 04:14 PM, said:

There are many different problems and the symptoms don't always point to the problem. If it gets bad enough before you figure it out the commutator turns into a fireball and its all over.
<{POST_SNAPBACK}>

I know from whence you speak and it's no fun to remove and replace a large, heavy DC motor in the winter.