10-30-2011, 11:21 PM
Lots of unexplained variables in this problem but focusing strictly on the 120 HZ and motor speed-up at full HP,from single phase, I've done it to make a direct drive wood shaper.
Starting from 230 single phase. Step the volts up to 460 single phase with a dry type distribution transformer. Connect the 460 Volt transformer output to a suitably rated 460V VFD. Connect the motor for 230 volts and connect its leads to the output of the VFD.Connecting a 230 volt motor to 460 sounds counter intuitive. The secret is Volt per Hz. Set the VFD parameters Volts / Hz to 3.8333 so the motor sees 60zh at 230 Volts. As the freq is ramped up past 60 Hz the motor will over speed to 2x ratings at 120 HZ. This works well and reliably on single and three phase input the the VFD and all parts of the system are run within parameters and ratings. The motor can be oversped to 180 Hz without significalt loss of rated HP.
Here's one additional wrinkle: size the VFD to suit the motors's FLA @ 230 volts and the motor will deliver full torque at 120 HZ - that is double the rated HP.
The question arises: will an induction motor withstand the overspeed. First, an induction motor is not like a DC motor where windings of copper wire are connected to a commutator ro form a kind of centrifugal grenade. OHT an induction motor stator has a squirrel cage winding that's essentially a big shorted circuit die cast into hose and slots punched in the armature laminations. Aluminum is 1/3 the density of copper thus better suited for service when oversped.
I once conducted a few experiments overspeeding induction motors. I oversped a 10 HP 3600 RPM three phase Hitachi motor to 10,800 RPM and held it at speed for over an hour. I dismantled the motor and dye checked the die cast armature and found no indications except for the poroustiy to be expected at the parting line. A run through a growler (ever heard of this diagnostic tool?) and found all segments of the shorted circuit complete. The bearingare tupically rated very conservatively. After an hour at speed they remained near the motor frame temp. The biggest objection to overspeeding an induction motor is the fan integrally cast integal to the armature winding can really eat up HP throuh windage.
My conclusion based on these experiments is that overspeeding to 3X rated RPM poses little hazard to a healthy three phase induction motor.
So the basic question of how to overspeed a three phase induction motor can be answered fairly simply: get the properly rated stuff and hook it up.
Starting from 230 single phase. Step the volts up to 460 single phase with a dry type distribution transformer. Connect the 460 Volt transformer output to a suitably rated 460V VFD. Connect the motor for 230 volts and connect its leads to the output of the VFD.Connecting a 230 volt motor to 460 sounds counter intuitive. The secret is Volt per Hz. Set the VFD parameters Volts / Hz to 3.8333 so the motor sees 60zh at 230 Volts. As the freq is ramped up past 60 Hz the motor will over speed to 2x ratings at 120 HZ. This works well and reliably on single and three phase input the the VFD and all parts of the system are run within parameters and ratings. The motor can be oversped to 180 Hz without significalt loss of rated HP.
Here's one additional wrinkle: size the VFD to suit the motors's FLA @ 230 volts and the motor will deliver full torque at 120 HZ - that is double the rated HP.
The question arises: will an induction motor withstand the overspeed. First, an induction motor is not like a DC motor where windings of copper wire are connected to a commutator ro form a kind of centrifugal grenade. OHT an induction motor stator has a squirrel cage winding that's essentially a big shorted circuit die cast into hose and slots punched in the armature laminations. Aluminum is 1/3 the density of copper thus better suited for service when oversped.
I once conducted a few experiments overspeeding induction motors. I oversped a 10 HP 3600 RPM three phase Hitachi motor to 10,800 RPM and held it at speed for over an hour. I dismantled the motor and dye checked the die cast armature and found no indications except for the poroustiy to be expected at the parting line. A run through a growler (ever heard of this diagnostic tool?) and found all segments of the shorted circuit complete. The bearingare tupically rated very conservatively. After an hour at speed they remained near the motor frame temp. The biggest objection to overspeeding an induction motor is the fan integrally cast integal to the armature winding can really eat up HP throuh windage.
My conclusion based on these experiments is that overspeeding to 3X rated RPM poses little hazard to a healthy three phase induction motor.
So the basic question of how to overspeed a three phase induction motor can be answered fairly simply: get the properly rated stuff and hook it up.
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