Soft start for 220v electric motor - Page 1
Forum section: electronics / projects

author: dr_p
author_rank: Frequent Contributor
timestamp: November 10, 2012, 07:23:36 pm
content: I know there are soft start modules available, but i want to build my own. I googled around for schematics, but no real luck.I know of the seriesresistor/relaytype of soft start, I'm not looking for that. I want to have a module that you can plug a 1...3kW electric tool/motor into. Because I want to keep the tool's power switch and control, but get the soft start every time.I made some back-of-the-envelope calculations for a permanent seriesresistorto limit the peak current. It doesn't look too good - too much power dissipated in normal use(after start), too much voltage drop.Considered using a seriesPTC, but I guess after the first start, it would get very hot, and the second start would be hard, on full power. Consider the tool an angle grinder, so you have an idea of the usage cycle.As far as I can understand, the initial current surge is because the coil of the electric motor acts initially as a resistor, before there is a magnetic field to limit the current through it. So trying to limit the current peak by adding a seriesinductorwill most likely do nothing.I'm trying to avoid complicated current sensing, time controlled thyristor circuit. Or at least is seems complicated now.Any ideas?

author: Kremmen
author_rank: Super Contributor
timestamp: November 10, 2012, 08:53:46 pm
content: Proper soft start units do contain SRCs or triacs and control electronics to drive the firing angle and thus the supply voltage. I think a triac is the simplest switch element you can select but you would still need some electronics to detect the supplyphase and fire the triac according to some starting/aceleration scheme.

author: ejeffrey
author_rank: Super Contributor
timestamp: November 10, 2012, 09:27:43 pm
content: The inrush current is due to the lack of back EMF. An induction motor is really a transformer with a moving secondary. With the rotor stationary , you only see the resistance and the leakage inductance, which are both small. Once it spins up you have the back EMF generated by the rotating secondary. A permanently inline inductor would indeed limit the current, but would also limit the torque, so I wouldn't really recommend it. A proper soft start should, as the name implies, only work when starting, and do basically nothing while running. For that you need a switching element. Either simple passive switch like an NTC (not a PTC!), a resistor/inductor + relay, or (the best solution), a triac with a controller that gradually increases the conduction angle. The simple solutions might be fine for something like a power transformer, but for your application I think you need the more intelligent controller.I am not an expert on motor control by any means, but I don't think it needs to be that complicated. You are driving a normal AC motor, not a BLDC motor, and the motor is capable of handling the full inrush. Also, you will presumably always start the motor at no load, so you don't have to handle widely varying conditions. I don't see why you couldn't get away with a circuit that just detects startup and increases the conduction angle to 100% smoothly over a fixed number of line cycles. You might need to tweak them minimum conduction angle and the ramp-up time to get optimal behavior for your tool, but other than sensing start/stop, you don't need to do any fancy sensing, and even if you don't have the 'optimal' ramp-up, you know the motor can take it. Again, don't take my word for it: do some research on soft-start motor control or hope one of the guys here who knows more about power electronics chimes in -- but this seems pretty straightforward to me. Obviously all the normal considerations about not working on mains voltages unless you know what you are doing apply.

author: dr_p
author_rank: Frequent Contributor
timestamp: November 11, 2012, 02:26:35 am
content: Thank you for your replies. OK, seems I have to roll up my sleeves, build a circuit and pop some fuses in the process. I found a schematic for variable speed. So how do I modify it into a soft start?So far I get this: on every half cycle, C1 gets charged faster or slower depending on R1 value. When the voltage on C1 is greater than ~30V, the diac breaks down, and conducts a current into the gate of the triac, witch remains on until the next zero crossing.What I don't get:- why use the diac and not directly connect the triac to R1-C1?- R2-C2 is supposed to protect the triac from fast voltage rise when in turns off. Is this necessary only when driving inductive loads?Now for modifying it to serve my soft start purpose:How about a diode in series with a fixed high enough value of R1. This should allow C1 to slowly charge, but never discharge, therefore increasing the voltage across it, and thus the conduction angle to 100%. Then maybe a 10Mohm across C1 to discharge it when the circuit is off.

author: Kremmen
author_rank: Super Contributor
timestamp: November 11, 2012, 07:53:09 am
content: OK, answers:- the diac is there to prevent premature ignition of the triac. Especially if the triac is what is called a "sensitive gate" one, the triggering current may be miniscule. The diac holds the current off until the RC circuit voltage is above the diac breakdown, and this creates a well defined ignition threshold for the triac.- The second RC circuit across the triac is there to protect from excessive dv/dt across the main terminals. The reason is this: if the voltage across the main terminals rises very fast, the gate in some cases cannot float/follow at the same rate and is "left behind". This is equivalent to a gate voltage pulse and will cause spurious triggering. You definitely don't want that. You should check from the device datasheet what the value is for max dv/dt and design accordingly.I am afraid your control scheme is too simple and won't work. You see, a triac is a kind of SCR or thyristor, and with these devices the voltage control is based on the phase angle of mains voltage. This in turn means that you need to generate the firing signal for each half-wave individually.In your scheme there will initially be no conduction until the diac threshold is achieved. Then the triac will fire. However, the control voltage in the cap will maintain its level (and in fact rise further) so when the ongoing half-wave ends and the next begins, the diac is already conducting, firing the triac at 0 phase angle i.e. full voltage. This will continue indefinitely. So, what you have created is not a soft start, but a delayed hard start.Referring to your circuit, the proper way of course is to decrease the value of the adjustable resistor until full voltage is achieved. The cap must discharge and recharge for each mains half-sine wave separately and this creates the relative complexity of the soft starter circuits. If you google some commercial implementations e.g. from my employer back in the day, ABB, you will see that they are "proper" gadgets, not just a couple of components soldered together. You will of course manage with less, subject to your specific requirements, but there will be some designing to do.

author: dr_p
author_rank: Frequent Contributor
timestamp: November 11, 2012, 08:37:37 pm
content: Great explanation! It's clear now and I see where i was wrong.But my quest is getting complicated because I don't fully understand AC and related elements, and I should in order to design the circuit properly. I'm clearly in over my head here, so I'll bail out. I don't think working with kW of power on mains potential is the way to learn the basics.I will look for more info on the web, build some more knowledge, scale it down and try again.Thank you for your involvement.

author: SeanB
author_rank: Super Contributor
timestamp: November 11, 2012, 08:49:53 pm
content: There was one on Silicon Chip a few months ago.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 12, 2012, 03:10:17 am
content: What about a high power dummy load (like a hair dryer), a large relay, and some sort of current sensor? The dummy load and relay would be connected like in a usual soft start circuit, but with the extra current sensor switching the relay. The relay would be open if there's no load, but as soon as the load is on, the current sensor turns on the relay. The delay is determined by the sensor and the relay. Note that such a circuit would only work where the motor has little or no load at starting.

author: Zero999
author_rank: Super Contributor
timestamp: November 12, 2012, 12:34:09 pm
content: Quote from: dr_p on November 10, 2012, 07:23:36 pmHow about using a current activated relay with normally closed contact shorting a resistor?When the power is applied the relay will open immediately so the current will have to flow through the resistor, once the current declines below the relay's threshold, it will close, bypassing the resistor.QuoteYou've got that the wrong way round. You'd need NTC, the resistance drops, as it gets hotter and is used as a soft start. PTCis used as a fuse.

author: dr_p
author_rank: Frequent Contributor
timestamp: November 12, 2012, 01:52:33 pm
content: @SeanB:I found the article in Silicon Chip, but I'd have to pay to see it.However, this application note from STM looks promising:http://www.datasheetcatalog.org/datasheet/SGSThomsonMicroelectronics/mXyzxsu.pdf@Hero999:I meant NTC. Resistance decreases as current increases. But I scrapped that ideaI don't know about the relay. Flashes, arcs, mechanical shocks, and same current sensing circuit as in the article above. Solid state looks more reliable.

author: Zero999
author_rank: Super Contributor
timestamp: November 12, 2012, 02:17:36 pm
content: Quote from: dr_p on November 12, 2012, 01:52:33 pmThat's a generalisation.Solid state is better in some cases and inferior in others.The relay is simpler to implement (no PCB required, just two components), is less sensitive to temperature variation and over voltage but a relay is sensitive to vibration and the contacts wear out.The solid state solution is mechanically more robust (no moving parts to wear out) but it's more sensitive to over-voltage and high temperatures.

author: dr_p
author_rank: Frequent Contributor
timestamp: November 12, 2012, 02:34:36 pm
content: Quote from: Hero999 on November 12, 2012, 02:17:36 pmI don't understand, how do you intend to use just the resistor and the relay?

author: quarks
author_rank: Frequent Contributor
timestamp: November 12, 2012, 03:17:48 pm
content: Because you are looking for a soft start 220V. A very easy way could be the ELV ESB 54, but I do not know if it is available in your country. If it is available, there should be a downloadable description with schematic or you can buy a whole kit.I use a few of them my self and for me they work very well.

author: Zero999
author_rank: Super Contributor
timestamp: November 12, 2012, 09:40:54 pm
content: Quote from: dr_p on November 12, 2012, 02:34:36 pmAs I suspected, you didn't understand what I was saying.A current relay is activated when the current flow through the coil exceeds a certain level. The coil is connected in series with the load, rather than in parallel. You could make one with a small relay and a current transformer.How much current does the motor normally use?

author: T4P
author_rank: Super Contributor
timestamp: November 14, 2012, 05:48:33 pm
content: I'm not sure if he said 1 or 3kW but 3kW is quite a lot for a angle grinderI guess about 4-5Arms

author: dr_p
author_rank: Frequent Contributor
timestamp: November 14, 2012, 07:39:16 pm
content: The angle grinder needs 1200W, but I have other tools around that go closer to 2kW.So 10A as a rough figure.