AC to DC rotary converter

I'm aware of some elvators that still use a rotary converter setup exactly like a single phase to three phase or changing frequency. You
can't easily get three phase out of single phase, but if you have three phase AC, then getting DC from it is easy, isn't it?
This rotary converter I have seen is a Westinghouse 25kW "MG set" that is made of a 40hp 460v 3ph motor and a 240v DC 105A generator in one case. Why do they continue to use these as an elevator power supply rather than converting to diode rectifiers?
AC motor -> DC generator -> DC motor seems inefficient. The MG set itself is probably maintenance intensive too. I can't think of a reason why they'd continue using it.. Any idea?
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ITSME.ULTIMATE wrote:

One of the advantages is that they can control the DC motor used to operate the lift. (The methods are many, mainly in the fields of the motor and genny :-). Of course many cheaper or commercial units use AC motors and also the control of AC motors has come a long way since the original concepts were formed. The MG sets are pretty reliable and dont take that much maintenance.
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Regards ......... Rheilly Phoull



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Early in my career I was headhunted by a local elevator company to do PLC programming. Much of their work was replacing relay controllers with PLCs, but retaining the MG sets and DC motors. They took a lot of measurements to learn how each installation controlled acceleration and deceleration by switching field voltages and polarity.
They were also an early user of VFDs and AC motors in elevators. It took a while and a number of fried braking resistors to convince Toshiba how much power was created by regenerative braking on an elevator car.
Mike
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"ITSME.ULTIMATE" wrote:

That's the way they used to do it back when solid state equipment wasn't available. Elevators designs, having life safety considerations, tend not to change rapidly. Newer systems use VF AC drives, but its not economical to retrofit older equipment, so you'll still see MG sets.
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Not sure its important for elevators, but MG sets are inherently regenerative! I've rescued more than one customer who replaced an MG set with diodes without realizing this!!
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BFoelsch wrote:

So are variable frequency drives, with the correct algorithms.
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On Tue, 23 May 2006 19:17:18 -0700, "Paul Hovnanian P.E."

I worked for a summer as an elevator tech trainee in downtown San Francisco on a 52 story building. They were considering replacing the MG sets with solid state DC drives at that time (AC drives were not that capable yet). The captitol cost was excessive, not because of the DC drive cost, but because of the cost of removing the MG set! It had to be taken out of the roof, and the only preactical way to do it was by helicopter. The insurance alone for that operation was 10 times the cost of the new controller!
Generally though, as the MG sets fail or need to be removed for service anyway, they are being replaced by solid state technology. It's just that they are so reliable that they tend to stay in service for a long long time.
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