Has anyone ever heard of a PWM controller for museum operation of huge 3 to 6 foot diameter DC motors and rotary converters?
It would be nice to put some of these old antique motors back into service in museums, rather than having them just sit around collecting dust.
This is probably an impossible request but I'd like to know if anyone has developed a DC motor controller capable of safely operating historic DC commutated motors and rotary converters from the early 1870 to 1930 period.
Usually these giant DC motors were started by hand using a huge resistance box starter that put lots of resistance in series with the motor, and then slowly the resistance is reduced by sliding a contactor handle from one terminal to the next over a period of about 10 seconds as the rotor increases to running speed. If supply power is cut off, the starting control handle immediately snaps to the Off position so the motor cannot start in the fully powered state.
This is because when a DC motor is not spinning it has almost no resistance to current flow and applying full power to a huge DC motor before it is at full RPM can burn up and destroy the motor windings. As the rotor speed increases the rotor produces back-EMF which fights against the input power. This then allows the starting resistance to be reduced until the DC motor has reached running speed.
These back-EMF resistance effects are generally unnoticed for modern small 5v and 12v DC motors but can be seriously destructive for huge old motors that ran on 250 volt, 25+ amp DC service.
Even though the hardware tends to be 100 years old I would expect they would work just as well with PWM in place of the manual starting resistance box, plus it seems that a PWM controller could provide for safety measures like detecting sudden unexpected brush sparking, field-coil shorting detection and overspeed detection (using a shaft speed sensor) that could protect an old motor just spinning unattended and on display in a museum.
The controller design would not necessarily have to be rated for 50+ amp DC output since a museum piece would just spin unloaded, with the only load coming from the rotor bearings and the commutator brush gear. Though, I do not know specifically what sort of amperage would be needed to gently spin up a large antique motor like this.
- Dale Mahalko
My Wikipedia user page:
http://en.wikipedia.org/wiki/User:DMahalko
It would be nice to put some of these old antique motors back into service in museums, rather than having them just sit around collecting dust.
This is probably an impossible request but I'd like to know if anyone has developed a DC motor controller capable of safely operating historic DC commutated motors and rotary converters from the early 1870 to 1930 period.
Usually these giant DC motors were started by hand using a huge resistance box starter that put lots of resistance in series with the motor, and then slowly the resistance is reduced by sliding a contactor handle from one terminal to the next over a period of about 10 seconds as the rotor increases to running speed. If supply power is cut off, the starting control handle immediately snaps to the Off position so the motor cannot start in the fully powered state.
This is because when a DC motor is not spinning it has almost no resistance to current flow and applying full power to a huge DC motor before it is at full RPM can burn up and destroy the motor windings. As the rotor speed increases the rotor produces back-EMF which fights against the input power. This then allows the starting resistance to be reduced until the DC motor has reached running speed.
These back-EMF resistance effects are generally unnoticed for modern small 5v and 12v DC motors but can be seriously destructive for huge old motors that ran on 250 volt, 25+ amp DC service.
Even though the hardware tends to be 100 years old I would expect they would work just as well with PWM in place of the manual starting resistance box, plus it seems that a PWM controller could provide for safety measures like detecting sudden unexpected brush sparking, field-coil shorting detection and overspeed detection (using a shaft speed sensor) that could protect an old motor just spinning unattended and on display in a museum.
The controller design would not necessarily have to be rated for 50+ amp DC output since a museum piece would just spin unloaded, with the only load coming from the rotor bearings and the commutator brush gear. Though, I do not know specifically what sort of amperage would be needed to gently spin up a large antique motor like this.
- Dale Mahalko
My Wikipedia user page:
http://en.wikipedia.org/wiki/User:DMahalko