elevator DC motors

[Mineralacc]

Why do some elevators use DC motors ?

 

[dereckbc]

Economics and Physics. Because large power motors are generally DC and less expensive than AC. DC motors are also much easier to control with precision using much simpler less complicated controls. And in places like New York City up until a few years ago, the electrical distribution system in the city was DC. To this day just about all of NYC elevators are still DC.

 

[Besoeker]

Economics and Physics. Because large power motors are generally DC and less expensive than AC.

I respectfully disagree.
I have dealt with both.
Tomorrow, I'm going to look at a problem on a VSD. It's 6350kW. About 8,500 HP.
It's an AC drive. As it happens I'm also bidding for a DC drive refurb. It's rated at 2,000 HP, about 1500kW.

DC motors are also much easier to control with precision using much simpler less complicated controls. And in places like New York City up until a few years ago, the electrical distribution system in the city was DC. To this day just about all of NYC elevators are still DC.

Sure, the power electronic control of DC motors is simpler.
But are the motors cheaper? I don't think so. And maintenance with the brushgear is an on going cost.

Better precision?
Um....well.... we manufacture specialist motor/drive systems that can get from 10,000rpm to zero within half a second ant to within half a degree of position accuracy.
AC systems.

 

[iceworm]

Why do some elevators use DC motors ?

JMO
Elevator and hoist motors don't need much for speed control or precision positioning - but they need some. But they have to have really high starting torque.

30+ years ago, maybe 20+ years ago, and maybe even today, it is (was) easier to get really high starting torque with a DC motor than with an AC motor. Design D and wound rotor AC motors are pretty good, but the starting torque is on the order of half that available from a similar sized DC motor. And the speed and position control of a DC motor is sufficient

And, of course there is always the money. Pulling the other direction, DC and wound rotor motors are expensive to build and maintain compared to a squirel cage induction motor. One might say that anyplace there is space, a 100hp Design B with a drive is a better deal than a 30hp series DC.

Don't hold to tight to the numbers, but I think the concepts are right.

ice

 

[dereckbc]

I respectfully disagree.

Np problem, I am aware both are out there. The OP asked why are DC motors used. Torque, cost, and control are my answers. If I had to give another reason is a lot of the older designs use DC. Some of our NYC friends correct me if I am wrong but I think most in NYC are still DC.

 

[brian john]

Np problem, I am aware both are out there. The OP asked why are DC motors used. Torque, cost, and control are my answers. If I had to give another reason is a lot of the older designs use DC. Some of our NYC friends correct me if I am wrong but I think most in NYC are still DC.

In Washington DC all older motors were DC, even with some retrofits only the generators are replaced (in some cases), in quite a few of the total redos they are going AC..

 

[Jraef]

There are other issues that are only recently addressable with AC motors and drives. The main one is regeneration when lowering on a traction elevator. In a DC 4 quadrant drive, turning the motor into a generator and pumping the regen energy back into the line for continuous braking is very simple. Until recently this was very difficult for AC VFDs, which has limited their use to shorter buildings where the regen energy can be safely dumped into Dynamic Braking resistors. DB is a much simpler and cheaper process, but one with duty cycle limitations. That means that the places where AC VFDs with DB have to complete head-to-head with simpler hydraulic elevators, and they usually lose. So that leaves the high-rise business, where they have to compete with DC tried-and-true systems.

Many of the major mfrs have been developing lower cost line regen solutions, but until the reliability is more proven it will take a while for it to filter into the elevator world. There are VERY stringent reliability issues related to passing the necessary safety standards for elevator controls; most common AC VFDs would never pass. Plus there are VERY expensive product liability issues that smaller mfrs cannot afford to take on. That's why so far, the only VFDs I have seen sold in elevator systems are brand-labeled by the elevator mfrs, like Otis, Thyssen etc. and of course because they are in BOTH the elevator and VFD business, Mitsubishi.