I was ask how to size a microproccessor replacement for a MG-set. The problem as I understand it to be is as follows: There is an elevator that ran @500 ft per min, the MG-set has been replaced by a microprocessor unit, the elevator now runs somewhat less than 500 ft per min., and the weight is now 3500 lb and originaly it was 4500 lb. One thought is to replace the dc motor with a larger one. It seems to me that when the microprocessor was sized, knowing that the motor was to be reused, it should have been noted that the motor needed to be replaced or that the microprocessor would not do the job, or used a microprocessor that would handle the job. Any way, what I need help with at this point is HOW DO YOU SIZE A MICROPROCESSOR TO REPLACE A MG-SET

-- MIKE SAUNDERS (MSAUNDER@GSA.GOV), January 26, 1998

Answers

I take it that you are using the original hoist motor that ran the unit at 500 fpm with rated load prior to the control change alluded to in your question. If this be true the motor should be sufficient, therefor I would suggest that the inquiry be directed in another direction.

The micro processor is a decision making unit not a power device. the mP tells a "power device" (which appears to be a part of the control change) when to go and how it should handle the go signal. The power device should be examined to determine whether or not it is capable of an output in terms of wattage or horsepower to drive the motor to achieve the desired speed at the rated capacity of the elevator car. The problem may well be a matter of adjustment of the new control system. You may wish to contact the control manufacturer in this regard.

I would question the weights you comment on in terms of the new rated capacity of the elevator, what is the counter weight in relation to the empty car weight or some other rational relationship, what is the rated horsepower of the hoist motor and can the rated motor voltage and amperage be determined, who manufactured the hoist motor and is the motor data plate information (all of it) available? Then one should establish the rated horsepower of the control device.

Simply put, the work required of the motor to achieve rated speed at full load must be determined, and then does the new (presumably) SCR drive have a sufficient output to drive the motor to do so?

Assuming an 85% system efficiency (gearless machine), one may wish to apply the formula (actual load x rated speed divided by the quantity 33000 x .85) to determine the required horsepower from which the drive and the hoist motor could be examined to deterine as to whether or not they are properly sized. Note that there are other factors that may be involved, and the preceeding is merely a rule of thumb with which to get started. One may wish also to determine whether or not there is a true closed loop control to regulate speeds under the varying loads the elevator is subjected to in normal useage.

I hope this helps. More information would help me.

Your question infers that the rated capacity of the elevator was reduced from 4500 pounds to 3500 pounds as an element of the changeover to a mP/SCR based control system, and that the rated speed remained at 500 fpm. Is this true? VPK

-- Vernon P. Keller (vpkeller@earthlink.net), February 08, 1998.