I hope the following comments be helpful:
A- The most important problem in Large AC machines is cooling problem, which is critical in stator teeth and copper bars area. When you change the machine type from synchronous to induction it will increase heating in this area while the cooling will be lowered due to the following reasons:
A-1- In synchronous machines stator winding caries only current for electromechanical energy conversion, while in induction motors magnetizing current (the current which plays the role of field current in synch. machine) also passes through stator winding. This increases stator winding’s loading and because of thermal problems machine rating will be decreased.
A-2- Due to the 1st reason, the air gap should be smaller than which is in synchronous machine. (Large air gap absorbs more mmf and thus needs more magnetizing current, which will reduce power factor of the motor). Due to the smaller air gap 3 thermal problems will be arises:
A-2-1- Cooling will be reduced. Cooling in medium sized salient pole synchronous machines often performed by two side fans. Cooling air moves along interpolar section of rotor and enters radial stator ducts gradually.
In large induction motors cooling air passes through axial holes in rotor (and a little air gap) then passes through radial ducts of rotor, reaches air gap area and then enters radial stator ducts.
I think the air passages in induction motor cooling circuit have more hydraulic pressure drop than which is in synchronous motor thus when you replace the salient pole rotor by induction rotor should consider more cooling pressure for fans. [1]
A-2-2- There is also another important reason to increase cooling fan pressure: there will be no fanning effect of rotor when you use an induction rotor. (The rotors of salient pole machines produce hydraulic pressure same as a fan does).
A-2-3- due to shorter air gap and the presence of teeth on rotor, there will be a high frequency pulsation in the flux density of stator teeth, which will produce more hystersis and eddy current loss in stator teeth.
B- Due to small air gap and presence of teeth both on rotor and stator there will be reluctance change when rotor slots move in front of stator slots. These reluctance changes produce a high frequency pulsating torque and thus noise. With the number of stator slots equal to or an integral multiple of rotor slots it will be a higher reluctance change and may result in a starting problem which calls "COGGING". It's very important to pay attention to the relation of number of rotor and stator slots.
C- Increase of voltage level will decrease cooling and also decrease slot space usage these again will result in lowering the power rating of machine from those for which it had been designed.
D- As a general rule of thumb the output of a rotating machine can be calculated from bellow relationship [2]:
So = 11 Kw.Bav.ac.sqr(D).L.n [kVA]
where
So is machine kVA rating,
Kw is winding factor,
Bav is average flux density [T], (range: 0.5 0.65 T)
ac is specific electric loading [A/m],(range: 20000~40000)
D is inside diameter of stator [m],
L is the length of stator [m],
And n is rotation speed [rps]
In this case L, D, kW and n will not change when the type of machine changes, Thus the only factors affecting new output rating are "Bav" and "ac". (Typical values for a synchronous machine in this range are ac=30000[A/m] and Bav=0.63[T])
Both "Bav" and "ac" taking smaller values in the design of induction motors than synchronous machines but in this case due to increased voltage there will be an additional decrease in "ac". Thus when you change the machine type you should decrease machine rating considerably. That’s why low speed high power induction machines aren't use in industry. They are uneconomical.
A.R.Nag.
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References:
1-Crew,N.J. "Flow distribution and pressure drop in salient pole electrical machines" proc. instn. mech. eng. vol 184, pt.3E, 1969-70
2- Say,M.G. "Performance and design of AC machines" Sir Issac pitman & Sons , London 1962
