Issues regarding pump motors
Thread Content
I have recently been observing the pump protection system in the power distribution room; it features overload protection and stall protection. What is the difference between overload and stall? ? A simpler explanation
Lock-up is a special case of overload; when a motor is in lock-up, its power factor is extremely low. The current during lock-up (referred to as lock-up current) can reach 7 times the rated current, and if this condition persists for an extended period, it can damage the motor. Overload protection may simply involve setting a value that is above the rated current or a few times the rated current, and then stopping the operation after that period of time has passed.
Overload means being under excessive load, while stall means being completely blocked.
Stall: the shaft is stuck and won’t rotate. Overload: the shaft is still working, but it rotates with difficulty :D
According to the manufacturer’s claims, the motor shuts down as soon as there is an overload; so how can a \"stall current\" occur at 7 times the rated current?
The motor’s startup time is not too long; the current gradually decreases. It takes some time for the thermal relay to heat up enough to trip, and as long as the motor’s startup time is short, it generally won’t cause the thermal relay to activate. During stall, the current remains very high, requiring protection in a short time. Of course, the two situations are different.
Oh, I see. Thank you~~~
The main technical parameter of a thermal relay is the setting current. The setting current refers to the maximum current that can flow through the heating element over an extended period without causing the thermal relay to operate. When the current flowing through the heating element exceeds 20% of the set current value, the thermal relay should operate within 20 minutes. The setting current of the thermal relay can be adjusted using the setting current knob. When selecting and setting a thermal relay, it is essential to ensure that the setting current value matches the motor’s rated current. Since a thermal relay operates in response to heat, it has considerable thermal inertia; therefore, even if the current flowing through the heating element exceeds the set current by several times for a short period of time, the thermal relay will not operate immediately. Only in this way will the thermal relay not operate due to the high starting current when the motor starts; otherwise, the motor will not be able to start. Conversely, if the current exceeds the set current by a small amount, it will also operate over time. It can be seen that thermal relays and fuses serve different purposes: thermal relays can only provide overload protection and not short-circuit protection, whereas fuses can only provide short-circuit protection and not overload protection. In a well-designed control circuit, especially in motors with large capacity, both types of protection should be available.
Considerations for locked-rotor design in power systems.