In the last post, we reviewed how SubMonitor protects a three-phase motor against high or low voltage and how the default settings of plus or minus 10% nameplate voltage provide the optimum protection in just about every installation. However, if for some reason more voltage tolerance is required, we covered how SubMonitor’s DETAILED SETUP allows the voltage trip points to be set up to plus or minus 20% of the motor’s nameplate voltage.
In this post, we’ll look at how SubMonitor protects against overload and underload conditions. The classic case of an overload is a bound or dragging pump. In this scenario, the motor is being asked to do more than it has been designed to handle and must pull an excessive amount of current. This higher amperage overheats and damages the motor.
At the other end, an underload can be caused by a dry well, a broken coupling, a loose impeller, or a blocked inlet. In these cases, the motor is saying, “I’m not working nearly as hard as I should be. That means I’m not moving as much water as I should be and something must be wrong.” An underload condition won’t damage the motor, but it can indicate a dry well condition which will destroy the pump. An underload condition could also indicate a lack of cooling flow past the motor.
In any case, an overload or underload condition indicates that something is wrong with the installation and corrective action is needed. A reliable indication of both an overload and an underload is the amount of current the motor is consuming. SubMonitor measures current via three current transformers built-in to the unit. These are sometimes called the sensor coils. These coils continuously measure the amount of current in each motor leg. One can almost think of the three sensor coils as three separate Amprobes, one for each leg, continuously measuring current.
Using its three sensor coils, SubMonitor is always monitoring for an overload or underload condition. The default setting for an overload condition is 115% of the motor’s service factor amps. For example, from the Franklin Electric AIM Manual, a Franklin 6-inch, 460V, 40 horsepower submersible motor has service factor amps of 61.6. So, in the default mode, SubMonitor will take this motor off-line if any leg exceeds 70.8 amps (61.6 x 115% = 70.8). If this occurs, SubMonitor will leave the motor off-line for 10 minutes and then attempt a restart. SubMonitor will do this three times, but if after three attempts, the motor is still overloaded, SubMonitor will keep the motor off-line until the issue is corrected and SubMonitor is manually reset.
Just like the voltage protection, the above settings can be customized using DETAILED SETUP. The overload trip point can be set from 80% to 125% of service factor amps. The off-time can be customized from the default setting of 10 minutes to anywhere between 5 and 60 minutes. The number of attempted restarts can adjusted from 0 to 10.
Underload works the same way, but of course, with different numbers. The SubMonitor underload default is 75% of service factor amps. So, in the case of our motor above with service factor amps of 61.6, the underload trip point will be 46.2 amps (61.6 x 75% = 46.2). If SubMonitor senses an underload, it will leave the pump shut down for 30 minutes. In the case of a dry well, this may allow the well to recover. Once again, three restarts will be attempted before a manual reset is required. If a shorter or longer off-time is needed, it can be adjusted from 10 to 120 minutes, and the number of restarts set from 0 to 10. In terms of the underload setting itself, it can be adjusted all the down to 30% of service factor amps (less sensitive) and up to 100% of service factor amps (more sensitive).
Even though SubMonitor offers a great deal of customization on the overload and underload settings, note that in nearly all cases, the default settings are the optimal settings and will do the best job of protecting the installation while minimizing false trips.
SubMonitor’s overload and underload monitoring deals with the values in each leg of a three-phase motor. But there’s another current parameter here that is just as important, and that’s the balance between the three legs. We’ll look at that in the next post.