Hello All,

Good day. On one of the 15KW A.C Motor, we notice that the Motor Slip increases with increase in load. This is a 4 Pole Motor directly coupled to a Centrifugal Pump. At normal load, the motor was running at 1444 RPM with a slip frequency of 56 RPM. But with higher load, the speed can go as low as 1390 RPM. We also note that Motor DE Bearing temperature rises to 80 degree C.

Can high slip affect Motor Rotor in long run?

Any suggestions to overcome this problem?. Do we need to use higher KW Motor?

Thanks in advance for your suggestions

Maha

Generally higher than normal slip will result in an overheated motor, and in time the motor will fail.

Some questions you need to ask is has this motor pump combination always been this way or is it just recent? If it is recent it could be an electrical supply issue, not the motor. If it has always been this way then it might point to the motor being undersized. You should check your pump curves and establish the motor size needed for the type of service and hydraulic characteristics of the system. Then obviously compare that to the motor you are using.

John from PA

Hi John,

Thanks for useful inputs. This Motor Pump combination has always been running this way past one year or so since we started monitoring it.

Please refer to the attached spectrum. The sidebands around running speed harmonics matches with Pole passing frequency. I have started seeing this only recently.

How do we assess the severity?. Does mere presence of these sidebands indicates Motor Rotor problem or we just need to trend further and look for more sidebands and amplitude increase of these sidebands.

Thanks for the suggestion regarding Motor sizing. We have started working on this direction.

Maha

Motor_Spectrum.ppt (62 Kb, 30 downloads)

Slip is expected to increase linearly with load. Double the load -> double the slip.

For US NEMA motors (which I doubt this is since you have 50hz supply frequency), the max slip at full load is 5%. For 4 pole 50hz motor (1550 rpm sync speed), that means at full load it wouldn't exceed 75 rpm slip.

You are seeing more than 100 rpm slip. So some possibilities:
1 - International specs differ from NEMA and allow higher slip.
2 - The motor is overloaded.
3 - Severe undervoltage or unbalanced voltage can cause higher clip.
4 - Rotor defect can cause higher than normal slip.


Checking the nameplate might shed light on #1 (what does it list as full load rpm?)
Checking current would shed light on number 2 / 3 (is current above nameplate FLA, or is it unbalanced).

The pole pass vibration mentioned is consistent with 4 (rotor bar defect), but normally should be cautios to make a call without confirming by current signature analysis.

Hi Electricpete,

Thanks for useful inputs. The synchronous speed is 1500 RPM and the nameplate RPM is 1465 and Amps - 29.4

The feedback from our Electric section is that there is no problem with running amps.

The Motor Standard is IEC 34-1

Thanks

Maha