12VDC Motor Control Problem

  • Thread starter Kirk S. Hegwood
  • Start date
K

Thread Starter

Kirk S. Hegwood

Hi All,

Being a "simple" system integrator who normally uses stock parts, I've come across a problem. I'm developing a machine which requires using 12VDC control voltage (no problem - using a 12VDC switched power supply suitable to 12Amps) and a 12VDC chemical pump. Don't know why, but I thought the power supply would be sufficient to drive the motor, it wasn't. The start current seems to be overloading the power supply. Does anybody know of a simple and inexpensive solution for this? Do I use some type of H-Bridge or
MOSFET configuration between the power supply and pump?

Thanks in advance.

Kirk S. Hegwood
President
Signing for Hegwood Electric Service, Inc.
[email protected]
 
R

rednaxela 66

Hi!

You might want to increase your amperage because you have two items that needs the power. The 12VDC motor and the 12VDC pump. :) The initial charge that seems to overload the gauge but it'll be fine.
 
K
>would be sufficient to drive the motor, it
>wasn't. The start current seems to be
>overloading the power supply. Does anybody know
>of a simple and inexpensive solution for this?
>Do I use some type of H-Bridge or
>MOSFET configuration between the power supply
>and pump?

I ran across a similar problem with a switched
power supply once. The power supply would try
to supply an overload current on startup (greater
than rated), then shut down for its own self
protection. It would wait for a short period of
time, reset, and repeat the cycle, since it could
not get through the startup peak. The earlier
linear power supplies had no problem since they
would simply saturate, supplying limited current,
until they got through the peak demand instead
of shutting down.

The "smart" power supply was too smart for its
own good. The "dumb" linear power worked as
expected.

You may want to try to find a vendor whose
supplies simply saturate at the rated current
rather than trying to supply overload current
for a limited time.
 
W
Kirk,

Digikey has some surge limiting devices that might work, and you may even solve the problem with a (for example) low-ohmage resistor (less than one ohm). The problem is that the switcher shuts down almost instantaneously from too much current; I've seen this happen with some switcher-based solenoid control circuits, which may draw several hundred amps of current (but only for a microsecond or so) upon powerup.

You could also use an analog or even an unregulated 12V power supply.

Regards,

Willy Smith
Numatics
Costa Rica
 
B

Brian Kukulski

Try putting a 12 VDC battery in parallel with the power supply. the power supply will keep it charged, and the battery will provide the inrush
requirements.
 
C
With larger motors, it's common to use a "soft-start" circuit to prevent this kind of problem. A motor that isn't moving is almost a short circuit, with just a very low winding resistance impeding current flow. Once it is spinning, the "back EMF" voltage it generates keeps too much current from going through.

A soft-start circuit basically consists of a current-limiting resistor in parallel with a switch (either solid-state or hard contact). These two are in series with the motor winding. At start-up, the switch is open, and the extra resistance of the resistor keeps too much "in-rush" current from flowing. The circuit also has some kind of sensor -- usually measuring the voltage across the motor -- that decides when the switch can be closed, effectively taking the resistor out of the circuit.

I don't know if there is anything commercially available for this low voltage. It's worth a look, though.

Curt Wilson
Delta Tau Data Systems
 
M

Mark Salerno

I am using a simple low side drive mosfet switch to control two types of pumps. One at 15 amps and one at 25 amps continous. Two tricks

1 - Use a simple unregulated power supply- transformer bridge and large cap.

2 - Very low RDS .001 on mosfet optimized for 5 volt gate operation. ST thomson maks a line of them. Put a fast schtokey catch diode across the motor windings. if you dont't need speed control you probably dont need a heat sink.

I am using PWM at about 3Khz and ramping up the speed on start up. This will also help any large start up current spikes.

The low voltage drive fets also makes it easy to connect to micros.

Hope this helps.
 
V

Vitor Finkel

Kirk,

Probably your motor requires more than 12 Amps to start. For brief periods your power supply can probably supply this. But, it was (probably) designed with a current limit to avoid damage in
case of overcurrent. Current limit circuits may be modified to carry over over-currents for
limited time periods. You need a good electronic engineer to modify that power supply, or maybe a
lower cost solution would be just to buy one with a larger output capacity. Try to find with the pump manufacturer what is the starting current, and, with the power supply manufacturer if he has a standardized solution for this problem.
It is quite possible he already had a client asking him to solve a similar problem before.

Vitor Finkel [email protected]
P.O. Box 16061 Tel (+55) 21 285-5641
22221.971 Rio de Janeiro Brazil Fax (+55) 21 205-3339
 
C

Clark Southoff

Hi Kirk:
I have run into similar problems with 24 VDC systems. The simple "try it" solution was to attach very large value capacitors (250,000 uF) to the output of the linear supply. The caution is the in rush current to these capacitors, which may trip your existing PSU. This required some resistors to be put in series with these capacitors.

I am not sure how these would work with the control loop used in the PSU control circuits.

Best of luck
Clark Southoff
Technology Wranglers Inc.
Calgary, Alberta
 
J

John Wheatley

A mickey mouse but simple solution might be to put a battery in as a low impedance source for starting the motor. A lbattery makes a large
capacitor! Your problem might be that the power supply is going into fold-back over current limit- can you adjust the current protection or turn it off?
JohnW
 
Use a big capacitor (> 10,000 uf) to buffer the inrush power. Use a small power resistor ( 1k ohm, 5W) across the cap to bleed off voltage when the power is shut off.

I have seen this used when stepper motors are driven from switched power supplies.

My numbers may not be right, do the math with ohms law to make sure that the currents are in the ballpark for the bleeder resistor. I am relying on memory.

You could also use a linear power supply, such as a Condor or a Sola. These can handle the inrush loads much better.

Bill Sturm
 
R

Robert R. Stephens Pennzoil Products

Switching power supplies can't handle short circuits or the high inrush current required to start a motor. I'd suggest trying a different type of power supply. The old style transformer, full wave bridge rectifier and a filter network of the same output rating should work assuming the full load current is below the rated continuous current of the power supply.
 
M

Michael Griffin

There are several items to be addressed here. If this is a small 12VDC conventional brush motor, a starting current of 10 times or more full load is not inconceivable. It is likely the power supply is going into fold back or shut down (as has been mentioned in other letters) on start up.

First, you need a power supply that can operate in a current limiting fashion, not one which shuts down when the full load is exceeded.
Despite some other comments on this point, this is not simply a function of whether it is a switcher or not. Both linear and switching supplies can operate either way, depending upon their design.
A power supply which shuts down on overload may work provided it reacts slowly enough. However, this is a race between your motor and the power supply. You would need to be fairly sure of the characteristics of both before you could say this would be reliable under all operating conditions.

Second, you need the power supply to be able to deliver enough current to provide the required starting torque for your application. On a fan load or something with similar characteristics, this is not a problem. Whether this is a problem with your pump is not something I can answer. A series resistor to limit the starting current may also limit your starting
torque by too much, so be careful if you consider this approach.

You mentioned that you are using "12VDC control voltage". Are you powering anything besides the pump motor with this same power supply? If you are, then when the power supply goes into current limiting mode, the voltage
supplied to these other devices will drop until the motor gets up to speed. The best thing is to make sure there is nothing besides the motor on this power supply.

I don't know if there is something exceptionally sensitive about this motor, but have you considered just using a transformer and rectifier to drive the motor? I will note that 12V times 12A is only 144 VA. You could size the transformer/rectifier combination much larger than this and it should still be fairly cheap provided you can find a transformer with a
suitable ratio.
The increased ripple will likely cause a bit more motor heating (more losses due to dV/dt), but again I could not tell you if this would be significant in your application (probably not). Most simple DC drives don't put out a supply even this clean to the motor so I don't see why this one should be a problem.


**********************
Michael Griffin
London, Ont. Canada
[email protected]
**********************
 
R
If you do this, see if you can tweak the output voltage to 13.7V rather than 12V, this is the correct voltage for a float charged lead acid battery.

You will only need the battery if your motor actually needs that starting torque, if you require to limit the start current (something that an SMPS does not necessarily do), you may just require an inrush limiting resistor. These are NTC thermistors who start with a low resistance, but straight away heat up and the
resistance virtually dissapears. They are rated on nominal current.
 
L
i have measured test racks with hundreds of disk
drives being burned in, driven by 5 and 12 Vdc
switching supplies. the inrush current is high for each drive, what they do is instal large value
capacitors at the output of the switching supplies. these discharge current into the load. for example u might try caps with values like 100,000microF and keep adding in parallel. u can get used caps pretty cheap.

rgds
leelock
 
K
There are actually "inexpensive" dc controllers out there that would behave as soft starts and/or speed regulators. I'm trying to stay away from this step.

Kirk S. Hegwood
President
Signing for Hegwood Electric Service, Inc.
[email protected]
 
K
See below.

Kirk S. Hegwood
President
Signing for Hegwood Electric Service, Inc.
[email protected]

Michael Griffin wrote:

At 14:07 08/12/00 -0500, Kirk S. Hegwood wrote:
<clip>
>I'm developing a machine which requires using 12VDC
>control voltage (no problem - using a 12VDC switched power supply suitable
>to 12Amps) and a 12VDC chemical pump. Don't know why, but I thought the
>power supply would be sufficient to drive the motor, it wasn't. The start
>current seems to be overloading the power supply. Does anybody know of a
>simple and inexpensive solution for this?
<clip>
There are several items to be addressed here. If this is a small
12VDC conventional brush motor, a starting current of 10 times or more full
load is not inconceivable. It is likely the power supply is going into fold
back or shut down (as has been mentioned in other letters) on start up.
[Kirk S. Hegwood] It's not a brush motor.

First, you need a power supply that can operate in a current
limiting fashion, not one which shuts down when the full load is exceeded.
Despite some other comments on this point, this is not simply a function of
whether it is a switcher or not. Both linear and switching supplies can
operate either way, depending upon their design.
A power supply which shuts down on overload may work provided it
reacts slowly enough. However, this is a race between your motor and the
power supply. You would need to be fairly sure of the characteristics of
both before you could say this would be reliable under all operating
conditions.

Second, you need the power supply to be able to deliver enough
current to provide the required starting torque for your application. On a
fan load or something with similar characteristics, this is not a problem.
Whether this is a problem with your pump is not something I can answer. A
series resistor to limit the starting current may also limit your starting
torque by too much, so be careful if you consider this approach.
[Kirk S. Hegwood] I'll keep this in mind.

You mentioned that you are using "12VDC control voltage". Are you
powering anything besides the pump motor with this same power supply? If you
are, then when the power supply goes into current limiting mode, the voltage
supplied to these other devices will drop until the motor gets up to speed.
The best thing is to make sure there is nothing besides the motor on this
power supply.

I don't know if there is something exceptionally sensitive about
this motor, but have you considered just using a transformer and rectifier
to drive the motor? I will note that 12V times 12A is only 144 VA. You could
size the transformer/rectifier combination much larger than this and it
should still be fairly cheap provided you can find a transformer with a
suitable ratio.
The increased ripple will likely cause a bit more motor heating
(more losses due to dV/dt), but again I could not tell you if this would be
significant in your application (probably not). Most simple DC drives don't
put out a supply even this clean to the motor so I don't see why this one
should be a problem.
[Kirk S. Hegwood] This might be the best all around approach. Thanks.
 
Don't rule a battery out Kirk it'll absolutely solve your problem. You can find very small (cheap) gel lead acid batteries which provide tons
of short duration current. If you only need a few amps for a short period of time, you'd be surprised how small and inexpensive they can be, and they solve all sorts of "poor power" issues.

Mark Hill
[email protected]
 
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