MRL elevator motor sizes

[lielec11]

Does anyone have a rule of thumb for machine-room-less (MRL) elevator motor sizes? Can be traction, hydraulic, or both...based on capacity and/or number of stops. This has come up on a few jobs lately during shop drawing review where what is being installed is actually higher than what our company has designed.

My company standard is "5-10HP per floor" but this doesn't seem to be accurate. I've brought it up to the owners but they seem to not care enough to change their ways.

I'm just wondering if anyone has a good way of estimating the motor sizes for reference on future projects. From my google search I cannot find anything on manufacturer websites but I have a few calls in to ThyssenKrupp and Schindler Elevators for more information.

 

[Ingenieur]

floors/stops don't matter
only capacity and speed

[ 0.4 x rated capacity (lbs) x speed (ft/sec) ] / (550 ft-lb/sec per HP x 0.9) = HP

assume capacity 4000 lb
speed 900 ft/min or 15 ft/sec
[ 0.4 x 4000 x 15 ] / (550 x 0.9 ) = 48 HP, call it 50

basically P = Work / time = (Force x distance) / time and (distance/time) = velocity
P = F x v
the 0.4 is to compensate for the counterweighting
550 lb-ft/sec = 1 HP is a conversion factor
0.9 is overall losses, friction, etc.

 

[lielec11]

Thank you!

:thumbsup::thumbsup::thumbsup:


I am having trouble understanding the proof behind the bottom of the equation. Anything else you can expand upon?

 

[Ingenieur]

Thank you!

:thumbsup::thumbsup::thumbsup:


I am having trouble understanding the proof behind the bottom of the equation. Anything else you can expand upon?

no problem

not really
basic Newtonian Mechanics

 

[lielec11]

no problem

not really
basic Newtonian Mechanics

Nvm, I looked at it again and the "550 x 0.9" is really just the conversion to HP. Everything else I'm good on. Thanks!

 

[Jraef]

I've never been asked to size the motor, that's the elevator suppliers job. But I have had experience in rating control components for elevator duty and duty cycle ABSOLUTELY matters. To my thinking that would extend to the number of floors as well. A building with 15 floors and an elevator that can be told to stop at every floor in the morning or at closing time will see a LOT more starts per hour than a building with 3 floors. So when the duty cycle is higher, one way to handle that in a motor is to de-rate it.

 

[steve66]

In my experience the motors for the MRL traction elevators are much smaller than the motors for hydraulic elevators.

Maybe they were just really, really slow elevators?

 

[lielec11]

I've never been asked to size the motor, that's the elevator suppliers job. But I have had experience in rating control components for elevator duty and duty cycle ABSOLUTELY matters.

I agree but the problem is the elevator manufacturer doesn't typically get involved until during construction when the contractor bids the job. At that point the design is already finished and we're sitting here hoping we allocated enough power for the elevator motor. I've also had it come in AFTER the switchboards and panels were approved only to have the elevator CB jump from 150A to 225A.

 

[Ingenieur]

trips/stops do not factor in

page 3-4 http://www.baldor.com/Shared/manuals/770-397.pdf
Elevator Motor Horsepower Selection
Selection of a motor and control for an elevator application is dependent upon several
variables. The primary variable is the overall mechanical efficiency of the elevator. The
efficiency of gear driven elevators varies from about 45 percent for slow moving cars to
70 percent for faster moving cars. On gear-less elevators, efficiency may be in the 90
percent range.

The horsepower required for a specific application can be calculated as follows:
US Measurement System

HP = [LBS x FPM x (1 - ocw/100)] / (33000 x eff/100)
my rule of thumb is a mod with typical values

Where:
LBS = Car capacity in pounds
FPM = Car speed in feet per minute (FPM)
OCW = Over counter weight in %(percent) of car capacity
%EFF = Elevator mechanical efficiency (decimal)

 

[lielec11]

One more thing... I am assuming the only thing that will vary between elevator types is the efficiency number? I.e. 0.9 may work for a gearless traction elevator but a hydraulic may be somewhere near 0.6-0.7 ?

 

[Ingenieur]

One more thing... I am assuming the only thing that will vary between elevator types is the efficiency number? I.e. 0.9 may work for a gearless traction elevator but a hydraulic may be somewhere near 0.6-0.7 ?

Correct
the 0.4 may change too
but if you use 0.5 you should be good
Look at a few jobs you completed, plug and see how close to the actual hp