Posted: 9/21/2020 11:39:20 AM EDT
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Got a 3 phase motor I want to run with a VFD. The motor is rated 4a at 220v. I measured the windings and they are all 5k ohms. Is there a way I can verify that the motor works before buying a VFD? The motor is 1100rpm. If a VFD does 0-400 hz does that mean it can run the motor up to 400/60*1100 rpm? That is faster than I need, if there an advantage to gearing the motor down? Ie will I get more power by running the motor faster? |
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You can wire the motor, sans VFD, to confirm rotation/operation. When you setup a VFD, you input the motor's name plate info (operating parameters). The VFD will then know the motor's max RPM, Amperage, Voltage, etc. Gearing down and running full rpm should provide the most torque. @cavgunner |
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Originally Posted By User55645: You can wire the motor, sans VFD, to confirm rotation/operation. When you setup a VFD, you input the motor's name plate info (operating parameters). The VFD will then know the motor's max RPM, Amperage, Voltage, etc. Gearing down and running full rpm should provide the most torque. @cavgunner You can bump start the motor But The in rush current could be very high I have an adjustable overload motor starter I use. Just as an example I started a 7.5hp 3p 480 motor with it. I put my amp clamp on one leg. At start up it drew 56amps. Operating current is 11 A You need OCP |
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Originally Posted By Morgan321: Got a 3 phase motor I want to run with a VFD. The motor is rated 4a at 220v. I measured the windings and they are all 5k ohms. Is there a way I can verify that the motor works before buying a VFD? As others have mentioned there are ways to do it, but really- you're going to be better off just taking the motor to an electric shop and having them test it. Might cost a couple bucks, but you're definitely not going to blow anything up by doing it that way, and they might even be nice and do it for free. Originally Posted By Morgan321: The motor is 1100rpm. If a VFD does 0-400 hz does that mean it can run the motor up to 400/60*1100 rpm? That is faster than I need, if there an advantage to gearing the motor down? Ie will I get more power by running the motor faster? Yes- that means that your VFD could, theoretically, run that motor at ~7,000 RPMs. It's also a supremely bad idea to do that, because the bearings and cooling for that motor are based around 1,100 RPMs, and chances are that you're not going to be able to determine what the true redline of the motor is until you reach it. Think of it like this: your car will comfortable cruise at 3,000 RPMs on the freeway all day long, right? That doesn't mean you can push the rev limiter up to 21,000 RPMs, though. You'll probably be able to get away with something like 90hz with no real issues, but I'd start off with a good long run at 60 while checking temperatures at both ends and in the middle. Then I'd start bumping it up 5hz at a time--still checking temperatures--until I found a maximum set point that I was comfortable using. |
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A VFD is one convenient way to convert single phase to three phase. However, if you want to use the VFD for variable speed, the motor needs to be inverter duty. Older non inverter duty motors do not like to be run at frequencies other than close to the 50 or 60 Hz that they were designed to run. The name plate will tell you if it is inverter duty or not. Like the advice above, take it to a motor shop and have them test run it on their panel. That test will confirm that the motor indeed runs, and runs with balanced current. At the shop I work, we frequently test walk-in motors, and depending on the individual we may or may not charge them for the service. |
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I have half a dozen motors running off VFDs in my shop and none will actually run at 400 hz. In fact, VFDs that come preset with a high limit of 400 won't even start the motor until I reset it to a max of 200 or less. I basically program them all to have a lower limit of 10 and upper limit of 120. |
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Is the torque output fairly linear at various speeds? Ie. At 550rpm I'm not going to get 1/2 the torque of 1100rpm - or am I? I hadn't thought about cooling - i just figured if you can run it twice as fast you can get about twice the power! Seems best to gear it to run close to rated speed? The tag says "condenser duty" or something similar. It had a flange on it that looked like it could've been for a fan and was with some industrial refrigeration scrap so I think it was a cooling fan. Are the cheapest VFD on eBay even worth trying? |
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I buy whatever is cheap on Ebay and they've worked fine for me. I lost one to a lightning strike. I run a large surface grinder, 2 drill presses, a 3hp mill, two 1hp disc grinders and 3hp knife grinder with them. They're fairly constant torque but when torque is required I find 4 pole or 6 pole motors have an advantage over 2 pole particularly at low hz. If your motor is 1100 rpm that must be a 6 pole and should perform fine over a wide range of hz. I don't typically exceed 3600 RPM on any motor. If I'm building something from scratch i buy an inverter duty motor but I'm also running OEM non-inverter duty motors with no issue. If they have longevity problems due to voltage spikes and a lack of insulation, I figure getting what life out if them I can is worth it. |
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Originally Posted By Morgan321: Is the torque output fairly linear at various speeds? Ie. At 550rpm I'm not going to get 1/2 the torque of 1100rpm - or am I? I hadn't thought about cooling - i just figured if you can run it twice as fast you can get about twice the power! Seems best to gear it to run close to rated speed? The tag says "condenser duty" or something similar. It had a flange on it that looked like it could've been for a fan and was with some industrial refrigeration scrap so I think it was a cooling fan. Are the cheapest VFD on eBay even worth trying? I've never actually measured the torque at different frequencies, but I don't *think* that the torque is really linear. I'm pretty sure you're not going to get 50% of torque at 30 hz. Also- if your motor needs a condenser (edit: I may have had a brain fart here. I tihnk I might have read "condenser" and thought "capacitor" rather than "thing in a reefer with a fan on it") to start, I don't think it's going to work at all with a VFD. I could be wrong, but I've never had good luck with condenser motors and VFDs. As for cheap VFDs? You can get *good* ones pretty inexpensively. Hitachi makes some nice ones to run 3-phase motors from single-phase 220. I've got one of those on my mill. It's got 6 or 7 entirely trouble-free years so far, and I think it was $199 at the most when I got it. You can get Fuji relatively cheap as well. Possibly even an already-superseded Allen-Bradley Powerflex, too. |
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Originally Posted By jh1990: You will still have 100% rated torque below 60Hz but will not have all of the rated HP since the power output is essentially torque*rpm. Above 60Hz your theoretical power output will be 100% of the motor rating but your torque will begin to decrease. On something like a mill or drill, I rarely go below 30hz because it becomes more of a problem. |
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| It's been a really long time since I messed with a 3ph motor, but my rule of thumb was check the bearings and check continuity between the frame and each leg and between legs. Any continuity was no Bueno. Please correct me if I'm wrong. We didn't have them fancy vfd's, if we needed 3ph and only single phase 220 was available, we'd build a phase converter. I ran a Miller cp200 for years on a 7.5hp rotary I built out of junkyard sourced switches, relays and capacitors. We balanced the phases within 1% with run caps. |
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Originally Posted By ArimoDave: ...However, if you want to use the VFD for variable speed, the motor needs to be inverter duty. Older non inverter duty motors do not like to be run at frequencies other than close to the 50 or 60 Hz that they were designed to run. ... |
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Originally Posted By rwilkins01: It's been a really long time since I messed with a 3ph motor, but my rule of thumb was check the bearings and check continuity between the frame and each leg and between legs. Any continuity was no Bueno. Please correct me if I'm wrong. Infinite resistance from each leg to ground. No continuity at all. < 50 ohms between legs. Usually less that 10 ohms. All 3 should be the same. |
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