Long before variable frequency drives became popular elevators were often driven by dual-speed asynchronous AC motors. Switching speeds was done by changing the number of poles (which is done by changing how stator coils are interconnected) - the motor would first run on lower speed, then switch to higher speed, then back to lower speed and then stop.

I've read many times that asynchronous AC motors have very low torque when starting. Looks like low starting torque would be a serious problem for driving an elevator - to such extent that a fully loaded elevator could stall the motor.

How is this problem resolved?

  • \$\begingroup\$ I've just read some days ago a question about DC distribution being shut down for old elevator motors... Maybe the answer is "they used DC motors". Or gearing. \$\endgroup\$ – Vladimir Cravero Mar 24 '15 at 11:51
  • \$\begingroup\$ @VladimirCravero DC motors were much, much earlier. I'm referring to current installations that still often use AC motors with pole switching. I guess you're referring to this cityroom.blogs.nytimes.com/2007/11/14/… \$\endgroup\$ – sharptooth Mar 24 '15 at 11:52
  • \$\begingroup\$ Yes that's what I was writing about. I really can't help but my bet is gearing, or "belting". \$\endgroup\$ – Vladimir Cravero Mar 24 '15 at 11:56
  • \$\begingroup\$ I don't understand the question. Why wouldn't you avoid the issue altogether by picking a motor whose starting torque is high enough for the load rating of the elevator? \$\endgroup\$ – Dave Tweed Mar 24 '15 at 11:56
  • \$\begingroup\$ @DaveTweed Maybe that's the answer, however I've seen a smaller AC motor (perhaps 200 watts or so) which drove a grinding wheel no problem except I had to start it by hand so I assume the starting torque is extremely poor. \$\endgroup\$ – sharptooth Mar 24 '15 at 11:59

Large elevators were and many still are, DC. There are a bunch of reasons.

A fully loaded elevator only has to move half the apparent mass because there is a counterweight. At half mass there is very little work done going up or down.

A DC motor can be locked magnetically. Polyphase variable frequency drives use DC injection ito hold a position.

However the answer you seek is that the winding motors operating on AC were slow. They had/have many poles, unlike the grinder mentioned above. The winding drum is slow. Why use gearing when many more poles will slow things. Think of a 720 RPM motor, or 350.

Also remember that a motor frame size reflects the number of start cycles per hour so an elevator is 'heavy duty'. The reason is cooling. So the motors are wa-ay over spec'd, many poles, big diameter for the kW and extremely reliable.

I am sure no one here has heard of an elevator motor burning out from overheating.


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