Motor Sizing 1

[signor2004]

Can someone help on how to select and size ac electric motors? Assuming you want to select a squirrel cage induction motor, how do you go about getting the actual size. You may assume any mechanical load, please.

 

[Barry1961]

The hard part is calculating the load of the system. In a extremely simple example of just pulling a cart there are several variables. Some of these are, co-efficient of friction of the wheels, how level the surface is, how fast you want to start and stop, efficiency of the drive/gearbox, type of surface(mud) and speed.

Once you have calculated the load there are other factors like enviroment(heat), speed range required, response required(inertia matching), number of starts per hour, if you need to produce torque at low or high speed, the type/condition of the voltage you have available and the type of drive or starter you use.

But if you ignore all the above and have put a torque wrench on a wheel of your cart and have measured 40 in-lb of torque to move the cart and you know the wheel needs to spin 1200 rpm the equation would be.

Torque(in-lb) x RPM
------------------- = HP
63,025

40 x 1200
--------------- = .76 HP
63,025

Most likely the motor would have a very hard time starting in the above example and would trip the overload relay.

Hope this helps.

Barry1961

 

[aolalde]

Get the speed-torque curve of the load. ( rpm vs Lb*Ft)
Define the load operating speed in rpm and get the torque required (Lb*Ft).

BHP= rpm*Lb*Ft/5252 (break horse power)

Assume a service factor around 1.10

Motor required HP= 1.1*BHP

After a motor HP is defined, the speed-torque curve of the motor is required as well as the reflected inertia of the load to the motor shaft (WK^2). Whit that information the starting analysis is done to ensure satisfactory motor life and performance.

Most motor manufacturers will provide support to potential customers.

 

[rbulsara]

See this link:

http://www.patchn.com/motorformula.htm

 

[rbulsara]

also eventually you need to check that the motor produces adequate torque to reach the rated speed for a given load in specified time, usually less than 10 seconds. For low inertia loads the acceleration time will 2-3 seconds.

 

[Farkel]

The 1st thing one does, is identify the application for which
the motor will be used. This identifys the Design of the motor, which will be used. This is an addition to what has all ready been said.

 

[advidana]

If in the USA, It's simple just Follow the National Elerctric Code Or get an electrician to wire it up for you.