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I have been experimenting with motors on my milling machine. I currently have a very temporary lash up of a 300 watt brushless motor installed. It has an integral controller that works much the same as a standard treadmill motor control. It uses a 5k external pot to set the rpm of the motor with a three wire connection, ground, tap and +15 volts.
The problem is that while it does a beautiful job of adjusting the rpm over a full range from crawl to maximum of 3000 rpm the motor has no effective governor that maintains the set rpm under load. That makes the speed control close to useless if any real load is imposed.
I sent the problem to the design committee to work on and they informed me yesterday that a possible solution exists that A: costs nothing B: Is dead simple C: requires no knowledge of electronics beyond that required to solder a few connections.
The answer to maintaining motor rpm under load is to use a "closed loop" feedback system that senses motor rpm and adjusts the speed control automatically to maintain the set rpm as the load changes.
The motor has attached to the drive pulley a plastic disk with six neo magnets embedded. This is located so that it passes close to the core of the transformer as the pulley rotates. This generates an AC voltage via the transformer that is rectified to DC.
My brushless motor operates so that turning up the rpm follows a decreasing voltage on the potentiometer tap. The sensor consists of a small transformer with a secondary rated at 12 vac from a 117vac input. The core is cut off to leave only a centre core, making the transformer into a solenoid.
The output of the transformer is rectified and filtered by a small capacitor and connected to the speed pot. More on that in a bit.
The disk of magnets is very non critical in construction, diameter or magnet size. The six magnets are installed with alternating north/south polarity and are a press fit to the plastic disk.
The transformer is mounted so that the magnets pass close to the face of the core. This should be adjustable for location and spacing so the system may be tuned. There are other possible configurations that will work just as well such as a radial mounted set of magnets on a smaller drum on a shaft with the transformer core perpendicular to the shaft.
The object is for the sensor to provided the opposite signal to that required to adjust the rpms of the motor. This is injected by connecting the plus and minus output according to how your speed control functions.
If the center tap of the potentiometer increases the voltage to make the motor run faster then the Sensor plus lead is connected to the tap and the minus lead to the plus volts side of the potentiometer.
If increasing the voltage at the tap makes the motor run slower then the minus side of the sensor output is connected to the zero volts side of the potentiometer.
The circuit:
The installation: (this is temporary)
The movie: 1 meg download
Watch it take a .1 x .1 DOC in aluminum while just barely turning a 3/4" end mill. You can count the rpms.
The problem is that while it does a beautiful job of adjusting the rpm over a full range from crawl to maximum of 3000 rpm the motor has no effective governor that maintains the set rpm under load. That makes the speed control close to useless if any real load is imposed.
I sent the problem to the design committee to work on and they informed me yesterday that a possible solution exists that A: costs nothing B: Is dead simple C: requires no knowledge of electronics beyond that required to solder a few connections.
The answer to maintaining motor rpm under load is to use a "closed loop" feedback system that senses motor rpm and adjusts the speed control automatically to maintain the set rpm as the load changes.
The motor has attached to the drive pulley a plastic disk with six neo magnets embedded. This is located so that it passes close to the core of the transformer as the pulley rotates. This generates an AC voltage via the transformer that is rectified to DC.
My brushless motor operates so that turning up the rpm follows a decreasing voltage on the potentiometer tap. The sensor consists of a small transformer with a secondary rated at 12 vac from a 117vac input. The core is cut off to leave only a centre core, making the transformer into a solenoid.
The output of the transformer is rectified and filtered by a small capacitor and connected to the speed pot. More on that in a bit.
The disk of magnets is very non critical in construction, diameter or magnet size. The six magnets are installed with alternating north/south polarity and are a press fit to the plastic disk.
The transformer is mounted so that the magnets pass close to the face of the core. This should be adjustable for location and spacing so the system may be tuned. There are other possible configurations that will work just as well such as a radial mounted set of magnets on a smaller drum on a shaft with the transformer core perpendicular to the shaft.
The object is for the sensor to provided the opposite signal to that required to adjust the rpms of the motor. This is injected by connecting the plus and minus output according to how your speed control functions.
If the center tap of the potentiometer increases the voltage to make the motor run faster then the Sensor plus lead is connected to the tap and the minus lead to the plus volts side of the potentiometer.
If increasing the voltage at the tap makes the motor run slower then the minus side of the sensor output is connected to the zero volts side of the potentiometer.
The circuit:
The installation: (this is temporary)
The movie: 1 meg download
Watch it take a .1 x .1 DOC in aluminum while just barely turning a 3/4" end mill. You can count the rpms.
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