12 Volt power Supply

Phil

It must be the 'surge' current thats blowing the rectifier. If the motor draws 20Amps with a moderate load, then I bet it draws

You'll need a more suitable bridge rectifier.

Steve Balstone

Cheers for that , Sounds like that is the problem, is there any way to limit the intial current draw, a coil or somthing like that ?

Phil

Mmmmmmmm

To keep it really simple, you could build a crude 'soft start' circuit. Get a 0.5 ohm very high wattage (50W). Connect this in series with the transformer output and the rectifier ac input. Now put a high current (20A) switch ACROSS the resistor. To start..... Turn switch off, turn on mains, motor will start slowly then throw on the new switch after a few seconds Putting these on the AC side NOT the DC side and that will save the switch contacts from weilding themselves together!!

Steve Balstone

Cheers sounds like a nice easy way of getting it going, will put it on a rely and push button to short out the resistor, so cannot be started with the resistor shorted, and put a thermal switch on the relay so if button not pressed then unit shuts down before getting too hot

Cheers again

Phil

snipped-for-privacy@hotmail.co.uk wrote:

If you move that relay to the primary side, you will not need such large contacts. The resistor will need to be about 200 ohms, in that case. If left on, it could produce almost 300 watts of heat. But this reduces the contact current to about 1 amp.

The bridge rectifier shorts out any motor inductive current, so that is no problem. I don't that the transformer will generate anything damaging with the motor connected across the rectifier. The main power switch will have no inrush spike (caused by transformer core saturation from the last power off half cycle being in the same direction as the first one on power up) if there will be a 200 ohm resistor in series for some time period. The contact that shorts that out always has 200 ohms across it to act as a spark suppressor. I see no problems if the primary side contacts are rated for 5 amps or so.

I would rather see an X capacitor of a microfarad or so across the primary.

switches on the primary side of the transformer can be dirty and thus generate some high voltage spikes on the secondary side when switched. i guess the question here is, does this happen if you simply turn on the supply with no motor load on it? also, motors can be very inductive and having no capacitor in the circuit can generate some nice HV energy if you are getting arcs in the switching process. i would try 2 things., a small non polarized cap on the switch contacts and one on the secondary side ! something like a .1 ceramic with the proper handling voltage. and for the output of the bridge a larger cap to help suppress the pulse.

we had the same problem at work with a simple DC motor on a Varistat with a bridge rectifier. , some times just turning on the unit if the motor was on a light or heavy start, it would short the bridge. you may also want to look at getting some TVS diodes (transient voltage suppression), don't use MOV's because they only work x number of times before they short. diodes/rectifiers have 0.6 volt on the average cutoff of no current flow, this lack of load can allow for the xformer to generate some nice HV with noisy switches and out of phase switching. because the pulse width is generally short and wiring in your application after the bridge produces some nice inductive reactance compind with the motor, HV could simply be able to jump the diodes voltage limits and short for that moment. the only other thing i can think of is that you have a heavy starting load on your motor? if the motor starts quickly then the rectifier should handle that. i would at least try to use a 20 amp fast blow fuse on the output. if you blow that on start up, then you have way too much starting torque in which case you should have a bigger unit or current limited supply. but try the caps first. i think that will cure your problem.

say what you want, that was a cure we did to solve a long nasty problem that was given to us after to many times of that part of the equipment being down. used a set of HV probes on the scope to monitor this problem and it was exactly that. out of phase/noisy contacts on the primary side randomly generating HV pulses combined with motor the bridge didn't like. the other guys try to keep a note on that equipment to instruct the operators to make sure the variac was down to zero before starting ! , but like any one else that works in a production type job they don't give a crap about machinery. when testing with just the motor on line with no caps, i was getting aprox 2000 V or more pulse generated when testing across the bridged. that is the AC-DC leg of the bridge., remove the motor load and i only saw a aprox 1/4 of that which the bridge seem to handle ok. put the .1 mica caps on the switch, secondary side and one on the +&- output along with a small DC cap, switching noise gone, no HV over that which was suppose to be there. and that unit has been running with the same bridge for at least 3 or more years now.

if you read a lot of the instruction manuals for drives or sensitive electronics, they will tell you to not! switch the primary side of a transformer feed because of this problem. switching the secondary side is much better to avoid this damaging noise. higher end electronics have suppression components on the line to remove that noise but a lot of the low end simple mini drives do not go to far when protecting it self from this kind of damage in which case you should be using a set of contacts to feed the drive at its shortest point. many supplies also use series inductors/chokes with a cap to remove this problem.

Yes there is, as discussed. However, it may well be that the limited current will prevent your motor from starting. Then it's cook the motor, cook the circuit time.

Ed

On Thu, 22 Jun 2006 10:51:28 -0400, in message , John Popelish scribed:

This brings to mind an interesting story. I used to write and maintain software for automatic testing of electronic circuit cards. One particular card had a "soft start" circuit similar to this, where a pair of resistors were in the path at initialization, then shorted by relay action some milliseconds later. The original test software author neglected to program the shorting relays after turn-on. Cards were being sent to software as defective, with comments such as "smoked resistors," "resistors glowed red" and so on.

The resistors were hardy wire-wound types, so showed no signs of damage when the circuit was cold. The cards routinely were returned to the system with no defect noted. This started a circular process that took a bit of time to figure out!

On 22 Jun 2006 04:25:47 -0700, in message , snipped-for-privacy@supanet.com scribed:

I agree with what's been posted so far, that you could either beef up your bridge diodes, or create a "soft start" circuit at the primary. But I also notice that you haven't indicated that you have any filter capacitors as part of your design. I wonder if some smoothing at the DC output might be a good idea?

On Sat, 24 Jun 2006 00:18:31 GMT, in message , ehsjr scribed:

Yeah, this could depend on how much torque the motor is required to have at startup. If there was a way to limit the torque load until the motor is turning, i.e. a clutch on the load, this might both allow a soft-start circuit to work, and at the same time eliminate the need for it.

(snip)

He has an inductive filter. The motor inductance averages the effect of the voltage wave.

On Sun, 25 Jun 2006 16:32:49 -0400, in message , John Popelish scribed:

OK, something to study up on. Motors have always been spooky to me.

When you first start a motor, it draws a *huge* amount of current with respect to the current it draws once it is up to speed. At start up, you want to be able to deliver that relatively hugh current - and soft start does the opposite. The motor needs that current to get started properly. Restricting it with a soft start may cook the both the motor and the powersupply.

Build or buy a supply that can deliver the needed start current. Maybe an auto battery charger - some of them have a very high current spring return "start" position on the charge selector switch. You could wire in a relay that transfers for a second or two to provide that "blast" of current. I say maybe, because I don't know how those chargers provide that starting current, so further investigation would be needed.

Even better would be to use an auto battery which can supply hundreds of amps for the brief startup period. There's no maybe in that solution.

Ed

it seems to me that doing that would soak up some of those voltage spikes that the experts have been blaming for the rectifier failure. it probably doesn't need capacitors of a size to fill the valleys of the rectified AC, just something to soak up the spikes.

the experts were saying to put the capacitirs on the primary... I don't see why they couldn't go after the rectifier...

Bye. Jasen

(snip)

The motor torque is roughly proportional to the current, so limiting the current to no more than full speed current just slows the initial acceleration, unless there is some break away static friction or other starting torque peak added to the inertial one.

The O.P. said nothing about being in a hurry to get the motors moving.

You need some minimum torque to get the motor going, at all. If there is insufficient torque, it just sits there cooking. Full speed current is a small percentage of starting current, and isn't even a valid figure. An unloaded motor vs a loaded motor comes to mind. The valid figure is FLA - full load amps. Compare FLA with LRA. LRA (locked rotor amps) is the startup current - the current the motor draws when it is stationary - and can be huge with respect to the FLA (full load amps) - as much as 6 times FLA. And who knows how much greater it will be vs full speed amps, which is meaningless.

If the motor sits there drawing full speed current and not moving, there's no airflow, so not only is it drawing LRA, it is not being cooled by air movement its spinning would cause via the attached fan, or even just the armature if there is no fan incorporated with the motor.

The posts recommending soft start - and not pointing out the above - are playing fast and loose with the OP's motor. Soft start, *properly* applied, is a good thing. It may well be that something posted might work. But the warning that it could cause the motor to cook stands. The issue is not about getting the motor up to speed quickly, it is about avoiding the situation where the motor doesn't start at all.

Ed