A stall occurs when the motor torque cannot overtake the load torque and the motor cannot rotate. When this happens, the motor will draw more current than the FLA to try to turn the load, again, defeating the purpose of the soft starter. Motor stalls are possible if the motor torque dips below the load torque; for a high inertia load, the starting load torque is quite high and stays relatively high. Fig. 26-27 show possible stall conditions (highlighted by arrows) for a soft start or a current limit start. Fig. 26-27 and 32 may be slightly misleading because once the motor stalls the speed will not increase.
Soft Starter Settings Starting Type Soft Start Initial Torque % 40%
Figure 26: Soft Start Stall Soft Starter Settings Starting Type Current Limit Initial Current% 300%
Figure 27: Current Limit Stall
- of Poles 4 Motor Inertia 100
Conclusion
other soft starters
The pump application simulation uses the same motor characteristics as the previous high inertia application but is connected to a pump. A pump load starts with almost no load torque and it increases with speed. The red line in Fig. 28 shows the load torque. Motor Information Load Information Motor Type NEMA B Load Type Pump Rated HP 200 Load Inertia 36000 Rated Speed 1500 Load Speed 605 Frequency 60 %Load Factor 80%
LRA % 600% %Inefficiency 30% LRT% 180%
The closer the motor torque can get to the pumps torque with out dipping below it, the more potential for energy savings. With almost no starting load torque a soft start can be set much lower than in a high inertia load. Fig. 28 is set to 2% of locked-rotor torque.
Figure 28: Pump Soft Start
Using a current limit start with this application, the motor torque is kept closer to the load torque (highlighted by arrows) than with the soft start above (highlighted by arrows), allowing even more energy savings. Fig. 29 is set to 225% of the locked-rotor torque.
Figure 29: Pump Current Limit
Pump Application Speed Characteristics Fig. 30-31 are speed vs. time curves for comparison to the across-the-line start.
Figure 30: Soft Start
Figure 31: Current Limit
With a pump application there is almost no starting torque, and the soft start option almost cannot fault. However, the current limit has an inherent dip in its starting profile, and again, caution must be taken to avoid stall conditions. Fig. 32 has a current limit set to 200% of the FLA and the motor starts to stall at around 50% of its full speed (highlighted by arrow).
Figure 32: Pump Stall
A soft starter is a versatile starter that can take many forms and be used to start many different applications. Along with protecting applications such as belt conveyors and saw mills, it can help save a great deal of energy by reducing the starting current and starting torque and help stop numerous conditions that are damaging to motors. Appendix
Figure 33: Kickstart Profile
Figure 34: Low Speed and Low Speed Revering Profile
Figure 35: Speed/Current vs. Torque Curve Explanation
Soft start circuits are useful in many applications, even ones that don't use motors.
For example, when a simple switching regulator is first connected to a simple solar array, the array voltage drops, causing the regulator to pull more current, causing the array voltage to drop further, causing the regulator to pull even more current, and so on. The result is "voltage collapse" with the system locking up in an undesirable state.
Soft-start circuits are one solution to this latchup problem.
Comments
Post a Comment