DC Shunt Electric Motor Max Speed Question.

[BrassCat]

Hello the group. I am considering using a 1HP DC 240V shunt electric motor on my lathe, It is GE, USA made. The label states speed at rated armature and full field current (voltage) of 1725 rpm. As these motors can run at fasted speeds with field weakening, What is the safe limit. I understand much higher HP motors will state that speed but this motor does not. Is there some expected safe speed increase for these low hp motors. I would think it is a mater of bearings and armature wind strength. I would like to get 2x to 2.5x speed increase. Yes, I want it for variable speed small lathe drive. With armature and shunt control I would have enough power for a 6 or 7 to1 speed range.

I would think brush maintenance would be important.
Would an upgrade of the shaft bearings be needed ?
What speed increase were these motors designed for ?
I see this motor is not rated for continues duty. I would consider
providing adequate cooling. This is hobby / garage use. Also, back
away from full loading of motor.

Comments appreciated.
Thanks, Stan

 

[Ulma Doctor]

The bearings are capable of 3450 rpms, i don’t know how much further I’d want to push it past there without upgrading bearings.

 

[markba633csi]

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[homebrewed]

Long back in college I had a lab where we worked with different kinds of electrical machines. When we worked with the shunt motors we were very careful because if the motor is under power and the field coil gets inadvertently disconnected, the theoretical RPM limit is inifinity! Of course, the motor would blow up at some point due to over-revving. These were _really_ big motor-generator sets so quite capable of causing a LOT of damage.

To demonstrate the danger, the professor teaching the lab actually unplugged the field winding very briefly, and the motor's RPMs started to wind up -- FAST. Once he reconnected the field winding the motor briefly became a generator, pumping that excess kinetic energy back into the power grid until it had slowed down to its normal RPMs. Like the nuclear physicist using a screwdriver to adjust a plutonium core learned (later named the "demon core"), probably not a recommended practice. Impressive though.

There were some other things you could do with those machines that would cause mayhem, like putting a dead short on the generator side. All the torque from the armatures would try to transfer itself to the field coil, causing a several hundred-pound motor/generator set to go rolling across the floor.....

 

[BrassCat]

Thanks for the responses. Ulma Doctor, At first 3450 rpm seemed like a funny number, then realized that is 2x the stated speed. Good deal.

homebrewed, thanks for pointing out the safety issues for a dc shunt motor. I am aware of those, That's why in the Monarch 1020 lathe there is a field current detector so to be able to shut the armature current off. I would figure most all (?) shunt drives would do that. I plan to insure my setup include that protection. Pictures of an armature that "exploded" from over-speed don't look good, sort of like a steam engine boiler that has exploded.

I do not know the bearing specs on the motor. Seems to be a lot of US made (older) that I am not finding specs for.

Any other comments most appreciated.
Thanks, Stan.

 

[BrassCat]

Ulma Doctor. Just wondering, to what extent can a PM dc motor run over speed ? This by raising the armature voltage ? There is a point of insulation breakdown and winding shorts if gone too far. Would bearings still go 2x speed ? Never thought about this before. Thanks.

 

[benmychree]

II used to run a cylindrical grinder that used a shunt wound DC motor to run the workhead (turn the part being ground) My observation of its operation was that speed was adjustable at speeds slower than the rated speed, not faster.

 

[edgar10937]

Depends on how good your field controller is, if you have a field current control you should be able to at least double speed but no telling where it will go unstable. Remember it needs an inertia load or if you get too low in field current speed will increase faster than you think.
Also as you decrease field current armature current will rise and you wll lose torque horsepower remaining constant in field weakening.
Edgar

Sent from my SM-G973U1 using Tapatalk

 

[Ulma Doctor]

Ulma Doctor. Just wondering, to what extent can a PM dc motor run over speed ? This by raising the armature voltage ? There is a point of insulation breakdown and winding shorts if gone too far. Would bearings still go 2x speed ? Never thought about this before. Thanks.

I’m not sure how much you can safely overspeed, but 2x rated speed is generally pushing limits. Over voltage can make dc motors run faster, as can field and armature voltage manipulation . In pm dc you can really only control voltage and resistance. There’s always the possibility of failure when testing the limits, i don’t think that putting double the voltage directly to the motor is advisable- maybe work up to maximum speed though experimentation. The motor may spin at double speed but the windings may run hot due to overexcitement . This will considerably reduce the motor’s service life if the motor is required to operate for extended periods of time or under heavy load.

 

[BrassCat]

Well thought out, makes sense. Thanks.