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Hi Everyone,

I am seeing 300 Hz in the spectrum on an 50 Hz LF AC motor with soft starter. I'm suspecting it is from SCR Firing rate but would it be possible to have SCR on an AC motor? Hoping to hear from the expert on this topic

Tags: SCR, Firing Rate

Soft starter does not use a VFD after motor is up to speed in most cases

SCR firing frequency is normally seen on DC or direct current motors, not AC induction, wound rotor or synchronous motors

Unlikely you are seeing SCR firing frequency, it MAYBE rotor bar frequency depending on speed of the motor, could be many things without knowledge of your machine.

You could provide vibration data to help troubleshoot further if needed.

Dave

Hi Dave,

Thanks for the reply. Yes the motor has no VFD only soft start AC motor. However, I am confused because I read some articles that says SCR can also be used both for AC and DC motors. If I may calculate the peak frequency of interest for the sake of representation it coincides with the full-wave rectified 6 SCR circuit on a 50 HZ LF. It may not pose a potential problem but it's good to know from my perspective when troubleshooting Electric motors. I am not an electrical guy but really keen to learn from the experts.

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Some soft starters keep the SCR's in the circuit at all times. Some utilize a start/run contactor dropping the SCR's out after startup. Yours may keep the SCR's in the circuit at all times and yes you are correct full wave rectified with 6x50Hz SCR's=300Hz or 18,000 cpm exactly. I see in your plot a small peak to the left of SCR frequency, I assume that is 4x run speed?

As a rule of thumb I have used .1 in/sec/pk to fix or trend, I believe this same value is acceptable for DC or AC motors. Others on this site may have more knowledge than myself regarding these values.

Have seen soft foot on motor cause 2xLF to increase. If you have the ability to monitor motor live time, loosen one motor foot at a time and watch to see if 2xLF decreases.

Dave

Thanks Dave at least I can now confirm that SCR is also present in AC motor depending on what system you're using and what for. The small peak on the left of SCR is the 6x RPM. The motor is running 1500 RPM. You're right, I am suspecting also airgap dissymmetry caused by soft foot, unfortunately the company is hesitant to do what you've suggested. Since the amplitude level is still very low ~0.7 mm/sec I don't think I will make a call.

Can you stick a band alarm and trend it on the peak of interest? If it stays static and doesn't increase might Ben just a matter of keeping an eye on it.

@Registered Member thanks for the reminder I put the alarm threshold just 50% above max peak. I am monitoring it on a monthly basis. We'll let you know it something goes wayward. Cheers!