Protecting three-phase submersible motors

Get an understanding of why it is important to protect your submersible motor, as well as how to do it.

With a motor protection unit, you can protect your submersible motor from certain failures. However, in order for the motor protection to work properly, you need to set it up correctly.

In this module, we’re going to introduce you to the importance of motor protection by highlighting some of the most common issues that can happen if your motor is unprotected.

We will also offer a couple of Grundfos recommendations on how to optimise your motor protection.
First of all, however, it’s important to mention that motor protection is based on local legislations and various installation standards across the world. The following is simply an example of a generic installation and not necessarily the right solution in your country.

You might be asking yourself why you should invest in motor protection for your submersible installation. Well, there’s a simple answer to that.
As these installations are submerged, they are very expensive to get access to if something goes wrong.

And therefore, it is recommended that you invest in a good motor protection unit, as in the long run, it will save both time and money.
In order to get the optimal result from the chosen motor protection, it is important to use the correct settings for the given installation.

In the following, we will highlight 3 parameters that are essential in gaining an optimal motor protection:
motor current
and motor temperature.

An increased current consumption can be caused by a higher demand for output, if the duty point for the submersible pump set changes,
or if the liquid density or viscosity in the installation has changed
– but it can also be a sign that the pump set has malfunctioned.
If the pump is seized and cannot be started, the current will remain at the level of the starting current – five to six times the value of the nominal current.
As the pump does not discharge in this situation, it will cause a rapid build-up of energy in the submersible motor, which could lead to overheating and deterioration of the submersible motor.
Another reason for unwanted heat generation in a three-phase submersible motor is voltage asymmetry. Voltage asymmetry is usually caused by problems in the supply net, such as defective transformers, or uneven load due to badly balanced single-phase consumers

Data for the initial motor protection setting can be found on the motor name plate. This data relates to a given submersible motor being fully utilised in terms of output. This will rarely be the correct setting as the submersible motor is coupled to a submersible pump, running at a duty point in which the maximum output of the motor is not utilised.

There are two ways to obtain the part load data. You can use Grundfos Product Center,
or you can find the current value simply by reducing the current setting on the motor protection device until tripping occurs. Then, increase the current level until nuisance tripping no longer occurs. Be aware that the more overcurrent you allow, the less protection of the submersible motor you will have.

Overvoltage poses a potential risk of damaging the submersible motor by generating far too much heat in the motor. Eventually, this will break the windings.
Undervoltage, on the other hand, typically happens when the drop cables are too small, meaning that the voltage drop from the power supply to the motor becomes too high. Therefore, it is important to size the cables correctly.

All Grundfos submersible motors are water-filled and they all have journal bearings. The journal bearings run on a hydrodynamically created water film, meaning that they do not wear out.
The robustness of this water film, however, depends on the temperature. If the water film is not built in a journal bearing, it causes friction, which can rapidly damage the bearing.
The motor temperature can increase due to different reasons, such as increasing media temperature, a build-up of scaling on the motor, or if the cooling flow is too small.
In some cases, a proper cooling flow can be applied by using cooling sleeves that force the flow along the motor.

These are typically used in any installations in which the pump is installed in a reservoir, tank, or a borehole which is too big. You can find the required cooling level on the motor nameplate.
The protection level for the motor temperature should be set during the first few days of operation, immediately after you have established a stable motor temperature. And if the temperature level increases significantly, it may be due to one or several of the issues we have just covered.

When it comes to the overall protection of a submersible motor, the traditional way of protecting it is by means of an overload relay as they protect the motor and motor conductors from any damage caused by prolonged periods of overcurrent circuit conditions.
Overload relays are rated by a trip class that defines the length of time that it takes for the relay to trip in an overload condition. If you choose an overload relay with any Grundfos products, it must be a trip class 10 or lower. It is recommended that you always choose overload relays with phase-failure monitoring.

The ideal way to protect your submersible pump is by means of an MP204, an electronic motor protection unit designed for optimal protection of a submersible motor.
MP204 safeguards against electrical issues such as voltage unbalance, or overvoltage and undervoltage, while it also has a so-called adjustable pump trip class enabling even faster switching in the case of a blocked pump. This reduces the risk of motor burnout.

Compared to a standard overload relay, the MP204 also offers monitoring, which can be used to monitor, for instance, the temperature of the submersible motor over time. Temperature monitoring can be used to define intervals for preventive maintenance.

Combining the MP204 with Grundfos submersible motors and a built-in Tempcon MS4000/6000 sensor allows you to monitor the submersible motor temperature without extra cabling, as you can use the power cables to transmit the temperature signal from the submersible motor to the MP204.

So, that concludes our module on the protection of three-phase submersible motors. Let’s recap.
First of all, the motor current, over/undervoltage, and the motor temperature are all important parameters in ensuring optimal motor protection.

Secondly, you should set the motor temperature protection level within the first few days of operation, immediately after you have established a stable motor temperature.
And finally, a Grundfos MP204 electronic motor protection unit not only offers motor protection, it can also help to monitor motor temperature.

Course overview

Modules: 3
Completion time
Completion time: 25 minutes
Difficulty level
Difficulty level: Advanced