marka12161
Stainless
- Joined
- Dec 23, 2016
- Location
- Oswego, NY USA
The insulation on the leads on the 7 1/2 hp motor in the 1941 monarch are cracked and brittle so i investigated the cost of a repair vs the cost of a new motor. In doing so i learned some things regarding the advances made in the last 78 years.
First of all, the old motor is huge by today's standards for it's rated hp. The guy specing a new motor for me told the the reason motors got smaller is the insulation is more efficient. Said another way, for a given resistance at a given voltage, the insulation material occupies less volume today than in the old days. He didn't say anything about performance at elevated temperatures or mean time between failure but i can imagine that got better as well. On the down side, the new motors don't have the same inertia due to the lower mass. This means that they don't handle sudden load changes as well as an old motor of equal hp. For this reason, the motor tech recommended i replace the 7.5 hp motor with a modern 10 hp motor.
I've heard it said that because old motors were bigger than new motors, they developed more torque. I've never been able to reconcile this since power is proportional to the product of speed and torque. Since the speed of an induction motor is basically set by the operating frequency, two motors of the same rated hp operating at the same speed must develop roughly the same torque. However, as i mentioned above, the old motors had more running inertia so i think the assertion that old motors had more torque is really that old motors had more running inertia which is really the ability to provide instantaneous torque.
A second thing i learned was that the frame cross reference charts are a bit counter intuitive. The old motor has a 284 frame and the cross reference charts call out 213T as the "equivalent" frame size. The hole pattern, shaft diameter and shaft distance to base dimensions of the 213T vs the 284 are all different. However a modern 284T motor has all the same dimensions as an old 284 frame. When i saked the motor tech why he couldn't give me a motor with a 284T frame, he indicated it would be much larger than 10hp. Bottom line is "equivalent frame size" really means "frame size for equivalent motor hp".
At any rate, my RPC has a 10 hp idler so i declined the new 10 hp motor. the motor shop is evaluating a repair to my old motor now. Hopefully it'll cost less than the new motor.
At this point, i'm not sure what i'll do. I do have a spare 5hp 3 phase motor kicking around so i may rig a second idler to start & run the monarch when i get it all together.
First of all, the old motor is huge by today's standards for it's rated hp. The guy specing a new motor for me told the the reason motors got smaller is the insulation is more efficient. Said another way, for a given resistance at a given voltage, the insulation material occupies less volume today than in the old days. He didn't say anything about performance at elevated temperatures or mean time between failure but i can imagine that got better as well. On the down side, the new motors don't have the same inertia due to the lower mass. This means that they don't handle sudden load changes as well as an old motor of equal hp. For this reason, the motor tech recommended i replace the 7.5 hp motor with a modern 10 hp motor.
I've heard it said that because old motors were bigger than new motors, they developed more torque. I've never been able to reconcile this since power is proportional to the product of speed and torque. Since the speed of an induction motor is basically set by the operating frequency, two motors of the same rated hp operating at the same speed must develop roughly the same torque. However, as i mentioned above, the old motors had more running inertia so i think the assertion that old motors had more torque is really that old motors had more running inertia which is really the ability to provide instantaneous torque.
A second thing i learned was that the frame cross reference charts are a bit counter intuitive. The old motor has a 284 frame and the cross reference charts call out 213T as the "equivalent" frame size. The hole pattern, shaft diameter and shaft distance to base dimensions of the 213T vs the 284 are all different. However a modern 284T motor has all the same dimensions as an old 284 frame. When i saked the motor tech why he couldn't give me a motor with a 284T frame, he indicated it would be much larger than 10hp. Bottom line is "equivalent frame size" really means "frame size for equivalent motor hp".
At any rate, my RPC has a 10 hp idler so i declined the new 10 hp motor. the motor shop is evaluating a repair to my old motor now. Hopefully it'll cost less than the new motor.
At this point, i'm not sure what i'll do. I do have a spare 5hp 3 phase motor kicking around so i may rig a second idler to start & run the monarch when i get it all together.