Soft start a 12V DC motor, max 250A, possible? - Page 1
Forum section: electronics / beginners

author: Bidda
author_rank: Newbie
timestamp: July 05, 2024, 03:03:54 pm
content: HiI have a boat (20ft) with an bow thruster that is to big, I want to soft start motor, aprox 2-3 sec.It is an 12V DC motor and in tecnical data it says max 245A.Is i possible to find this soft starter in stock, enyhvere??BRBrede

author: Benta
author_rank: Super Contributor
timestamp: July 05, 2024, 04:59:45 pm
content: Ah? Do you mean A? There's a hell of a difference.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: July 05, 2024, 05:23:51 pm
content: Even if it is A,(Amps), 245 is a lot.

author: themadhippy
author_rank: Super Contributor
timestamp: July 05, 2024, 05:38:56 pm
content: QuoteIs it? T'is not even 3KW

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 05, 2024, 05:45:19 pm
content: What type of motor is this? Does it not already have a speed controller? I would think that would be where any soft start/limited ramp up rate would be implemented in such a system.

author: johansen
author_rank: Super Contributor
timestamp: July 05, 2024, 06:17:07 pm
content: Find a 500 amp carbon pile battery load tester, with a knife switch to bypass it.

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 05, 2024, 06:23:38 pm
content: They make speed controller for bow thruster. A speed controller would be a soft start.

author: Bidda
author_rank: Newbie
timestamp: July 06, 2024, 12:40:57 pm
content: Quote from: Benta on July 05, 2024, 04:59:45 pmYes, A, my bad

author: Bidda
author_rank: Newbie
timestamp: July 06, 2024, 12:42:03 pm
content: Quote from: BrokenYugo on July 05, 2024, 05:45:19 pmNo speed controll, only on/off

author: Bidda
author_rank: Newbie
timestamp: July 06, 2024, 12:47:47 pm
content: Quote from: BeBuLamar on July 05, 2024, 06:23:38 pmYes, Sleipner have a set I can buy, it is expensice (2500 EUR). A softstarter should be less costs and work.

author: Bidda
author_rank: Newbie
timestamp: July 06, 2024, 05:35:12 pm
content: Quote from: johansen on July 05, 2024, 06:17:07 pmThx, can you make me a drawing of how you think? Link to equipment?

author: mikerj
author_rank: Super Contributor
timestamp: July 06, 2024, 07:41:42 pm
content: If it's a regular brushed motor you should be able to find something much less expensive e.g.https://www.4qd.co.uk/product/pro-160/

author: Bidda
author_rank: Newbie
timestamp: July 06, 2024, 07:57:03 pm
content: THX, Yes it is a brushed motor, I will contact them to ask for a solution :-)

author: johansen
author_rank: Super Contributor
timestamp: July 06, 2024, 10:41:40 pm
content: Quote from: Bidda on July 06, 2024, 05:35:12 pmcarbon pile load testers are a stack of about 25, 1/8" thick carbon disks 3" diameter. flat plates on both ends connect to the battery terminals.the adjustable knob tensions a spring which compresses the stack together. works from about 0-500 amps somewhat non linearly with time because as far as i remember, the carbon shrinks when it heats up.you could make your own using two flat plates, spring tension etc.. but instead of carbon disks you could strip the copper off of arc gouging rods and cut them and lay them like cordwood in an alternating cross pattern.or just buy 50$ worth of NTC resistors.

author: WokeJordon
author_rank: Newbie
timestamp: July 09, 2024, 07:42:09 am
content: Do you means Max 250mA?

author: Tim S
author_rank: Contributor
timestamp: July 09, 2024, 08:46:23 am
content: If it's too powerful, then elctronic PWM is one way to go, but you could always change the propellor for a smaller diameter or a finer pitch one. PWM and marine are not good bedfellows!Essentially the thruster motor is a DC starter motor and will easily pull hundreds of amps. In a yacht there is often a load battery next to the motor, long cable runs cause voltage drop when trying to pull 200A. If you have a battery next to the motor, try disconnecting the battery - the long cable run from the engine battery will act as a ballast resistor. Beware - the cables will get hot! If this is too much, then a delayed switching in of the load battery could work. When you press the 'thrust' switch, the motor can start on the long cable then a relay/solenoid can switch in the load battery after - say 2 seconds to save overheating the cables.Tim.

author: Bidda
author_rank: Newbie
timestamp: July 10, 2024, 09:20:07 am
content: Quote from: WokeJordon on July 09, 2024, 07:42:09 amNo, 250A is max

author: jpanhalt
author_rank: Super Contributor
timestamp: July 10, 2024, 09:44:33 am
content: Golf carts and other 12/24V vehicles give soft start. Investigate a controller from one of them.If you want to DIY, there are single chips like the old TI TPIC2101 to start with. Use that with a mosfet gate driver and several paralleled mosfets. I did that to control a 6V Ford long-shaft starter running at 12V. (That starter was used by Ford for cars, trucks, and tractors. It is old fashioned and not geared.)https://www.ti.com/lit/ab/slit110/slit110.pdf?ts=1720604361075&ref_url=https%253A%252F%252Fwww.google.com%252F

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 10, 2024, 07:30:10 pm
content: Quote from: jpanhalt on July 10, 2024, 09:44:33 amSoftstart? and not variable speed?

author: jpanhalt
author_rank: Super Contributor
timestamp: July 10, 2024, 07:34:04 pm
content: Quote from: BeBuLamar on July 10, 2024, 07:30:10 pmIsn't that what the TS asked?

author: Siwastaja
author_rank: Super Contributor
timestamp: July 11, 2024, 10:53:06 am
content: Quote from: jpanhalt on July 10, 2024, 07:34:04 pmIn this context, it's a feature name (not a product class name), and a meaningless meta-feature name. It's just that customers are calling motor controller manufacturers: "but does this have a soft start?" Instead of explaining: "In context of current-regulated (torque-limiting) motor controllers, there is no concept of 'soft start' at all, soft starting is an implicit side effect in limiting torque in all other edge cases (startup being only one) too", they found out it's easier to write "built-in soft start" to stop stupid questions.(Or, it could just mean that the controller handles precharge internally.)"Soft start" as a product can mean anything, but is usually some relatively simple and usually very inefficient device for rough current limiting in some cases (often limited to some fixed time during startup only). For OP's case, due to the sheer power level required, a real motor controller is definitely the best answer. The defining factors for real motor controller is ramping motor current in triangular waveform by chopping the input DC voltage on/off while providing freewheeling path for the current during off-time; and sensing motor current so that the duty cycle can be adjusted accurately for current (hence torque) limitation; important for protecting the controller (its semiconductors) itself, but also to protect the motor and mechanics. This active current limitation works at nearly 100% efficiency and is what provides the "soft start", but is not limited to working during "start" only.Golf cart and forklift controllers are well available in this power rating; see Curtis for a typical manufacturer which AFAIK makes decent products.12V 250A is not that weird for a motor, although 24V 125A motor would be at a sweeter spot. But remember, motor voltage and controller input voltage are two different things; once you get a decent motor controller and configure it correctly, you can use e.g. a 24V battery instead, halving the battery current (while retaining the motor current), which is great if you need wiring longer than a few feet; the controller would be placed closest to the motor, then.

author: SteveThackery
author_rank: Super Contributor
timestamp: July 11, 2024, 01:01:56 pm
content: Quote from: Siwastaja on July 11, 2024, 10:53:06 amThere is a learning opportunity here for me! I don't really understand what you are saying in the above paragraph when you imply that soft starting isn't even a real thing in current-regulated controllers.To my mind, a soft start is when the motor increases its speed from 0rpm to its normal running speed over a controlled period of time. Imagine a chart with time along the x axis, motor speed along the y axis, and the required speed vs time curve is plotted on the graph. It is themotor speedthat is of primary interest, rather than the motor torque. Of course the torque determines the rate of speed increase, and the torque is controlled by the current in the motor. So obviously controlling the current controls the acceleration of the motor.As I see it, though, the current isn't the thing of primary interest - it's the motor speed. So I envisage a control circuit whereby the instantaneous speed of the motor is monitored (NOT the motor current), and the current is adjusted in real time to give the required motor speed at every point in the start-up curve. Depending on the load on the motor, different soft starts might require different current ramps in order to achieve the wanted speed ramp.This sounds to me like a distinct function that does not automatically "fall out" of a constant current controller. It requires a couple of extra things: a speed feedback from the motor; and an internal representation of the required start-up curve.Obviously a constant current controller has thepotentialto provide a soft start just by limiting the current in the motor. But surely when you want aspecificstart-up curve you need the extras I've described in the above paragraph. The controllermighthave the facility for a speed/time start-up curve to be stored internally and invoked at motor start-up, but equally it might rely on external circuitry to do that and send the appropriate control signals to the motor controller.Now, I'm an amateur and a hobbyist, so I might be completely wrong about all of this. If so, could you explain a bit more about your thinking behind your statement?Thank you! I always appreciate people taking the time to teach us beginners.

author: Ian.M
author_rank: Super Contributor
timestamp: July 11, 2024, 01:19:35 pm
content: All brushed DC motor controllers have to be capable of handling the locked rotor stall current of the motor as it will initially draw the same current when starting from stationary. For all except the smallest motors it is uneconomic to size the output transistors to handle the stall current, so dynamic current limiting is almost invariably used. The current limit directly determines the max. torque and thus the max. acceleration at startup. As Siwastaja explains above, that inherently and unavoidably provides a soft start characteristic. It may not be the soft start characteristic you want, but once you have a motor controller, it doesn't take much to low pass filter the low voltage/energy control signal if you want to further reduce the motor acceleration.

author: jpanhalt
author_rank: Super Contributor
timestamp: July 11, 2024, 02:38:36 pm
content: I think the context of the TS is clear. He does not want the bow thruster to turn on at the full set speed.QuoteI used that chip with an LT1158 for gate driver, and it worked as expected. (See attachment for Figure 5.) Start-up can be set slower than needed by the output mosfets), and you can set the target PWM for the brushed DC motor. It is not simply a marketing ploy, and it is a valid search term both in datasheets and on Google.I don't disagree that one purpose for a soft start is to avoid high inrush current, but you can also size the output mosfets to handle that. Another purpose is to provide a smoother start for whatever is being controlled. Apparently, the TS was able to start the bow thruster without soft start (or he didn't notice it happening), but still wanted a more gradual application of power.

author: johansen
author_rank: Super Contributor
timestamp: July 11, 2024, 07:01:09 pm
content: does anyone here really think OP can diy a 250 amp PWM motor controller?most of the time they have no idea how much inductance is in their battery bank and they blow them up at turn on, then when they add capacitors, it isn't enough and they blow up a second set of mosfets. i've seen this more than 5 times in a row.mosfets often fail ON and op doesn't need this on a boat.stick to simple rhreostat.

author: jpanhalt
author_rank: Super Contributor
timestamp: July 11, 2024, 07:21:04 pm
content: Quote from: johansen on July 11, 2024, 07:01:09 pmI did. It was my first project when I got back into the hobby in my late 50's. The aforementioned controller for a 6-volt long-shaft Ford starter at 12V drew a lot of amperage (not legal for contests). It was nowhere near adequate for continuous use, but quite capable for winch launching sailplanes of several kg and wingspans of 4m.

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 11, 2024, 08:20:55 pm
content: I suggested a motor drive (speed controller) but the OP said that it's too expensive and he only wants a soft starter. While I don't know of a soft starter for 12VDC motor I have used them for 480VAC 3 phase motor often. The application of soft starter is for the case when you would want to run the motor at full speed all the time and just want to limit the inrush current. Golf cart and most battery operated vehicles have motor drive to control their speed. You wouldn't want a golf cart to run only at full speed do you?So when someone said to use a controller from the golf cart I asked do golf carts have soft start and not speed controller?

author: jpanhalt
author_rank: Super Contributor
timestamp: July 11, 2024, 08:42:17 pm
content: I am not above scavenging junk yards, aka recycling centers. I still think the TS's best option is the controller from a used or junked golf cart. Here's the very first hit on ebay for golf cart controller:https://www.ebay.com/itm/276545575375?itmmeta=01J2HRC589DN8WVMGVQ5EC246T&hash=item40636699cf:g:Jx8AAOSwPABmjmqn&itmprp=enc%3AAQAJAAAA0CuaI5rCM41oRaddcR%2B8amB1BhH4N8HiXokUhC3Y3pPz0wNwsnjOdLrEm51eFIc7NaCEAzhyVeYLAsQ350q0lP1yTAZItSEFg4ErHBYiTjXd%2BfhNelfyNBXm6pkjHlxs%2BoBDdFIVHymItt4mUG8WShJQOrkgDiriAZXjIfZxn%2BUfWnTWYb9F894d3V2hAIbZ3wfV0OALQOaO1K9jI6CRxUKouzoraUTG9QrMI7fKehwcXbQ1cpMMZqbGG1P6c2YDQCdJSmDF%2B908hrGB25C8z1U%3D%7Ctkp%3ABk9SR5jUsLiUZAThat's new and less than $200 USD. What would one from a junked cart cost?Edit: Spelling

author: Smokey
author_rank: Super Contributor
timestamp: July 11, 2024, 09:21:42 pm
content: The initial inrush on a brushed motor is huge. It (often) quickly (25ms ish) tails off to the run current based on the load. Go put a current probe on a car starter and watch that on the scope when you crank over your engine. It's shocking the first time you see it.The "simple" solution is you need an actual motor controller. Something that does torque (current) control at a minimum. This will limit the huge inrush peak until the thing gets spinning by actually regulating current. Measure your "run" current and size the controller for that, not the uncontrolled peaks.There is some stuff in the EV space that would probably work. Check these out.https://kellycontroller.com/controllers/brushed/