In the last several years, more importance has been placed on increased reliability of downhole equipment. One way this is being reinforced today is by function testing a motor on a dynamometer, or dyno, before it is used downhole. A dyno is essentially a flow loop used to measure motor output. Most dyno tests are performed to determine the proper functionality of a motor. However, longer testing can be done to prove the endurance of a motor and its elastomer.
Today, I will be discussing the ongoing trend of function testing motors on a dyno. There has been ample research on why motors should be dyno tested, how to interpret the dyno test results, and what causes a motor to fail the test.
First, let’s discuss what is involved in a function test. When function testing a motor, the dyno operator turns on pumps using water at an ambient temperature. The pumps are brought up to maximum flow based on the spec sheet provided by the motor or power section company. If maximum flow is not achievable based on the limitations of the dyno, then the operator reduces the flow to what the dyno can reach.
Once the maximum flow has been reached, the operator applies torque to the motor through a brake or clutch. This torque is translated into differential pressure. More and more torque is applied until the motor’s specified maximum is reached or until the capabilities of the dyno have been maxed out.
When the maximum torque has been reached, the operator reduces the pump flow back down to end the test. From start to finish, the test takes approximately one minute.
Now, let’s discuss why motors should be function tested. There are two main objectives when running a function dyno test. The first objective is to test the bottom-end components and make sure everything is operating properly. The second objective, which we will spend more time discussing, is to ensure the power section is in proper condition to perform downhole. This short test will ensure that the fit of the power section is optimal by comparing the theoretical RPM curve versus the measured RPM curve. If the fit is too loose and not compensated in the results, then the test will fail due to excessive RPM drop. This must be considered if the motors are going to be sent to an application that requires a looser fit for optimal performance based on BHT. It will also verify that the theoretical torque slope provided is accurate by comparing the maximum torque from the spec versus the torque seen at maximum pressure.
Next, what does each output tell us? Below is a copy of a function test. Let’s go through each parameter and what they represent.
No Load RPM: This measurement is recorded once the pumps reach the maximum flow for the test before any torque load is applied.
Torque at Full Load: The amount of torque measured once maximum differential pressure is reached.
RPM at Full Load: The RPM output of the power section once maximum differential pressure is reached.
Peak Power: The maximum horsepower measured of the power section.
Mean Flow: The average flow during the test.
Off-Bottom Pressure: The amount of pressure seen at maximum flow before any torque is applied.
Test Differential Pressure: The amount of pressure seen at full toque load.
Now that we know what each measurement means, how are those measurements relevant to us? The Operator or motor company passes or fails a motor based on a percentage of the “Peak Horsepower” achieved by the motor. This is calculated based on the “Torque at Full Load” and the “RPM at Full Load”. The dyno operator inputs the theoretical numbers from the spec sheets provided by the power section company or motor company. The dyno company’s internal software calculates the difference between the “Peak Horsepower” and the theoretical maximum horsepower. If the percentage of difference is more than that required by the operator or motor company, the motor fails the test. If the percentage of difference is within the acceptable parameters, the motor passes.
What would cause a power section to fail this test? In theory, if the power section is properly designed by the OEM, the theoretical numbers input by the dyno operator are correct, and the test is run effectively, then the “Torque at Full Load” should always pass. This is due to the theory that the amount of differential pressure applied will have a direct correlation to the amount of torque being put out.
With that being said, we are left with the other half of the Horsepower equation, which is RPM. This is the primary reason motors fail a dyno test.
The main cause for this is fit. When discussing fit, usually the conversation stems around the compression between the rotor and stator provided on cert from the motor company. However, what is not discussed is the actual profile of the rotor and stator. What I mean by that is, what are the conditions of the flanks of each piece? This is something that cannot be measured by most motor companies.
The biggest potential issue is the rotor. Every time a rotor is stripped and recoated, the flanks of the rotor start to lose their original shape. This increases the amount of bypass, which will lead to an increase in RPM drop. This can be seen in an RPM curve that drops off much steeper than the theoretical RPM curves provided.
Below is an example of a full scan of a rotor profile after several recoats. Notice how the majors and minors are within tolerance, but the flanks have become modified due to polishing a stripped rotor.
At the end of the day, the goal of 100% success is always going to be the objective. A function test will not fully guarantee that the motor will be successful, however, it does provide an extra piece of quality inspection before use to reduce the risk of a downhole failure and sub-optimal performance.
