Here You Go.


Can your electric motor shop test run your motor or properly power it?



It’s interesting how rarely this question is ever discussed. Can the electric motor shop that you use, run your motor or properly power it with full voltage? Or do they simply spin the motor up to full speed/volts, “for a moment”, but then have to reduce the voltage because their power supply can’t handle the load?

What exactly is meant by this?

(For ease, we are going to assume the required repair specification requires an unloaded test run)

Here’s an example:

You send out a 2500HP, 445 RPM, 4000V, 348 FLA, Siemens-Allis motor to be rewound. The shop goes through its entire process and completes the rewind and then re-assembles the unit. They go through a series of electrical testing to ensure the new winding is “good to go”. Now it’s time for the final test, a test run at full voltage.

You put this on your 750KVA test panel and start the motor. If you can get it up to full speed and voltage you realize the transformer is humming loudly and before you know it the test panel either trips in current limit or burns up or damages your motor (or their panel) in the process! Then, you realize the only way you can run this motor is by reducing the voltage and resultant load on your transformer.

So what happens when you run a motor at half voltage? You can in many cases get the motor up to full speed …. Isn’t that all that matters? No not really, what you miss is the motor needs to operate at full voltage (I.e. have a test panel with large enough capacity (KVA) to do that) if you expect to get an accurate test run with real vibration readings. When a motor is operated at half voltage it is not magnetized fully, therefore the magnetic noise is much less and resultant vibration will be too! You need to run the motor at full voltage for the whole test run to ensure the magnetic vibration (which will be the same on site when in operation) is at an acceptable level. What’s acceptable? Today most customers have vibration standards that require various “bands” (frequency ranges) and amplitudes to be under a certain maximum value. Magnetic vibration will show up at 2X line frequency or 120Hz (in the USA). If the voltage is half, the magnetic field is proportional to voltage, so the magnetic “2 X line frequency” will be much lower in amplitude and give a false sense everything is ok. In other words, your vibration readings and accuracy of the test run will not be correct.

Now the motor (At another repair shop) is being put on a 2500KVA test panel to run the motor at full voltage. The motor runs at 4000 volts and the current settles around 145 amps (no load for each phase), which is well within this transformers KVA capabilities (I.e. this would equate to about 1000KVA needed at this voltage and current).

Why does this matter?

A motor of that size requires a lot more manpower and equipment to remove and then to install it. If it is being rewound, do you want it fully energized at full voltage, for the first time, when it is returned, installed and sitting on your base? Or do you want it to be fully energized at full voltage at a repair shop prior to returning to you, ensuring the repair was performed successfully?

The choice is yours to make! Just understand the difference between powering a motor up at full voltage for the whole test run verses just running it up to full speed at reduced voltage. Ask your electric motor repair shop what their KVA rating of their test panel is, and it can answer this question for you, before sending it out.

 Download What To Look For In An Electric Motor Repair Shop


Justin T. Hatfield & Todd A. Hatfield

HECO - All Systems Go

269-381-7200 & 


About the authors:

Justin T. Hatfield  is Vice President of Operations at HECO. He is responsible for Electric Motor & Drive Sales, Electric Motor & Generator Repairs, Spare Solutions, and Predictive Services. Justin was instrumental in developing HECO MAPPS (Motor and Powertrain Performance Systems)  which focuses on “why” you have a motor problem instead of simply “What” product or service should be recommended. HECO is an EASA Accredited Service Center for Electric Motors as well as a provider of predictive maintenance services and products throughout the United States.


Todd Hatfield is a Senior Electrical Engineer at HECO. He has over 35 years' experience in generator and electric motor repair and engineering. Todd has a BS in Electrical Engineering, and his areas of expertise are: electrical and mechanical motor redesigns and engineering, root cause of failure analysis, and quality electric motor rebuilding.

Subscribe to Email Updates

Posts by Topic

see all

Recent Posts