What is it I'm looking for in a current spectra?
Is it sidebands of 2 x slip frequency or 2 x slip x # of poles?

thanks in advance

It's sidebands spaced at the number of poles times slip frequency which is referred to pole passing frequency.

As was mentioned, in current spectrum you are looking for pole pass frequency sidebands around LF. There are two ways to calculate pole pass frequency Fp and of course give the same result:

equation 1: Fp = 2* s * LF
equation 2: Fp = p * (N - Nsync)
where
N = speed
LF = line frequency (i.e 60hz in US)
p = number of poles (is 4 for 1800 rpm motor in US)
Nsync = sync speed = 2*LF/p
s = slip = (N-Nsync)/Nsync

To prove they are equal, start with equation 1 and plug in in LF = p*Nsync/2 and s = (N-Nsync)/Nsync:

Fp = 2* [s]*[LF] = 2*[(N-Nsync)/Nsync]([p*Nsync/2]
cancel 2*Nsync from numerator and denominaotr
Fp = (N-Nsync) * p
This is the same as equation 2

For example 4 pole motor at 1785rpm, calculate using equation 2:
Fp = 4 * (1800-1785)=60 cpm = 1hz
You'd be looking at the magnitude at 59hz and 612hz compared to the magnitude at 60hz

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