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| Rewiring old 3 phase motor for low voltage |
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| David Jupp:
--- Quote from: loply on October 27, 2012, 10:27:15 AM ---Oddly, this particular motor doesn't have a cooling fan of any kind, which I found odd. It does have several kilos of metal enclosure though, perhaps they expect it to simply conduct away. --- End quote --- It probably has some small 'fins' on one end of rotor ?- which will act as a very crude centrifugal fan. I have a similar one and the fins really don't amount to much. As you say, all that iron will help! |
| Noitoen:
If you have to pay to get a motor rewound, you can ask them to calculate the winding to a even lower voltage than the original. A 1 HP motor 3~220V/50Hz, rewound for 3~110V/50Hz, will do, on a 2Hp/220V inverter, configured for a 110V/50Hz motor, 2Hp at 220V/100Hz, without any derating. It's usually wise to remove the fan from the motor's shaft and assemble a forced cooling fan. Motors have a linear V/Hz curve and although you can "over-speed" a standard 220V/50Hz motor on a regular 220V inverter, as soon as you overshoot the original motor's design frequency, if you don't increase the voltage in proportion, you will lose torque. |
| hopefuldave:
--- Quote from: BillTodd --- I had a go at finding the star point on my HLV-H speed control motor, a small 1/8 hp device. The windings were so tightly bound and bonded with epoxy it would have simply broken the copper wire if I had tried to pull it apart. In the end I choose to drive it in star mode at 415vac from the main motor's VFD In the case of my Hardinge 2 speed (dual windings) main motor, the rewiring would have been so complicated by extra wire, that it just isn't worth it; Stepping up the 240 mains to 415 with a transformer and using a high voltage (=cheaper on ebay) VFD was easier and meant I didn't need to touch the, awkward to get at , motor at all (nor did I need to disturb the motor's 0.0003" balance!) . Stepping up the mains voltage and using a high voltage VFD is easy :) Bill --- End quote --- Worse yet, my Holbrook has a 3-speed 415V motor with 9 connections (a Dahlander-wound BTH with a separate medium-speed winding - the 3-speed switch is a thing of Frankensteinian beauty)... I too chickened out of a rewound stator (and the local motor rewinders quoted rather more than I'd paid for the complete lathe) but rather than step up the mains I went with a "voltage doubler" mod to the 415V-input VFD (find the common point between the two banks of DC bus capacitors, move the Neutral there, away you go) and derated the VFD somewhat (5HP to 3HP as suits the motor). The motor (precision balanced like those in Hardinges, Harrisons etc.) seems fine up to 75Hz (giving an useful spindle speed increase from 1500 to 2250 RPM) and down to 25HZ, below which torque seems to start dropping off a bit - I've taken it as low as 5Hz (the "jog" speed I configured), which gives 2.2RPM in the lowest-speed gear/backgear/winding but wouldn't expect it to do much of a cut! Handy when setting up or toolpost grinding, though. The VFD I use (an old ABB ACS300 series) has a relay output that can be configured to indicate above/below a preset frequency and I've used this to switch *off* an external fan when it goes above 30HZ so it keeps cool at low speeds and cools down in idle periods when the motor stops - seems to work OK! Dave H. (the other one) |
| BillTodd:
--- Quote --- The VFD I use (an old ABB ACS300 series) has a relay output that can be configured to indicate above/below a preset frequency and I've used this to switch *off* an external fan when it goes above 30HZ so it keeps cool at low speeds and cools down in idle periods when the motor stops - seems to work OK! --- End quote --- Neat :) I've a ABB ACS300 series VFD on my mill - very nice device (albeit with an almost impenetrable manual! - although I did manage how to get it to measure [with an external sensor] and display spindle speed in RPM ) |
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