Is there a rule of thumb for what is considered excessive slip in a motor?
I have a 2 pole motor that is turning 3242 RPM at full load.

If it's a motor purchased to NEMA specifications (as most in the US are), it should have full load speed listed on nameplate. Speed shouldn't be below nameplate speed unless:

• 1 - the motor is overloaded
  
• 2 - the voltage at motor terminals is below nameplate
  
• 3 - the voltage is unbalanced
  
• 4 - the motor is connected improperly for the system voltage. For example a dual voltage 460/230 motor may have two winding groups per phase: should be connected in series for 480 volt system and in parallel for 240v system. If you connect it in series while connected to 240v system, it is equivalent to applying half of system voltage and slip will be high by roughly a factor of 4 for the same load. Also many motors can be configured for wye-start/delta-run. Leaving the motor in wye when running is equivalent to voltage low by factor of sqrt(3) and slip will be high by a factor of roughly 3 for the same load.
  
• 5 - (unlikely) rotor resistance is high, for example due to rotor defect or unusually high rotor temperature.
  
• 6 - motor is driven from a vfd (not really a cause of "high slip" but can mess with the slip calculation if you use regular line frequency).
  
• 7 - stator fault (unlikely - usually results in trip)
  
• 8 - stator miswired during rewind

If it is further a NEMA "Design B" motor (general purpose), then the full load speed listed on the nameplate should be no more than 5% below sync speed (i.e. less than 5% slip at full load). The vast majority of NEMA Design B motors have nameplate speed only 1% or 2% below sync speed... only a few of the old small 2-pole motors have nameplate speed close to th 5% limit.

Your motor is 10% below sync speed. Assuming it is NEMA Design B, you have one of the 7 conditions listed above.

Further possible investigation could include:

• measure current all three phases.
  
• measure voltage at an accessible location (closer to motor is preferable but not always possible)
  
• possibly attempt to estimate loading based on driven load parameters
  
• high resolution FFT check of vib for pole pass sidebands around 1x, or even better current check for pole pass sidebands around 1*LF.
  
• check the actual connections against the nameplate instructions
  
• check for abnormally high temperatures at the motor (skin temperature, winding temperauter if accessible)
  
• thermography scan of accessible connections feeding this motor in the MCC
  
• compare your current to your thermal overload setting and check your thermal overload setting.
  
• check if the motor is driven from vfd
  
• off-line stator tests (megger and bridge) are possible to check for stator fault, but if you measure balanced currents, stator fault is unlikely. If motor was recently rewound, then possibly investigate with above off-line testing and consider more detailed off-line testing (surge test, inductive balance test).

By the way, has it been running this way a long time or something changed?

This message has been edited. Last edited by: electricpete,

And wiring errors do happen. Attached is a tiny case study of wiring error at our plant resulting in low speed very similar to yours. In our case the problem occurred just after motor swap, and was accompanied by tripping of the thermal overload.

ExcerptLowSpeedDueToMisconnection.ppt (277 KB, 21 downloads)

Drivewizard, by what means are you measuring the 3242 RPM?

John from PA

Electricpete, and John: Sorry for the bad data, it should have been 3421.9 CPM, not 3242. It's been a hectic week.

It is a Centrifugal Water Chiller ( Trane M#: PCV) This chiller is approx 500 ton capacity, it has been in operation for at least 25 years. Wye-Delta, 480 volt motor. Has old dash pot OL.'s. I haven't checked all 6 legs for imbalance.
Chiller was just overhauled by one of our guys, Rotor was sent back to our Factory for rework as shaft was bent(out of tolerance).
The stator has not been out of chiller and tested.
I was just getting some baseline readings after the overhaul. No reason for taking data other than getting a baseline. We/I have never taken vibe readings on this chiller prior.
I was looking over data and noticed that turning speed(1X?) seemed a little slow.
I did not get any motor nameplate data at the time, as I didn't anticipate needing it. (Lesson learned!)
I will try to attach some of the readings I took, I am assuming the high peak at 3421.9 CPM is turning speed.
Chiller was close to 100% loaded at the time readings were taken.
This may be perfectly normal, I am just trying to learn if that is the case.

Motor_problem.doc (40 KB, 18 downloads)

I think the reason for the strange current TWF is that it is not sampled fast enough.

Looking at current log spectrum:
LF = 3600 cpm. Magnitude = 218.6
Sideband = 3252cpm, Magnitude = 0.31

Assuming this is a pole pass sideband, the magnitude is not at a concern level. It is around 57dB below 1*LF

Based on the frequency of the assumed pole pass sideband, we calculate running speed RS = 3600 - (3600-3252)/p = 3600 - (3600-3252)/2 = 3426. This is not too far from what is in your vib spectrum labeled 3421. This is right around 5% below sync speed. May or may not be normal for a full load condition (as I mentioned 5% is max allowed for NEMA design B motor) - really have to check against nameplate speed. If this is a larger motor 100hp or above I would be surprised to see nameplate slip that high (5%). You mentioned you have wye delta starting sequence - brings another possibility - starter malfunction remains in wye and never switches to delta. But again the very first thing to do is check nameplate speed (and get other nameplate data while you're at it).