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'Soft starting' a DC motor

D

djgyro

Hi all, fairly new to messing about with electronics so please bear with me
I need a control device to in effect 'soft-start' a 24v DC permanent magnet motor rated at 375w and 22.5A and don't really know how to go about it.
If the motor is rated at 22.5A, what will it draw on start-up? To my thinking current will be higher than the run current as with AC?

SK Pang do a Pololu High-Power Motor Driver 18v25 CS, the link is below. I have contacted them but they are rubbish at replying and various searches on Wikipedia haven't helped me understand anymore about the electronics in the unit.
Do I need some extra equipment to 'program' this board or is it done with dip switches?

http://www.skpang.co.uk/catalog/product_info.php?cPath=159_161&products_id=662

If anyone has other thoughts about what I could use instead I would be very grateful, I assume PWM is the only way to 'soft-start' DC?

Rgds, Ben
 
D

djgyro

Hi,
The motor is to move a load weighing 600kgs but I would like it to have capacity to move 1200kgs. With help from a uni-buddy we have worked out that based on rolling resistance and the size of drive-wheel, it will require about 22Nm of torque to move the load. I have included the motor url below. I have asked the supplier for motor performance graphs but they have as yet failed to supply anything, even when they sent the motor!

http://www.alibaba.com/product-gs/324828797/24VDC_375W_DC_Gear_Motor.html

It has been pointed out that 375w at 12v is much less than 22a. Any ideas why the two do not correlate?

Rgds, Ben
 
poor mystic

poor mystic

Yes the current at start-up might be very large, especially if there is significant stiction. I believe (though I am not a motor specialist) that it is during start-up that motors are most often damaged, because there is little back-emf to limit current, as there would be once the motor is moving. So there might be lots more torque required at start-up, and hugely more current.
I hope a motor specialist comes along to help you with this one.
 
D

djgyro

poor mystic, yes the problem with back emf does appear to be critical to successful application. I am convinced that there is a unit out there that will do exactly what I want to achieve. RS advertise a circuit but apparently it is out of stock and may not actually come back into stock..........which is useful!
 
poor mystic

poor mystic

Hi again Digyro
It's a well-known problem, that the stiction part of the load cycle comes at the worst possible time.
I couldn't get anything from the link you posted, but it sounds like you've spent some real money already, and to protect your investment (and get the job done) I think it might be good to consult a mechanical engineer with experience in electric motor driven plant equipment.
 
D

djgyro

Think it will get to that poor mystic. Another electronics forum has suggested a PWM driver that should do significantly more than what I want it to do and is rated at 50amps so I'm going to invest in that, do some trials and see exactly what my start and run currents are and then I will have more defined parameters to either get something off the shelf or custom made.
Incidentally the SK Pang unit appears to need a module that sets the PWM frequency so I've ruled it out.

Thanks all for your input, gratefully appreciated

Rgds, Ben
 
duke37

duke37

The motor will have an effective resistance when under full load of about 1 ohm, If a resistance of about 0.5 ohm is placed in series, then the motor should still start. The resistance could be by-passed with a relay after a suitable time. Alternatively, A powerful MOSFET (IRF540?) placed in series with the motor could be gently turned on with a resistor feeding a capacitor. There will be significant power dissipated in the FET during start up unlike a PWM circuit and it may be necessary to use a heat sink.
 
R

Resqueline

djgyro said:
The motor is to move a load weighing 600kgs but I would like it to have capacity to move 1200kgs.
It has been pointed out that 375w at 12v is much less than 22a. Any ideas why the two do not correlate?
Click to expand...

Is the load linear or rotating, and what speed is it to attain? If linear load what pulley diameter, and if rotating mass what figure of inertia does it have?
The 375W is the shaft output (mechanical) power. Divide this by the 528W electrical power that the motor draws and you get a 71% efficiency figure.
 
D

djgyro

Ahhh thanks Resqueline, that makes sense, there is a 4-stage epicyclic gearbox between the motor and output shaft, the efficiency loss will be due to this.
The load is linear and I have calculated that the load requires less than 400N (using several 'worst-case' scenarios) to accelerate the load to its constant speed of approx 1mph. The pulley diameter is effectively 300mm so I calculated the motor would require 60Nm of torque.
I hope I went about all that correctly!

To be honest I looked more at the output shaft diameter of 22mm and reckoned allowing for a suitable safety margin, the motor looked 'beefy' enough to do the job!

Rgds, Ben
 
M

motronic

SS-1 DC Soft Starter

djgyro said:
Hi all, fairly new to messing about with electronics so please bear with me
I need a control device to in effect 'soft-start' a 24v DC permanent magnet motor rated at 375w and 22.5A and don't really know how to go about it.
If the motor is rated at 22.5A, what will it draw on start-up? To my thinking current will be higher than the run current as with AC?


If anyone has other thoughts about what I could use instead I would be very grateful, I assume PWM is the only way to 'soft-start' DC?

Rgds, Ben
Click to expand...

I can help you there. But looking at the date of this message I suspect the window of opportunity has well and truly passed. Check out our Web Site and see the icon for 'DC SOFT STARTERS' and it will take you to what you need.
Regards
Motonic.
 
J

johntucker

Hi Motronic,
what is the URL of your website?
I need soft start for a Motenergy ME0708 DC motor - a PWM unit that will allow speed control would be great. Can you help?
Cheers,
John
 
J

johntucker

I found the PUP20 - thanks for that.
The motor I wish to control is a Motenergy ME0708 at 48 - 52V. It will draw 200amps at full load - 10kW and 800 at startup. I will only need half power. Is there some way I could modify your product - like swapping its power mosfet with an IRFP2907 so that it could work in this situation?
 
M

motronic

Hi John, no, this model PUP20 is not designed for such large currents or that voltage.
We have another model that can do 48v but not at that current. (SDX50). Sounds like you need to talk to Curtis Controls for your need.
Cheers mate.
 
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