I know you've said you do not want any more emails, so please do not feel under obligation to replay.
I was on holidays and missed the whole discussion
Apologies to jbartos, electricpete,dougmsoe, gsimson if I repeate what you have already said; you guys provided v.good replays.
OhioAviator- you did not state at the beginning what is the application you are looking for, speed control or soft start ?
Anyway, here is my contribution:
Solid State Soft Start Systems reduce the available full voltage start torque when applied to SQ or WR induction motors. Hence applications requiring a maximum starting torque cannot be used with a soft starter. However, where the WR motor has been used to reduce and control the starting torque application of soft start control can be readily adopted.
The characteristic of increasing torque with decreasing current frequently determines the selection of the WR motor when starting current limitations are severe. I.e if a power company regulations limits LRC to 1.5 FLC , WR motor would still produce a LRT of 1.5 FLT.
Each addition of R in the rotor circuit reduces motor speed . Speed reduction is practical only to 50 % of synchronous speed. Beyond that speed becomes unstable because high slip characteristics are produced in the a rotor that operates with a high resistance in its circuit. For this reason application where speed is to be reduced <50% are used only where a constant loads are involved( bridge, trolley, crane)
Normally, a WR motor is designed to operate with small slip and high efficiency at full load. So, when speed is reduced , slip increases and efficiency decreases.
When operating at anything other then max speed the resistance control of WR motor consumes power that would otherwise be used to move the load. This inherent inefficiency of WR design has lead to its obsolescence in variable speed application.
Consequently, applications requiring variable speed 3 ph motors now use a variable frequency drive (VFD) to adjust the speed of a standard SQ motor. The VFD electronically changes the frequency of the AC current supply , thus changing the synchronous speed of the motor. This has a huge benefits in power consumption over the resistance control.
The other problem with application of WR motor in variable speed application is that if the secondary resistance value of each phase become unbalance, the vibratory torque is generated.
WR motor can be applied on either constant speed or adjustable-speed drives. They are particularly suitable for smooth acceleration of loads in application which require high starting torque with low starting current or impact load (elevators, ventilating fans, printing press, pumps, compressors, conveyors, pulverizers, stokers, positive pressure blowers, crushers, shredders .
In summary major reasons for selecting WR motors are:
1. The load can be started at the max torque
2. Large starting torque can be obtained in comparison with low starting current
3. In case when starting frequency is too high for the thermal resistivity of the an ordinary SC motor
4. In case GD2 of the load is too large for the thermal resistivity of an ordinary SC motor
5. In case load requires cushion starting
Today, owning to their complicated construction and hence high maintenance and cost, WR motors are used mainly in applications when relatively high starting torque is required, but in which the starting current may not exceed the FLC much-application 2 above.
Note – in very large applications 8000 + kW a solid state slip energy recovery (SER) system in conjunction with the conventional slipring resistors is used to perform variable speed control.
