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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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#1
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Maximum speed for 3 phase motor?
In a thread here someone thought that a 1725 motor probably has the same mechanical structure as a 3450 & hence could be run at 120 Hz. My question is: if a 1725 motor is mechanically the same as a 3450 motor, do you think that it could be run at 5175 rpm? The only way to know is to test to destruction. As a data point, I have several days of run time on a 7.5 hp 1725 at 4000rpm. Karl |
#2
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Maximum speed for 3 phase motor?
"Karl Townsend" wrote in message anews.com... In a thread here someone thought that a 1725 motor probably has the same mechanical structure as a 3450 & hence could be run at 120 Hz. My question is: if a 1725 motor is mechanically the same as a 3450 motor, do you think that it could be run at 5175 rpm? The only way to know is to test to destruction. As a data point, I have several days of run time on a 7.5 hp 1725 at 4000rpm. Karl Hi Karl I like your suggestion that we dont have much real data on how fast we can run motors before they fail and actually destroying one is probably the only wat to learn its limit. I wanted to suggest that the relationship between RPM and the force that throws the rotor windings out into the field is not linear. The force is related to the square of the angular velocity. So, some low RPM variations may not be a good guide for predicting for other high RPM ranges. Jerry |
#3
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Maximum speed for 3 phase motor?
On Sat, 9 Feb 2008 07:12:57 -0500, "Karl Townsend"
wrote: In a thread here someone thought that a 1725 motor probably has the same mechanical structure as a 3450 & hence could be run at 120 Hz. My question is: if a 1725 motor is mechanically the same as a 3450 motor, do you think that it could be run at 5175 rpm? The only way to know is to test to destruction. As a data point, I have several days of run time on a 7.5 hp 1725 at 4000rpm. Karl I've run a lot of motors on VFD's at work. Inverter rated motors quite often have the maximum rpm on the name plate. A lot of them have been in the 4300 -- 4800 range. I doubt you're going to find the data on a non-vfd rated motor, so personally I'd be hesitant to run over 4800. Below is a link to the manual for Marathon Black Max motors, which is the type I use most frequently. Look at Table 3.4 for the recommended maximum for those motors. Max speed is related to frame size, which is no surprise due to the increase in rotor size. I doubt the numbers are valid for your motor, but the trend should hold. http://www.marathonelectric.com/moto...uals/5554E.pdf Pete Keillor |
#4
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Maximum speed for 3 phase motor?
Pete Keillor wrote:
.... Below is a link to the manual for Marathon Black Max motors, ... Table 3.4 for the recommended maximum for those motors. ... Thanks - good link! A couple of interesting things the - for *belted" loads, their limit is 125% of rated speed. I'd guess that's because of the radial bearing load. I'm going to go ahead and ignore that. otherwise (direct load), for my frame: - a 3600 rpm motor has a 7200 (120 Hz) limit - a 1800 rpm motor has a limit of 5400 (180 Hz) I think that I'll compromise between the 125% limit and the 200/300% and limit the 3600 to 150% (90 Hz, 5400 rpm) & the 1800 to 200% (120 Hz, 3600 rpm). Thanks, Bob |
#5
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Maximum speed for 3 phase motor?
On Sat, 9 Feb 2008 19:46:04 -0600, "Robert Swinney"
wrote: This whole discussion reminds me of the old saw about giving a monkey a loaded gun and wondering how long it will be before he shoots you in the ass with it. Bob Swinney Bob, you can do what you want of course, but I'll reiterate: I doubt you're going to find the data on a non-vfd rated motor, so personally I'd be hesitant to run over 4800. I doubt the numbers are valid for your motor, but the trend should hold. I know inverter duty motors are insulated better due to the issues of cooling at low speeds and high voltage transients from the VFD. It is also entirely possible that the rotors in inverter duty motors are built to withstand overspeed conditions, given the likelihood they'll get used that way. Also, your application involves the side load of a belt, although that would only relate to bearing failure. The overspeed failure normally includes unwrapping of the rotor with consequent sudden stoppage, and in big motors, likelihood that the motor is going to wind up in a different location. I heard of one instance in one of our plants in Texas of a 100 Hp. motor making it clear across the ground level of the plant when the pump it was driving seized. Yikes! I personally would be cautious. Like one of the posters said, a failure can be catastrophic. "Bob Engelhardt" wrote in message ... Pete Keillor wrote: ... Below is a link to the manual for Marathon Black Max motors, ... Table 3.4 for the recommended maximum for those motors. ... Thanks - good link! A couple of interesting things the - for *belted" loads, their limit is 125% of rated speed. I'd guess that's because of the radial bearing load. I'm going to go ahead and ignore that. otherwise (direct load), for my frame: - a 3600 rpm motor has a 7200 (120 Hz) limit - a 1800 rpm motor has a limit of 5400 (180 Hz) I think that I'll compromise between the 125% limit and the 200/300% and limit the 3600 to 150% (90 Hz, 5400 rpm) & the 1800 to 200% (120 Hz, 3600 rpm). Thanks, Bob |
#6
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Maximum speed for 3 phase motor?
On Sun, 10 Feb 2008 09:44:27 -0600, "Robert Swinney"
wrote: Thanx Pete ! You did well to repeat the warning for those that might benefit from it. The "monkey & loaded gun" analogy was aimed at anyone stupid enough have no regard for the simple physics involved. I appreciate your reiteration although doubt those that can really use use it even bothered to read it. Are you going to N.A.M.E.S. this year? Bob Swinney I haven't decided yet, too much stuff going on. If I do, I'd probably only spend one night or drive down and back same day (ugh). Pete snip |
#7
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Maximum speed for 3 phase motor?
Jerry wrote:
I like your suggestion that we dont have much real data on how fast we can run motors before they fail and actually destroying one is probably the only way to learn its limit. Hmmm, I don't recommend such an experiment without the appropriate test site. While it is likely the first thing to fail would be a copper bar in the rotor push outward enough to make the rotor iron hit the stator and stop the motor, there is the possibility the entire rotor would burst completely! I wouldn't want to be annywhere near one of these rotors coming apart at well above 5000 RPM. When discussing these overspeeding topics some time ago, I believe on this same list, somebody from the UK told about a shop that had modified motors for operation on 800 Hz in a woodworking shop. They had 2 diesel generators, one wound for 400 Hz, one for 800. They started the whole shop on 400 Hz, then switched to 800 Hz, then shut off the 400 Hz generator. I don't know what number of poles they used on the motors, I'm pretty sure they couldn't be 2-pole, 4-pole, or even 6-pole, that is just too fast for standard rotors. Guessing an 8-pole winding at 800 Hz, that would be 12,000 RPM, the next possibility is 12 pole, for 8000 RPM. (I am not recommending doing anything like this, I tend to be REALLY conservative when overspeeding anything.) Jon |
#8
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Maximum speed for 3 phase motor?
Pete Keillor wrote:
The overspeed failure normally includes unwrapping of the rotor with consequent sudden stoppage, and in big motors, likelihood that the motor is going to wind up in a different location. I heard of one instance in one of our plants in Texas of a 100 Hp. motor making it clear across the ground level of the plant when the pump it was driving seized. Yikes! I find this hard to believe. I would expect the shaft coupling to shear easily and let the motor free. I would expect this more catastrophic scenario would require a locking-up of the rotor to the stator. Even if the bearings seized, there would be huge shrieking and either the bearing burns up white hot, or the motor protector cuts off. But, if a rotor bar came loose, the rotor can seize to the stator very catastrophically. This is pretty RARE. I personally would be cautious. Like one of the posters said, a failure can be catastrophic. Yes, the faster you run it, the slimmer the safety margin gets. Jon |
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