Marc:
Regarding variable frequency AC drives (The Spindle Drive).
These are "Pulse Width Modulated" drives and yes they put out a square wave. But, when the drive is puting out (For Example) 30 hertz to drive the motor, it's important to remember that this is 30 hertz modulation of a much higher frequency carrier. These carriers are in the range of tens of kiloherts!
To put it another way, the drive is puting out a 12 KILOhertz signal that is modulated by a 30 hertz signal.
Most (Maybe All) of the PWM drives I've used had a parameter that could be set that gave you a choice of several carrier freqencies. For example, on one of the drives I've used recently, you could set the carrier to 3, 4.5, 6, 9, or 12 kilohertz. There are disadvantages to setting it low and disadvantages to setting it high. It creates a compromise.
If the carrier frequency is low enough, the motors and drives can get noisy (Physicaly not electricaly). You can actually hear the carrier as the motor runs, it makes kind of a whining noise. Depending on the application/environment this may or may not be a concern.
If the carrier frequency is set high to quiet the motors down, it "may" cause heating of cables, conections, and motors. I was always more concerned with this as the environment I worked in was allready noisy and you could never hear the motors anyway.
When you consider that the drive is using a carrier that is, for example, 12 kilohertz you realize that the drive is creating harmonics that are multiples of that 12 kilohertz such as 24 khz, 36 khz, etc. This is what ESjaavik was refering to and, yes it is important to be meticulous with the shields and grounds to be sure that this doesn't get "transmitted" throughout the cabinet.
I've never twisted three phase conductors ( cause I'm not man enough to twist the sizes I regularly use
) but I do make sure that they are run bundled and tied together in groups of three. And like Al The Man said I ALWAYS use an inductive device (I don't think it's a choke, I think it's just a special isolation transformer) on the output of PWM drives. This not only protects against noise, but also protects the output stage of the drive from high frequency energy being reflected back into it causing damage to its IBGTs. The drive manufacturors and transformer manufacturors have recomendations for this. I also ALWAYS use chokes on the input side of the drives.
And Finally, When you are looking at a signal on the scope, if there is some distortion or noise that is finding its way into BOTH the signal line you are looking at and the reference you are using (In other words: Whatever you have your scopes probe hooked to and whatever you have your scopes ground hooked to), AND this distortion/noise is the same polarity in both, (for example, the signal is raised by 5 volts, and the reference is raised by 5 volts) it will NOT show up on your scope even though it is there.
I know that may be hard to follow but I don't know how else to describe it. This distortion has a name but I can't for the life of me put my finger on it. Anybody else know?