Correct me if I am wrong, but I thought that there were special motors designed to accommodate the added heating from the high frequency components and for the shorter insulation life because of the heating.Tom
Hi Tom!
There are motors that are DESIGNATED to be avertised to accomodate high-frequency input and transients. As far as design, there's only some things they CAN change... but the biggest difference from what I see, is the amount of focus on insulation quality.
See, the motor is naturally an AC electric device... you have a pair of coils which alternate between attraction and repulsion. Even a DC motor has alternation. That being the case, there's high inductance... and therefore, some frequency at which the coils simply cannot respond above. Likewise, the core of an inductor (if it has one) contributes to the inductance, hence, further limits the frequency roll-off point. Transformers are the same way... you can't put 1.3Mhz into a 60hz coil and expect it to come out the other end... it won't make it.
What DOES happen, is that whatever you drive IN, that cannot come out, is either reflected, or dissipated as heat.
Now, consider the motor to be a 60hz inductor. If you're slamming 20kw of 400hz into a 60hz inductor for one hour, and only 10kw is coming out, you'll have 34121 BTU of waste heat to warm your shop. That'd be nice today, but call me back in July and I'll have less delicate words...
In reality, you'll have melted motor insulation before you ever get to that level of inefficiency, but also, there aren't many situations where you'd slam 400hz into a 60hz inductor. Highest I run my machines... lessee... the radial drill has an ordinary 1800rpm C-face motor, and I run it to around 215hz. I don't get a whole lotta torque with it whirling away in the realm of 6500rpm, but usually that only occurs when I'm drilling small-diameter/low torque holes, so it doesn't matter. The real result, is that there's not much energy going in, not much load, not much waste heat, hence not much heating in the motor. I HAVE removed the motor's shaft-mounted fan, and replaced it with a constant-speed 'muffin' type fan, but not because the motor gets any warmer at any speed... it's because when that things gettin' it on at 6k, the shaft-mount fan may-as-well be an air-raid siren... the neighborhood heads for the storm shelter. I like music in my shop... that means not having to live in earplugs.
The manufacturer's first and foremost concern, is that the windings don't fail. There's two aspects of this... first being, do they get hot enough for the insulation to break down. Second, is that when you apply current to a wire, it generates a field. When the field builds, anything that attracts or repels that field, will want to MOVE THE WIRE. That means the wire could vibrate itself to death... or it's insulation could fail. This isn't a phenomenon limited to motors on VFDs... it happens with ALL motors, so manufacturers wrap the coils in cloth tape, or pot them in epoxy or goo, whatever they could, to keep the windings from vibrating.
Modern motors, they use vacuum and pressure, heat-curing epoxies, and all sorts of dandy stuff to make them stay down tight. Old motors were made using the technology THEY had on hand... and they built them as tough as they could, and frequently did very well, judging by how many are still in service.
If you run a VFD, or any other solid-state drive concept into a motor at 60 hz, you'll likely find that even with a substantial amount of harmonic distortion and RF hash, that ANY good industrial motor simply won't care... that any noise emitted, will be reflected back out of the motor with very little heating... and as I noted (and others agreed), that if you're listening to an AM Oldies channel, Buddy Holly will sound like Jimi Hendrix...
