Limitations of overdriving a motor with a VFD - Page 1
Forum section: electronics / beginners

author: Leuven
author_rank: Regular Contributor
timestamp: August 07, 2014, 02:09:00 pm
content: I have a milling machine that originally came with a 3 phase 415V motor in Dahlander configuration (2 speeds - 1400 and 2800rpm).The fact that it is wound for 2 speeds, means it can only run at 415V. If I want to use a VFD at 240V mains, only solution is a rewinding the motor for 240V.The original specs are as follows:Speed 1 - 1400rpm, 0.6kw, 1.8ASpeed 2 - 2800rpm, 0.9kw, 2.3AThe goal is to replicate the original design, with the use of a VFD and a rewinding. So here are my options:Option 1:Rewind at 240V, 2800rpm. Judging by the original specs, say I get about 0.9Kw at 50Hz and 0.45kw at 25Hz. Even if I use torque boost on the VFD, the lower speed is still about 20% under spec. - probably worse if I consider that the new winding is 2 pole (less torque vs 4 pole to begin with).Option 2:Rewind at 120V, 1400rpm. Here I'm getting confused with the numbers (maybe somebody can help?), but in principle I will get standard operation at 50Hz and double speed, double torque, double amps at 100Hz. That is overdriving the original spec by a factor of 2. Some of this overdriving can be offset by uprating the winding wire to take more amps but obviously I am limited by slot space so it's hard to put a percentage on it.I don't consider a VFD with a step up transformer, they are much more expensive and rewinding costs me nothing.So I'm looking for the most elegant and efficient solution. If you were faced with this dilemma, what route would you go for? I'm particularly interested in option 2 so would be nice to hear from people with hands on experience on it. The electric vehicle folks are doing this on a routine basis to their motors.

author: KJDS
author_rank: Super Contributor
timestamp: August 07, 2014, 03:01:59 pm
content: Teco make VFDs with 240V in and three phases of 240V (which is 415V between phases) out

author: Leuven
author_rank: Regular Contributor
timestamp: August 07, 2014, 04:34:35 pm
content: Quote from: KJDS on August 07, 2014, 03:01:59 pmI'm not sure I follow, isn't that standard for all VFDs - 1 x 240V in and 3x 240V out? And how would you get 415V out of 3 x 240V? My original motor is 415V 3 phase.

author: IanB
author_rank: Super Contributor
timestamp: August 07, 2014, 04:54:57 pm
content: A three phase supply at 415 V line to line provides 240 V line to neutral.So any supply that provides 240 V x 3 in Wye configuration with a neutral also provides 415 V x 3 if you connect the load between the line conductors.It's therefore necessary to be precise about the description of the systems under consideration.

author: SeanB
author_rank: Super Contributor
timestamp: August 07, 2014, 05:23:10 pm
content: You do not have to rewind it, just connect the 2 half coils in parallel and then wire them in delta and use a regular 1kVA inverter to drive it. Then the inverter output will drive it happily and not kill the motor.

author: Leuven
author_rank: Regular Contributor
timestamp: August 07, 2014, 06:32:50 pm
content: Quote from: IanB on August 07, 2014, 04:54:57 pmDo you mind elaborating a little bit? How exactly do you connect between the motor and vfd, and what is the downside of this arrangement? What changes vs original plate spec - power, speed, torque, amp etc?On another note, VFDs don't like switches between them and the load and since I would have to switch between one coil arrangement and another, I'm not sure this option wouldn't be ideal. Unless I use 2 VFDs.Edit: I think I get it. VFDs voltage output is between the lines already.

author: Leuven
author_rank: Regular Contributor
timestamp: August 07, 2014, 06:35:38 pm
content: Quote from: SeanB on August 07, 2014, 05:23:10 pmHow would you do that, like on the attached diagram? The original configuration is delta for 1400rpm and double star for 2800rpm.What speed and power would I get with this method?

author: IanB
author_rank: Super Contributor
timestamp: August 07, 2014, 07:11:01 pm
content: Quote from: Leuven on August 07, 2014, 06:32:50 pmOK:In a conventional mains supply, the 240 V is measured between one of the phases and the neutral conductor, e.g. L1 - N = 240 VIn such a supply, the voltage between two of the phase conductors is 415 V, e.g. L1 - L2 = 415 VTypically a three phase motor will be connected to the three line conductors L1, L2 and L3.If there is a VFD, I imagine it will provide three output line conductors to be connected to the three phase motor.The question to be clear about is whether the output voltage from the VFD is measured between any pair of these line conductors? I imagine that is the case, but then it seems more likely to me (by pure assumption) that such a VFD would provide 415 V so it can be used with 415 V motors.Someone with more knowledge of actual VFDs would have to clarify.

author: SeanB
author_rank: Super Contributor
timestamp: August 07, 2014, 07:23:09 pm
content: No, most 220V VFD units provide 220V between phases, thus a motor has to be rated for 220V in delta or 380v in star, but must be connected delta for low voltage operation. 380V units the motor will have to be rated for 380V in either star or delta, and wired for 380V operation. I have not seen a 240V VFD that has an internal boost converter to allow operation with a 600v DC bus to drive a 380V motor.Just be careful with large rotationg loads and trying to stop them with the VFD without either a brake chopper resistor or some other energy dissipation method, otherwise you will trip the inverter on overvoltage on the main DC bus. I had to add one on an inverter that I needed to stop the motor in under 2 turns of the motor. It had to ramp up to speed in the same time, so the inverter ended up very much oversized for the motor.

author: Leuven
author_rank: Regular Contributor
timestamp: August 07, 2014, 10:58:47 pm
content: Quote from: IanB on August 07, 2014, 07:11:01 pmThanks for the clarification. There is a VFD with a step up transformer inside but much more expensive and not readily available. I certainly cannot get one for £50.Quote from: SeanB on August 07, 2014, 07:23:09 pmThe end load has no real inertia, just a cutting tool. In fact, the motor is running a gearbox before the spindle which somehow acts like a brake on its own. I think I can safely run this without a brake resistor.Is my diagram correct?

author: BillWojo
author_rank: Regular Contributor
timestamp: August 08, 2014, 12:57:26 am
content: What make and model of milling machine is it? 2 speed motor sounds European to me.BillWojo

author: Leuven
author_rank: Regular Contributor
timestamp: August 08, 2014, 02:16:00 am
content: Quote from: BillWojo on August 08, 2014, 12:57:26 amIt's a swedish made Arboga U2508. I kind of understand the design considerations, the 2 speed motor allows a lighter gearbox.

author: BillWojo
author_rank: Regular Contributor
timestamp: August 08, 2014, 04:43:30 am
content: I'm familiar with that machine, when I worked for Bridgeport machine tools as a service tech we had the Arboga line for a few years. Very nice machines but that motor is built into the head casting. That's why I wanted to know the manufacture to see if it was possible to substitute a different motor. Not going to happen. My suggestion is to look around for a transformer from ebay or a surplus house and build a rotary phase converter to change your 240 single phase to 240 3 phase and buck boost it. As an added benefit to having the phase converter is now industrial 3 phase machinery such as a small lathe can be run from it also. Machine tools are like electronics gear, you never have enough.A good source of info on phase converters can be found over at Practicalmachinist.com , my other hangout. They have a sub forum devoted to that stuff.BillWojo

author: geppa.dee
author_rank: Contributor
timestamp: August 08, 2014, 04:58:03 pm
content: Quote from: Leuven on August 07, 2014, 06:32:50 pmAnybody could please educate me on why this is ?I run a lathe and a mill, both with three-phase motors from the same VFD by switching each of the three output lines through DIN SPST 10A switches. The whole thing has been running just fine for a couple of years now?I also run the movement steppers on these machines by switching the outputs from just one set of controllers. Also works just nice.Of course, I never switch loads with either the VFD or the stepper controllers started and like I said... everything works just fine so far. What am I doing wrong ?

author: SeanB
author_rank: Super Contributor
timestamp: August 08, 2014, 05:19:25 pm
content: The problem is switching with a load applied and the inverter running. Typically one pole either makes first or breaks first, and the inverter sees that as a fault at best and trips out on imbalance, or you switch it on into a motor standing still and you have a massive current spike and blow the output block to pieces inside the inverter.

author: Leuven
author_rank: Regular Contributor
timestamp: August 08, 2014, 06:14:27 pm
content: Quote from: BillWojo on August 08, 2014, 04:43:30 amYeah, it's a nice solid machine. Came out of a school department, virtually new. They haven't even bothered to clean out that protective goo the factory brushed on, which you're supposed to clean as soon as you get ownership of the machine. Too bad it's such a headache with the motor. I'd love a bridgeport but the weight of that thing is a bit too much for me now.I'm not that found of phase converters for some reason, probably because I love VFDs too much

author: Leuven
author_rank: Regular Contributor
timestamp: August 08, 2014, 06:33:27 pm
content: Quote from: geppa.dee on August 08, 2014, 04:58:03 pmIt's one of those things that works in practice (with a degree of risk of course) but manufacturers strongly recommend against it. Sean explained it very well, and there goes your peace of mind. Now every time you're about to switch between those 2 machines, you'll be acutely aware you might send that VFD to a better place

author: geppa.dee
author_rank: Contributor
timestamp: August 08, 2014, 08:02:02 pm
content: Quote from: SeanB on August 08, 2014, 05:19:25 pmThanks for confirming what I logically suspectedSean. That the problem is in switching between loads with the outputs switched ON. I never do that.I remember reading when I first rigged the machines (didn't know about EEVB back then), this exact warning: "don't switch VFD outputs". It didn't seem to apply to switching the loads with the VFD stopped. What's more, there are VFDs that have multiple profiles for just this situation (different accel/decel curves and power limits). So I concluded that those giving the warning were either not into thinking things through or not into giving correct warnings.Quote from: Leuven on August 08, 2014, 06:33:27 pmFar from losing my peace of mind, it's been increased and my VFD is in the best place that it could ever be already. Bolted to my wall.Thank you both and sorry for the mini thread hijack Leuven.To stir it back, I can add that back in the day, I did exactly what Sean suggested on the first page. Both motors came configured for three phase 380V from the factory and I had to change them for three phase 220V operation by changing from star to delta connection in the connection box as my supply is 220V nominal and that's what the VFD puts out.

author: Leuven
author_rank: Regular Contributor
timestamp: August 08, 2014, 08:21:47 pm
content: Quote from: geppa.dee on August 08, 2014, 08:02:02 pmYes, that's the classic way of running a VFD at 240V but that option is not available to me. Because of the 2 speeds, I can't connect in delta at 240. At least not without fishing out more coil ends buried inside the motor winding. This is what I'm trying got figure out at the moment. Rewinding doesn't cost me anything, but my contact is abroad and before I pay for shipping back and forth, I'll be down £70. I'd rather buy some tooling with that money, if I can help it.