Our plant has several Reliance DC motors ranging in size from 25 hp to 100 hp and I have been seeing vibration signatures that are not what I am told they should be. The drives are Reliance full wave rectified and I see from materials I have read that they should have a peak in the spectrum at 360 hz. and no other peaks at any multiples of line frequency. In fact, these motors show peaks at 180 hz. and harmonics of 180 hz. The electricians and the reliance drive technicians I talked to assure me that everthing is working like it is supposed to and all SCR's are firing but no one can explain why I am seeing 180hz. peaks on full wave rectified drives. One of these motors has recently started showing sidebands of runspeed around the 180 hz peak and is running hot. These drives have been in operation a long time and have shown this pattern always so apparently it is not a defect, I just want to understand it better. If any one has similar experiences or some knowledge that might be helpful I am curious to hear it. I am attaching a spectrum to illustrate.

_2_hyd_drive_spectrum.doc (26 Kb, 37 downloads) #2 hyd drive spectrum.doc

You might want to check to ensure that you have balanced voltage coming into the DC drive. There is the possibility that you might have unbalanced power which will effect some of the rectifier circuit and will result in a value below the (number of SCR's)x(Line Frequency), but will be a multiplier of line frequency. I would look to this, first, if it is occurring on multiple drives. (this can also happen when the transformer is designed with a 'b-phase' grounded secondary for lighting circuits - then each phase to ground will show something like: 277 V, 90 V, 277 V).

The sidebands will most likely relate to a bad card (ie: the comparator card) or speed fluctuation issues related to the 180 Hz peak. Check the connections.

How long has the issue been going on?

Thank you for your response Howard. I have been doing vibration analysis at our plant for 2 years but this same pattern has been showing up in several of our motors for as long as I can find data, at least back to the early 90's. I am having the electricians check the balance of the incoming current and will post it here when I find out.

At the previous plant that I worked at, we had 2 triristors firing to make up full wave rectification.As the triristors are half wave rectification we use to get 150Hz and multiples.In your case it would be 180 Hz and multiples.Readings taken in velocity is a good option as the amplitudes specfied on the Charlotte Vibration chart is a very good guide to evaluate your condition of your cards. Our average alarm levels were set at 2.5mm/sec to 3.25mm/sec.

hope this helps

PS : I forgot to mention. You should have your electricians check the comperator card at the back of the motor as well .Loose connection would lead to unstable motor speeds(Running speed harmonics).Comperator leads to motor running hot. You can check you speeds with a tacho or strobe.

Thanks to everyone for their input. I am waiting on our electricians, (who are stretched pretty thin), to check out the suggestions that have been made and when I get the results I will post them.

Our electricians have done the checks I asked and have given me some of the history of this system. I thought I would pass this along to those interested.
The motor speeds are constant.
The voltage is balanced although it is lower than it should be. It is 452 volts on a 480 volt system. The electricians said when the machine is down the voltage goes too high so they keep it lower to prevent this.
The fields in the motor have been weakened to achieve higher speeds. Process changes since that time have put more load on resulting in the motor running hot.
The setup of the drive is like Hadrian described with 2 thiritors per SCR which must be what gives us the 180 hz harmonics.

Thanks again to all who responded to my question.
Randle

Randle,
We use 6 Thiristors giving us 300Hz Full rectification.We found one not firing and gave us 50hz with harmonics. This was confirmed with an occiliscope on the drive, one was not firing.The 300Hz lost and gearbox very noisy.