Resistor in parallel with DC motor to increase current draw...(quasi-torque limiting)

Thread Starter

jhausch

Joined Jun 7, 2009
49
I did some searching on this and found the pages that deal with resistors and inductors in parallel on an AC circuit and did see DC referenced, but would still like some beginner help if that's OK.

I have a DC gearmotor in an application. The supply is 12.0 to 13.7V. The current draw when under the load as required by the application is between 5 and 6 amps. The duty cycle is on for 5 seconds, off for 10 seconds, on for 5 seconds, off for an hour. (the motor is rated for 7.5A)

My issue is dealing with jam conditions. (As you can probably tell by the voltage range, this is a mobile application and I am using ATC style fuses.)

If I fuse it at 5A, the force/torque in a jam is too high for the mechanics of the system. If I fuse it at 4A, it will blow the fuses in less than 2 seconds and the force will be within limits. The issue is that 4A ATC fuses are not common and we'd like to use fuses with the built in LED indication. (I know, I know, picky picky, right?)

I was thinking about putting a higher wattage resistor in parallel with the motor to increase current draw so a more common (5A or 7.5A fuse) would blow in under 3 seconds.

Could I have some help sizing that resistor, please? Any other ideas?

Thanks in advance for the advice.
 

GopherT

Joined Nov 23, 2012
8,012
I did some searching on this and found the pages that deal with resistors and inductors in parallel on an AC circuit and did see DC referenced, but would still like some beginner help if that's OK.

I have a DC gearmotor in an application. The supply is 12.0 to 13.7V. The current draw when under the load as required by the application is between 5 and 6 amps. The duty cycle is on for 5 seconds, off for 10 seconds, on for 5 seconds, off for an hour. (the motor is rated for 7.5A)

My issue is dealing with jam conditions. (As you can probably tell by the voltage range, this is a mobile application and I am using ATC style fuses.)

If I fuse it at 5A, the force/torque in a jam is too high for the mechanics of the system. If I fuse it at 4A, it will blow the fuses in less than 2 seconds and the force will be within limits. The issue is that 4A ATC fuses are not common and we'd like to use fuses with the built in LED indication. (I know, I know, picky picky, right?)

I was thinking about putting a higher wattage resistor in parallel with the motor to increase current draw so a more common (5A or 7.5A fuse) would blow in under 3 seconds.

Could I have some help sizing that resistor, please? Any other ideas?

Thanks in advance for the advice.
The resistor will be drawing all of the time.

A better option would be a pair if transistors that forms a current limit (or current source). Look up current source. Set to 4 amps and it will have limited impact on your project until you try to draw more than 4 amps.

http://en.m.wikipedia.org/wiki/Current_source#Zener_diode_current_source

Set the Zener voltage / emitter resistor = 4 amps in the Wikipedia Zener diode source system.
 

Thread Starter

jhausch

Joined Jun 7, 2009
49
The resistor will be drawing all of the time.
Not necessarily "all" of the time, just when the motor is in motion. When not in motion, there isn't any power going to the motor.


A better option would be a pair if transistors that forms a current limit (or current source). Look up current source. Set to 4 amps and it will have limited impact on your project until you try to draw more than 4 amps.

http://en.m.wikipedia.org/wiki/Current_source#Zener_diode_current_source

Set the Zener voltage / emitter resistor = 4 amps in the Wikipedia Zener diode source system
I'm not quite smart enough to fully grasp this recommendation, but I think I get the gist. Is the idea that above 4A (or however I tune the circuit with the R values), this circuit acts like a short and will cause the fuse to blow? Does it matter that I need to run the motor in both directions? It looks like the diode in those circuits are set up for current flow in one direction only.
 

GopherT

Joined Nov 23, 2012
8,012
I'm not quite smart enough to fully grasp this recommendation, but I think I get the gist. Is the idea that above 4A (or however I tune the circuit with the R values), this circuit acts like a short and will cause the fuse to blow? Does it matter that I need to run the motor in both directions? It looks like the diode in those circuits are set up for current flow in one direction only.
It doesn't short out, but instead, the transistor is essentially a valve that starts to turn off when there is too much flow.
 
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Thread Starter

jhausch

Joined Jun 7, 2009
49
Why not fix the weak mechanical parts problem instead? :confused:
Couple of reasons...

1) Too late in the mechanical design process

2) This is a gearmotor turning an acme screw, so the force multiplication is pretty impressive. Stall out the gearmotor and we have over 800# of force at the actuator. The force the application requires is around 175# and the travel speed requirement is being met. As you probably know, the speed/torque curve for the motor "is what it is" and usually gives you no torque at full speed and full torque at stall. We are meeting the application requirements at the "rated/nominal" speed and torque of the motor, but that stall torque is a killer..... Especially since the acme does not backdrive.

We've actually added a neat slip clutch from this company between the motor and screw: http://www.dynatect.com/polyclutch
It slips accordingly, but we'd like to not use it if we don't have to.
 

crutschow

Joined Mar 14, 2008
27,947
.......................
I'm not quite smart enough to fully grasp this recommendation, but I think I get the gist. Is the idea that above 4A (or however I tune the circuit with the R values), this circuit acts like a short and will cause the fuse to blow? Does it matter that I need to run the motor in both directions? It looks like the diode in those circuits are set up for current flow in one direction only.
If you need to limit the current in either direction you can place the limit current inside a diode bridge that is in series with the motor. The bridge wall cause a unidirectional current flow through the limiter regardless of the motor current flow direction.
 

Thread Starter

jhausch

Joined Jun 7, 2009
49
Thanks, guys. This is really beyond my current abilities/understanding. I need to spend some time walking through that option shown in "figure 4" (4, right?)
http://en.m.wikipedia.org/wiki/Current_source#Zener_diode_current_source

Zapper/crutschow, I don't follow what you mean by, "place the limit current inside a diode bridge that is in series with the motor". I'm picturing a 4 diode bridge in my mind and have no idea if I'd use all for connection points and what goes in the middle.....
 

MikeML

Joined Oct 2, 2009
5,444
I scanned the posts but didn't see an obvious suggestion. Put a low-value high-power resistor in series with the motor, This will reduce the stall torque, but not greatly reduce the running speed when it is lightly loaded.

Some reading for you...
 

Thread Starter

jhausch

Joined Jun 7, 2009
49
I scanned the posts but didn't see an obvious suggestion. Put a low-value high-power resistor in series with the motor, This will reduce the stall torque, but not greatly reduce the running speed when it is lightly loaded.

Some reading for you...
A resistor in series with the motor would decrease the current through the circuit, right? How does that help me? It seems like it would shift the whole speed torque curve over. What am I missing?
 

crutschow

Joined Mar 14, 2008
27,947
Actually, after reading your initial post again, I'm a little confused.
If the normal motor current is between 5 and 6 amps, how do you expect it to work if you limit/fuse the current to 4A? :confused:
 

Thread Starter

jhausch

Joined Jun 7, 2009
49
Actually, after reading your initial post again, I'm a little confused.
If the normal motor current is between 5 and 6 amps, how do you expect it to work if you limit/fuse the current to 4A? :confused:
Great question.

Check out the curve/chart for some ATC fuses. You can run a 4A fuse for quite a while (many seconds) at 5A. A 5A fuse runs a very long time a 5.5A.

If a 5a fuse blew "instantly" at 5.1A, it would either blow on inrush (not good) or work perfectly for my application.

I also tried some very fast acting glass tube style fuses (4 and 5A) in my attempt to limit overload force with fusing. Interestingly, the 4 and 5 lasted longer than the 4A ATC and I had the same high force issues.

As it sits now, the 4A atc fuse seems to survive the application duty cycle, and blow in a jam. However, I am concerned with nuisance fuse failure at higher ambient temps and 4A ATC fuses are much less common at your corner auto parts retailer.

On the one hand, the original request was to find a way to use a 5A fuse, but get fuse failure behavior like I am setting with the 4A (blow in less than 1-2 sec in a jam).

On the other hand, a faster acting (simple) current limiting circuit is a great idea. I wish I was smart enough to better understand the solutions offered.
 
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