Is it normal for the inrush current to be 290A when starting a 90 kW fire pump using a star-delta starter?

Question: We have installed a set of fire pumps with one in operation and one as a standby. The control cabinet uses star-delta starting. When Pump No. 1 starts, the instantaneous current reaches 290A, and the ammeter detects an overload, resulting in the automatic start of Pump No. 2. When Pump No. 2 starts, the instantaneous current also reaches 290A, and the ammeter detects an overload again, triggering the automatic start of Pump No. 1. This results in a back-and-forth starting situation. Both the pumps and the control cabinet have been inspected and found to be normal. Could you please provide some advice on how to handle this issue?

Answer: When initiating a 90 kW fire pump using a star-delta starter, the instantaneous starting current of 290A falls within the normal range.

As we all know, the direct starting current of a three-phase asynchronous motor is 4-7 times its rated current. If a star-delta starting method is employed for voltage reduction, the starting current is approximately one-third of that during normal starting, roughly 1.3-2.5 times the rated current. For a 380V three-phase asynchronous motor, 1 kW is approximately equivalent to 2A, so a 90 kW motor has a rated current of approximately 180A.

Three-phase asynchronous motors above 7.5 kW cannot be started directly and require voltage reduction for starting. Common voltage reduction starting methods include series resistance starting, autotransformer starting, and star-delta starting. For light-load equipment such as pumps, star-delta starting is generally adopted due to its significant effects, low cost, and ease of maintenance.

Since the question does not specify which voltage reduction starting method is used, we will calculate based on the most common star-delta starting method. For a 90 kW pump using star-delta starting, if we estimate the starting current as 1.5 times the rated current, the current would be 180 * 1.5 = 270A, which is very close to the observed value. Therefore, a starting current of 290A is quite normal.

What exactly is causing the pumps to start back and forth?

 Due to the complexity of the two star-delta voltage reduction starting circuits, let's simplify the scenario and use an example of a direct starting system with automatic switchover between one pump in operation and one as a standby. The circuit diagram is as follows:

As analyzed from the diagram, the most likely reason for the pumps to start back and forth is that the setting current of the thermal relay is set too low.

Diagram of the thermal relay：

On the thermal relay, there is a white circular knob that can be adjusted using a screwdriver to set the thermal relay's setting current value. When the actual current exceeds the set value for a certain period of time, the auxiliary contacts of the thermal relay will actuate—the normally open contact will close, and the normally closed contact will open.

Adjustment of the thermal relay's setting current

Here, I introduce two methods for adjusting the thermal relay

Set the thermal relay based on the motor's operating current. For example, if the motor's operating current is 100A, set the thermal relay to 1.05 times the operating current, which is 100*1.05=105A.

Note: This refers to the current during normal operation, not the rated current. The operating current of the same motor can vary under different loads. If the operating current is unknown, it can be measured using a clamp meter.

If a clamp meter is not available, use the most basic method. First, set the thermal relay to 1.05 times the rated current and let it run for a period of time. If the thermal relay trips, increase the setting slightly until it is just slightly above the critical value.

The above are some personal opinions. If you have other insights, please leave a comment for learning and exchange!