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Harrison 13x40 toolroom lathe, $1700, Chatham, ON

Darren said:
I took the advice of a motor/drive tech. It works great from 20-120hz limits that i set in the vfd. plenty of power at all speeds.
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Interesting. I would have thought that 120 on low speed windings would never fuss the motor bearings. But I would expect 20hz to push the cooling to the extreme limits for a non-VFD rated motor.

Conversely, running the motor at 20Hz on the high speed windings would not hurt cooling but 120Hz could kill bearings.

So it's that cooling vs bearings thing that would have driven my choice. I would have reasoned that the bearings would be fine at 60hz on high-speed cuz that's the design rating anyway. And I would have thought 20hz on high-speed would be just like low speed so cooling is fine too.

What were the rated speeds for the motor anyway?
 
I'm pretty sure its 1725/3450 at 60hz.

Cooling at low speeds hasn't been an issue. Its a pretty heavy industrial motor, not a hobby grade machine. Then again, I don't run it at low speeds all that often, or under heavy load at low speeds. I also don't run it at 120hz often. The lathe has 8 geared speeds, so the motor is usually around 60hz.
 
2 speed motors are run on drives however to maintain both speed range capabilities the VFD has to be sized at the amps required for the higher draw. And the contactor/pole switching setup has to remain functional. Common 2 speed motors switch from 2 to 4 pole or 4 to 6 pole.
 
Initially, I was going to wire in a set of contactors to switch the windings, but decided that it wasn't really needed, and my experiences show that its not. In fact, as I received the lathe, it was wired for it already. I would have only needed an interlock relay to prevent speed from being switched while the motor is under power. In the end, I replaced the 2 speed switch with a pot to control the speed.
 
Now that I look closer at the picture, this lathe is not fitted with the 2 speed switch beside the stop /start switch. A 2 speed motor must have been an option
 
Darren said:
I'm pretty sure its 1725/3450 at 60hz.

Cooling at low speeds hasn't been an issue. Its a pretty heavy industrial motor, not a hobby grade machine. Then again, I don't run it at low speeds all that often, or under heavy load at low speeds. I also don't run it at 120hz often. The lathe has 8 geared speeds, so the motor is usually around 60hz.
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So only a 2x speed ratio. Not as problematic as a 5 or 10 to 1 would be.

I did some temperature tests when I was setting up a regular 220 3ph non-VFD rated motor and was surprised at how well it handled low speeds at no load. But it went to hell in a hand-basket at higher loads. Temps were skyrocketing at 20hz with even relatively low loads. I ended up deciding that 45 Hz was a safer minimum. Similarly, bearing quality is an unknown I can't even test for, so I decided on a 90hz maximum. But I have seen pro recommendations at 75hz for ordinary motors.

The trouble is that 45 & 75 are just 25% each or 50% total. Not near 2:1. 40 to 80 isn't much more and that gets you 2:1. That's about when I decided that I would either keep and continue to use the existing gears or replace the motor with a VFD Rated Motor.
 
On the 2 spd 3ph motor front...

My First mill was available below market price because someone with a forklift destroyed the slo Fwd-fwd-stop-rev-slo rev switch. So badly that you couldn't piece it back to figure out the wiring.

After some head scratching I isolated the high speed windings and have run it on the VFD that way ever since. I considered the low speed winding config, but since it is a belt drive machine with back gear, the weight was on the high speed side.
 
My 3hp mill motor is a 4 pole 7200 rpm inverter rated unit. Its 1:1 driven spindle runs from 40-6000rpm in its current configuration. Of note is the giant electric pancake fan that runs full speed whenever the spindle is running. I have no idea of the HZ it sees. There's no display on the VFD. Even at low speed/heavy load, it never gets beyond warm. It was designed as a CNC mill, so i'm sure they designed it for much heavier usage than I put it through.

The motor on the Emco has a large fan , heavy finned case, and when i've checked it, heat wasn't a concern. The 5hp Baldor motor on my compressor gets pretty hot. I'll be adding a fan to it on a timer. The compressor runs a lot in my shop. But heat hasn't killed it yet.
 
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