Techs from two different drive companies were not big fans on operating motors for long periods above 60 Hz. I can not remember seeing a motor nameplate stating operating above 60 HZ. Right before I retired they had might have been twin 15 or 20 HP motors that VFD was programmed to run at a maximum of 66 HZ. The large hospital / research centers that I retired from appeared to oversize all AHU units. Just about every area had two supply fans from 40 to 125 HP supply fans and one smaller HP return fan. Even if one AHU was down to replace regular &'HEPA filters the other unit usually ran below 90%. They were smart to add crossover louvers that if an AHU unit comp!shot failed they would open louvers to have another AHU handle the air supply. In drive class what you termed starving input voltage they mentioned on most drives the output voltage & frequency are adjusted on a sliding scale. Example a 480 volt 60 HZ motor that speed is reduced where drive is at 30 HZ will have 240 volt output to motor. Of course we used to call drive outputs PWM Pulse Width Modulation. Believe drive IGBT'S may have at least a 5,000 HZ.
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Why is there an inverter on an air conditioner?
- Thread starter caribconsult
- Start date
fastline
Senior Member
- Location
- midwest usa
- Occupation
- Engineer
Techs from two different drive companies were not big fans on operating motors for long periods above 60 Hz. I can not remember seeing a motor nameplate stating operating above 60 HZ. Right before I retired they had might have been twin 15 or 20 HP motors that VFD was programmed to run at a maximum of 66 HZ. The large hospital / research centers that I retired from appeared to oversize all AHU units. Just about every area had two supply fans from 40 to 125 HP supply fans and one smaller HP return fan. Even if one AHU was down to replace regular &'HEPA filters the other unit usually ran below 90%. They were smart to add crossover louvers that if an AHU unit comp!shot failed they would open louvers to have another AHU handle the air supply. In drive class what you termed starving input voltage they mentioned on most drives the output voltage & frequency are adjusted on a sliding scale. Example a 480 volt 60 HZ motor that speed is reduced where drive is at 30 HZ will have 240 volt output to motor. Of course we used to call drive outputs PWM Pulse Width Modulation. Believe drive IGBT'S may have at least a 5,000 HZ.
I wouldn't know if these 'techs' were educated in the space, or just making assumptions, but generally speaking, when programmed, many configs set the drive up in a scalar or linear scale where full line voltage is reached at 60hz. Beyond that point you cannot serve up any more volts, only frequency. If a motor is not pulling it's full capacity at 60hz, overspeeding can get you in a better sweet spot, especially with a flan or blower, but you have to understand the dynamics of calculating for it. There is both an electrical and mechanical aspect the speed changes.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Buried in the NEMA motor standards are the standard allowed safe overspeed values for different size motors. But when you run a motor past 60 Hz you age your bearings more rapidly.
If you design your system from the ground up for > 60 Hz operation it is quite useful. I know that the minisplits I installed a couple of years ago run the compressor at > 60Hz.
A trick for pushing a motor in a 'get-er-done' situation is to take a 240/480V motor, connect it in the 240V configuration, and then run it on an inverter at > 60Hz.
-Jonathan
If you design your system from the ground up for > 60 Hz operation it is quite useful. I know that the minisplits I installed a couple of years ago run the compressor at > 60Hz.
A trick for pushing a motor in a 'get-er-done' situation is to take a 240/480V motor, connect it in the 240V configuration, and then run it on an inverter at > 60Hz.
-Jonathan
fastline
Senior Member
- Location
- midwest usa
- Occupation
- Engineer
You're exactly right on motor lead connections and something people don't realize with VFDs. At the end of the day, the amps are what matter. We even do wye/delta switching on the fly. It's also a neaky way to extract more HP from a motor, BUT then thermal monitoring and fans become a thing.Buried in the NEMA motor standards are the standard allowed safe overspeed values for different size motors. But when you run a motor past 60 Hz you age your bearings more rapidly.
If you design your system from the ground up for > 60 Hz operation it is quite useful. I know that the minisplits I installed a couple of years ago run the compressor at > 60Hz.
A trick for pushing a motor in a 'get-er-done' situation is to take a 240/480V motor, connect it in the 240V configuration, and then run it on an inverter at > 60Hz.
-Jonathan
Regarding speeds over base, this is what I was saying about the 'mechanical assessment', but in 99% of cases, it is bogus to think bearings are going to fail with a few extra hz. I know bearings well and 3600rpm is not even close to a design limit for most bearings. BUT, this is where the mechanical thing comes in because imbalance calculations can quickly get you out of radial load spec if you are careless.