Squirrel cage motors : frequently asked questions

Question 1
Doug,   In that exam question involving a square wave load which cycled between zero and 500 Nm, I assumed that provided the squirrel cage drive motor was started during the zero-load part of the cycle then I didn't need to consider acceleration. The marks I received were not impressive - why was this ?

Answer
In the context of motor drives, a 'load' is a device or machine for carrying out some duty such as pumping fluid, or raising a weight, or cutting metal etc. The 'load' machine absorbs mechanical energy from the motor and transforms that energy into hydraulic energy, or potential energy, or thermal energy - or whatever, depending on the duty of the 'load'. The torque required by the load to carry out this duty at a certain speed   n_L is the load torque   T of ( 1a).
Quite apart from the foregoing, the load machine possesses inertia   I and needs an external torque   T_accn to bring it up to speed from rest - the shorter the acceleration period, the more torque is required. This is basic Dynamics :   T_accn = Ia is the rotational analogue of the translational   F_accn = ma.
You must be able to distinguish between   T_L and   T_accn.
Think of a water pump by way of illustration.

-	To drive the pump at constant speed ( Taccn = 0 ) and impart hydraulic energy to the water, the pump needs an external power source equal to the load torque ( TL ) times the speed (with appropriate units!).
-	If the pump is started up without any water in it ( TL = 0 ) then it'll still need torque ( Taccn) to accelerate - the water doesn't enter into consideration.
-	When the pump full of water is accelerated then both torque components have to be supplied by the drive motor, that is   TM = TL + Taccn.

In general therefore the net torque available for acceleration is   T_net ( ie. T_accn ) = T_M - T_L.

In the majority of practical drives the interia is so small that   T_accn is negligible - but until you've got more experience under your belt you must consider acceleration issues.
Each case must be treated on its merits.
For example when the above pump accelerates with water in it, there is also a column of water which is being brought up to speed. The inertia of this water column is thus a component of the system inertia appearing in ( 4). But is it a significant component ? You'll have to be the judge of this when you meet up with such a problem.
Another example - a metal cutting lathe. The nature of this load is such that cutting is never carried out until the machine is up to speed - so   T_L = 0 while accelerating, though the machine will need a small ( presumed negligible here ) torque to overcome internal friction and windage.
The above cited approach to the exam problem is similar as acceleration occurs while there's no load torque.
But it should now be clear from the above that :