Performance of Star-Delta Motor Starting
Many questions submitted to the site are about motor starting, specifically star-delta. A more detailed study is recommended for all but the most basic applications. A software study allows for the evaluation of both the electrical and mechanical performance of the connected mechanical systems.
This note provides an example of one method for analyzing the performance of a star-delta motor starting circuit.
The Illustration
It will be investigated a relatively simple example of a 15 kW motor supplied directly from a source and with the load modelled by a simple inertia. By keeping the model simple, the principles will be easier to examine and comprehend.
The model’s technical parameters are as follows:
1 pole pair squirrel cage motor, 15 kW, 380 V, 50 Hz,
stator resistance, Rs = 0.0258 pu, and reactance Ls = 0.0930 pu
rotor resistance, Rr’ = 0.0145 pu and reactance Lr’ = 0.0424 pu (referred to stator)
Lm = 1.7562 pu stator zero sequence inductance magnetising inductance Lo = 0.930 kg.m2 connected inertia = 0.15 kg.m2
There are three stages to the modeling process that are necessary to fully grasp the motor starter’s operation:
1 model for starting directly on the internet
2 adding an open-transition starter to the model of (1).
3 modeling a closed transition starter for the star delta
Circuits for Starting Motors
The reader is assumed to have some knowledge of motor starting circuits when this post is written. If you are unfamiliar with these types of circuits, you can read “Motor Starting and Control Primer,” a short introductory eBook on the subject.
By first building a direct-on-line model, we can ensure that the output matches expectations and that the model is working properly. The direct-on-line model output also provides a baseline performance against which the results of star-delta starting can be compared.
Models for star-delta starting will include both open-transition and closed-transition implementations. When switching between star and delta in an open transition, there is a power break, whereas in a closed transition, resistors are used to eliminate any power break.
In practice, timer relays are used to control the switchover of star-delta starters. Timing signals are used in the model examples to simulate this behavior. When to switch from start to delta is frequently stated as the time when the motor has reached 75 to 85 percent of its operating speed. To investigate this rule, we will examine three scenarios with changeovers occurring at 70{8e0181f9b4047a7790fd20bbf2d43faff96926569f2820f3da0bb199a786bafe}, 80{8e0181f9b4047a7790fd20bbf2d43faff96926569f2820f3da0bb199a786bafe}, and 90{8e0181f9b4047a7790fd20bbf2d43faff96926569f2820f3da0bb199a786bafe} of full speed.
Circuit Simulation
Simulink was used to create the models, and the post only provides brief explanations of how they work. You can visit the Simulink website for more information on any aspect.
We can use the models to measure and analyze a variety of parameters. The three most important to engineers (and frequently the subject of text book discussion) have been chosen for consideration:
electrical torque speed of a stator
Starting Directly On-line
Direct online registration is simple. A line contactor closes, connecting the power supply to the motor directly. The windings of the motor are connected in a delta configuration.
The circuit used to model the behavior of direct-on-line starting is shown in the illustration (click for a larger image). Although the model is simple, I’ll explain the function of each element briefly:
asynchronous device Squirrel cage – simulates our motor’s dynamic behavior (via Park’s dq0 transformation).
A three phase voltage source provides power to the circuit, and resistance, R = 0.5, simulates the source impedance and transmission path. After 0.4 seconds (set by Ton), a switch (line contactor) operates to connect the supply to the motor.
The three phases are directly connected to the positive end of the windings (the first phase is also transposed in the ‘Delta Connection’ and connected to the negative end of the windings (the second phase is connected to the positive end of the windings).
The ‘Inertia’ element represents connected inertia, while the ‘Stator Current’ sensor and (pu) terminals allow measurements to be taken and displayed by the scope block.
The plot below depicts the results of the simulation.
The results are consistent with what would be expected of a direct-on-line starter based on foundation theory. Once the motor is up to speed, the speed gradually increases to full speed, and the electrical torque follows the expected profile of increasing and then decreasing. When the motor reaches full speed, the motor current drops to its normal operating value.
The plots of speed and torque are per unit (pu). Real values for current are plotted because they are most important to any engineer implementing a starting circuit. Furthermore, the instantaneous and rms current curves are plotted.
The following are the key conclusions drawn from the results:
The time to full speed is approximately 2.7 seconds.
Full load current is about 166 A, and starting current is about 21 A. (7.9 times the running current)
Timing (motor connected in star)
The direct-on-line model is run with the motor winding in a star configuration before moving on to star-delta starting. This is demonstrated in the image by connecting the negative windings (2) to earth. The goal is to find timing points for star delta transitions.
The output (not shown) is similar to that of the delta-connected scenario, but the starting currents (and thus torque) are lower and the acceleration times are longer.
By examining the speed plot, we can determine the durations (beginning at t=0) during which the motor has accelerated up to the switching points of interest to us.
70 percent of maximum speed in 3.13 seconds
80 percent of maximum speed in 3.36 seconds
90 percent of maximum speed in 3.48 seconds
Transition from Star to Delta
The power is first supplied to the winding in a star configuration in a star-delta open transition starter. The power is disconnected after the appropriate time delay, the windings are reconfigured to delta, and the power is reconnected. The time lag between disconnecting and reconnecting the motor in star is typically around 40 mS.
The timing events for our 70 percent star-delta design can be described using the results of the direct-on-line study (star connected winding) as follows:
Ts at 3.53 s (3.13 + 0.4) – star contactor opened Td at 3.57 s (3.53 + 0.04) – power connected, star contactor closed, delta contactor open – The delta contact was closed.
In short, the three scenarios to be considered are as follows:
70{8e0181f9b4047a7790fd20bbf2d43faff96926569f2820f3da0bb199a786bafe} – 0.4, 3.53, 3.57
80{8e0181f9b4047a7790fd20bbf2d43faff96926569f2820f3da0bb199a786bafe} – 0.4, 3.76, 3.80
90{8e0181f9b4047a7790fd20bbf2d43faff96926569f2820f3da0bb199a786bafe} – 0.4, 3.88, 3.92
Image(60)
Model of the Star-Delta Open Transition Circuit
The star-delta open transition model is similar to the direct-on-line starter model. The additional components are as follows:
a star contact to set the motor winding in a start configuration initially, as switched out after Ts a delta contactor to set the motor winding in a delta configuration after Td
We have the following performance plots after running the simulations:
The curves show that the motor starting current has been reduced. The primary reason for using a star-delta starter is to reduce motor starting current. Although the starting current has been reduced, the acceleration torque has also been reduced, resulting in a longer time for the motor to reach full speed.
In the 70{8e0181f9b4047a7790fd20bbf2d43faff96926569f2820f3da0bb199a786bafe} example, there is a significant spike at the changeover, resulting in voltage drops that are no better than using a direct-on-line starter. Depending on the di/dt and magnitude, it can frequently cause more problems than using the simpler direct on starting. This is typical of a poorly configured star-delta starter, which is why a closed transition starter is frequently preferred.
Closed Star-Delta Transition
Closed transition starting resistors are installed in the motor’s negative end windings to ensure that the motor is never disconnected from the power supply.
Resistor sizing can be difficult, and models like these can be extremely useful. The following resistor size is chosen for this example based on a 30{8e0181f9b4047a7790fd20bbf2d43faff96926569f2820f3da0bb199a786bafe} voltage drop across the resistor:
VLN is the voltage from the line to neutral (2220 V), and Ia is the starting current (81 A). Using the formula, the calculated resistance is R = 0.8148.
The star-delta closed transition model differs slightly from the open transition model in that:
Tr1 opens the star contactor and switches a delta-connected resistor bank (R1) into circuit (Tr1 = Td).
For the sake of brevity, we are only interested in what happens to the current spike in the 70{8e0181f9b4047a7790fd20bbf2d43faff96926569f2820f3da0bb199a786bafe} case.
As can be seen, the current spike has been significantly reduced, demonstrating how resistors can improve the characteristics of a poorly timed changeover.
Conclusion
As demonstrated, studying motor starting circuits is not difficult, but it provides detailed insight into how the motor and load work while starting and in steady state conditions.
While the example provided is simplified, it is just as simple to extend the model to represent real network conditions and/or run various ‘what-if’ scenarios, investigate alternative starting methods, or change other parameters (for example resistor I2t losses).
