3-phase AC motor soft start - Page 1
Forum section: electronics / projects

author: rodrigopires
author_rank: Regular Contributor
timestamp: January 31, 2016, 10:15:58 pm
content: Hi everyone! A friend of mine is building a wood chipper and he need a soft starter for the 2hp 3-phase motor. I've seen some circuits using caps and SCR's. Any circuit ideas and recommendations? Thanks

author: ConKbot
author_rank: Super Contributor
timestamp: January 31, 2016, 11:19:35 pm
content: As far as I know, most small motors like that would be direct online starting. A motor starter for that would just be a contactor with a thermal/magnetic breaker with a delay, which would trip if the motor takes too long to start.If you have a heavy load that can't be disconnected mechanically (heavy drum with teeth I'd imagine? ) then a vfd would be a good option if you need to ramp up.

author: AlfBaz
author_rank: Super Contributor
timestamp: February 01, 2016, 12:17:22 am
content: If the motor has 6 leads out, you could implement a star(wye for yanks)/delta starter

author: promacjoe
author_rank: Regular Contributor
timestamp: February 01, 2016, 03:14:42 am
content: first a question, Does your friend have three-phase available. Or are you trying to convert a single phase to a 3P. I know this is an engineering form, and you may want to make it yourself, but considering safety is an issue, I suggest using one of the variable frequency drive units that are available. Westinghouse Hitachi and others have great VFDs that can do exactly what you're asking along with having several safety features such as emergency stop and Breaking. They are available for single and 3 phase power input. although it would be a great learning experience, considering the expense, it is worth looking at these as well.also note: if you use a variable frequency drive, make sure you do not run it over the manufacturers specified RPM.

author: iromero
author_rank: Unknown
timestamp: February 01, 2016, 03:17:01 am
content: +1 on the star/delta starter (if even necessary 2HP is a pretty tiny motor). it's a pretty robust and simple solution. You usually wanna go with soft starters if you have many motors starting and stopping all the time and you can't tolerate the big starting currents. If you must use a soft starter you'll probably have a better time just buying one, doesn't need to be Allen-Bradley; ABB, Siemens and other brands may offer pretty affordable ones that have all the safety features and reliability built in, i've found cases that buying the soft starter was cheaper than the contactors and timer relays needed to implement star/delta. You might even find VFDs affordable.

author: johansen
author_rank: Super Contributor
timestamp: February 01, 2016, 03:42:19 am
content: Quote from: promacjoe on February 01, 2016, 03:14:42 amwell, you can't, it will be frequency limited. some stop at 120, 200, 240 or 400 hz. very few go to 600hz.A woodchipper at 4 times standard rpm, would be very interesting..anyhow, you don't need a soft starter for a 2 hp motor.

author: promacjoe
author_rank: Regular Contributor
timestamp: February 01, 2016, 04:22:09 am
content: in the US, The line frequency is 60Hz. A VFD normally range between zero and 120Hz. although I'm sure that there are special application VFD's that go that high.But we are not talking about a centrifuge used for nuclear fuel. were talking about a wood chipper. Funny thing about a wood chipper. When you drive them too fast, they tend to fly apart. Throwing parts everywhere. I read a story about a company that had a part come through the roof. at first they thought it was off an airplane since they were close to the airport. When they contacted the FAA to investigate it, they could not identify the part. Someone finally identified It as being a tungsten carbide chipper cutter. As it turned out, a tree company was using their industrial chipper down the street. When it came apart, It through the cutter up into the air and It landed on the roof of the factory. fortunately it did not injure anyone in the factory. I have no idea if it injured anyone else.

author: CaveMannDave
author_rank: Regular Contributor
timestamp: February 01, 2016, 05:16:04 am
content: Hi:In reality, 2HP is puny, as these things go. About the only motivation for soft-starting (other than your own desire to play around with the design, and have to replace undersized modules frequently), would be if this needed a VERY long, undersized portable cable (voltage sag), had some enormous rotating inertia (excessive LRA vs time), or that the motor / belting design was substandard for the load (different motor designs/classes for different load characteristics), or if the supply is only single-phase.When I worked @ a TI plant, in the 70s, the Powers-That-Be had specified Texas Instruments prototype solid-state motor starters (evidently as new product R&D) on most of the Plant's ventilation blowers. Even though conservatively rated, the Thyristors didn't seem to like living, and we went through a whole lot of replacements.If you MUST have soft-start, the Wye-Delta arrangement works well, or if the winding/lead arrangement precludes that, look into Resistor types.Both types require 2 Contactors, an overload relay, Timer relay, and misc. (enclosure, disconnect, fusing, control), and 3 Power resistors, or 3 extra wires. These things, available off-the-shelf, probably cost less, last longer, and are safer than anything you are likely to cobble together, have been time-tested, and are Certified and Approved, so insurance will be OK.They only LOOK expensive if you don't see the Time-Value-of-Money, or the maintenance cost of 1-off Prototypes. [Been there, Done that, Had to sell the tee-shirt to eat] Yeah, Yeah, it's LOTS of fun to roll-ur-own, but do you want to be Married to this thing, or really Help the guy?It's exciting as a pastime, but if you sell yourself cheap, you’re treated like a Whore.HTHCheers,Dave

author: Paul Moir
author_rank: Frequent Contributor
timestamp: February 01, 2016, 05:55:29 am
content: A 2hp VFD with a bunch of helpful bells and whistles such as overload detection is $164 with free shipping from automationdirect.com. To my mind, that's going to be impossible to beat in a DIY.

author: rodrigopires
author_rank: Regular Contributor
timestamp: February 01, 2016, 08:13:24 pm
content: Quote from: promacjoe on February 01, 2016, 03:14:42 amno he has three-phase output, if it was single phase it would be easier... As far as the VFDs, well that is very expensive and since the thing here is to make a homemade machine buying a piece that is even more expensive than the machine itself wouldn't make sense.Quote from: CaveMannDave on February 01, 2016, 05:16:04 amI get what you'r saying, but it not my project. I tell him about the VFD.So if i use the Wye-Delta arrangement i can power the motor with single phase, then can i use something like this diagram?Thanks!http://agonesyah.blogspot.pt/2014/04/adding-soft-start-to-water-pump-motors.html

author: AlfBaz
author_rank: Super Contributor
timestamp: February 02, 2016, 04:39:04 am
content: Quote from: rodrigopires on February 01, 2016, 08:13:24 pmYou cant run a 3 phase motor with 2 phases. The windings are physically distributed 120 degrees apart so it'll growl like a bitch and burn out.I'll ask again. Are there 6 leads out of the motor windings onto a terminal block? If not you'll have to use a soft starter. They come in contactor size and can usually fit where the main contactor or switch sits. If you only have 3 leads out then you're stuck with a vfd.Other options are available for soft start such as series resistance that is shorted out after a time delay but the kilowatt rating of the resistors plus protection in case the shorting contactor doesn't come in thereby frying the resistors will probably end up costing as much as a pre-bought VFD or soft starter. I saw a 1.5kW Siemens softstarter on RS components for about $300AUD (about $2 US) so you should be able to find something for half that price elsewhere

author: rodrigopires
author_rank: Regular Contributor
timestamp: February 02, 2016, 06:16:44 pm
content: Well... There's been a twist. Turns out he already has a VFD laying around in his shop, but its monophasic. Can he convert the three-phase motor to Delta and ,using single phase, connect it to the VFD? Also its a watt drive L2000-007 NFE.

author: cs.dk
author_rank: Unknown
timestamp: February 03, 2016, 04:50:32 am
content: Quote from: rodrigopires on February 02, 2016, 06:16:44 pmLooks like it's a 0,75 kW drive, it won't cut it. You can't overload a VFD that much, it'll simply cut off and throw an overload error.2,2 kW for a chipper sound like a really underpowered one. I've been working a place where we make chippers, 15 kW was the absolute minimum we did use on the smaller ones, and they were geared down. It was slow rotating chippers ~150 rpm at the rotor. For faster chippers, i haven't seen some with under 22 kW motor.

author: ndunnett
author_rank: Newbie
timestamp: February 03, 2016, 10:21:22 am
content: If you are going to use a 3 phase motor, regardless of other variables you need to feed it 3 phase due to the physical arrangement of the windings, otherwise it will stall and let all the smoke out. Star (wye) and delta are types of 3 phase winding connections, so no, that still won't allow you to supply it with single phase. Since you mentioned the cost of a VFD is too much, I'll support what AlfBaz is saying: how many wires do you have in the terminal box? If it is only three, then it must have an internal star connection, so you will be limited to series resistance starters and other soft starters of the like. If it has six, you will be able to use a star-delta starter and that will probably be your cheapest/simplest option. Have you considered looking for a single phase motor instead?

author: rodrigopires
author_rank: Regular Contributor
timestamp: February 04, 2016, 04:48:10 pm
content: yeah, a single phase motor might be the best option...