Test equip­ment

DEMM Engineering & Manufacturing - - EDITORIAL -

VOLT­AGE PULSES from a vari­able speed drive can cou­ple from a mo­tor’s sta­tor to its ro­tor, caus­ing a volt­age to ap­pear on the ro­tor shaft. When this ro­tor shaft volt­age ex­ceeds the in­su­lat­ing ca­pac­ity of the bear­ing grease, volt­age arcs or elec­tri­cal dis­charge ma­chin­ing can oc­cur, caus­ing pit­ting and flut­ing of the mo­tor bear­ing ar­ing race, dam­age that can cause a mo­tor to fail l pre­ma­turely. This ap­pli­ca­tion note ex­plains s how to use the MDA-550 Mo­tor Drive Anal­yser and a shaft volt­age probe to mea­sure mo­tor shaft aft volt­age dis­charge events.

SHAFT VOLT­AGE DIS­CHARGES

Ca­pac­i­tive cou­pling be­tween a mo­tor’s sta­tor and ro­tor can cre­ate a volt­age on a mo­tor shaft. For this rea­son, bear­ings in elec­tric mo­tors can suf­fer from wear and tear r caused not only by the ro­ta­tion of the shaft but also by elec­tric cur­rents flow­ing from the e mo­tor shaft to ground through the bear­ings. Mo­tors pow­ered by sine wave ac power may have shaft/bear­ing-to-frame volt­ages of about ut

1 to 2 V. Mo­tors pow­ered by the rapidly switch­ing ching wave­forms of vari­able fre­quency drives ( VFD) ) may, how­ever, have shaft/bear­ing-to-frame volt­ages ges as high as 8 to 15 V. Volt­ages at th­ese lev­els can over­come the in­su­lat­ing prop­er­ties of bear­ing g grease and the re­sult­ing sparks can cause pit­ting, flut­ing, fu­sion craters, and, even­tu­ally, pre­ma­ture fail­ure of the bear­ings and mo­tor.

SHAFT VOLT­AGE PROBE

Ca­pac­i­tive Ca­pac­i­tiv cou­pling be­tween a mo­tor’s sta­tor and ro­tor rot can cre­ate a volt­age on a mo­tor shaft. For Fo this rea­son, bear­ings in elec­tric mo­tors can ca suf­fer from wear and tear caused not only on by the ro­ta­tion of the shaft but also by elec­tric el cur­rents flow­ing from the mo­tor shaft to ground through the bear­ings. Mo­tors pow­ered po by sine wave ac power may have shaft/bear­ing-to- sh frame volt­ages of about 1 to 2 V. V Mo­tors pow­ered by the rapidly switch­ing wave­forms wa of vari­able fre­quency drives ( VFD) may, ma how­ever, have shaft/bear­ing-to-frame volt­ages vol as high as 8 to 15 V. Volt­ages at th­ese lev­els leve can over­come the in­su­lat­ing prop­er­ties of bear­ing b grease and the re­sult­ing sparks can cause pit­ting, flut­ing, fu­sion craters, and, even­tu­ally, even pre­ma­ture fail­ure of the bear­ings and mo­tor.

MEA­SUR­ING DE­VICE

The MDA-550 is de­signed to quickly and eas­ily test and trou­bleshoot typ­i­cal prob­lems on three-phase and sin­gle-phase in­verter type mo­tor drive sys­tems. Next to the four-chan­nel portable os­cil­lo­scope and recorder func­tion­al­ity, spe­cific mo­tor drive anal­y­sis func­tions of­fer step-by-step set-up guid­ance with on­screen in­for­ma­tion. This makes it easy to con­fig­ure the anal­yser and get the drive mea­sure­ments from the power in­put to the in­stalled mo­tor, in­clud­ing shaft volt­age mea­sure­ments.

Shaft-volt­age spikes can be ex­ceed­ingly fast, even be­low the mi­crosec­ond mea­sure­ment range. The Fluke MDA-550’s high 500 MHz band­width and fast sam­pling rate (up to 5 Gsam­ple/s) make it ideal for mea­sur­ing rapid changes in volt­ages.

MEA­SURE­MENT RE­SULTS

The de­fault mo­tor shaft volt­age mea­sure­ment shows volt­age wave­forms as mea­sured on the mo­tor shaft it­self. The volt­age peak-peak value dis­played at the top of the screen iden­ti­fies the max­i­mum level of the cap­tured wave­form, which is al­ready an in­di­ca­tion that high volt­age lev­els are present on the shaft. How­ever, the fre­quency with which th­ese fast dis­charges oc­cur can­not be de­ter­mined this way. Util­is­ing the “EVENTS ON” func­tion, the MDA-550 will dis­play wave­forms with dis­charges based on a pre-de­fined min­i­mum volt­age dif­fer­ence and max­i­mum time dif­fer­ence.

The dis­play is up­dated with each wave­form cap­tured that has a steeper rise- or fall-time, along with the num­ber of de­tected events per sec­ond. Dis­charge wave­forms will show an in­crease in volt­age to ground fol­lowed by a sharp ver­ti­cal line to ground at the mo­ment of dis­charge. For a de­tailed anal­y­sis of the last cap­tured wave­form, the MDA-550 of­fers the “RE­PLAY” fea­ture with a buf­fer of the screens for the last 100 wave­forms cap­tured. The screen can be in­di­vid­u­ally se­lected or dis­played as an an­i­ma­tion.

Volt­age dis­charges of > 15 V and tran­si­tion times faster than 50 nanosec­onds can point to dis­charges ca­pa­ble of dam­ag­ing bear­ings. How­ever, this alone is not enough to de­ter­mine po­ten­tial dam­age to a bear­ing as there are many other fac­tors in­flu­enc­ing this.

If ex­ces­sive shaft volt­ages are de­tected, it is a good prac­tice to see if volt­age dis­charges can be re­duced by adapt­ing the ca­bling, ground­ing, drive pa­ram­e­ters, or lu­bri­cant. If this is not pos­si­ble or does not help mit­i­gate the prob­lem, shaft ground­ing de­vices or an iso­lated shaft are use­ful al­ter­na­tives. The ef­fects of th­ese shaft volt­age mit­i­ga­tion ef­forts can eas­ily be un­der­stood by com­par­ing the num­ber of events recorded be­fore and af­ter changes were im­ple­mented.

CON­TACT FLUKE ANZ AT AUINFO@FLUKE.COM FOR MORE IN­FOR­MA­TION OR FOR AN ON-SITE DEMON­STRA­TION.

MEA­SUR­ING MO­TOR SHAFT VOLT­AGE WITH THE SHAFT VOLT­AGE PROBE KIT

FLUKE MDA-550 MO­TOR DRIVE ANAL­YSER

MO­TOR SHAFT VOLT­AGE SIG­NAL WITH EVENTS TURNED OFF SHOWS ALL CAP­TURED SIG­NALS WITH PEAK-PEAK VOLT­AGE READ­ING.

SHAFT VOLT­AGE DIS­CHARGE EVENTS WITH SPEC­I­FIED MIN­I­MUM VOLT­AGE AND MAX­I­MUM FALL- OR RISE-TIME IN TALE FORM.

THE RE­PLAY FUNC­TION SHOWS THE LAST 100 CAP­TURED WAVE­FORMS FOR AD­VANCED ANAL­Y­SIS.

WITH EVENTS ON, ONLY WAVE­FORMS WITH A SPEC­I­FIED MIN­I­MUM VOLT­AGE AND MAX­I­MUM FALL- OR RISE-TIME ARE DIS­PLAYED.

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