Alternatives to varistors as overload protection at DC motor

In summary, varistors are commonly used as overload protection for DC motors, but there are alternative options available. Thermal overload protection devices, such as thermal switches or bimetallic strips, can sense and respond to changes in temperature caused by an overload. Electronic overload relays use electronic circuitry to detect and interrupt excessive current. Fuses, circuit breakers, and shunt trips can also be used to protect against overloads. Each alternative has its own advantages and disadvantages, so the most suitable option should be chosen based on the specific needs and requirements of the application.
  • #1
marellasunny
255
3
Alternatives to varistors as overload protection at DC motor

DC Motor here implies 12V linear actuator w/DC Motor that consumes 2.8A under loading condition. Pretty costly equipment.

I'm using a L298N motor driver connected in parallel to run the linear actuator. The motor driver is run by a 12V 10Ah lead acid battery. An electrical engineer advised me to use varistors 'MOVs' instead of 10A blade fuses for overload protection at my linear actuator. He specified that I select a varistor rated at 1.5x12V=18V rms and 1000A surge. I've been able to source only 130V rated Varistors in my locality/country. Is there any other alternative cheap way to ensure surge protection of my costly linear actuator? Are the fuses engh?
From what I've read varistors start to conduct current only above the DC rated voltage.
L298N connected in parallel:
hb_l298.jpg


Varistor working:
VaristorCharacteristicCurve.gif

10A blade fuse:
http://s1097.photobucket.com/user/wlwltd/media/10_AMP_BLADE_FUSE_STANDARD_2.jpg.html?t=1324466963
 
Engineering news on Phys.org
  • #2
What sort of faults are you trying to protect against?

An 18V 1000A varistor would imply you are concerned about over voltage - perhaps due to a failure of the lead acid battery charger?

If you are concerned about something stalling/blocking the linear actuator/DC Motor causing excess current draw then I think a fuse is a better option. However there are Varistors intended for use as automatically resettable fuses...

http://www.littelfuse.com/products/resettable-ptcs.aspx

http://www.littelfuse.com/~/media/e..._dc_application_varistor_design_guide.pdf.pdf

Perhaps read up on the different types of varistor. There are some you put in parallel with the load and these clamp or absorb over voltage transients. There are others you put in series with the load that act like automatically resettable fuses and protect against over current faults.
 
Last edited:
  • #3
marellasunny said:
An electrical engineer advised me to use varistors 'MOVs' instead of 10A blade fuses for overload protection at my linear actuator. He specified that I select a varistor rated at 1.5x12V=18V rms and 1000A surge. I've been able to source only 130V rated Varistors in my locality/country.
That advice is not applicable to your situation.

The advice from CWatters to use a current limiting device is good. Resettable fuses are also marketed as “polyswitch” or “polyfuse” because they are based on a polymer. https://en.wikipedia.org/wiki/Resettable_fuse

If PWM control of the DC motor was being used then a current sense resistor on pins 1 and 15 of the L298 could detect load current and reduce duty cycle to limit maximum current.
 
  • #4
Thanks for the replies. FYI I managed to only get hold of 130V clamping voltage varistors. Nothing in the range of 18V is available. So, I'm thinking of connecting series-opposing 15V zener diodes instead. Is that okay?

CWatters- "If you are concerned about something stalling/blocking the linear actuator/DC Motor causing excess current draw then I think a fuse is a better option."

My DC Motor (Linear actuator) already has limit switches inside of them. So, this would mean that everytime my actuator shaft reaches it end limits, my motor no longer draws any more current (mechanical switches). Nevertheless I'm going to use a 10A fuse in series.

"An 18V 1000A varistor would imply you are concerned about over voltage - perhaps due to a failure of the lead acid battery charger?"

My lead acid battery is getting charged by solar power( regulated by a charge controller ). This battery in-turn powers my actuator dc motor. I'm concerned about over voltage from motor switch-off and the collapsing magnetic field that could spike voltage. .

Baluncore- "If PWM control of the DC motor was being used then a current sense resistor on pins 1 and 15 of the L298 could detect load current and reduce duty cycle to limit maximum current."

Great idea Sir. I'll see if I can use these pins for ordinary motor switch on/off also. I don't use PWM speed control, nevertheless...

Unfortunately, I've got to import polyfuses & 18V varistors. Lead time and money are constraints. I'm thinking of going with the zener diode idea/sense pins idea/try and rip-off spare ECU circuits to search for polyfuses.
 
  • #5
marellasunny said:
Thanks for the replies. FYI I managed to only get hold of 130V clamping voltage varistors. Nothing in the range of 18V is available. So, I'm thinking of connecting series-opposing 15V zener diodes instead. Is that okay?
That is quite unnecessary as maximum supply voltage should not be a problem.
The overload problem you fear is motor current when the motor is stalled.
 
  • Like
Likes jim hardy
  • #6
marellasunny said:
Nothing in the range of 18V is available
i guess D2 through D5 direct motor surges to the power supply rails ?
If somebody is worried that's inadequate

One of these might be reassuring
http://www.littelfuse.com/~/media/e.../littelfuse_tvs_diode_1_5ke_datasheet.pdf.pdf
or if you're protecting against lightning (1000 amps? sounds impractical to me)
here's a 548 amp 20 volt...
http://www.littelfuse.com/~/media/e.../littelfuse_tvs_diode_20kpa_datasheet.pdf.pdfand their whole line of them
http://www.littelfuse.com/products/tvs-diodes.aspx
i'd say pick one higher than your power supply tolerance but below absolute maximum voltage rating of the device
 

1. What are varistors and how do they work as overload protection for DC motors?

Varistors are voltage-dependent resistors that are used to protect electrical circuits from overvoltage events. When the voltage across a varistor exceeds a certain threshold, it begins to conduct electricity and absorbs the excess energy, protecting the circuit. In the case of DC motors, varistors are used to protect the motor from voltage spikes that can occur when the motor is turned on or off.

2. Why are alternatives to varistors needed for overload protection at DC motors?

While varistors are effective at protecting circuits from overvoltage events, they have some limitations. Varistors can only handle a limited number of overvoltage events before they degrade and need to be replaced. Additionally, they can only protect against overvoltage events and are not effective for other types of electrical faults.

3. What are some common alternatives to varistors for overload protection at DC motors?

Some common alternatives to varistors for overload protection at DC motors include fuses, circuit breakers, and surge protectors. Fuses and circuit breakers are designed to open the circuit when an overcurrent event occurs, protecting the motor from damage. Surge protectors use a similar principle as varistors, but are designed to handle a larger number of voltage spikes.

4. What are the advantages and disadvantages of using alternatives to varistors for overload protection at DC motors?

The main advantage of using alternatives to varistors is that they can protect against a wider range of electrical faults, not just overvoltage events. They also do not degrade over time and need to be replaced, unlike varistors. However, these alternatives may be more expensive and may not be as effective in protecting against voltage spikes.

5. How can I determine the best alternative to varistors for overload protection at my DC motor?

The best alternative to varistors for overload protection will depend on the specific needs and requirements of your DC motor. Factors to consider include the voltage and current ratings of your motor, the type and frequency of electrical faults, and the cost and ease of implementation of the alternative. It is recommended to consult with a qualified electrician or engineer to determine the best option for your specific application.

Similar threads

Linear actuator limit switch replacement/repair
    • Mechanical Engineering
Replies
7
Views
2K

Hot Threads

Recent Insights

Back
Top