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7 essential attributes of the fixturing and environmental control system in rotating electric testing

On Apr 11, 2019

Once you’ve finished the preparation stage of a rotating electrics test system, it’s time to further determine how you are going to handle your device under test (DUT). Here are the essential attributes to consider in a test system:

1. A rigid DUT fixture and support system – Without adequate stiffness, it’s impossible to ensure test quality. High stiffness is important in this environment, as it will minimize vibration by transferring energy from the rotating shafts directly to the underlying support structure with few losses. You don’t want the fixture to alter the amount of energy that’s transferred to the DUT. From a reliability standpoint, you want to minimize vibration associated with rotating shafts in a test system, which minimizes stress on attachment points and metal fatigue.

2. DUT shaft and drive system alignment – At high shaft speeds, misalignment can generate catastrophic failures (like shaft breakage or bearings seizing) during test. That’s why this attribute is critical. Fundamentally, you are trying to determine if your rotating shafts are well balanced. The best practice is to have precision balanced shafts, which minimize forces associated with shaft rotation. Typically, you want less than one-thousandth of an inch of misalignment between coupled shafts to maximize coupling life.

3. DUT mechanical and electrical load limits – The DUT specifications will ultimately determine the upper limits of the mechanical and electrical load profiles. With your test partner, you will need to determine how best to mechanically and electrically achieve the desired test load characteristics. For example, in a belt drive application, evaluate belt tensions, slip ratios and thermal limits to specify appropriate belt designs and materials. Keeping in mind, high slip ratios can lead to premature belt failure.alternator tester

4. DUT Electrical loading – DUT specifications determine the upper limits of electrical loading as well. Here are four key considerations you need to determine for this attribute:

  • How precisely do you want to control the loading?
  • What are your ranges (max voltage, output power, max current, etc.)?
  • Have you identified the best cabling size and termination approach for your test application?
  • Do you have to be able to absorb power or produce power?
  • Are there efficiency requirements that will force you to do power regeneration?

5. Audible noise – There are two main considerations when it comes to audible noise in rotating electrics testing:

  • Undesirable DUT emitted noise during testing – In durability testing, you don’t want the DUT to be disruptive in the testing facility. If your business has defined maximum allowable noise levels, it’s easy to relay this information to your test partner and avoid the problem (for example, we were given a 80 dba limit in the dynamometer-based durability test stations we built for a customer). However, if your company has not established its allowable noise levels, ask your test partner what they have done for similar systems and what is typical of the industry. For example, if the system threatens to create too much noise, your test partner might provide an insulated cover around the DUT to isolate DUT noise from the surrounding environment.
  • Characterization of DUT emitted noise – Sometimes it’s important to measure the DUT noise output in a test scenario. Your test partner will help you determine the equipment needs in a test system to characterize the noise output of the DUT (frequency range, anechoic chamber, calibrated microphone, etc.).

6. DUT parameter measurements – Typically, these four variables are measured in a rotating electrics test scenario: torque, speed, voltage and current. These DUT parameter measurements can be used to characterize the device’s efficiency. Here’s what you need to know about each:

  • Torque: It can be measured directly or indirectly. In the case of a direct measurement, you need an external torque transducer directly coupled to the DUT. In the indirect case, you can measure the input power to the DUT by interrogating the relevant motor drive system while accounting for drive system inefficiency.
  • Speed: Normally, speed is measured via direct sensing of rotating a shaft. There are a variety of ways to capture this measurement (optical, inductive or Hall Effect). Indirect speed measurement can also be accomplished by capturing the frequency of the DUT output voltage and deriving speed based on a DUT pole configuration.
  • Voltage: This is one of the easiest measurements to take. Typically, a DMM (digital multimeter) instrument is used to measure voltage. Specify to your test partner whether you want an average value, peak value or RMS value.
  • Current: A DMM instrument is typically used to measure current but to do it requires much more resolution. There are two ways to sense current: use an in-circuit sense resistor high current shunt or employ an inductively coupled current transformer.

7. Thermal environmental control and monitoring – Rotating electrics can generate a lot of heat. You need to manage it, monitor it and be able to add or subtract it as needed in your test system. In some test systems you will be trying to emulate an automotive engine compartment thermal behavior. You want to develop an understanding of the DUT power levels and component specifications, which will define its thermal operating limits and allow you to understand how to keep it within its safe operating range. Fluid is sometimes used to maintain DUT operating temperature. If so, a fluid conditioning system will be required. But in many cases air is used to keep the DUT cool, so an air conditioning system will be necessary.

As far as temperature measurement goes, DUT environmental, and fluid temperatures need to be monitored, and controlled. Normally, standard thermal couples are used but in certain cases optical measurement may be appropriate.

Evaluating these seven attributes is almost always part of our process when developing a test system. For more information on our work in rotating electrics, check out how we created a custom, open-architecture alternator tester using NI LabVIEW and TestStand.

 

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