variable frequency drive conversions

[tonyfoale]

Thanks for the fast reply, Tony. My 2HP, 3ph lathe came with a new (at the time) RotoPhase converter and 5 HP motor. The lathe also has an antiquated mechanical, infinitely variable drive. Everything has been in storage for the past 30 years and not used. However, my understanding is that the old phase converters were quite noisy. Since I need to convert the lathe to single phase it would be nice to simplify the variable RPM drive. I was considering a VFD because it may be quieter and more efficient than the RotoPhase but also because it would simplify RPM changes. Your thoughts . . .

My thoughts. Firstly, many VFDs are noisy, those have a cooling fan which makes a lot of noise and is on all the time. I cured an ABB VFD by substituting a quiet computer fan which still runs all the time. More recently I got a Huanyang VFD which was even louder, I changed that for a computer fan, the original was 24 V but the computer fan is 12 V, so I used a series resistor. This is a very easy modification. Only last night I accidentally came across a video by Mark Presling (he also inhabits this forum so he might chirp in) about silencing a Huanyang VFD. His approach was somewhat different. He added a simple 45 deg bimetallic temperature switch such that the fan only comes on when the temperature of the internal heat sink gets to 45 deg. This morning I ordered some thermal switches, I think that the ultimate solution is Mark's switching and a quieter fan.

There is another source of noise which generally afflicts some older motors. The PWM carrier frequency of VFDs ranges from around 2 kHz to over 10 kHz. Some motors will make a noise at that frequency which can be quite annoying. My ABB VFD drives a very old 1950s 5 hp motor on a shock dyno and there is no audible noise, I have not checked the carrier frequency. The Huanyang drives a 5 hp modern motor on my mill and there is no noise that I hear. The default frequency on that is 7 kHz but that is programmable up and down. However, if I use the Huanyang with the newly fitted 1990s motor on my lathe it whines. I made what is called a dV/dt filter to put between the motor and VFD which has cured it. In his video Mark mentions the same problem which he fixed by programming the carrier frequency to be higher. This is a very common "fix" but a word of warning. In general terms a higher carrier frequency is good for the motor but bad for the VFD because the IGBTs inside (electronic switches) generate more heat due to the increased switching.

About swapping the mechanical variable speed for a VFD, that should be fine unless you need to do heavy work at low speed, I have commented on this in earlier posts. Like my recent lathe motor change it looks like your lathe motor has gone from 2 hp to 5 hp which will be a big help.

About replacing the rotary converter with a VFD, I would not hesitate. the rotary converter will have a 3 ph motor inside which might be useful to power something else.

PS. here is a link to Mark's video https://www.youtube.com/watch?reload=9&v=x882vvX_L7Y

PPS. I will make post soon about making the dV/dt filter and generally filtering VFD/motor combinations.