Open-Loop and Closed-Loop

An example of open loop control is the voltage ramp (Fig. 14), the voltage ramps from an initial voltage to full voltage in a linear fashion without regards to the load. Pump start is another form of open-loop control. The pump starter’s firing circuit ramps up the voltage with a profile that allows the speed/torque to ramp (Fig. 15) in a more efficient manner and helps protect against water hammering, a common problem in pump applications. Fig. 14-15 also show examples of soft stopping. Applications such as current limit (Fig. 16) use feedback from the motor or the line current/voltage to change the firing angle of the SCRs as necessary, hence closed-loop. All control schemes listed monitor back EMF of the motor as to not become unstable.

Figure 14: Voltage Ramp 

Figure 15: Pump Start 

Figure 16: Current Limit 

Applications

Soft starters can be made for a reversing application by adding two extra SCR pairs that switch two phases. For example, line phase “b” is connected to load phase “c” and vice versa. L2 (“b”) is connected to T3 (“c”) and L3 (“c”) is connected to T2 (“b”) in Fig. 17. A delta configuration motor can also be controlled with a soft starter (Fig. 18), but it will see more current than a line connected motor. As a way to get around the larger current switching, a soft starter can be wired “inside the delta,” as can be seen in Fig. 19. Wiring in this configuration will allow the soft starter to control a larger motor than even line connected by a √3 advantage. For instance, an “inside the delta” soft starter can switch a 277A load versus a line connected soft starter needs to be able to switch 480A to control the same rated motor load. A disadvantage of “inside the delta” is that it requires six leads coming from the motor which can be an added expense with larger HP motors.

Figure 17: Reversing Soft Starter Topology

Figure 18: Delta Soft Starter Topology

Figure 19: "Inside the Delta" Topology

Above are some physical variations of soft starter applications. In contrast, kick starting and low speed ramps are some other applications that can be implemented via different programming of SCR firing angles.

High Inertia Load Characteristics

Below are some calculated simulations of different applications of soft starters with high inertia loads. Table 2 holds the properties the motor simulated. Fig. 20-21 shows a general full voltage start for comparison with a motor that has a locked-rotor current LRA that is 600% of the FLA and a locked-rotor torque LRT or starting torque that is 180% of the full load torque. See Appendix Fig. 35 for an explanation of the %Current & % Torque vs. Speed curves. Table 2: Simulated Motor Motor Information Load Information Motor Type NEMA B Load Type High Inertia Rated HP 200 Load Inertia 36000 Rated Speed 1750 Load Speed 605 Frequency 60 %Load Factor 80%

1. of Poles 4 Motor Inertia 100

LRA % 600% %Inefficiency 30% LRT% 180%

Motors must be sized appropriately to have enough torque with respect to its load or the motor will not be able to start, irregardless of starter type.

Figure 20: Full Voltage Starting 

Figure 21: Full Voltage Starting

As you can see in Fig. 22, the starting current is reduced from 600% to roughly 400% and the initial starting torque is set to 50%. The initial torque for this motor application cannot be any less than the load torque, or a stall condition may occur (discussed later).

Figure 22: Soft Start Motor Starting Characteristics 

In a current limit start (Fig. 23), the current is limited to a specified value, in this instance 450% of the FLA. The lowest level to which the current can be limited depends on the motor torque. Again, caution needs to taken in setting the current limit to make sure the motor torque does not dip below the load torque to avoid stall conditions.

Figure 23: Current Limit Motor Starting Characteristics  High Inertia Load Speed Characteristics Fig. 24-25 are for comparison to an across-the-line start.

Figure 24: Soft Start 

Figure 25: Current Limit Start