Calculation of Inertia Ratio

The maximum load that can be handled by the motor is given in the motor selection tables published by the manufacturer. There is very little connection between this and its Moment of Inertia.

[kg.m²]is a unit of the property Moment of Inertia.

You should consider method of starting(DOL,star delta,softstarter etc) as well as motor characteristics(design A,B,C,D etc and ratio of starting kVA/hp) and load characteristic (C.pump,blower,conveyor,compressor etc)

You should draw motor characteristic & load(eg:c.pump)characteristics on the same sheet of paper and study.

From first principles rather than vast experience....

Inertia is a property and is not usually expressed as a ratio. Inertia in the world of rotation is like Mass the the world or linear motion, and so just as Force = Mass x Acceleration in linear motion, Torque = Inertia x Angular Acceleration. Newtons law of motion apply equally in both the linear world and the rotational world (correct units assumed)

The correct units in the metric system for rotation are N_m for Torque, Kg_m2 for Inertia and Rad/sec for angular acceleration. Your motor selection will be determined by how long it can operate in the stall and semi-stall or at high slip conditions, till the rotational speed comes up to normal.

The calculation is basically about how much over current and heat the motor can soak up without getting too hot.

So....no body noticed that I wrote the incorrect units for angular acceleration !!

- they are of course rad/sec2 not read/sec, the later being angular velocity.

But to add to the comment before: there in area I did not cover, and that is in control systems were response time or frequency response characteristics are often important

The rotor inertia has nothing to do with the selection of the motor. This is in the domain of the motor maker. What you need to look for is the load inertia that the motor needs to handle at start until it reaches the required (designed) speed of the load connected to it.

Does the rotor act like a flywheel starting and accelerating the connected load?. If stored energy in rotor is low can it start a big load & will it stall?.

The Rotor inertia is determined or fixed at the design stage of the motor. I do not think that the maker will just increase the mass of the motor just to increase the inertia. There are several variables that are manipulated at the design stage for specific requirement or special duty motors. One of the important ones (I think) will be the initial torque (or starting torque). This is the answer to your STALL question.

To store energy for the purpose of starting a big load ... suddenly ... then you could add a flywheel on the drive to enable such loading (via a clutch? or ?). BUT be careful when designing such a method, no to SNAP your shaft or DISTORT the rotor material!

Forget using the rotor as a flywheel. It is done for low values only.

The inertia of a motor has nothing to do with starting, as at the moment of start, the angular velocity is zero and there is no angular momentum. At the moment of starting, whether the motor is able to spin freely or whether is it connected to a super-massive flywheel cannot be determined. Therefore, the conditions at starting are the same across all cases of mechanical load.

A motor will stall if the torque needed to sustain the mechanical load exceeds that which the motor is able to deliver. The motor overload protection device(s) need(s) to operate to protect the motor from excessive heating in such a case.

According to legend, the motor needed to rotate the restaurant about the axis of the British Telecom tower in London is a fraction of a horsepower; it is so because although the restaurant has several tens of tons of mass to provide inertia, the gearing in the drive is of such a high ratio that the rotation speed is very low (typically 1 revolution per hour) and consequently very little torque is demanded of the motor, both at starting and during continuous operation.

There are two types of starting conditions namely starting with no-load and starting with load connected. In a centrifugal pump at no load(low current) discharge valve is closed and at full load valve open(high current).

Where do you get this stuff?

Do you ever consider what you are about to type, before you start typing?