Hello everyone I'm new here and posting basically asking the same question as this thread
https://forum.allaboutcircuits.com/threads/dc-motor-overload-protection.94480/

I didn't want to hijack a 2014 thread so I created a new one.

I'm looking at protecting a 180w 11.3a DC brushed motor powered by 2 pcs. automotive 12v battery connected in series. Initially it had a complex control board that has now broken down. The control board had a remote control and it's purpose was to have an auto shutdown timer. I'm assuming some sort of protection for the motor was included too.

Right now we have removed this board and connected the motor directly to the battery and used the SPST foot pedal switch to turn the motor on and off. As a basic diagram here is how everything was connected
BATTERY > BREAKER > CONTROL BOARD > FOOT PEDAL > MOTOR

Since the control board broke down we have bypassed it by connecting the motor directly to the foot pedal switch. The breaker was always tripping so we had to remove it too.
BATTERY > FOOT PEDAL > MOTOR

What I was hoping to achieve is something to protect the motor from overload in case the motor runs too hot. Just like the youtube video below

BATTERY > OVERLOAD PROTECTOR > FOOT PEDAL > MOTOR

Any help would be appreciated. TIA

 

Why would a 180 watt motor get too hot?

Pour your cold cup of coffee over it and if it says: YUM it is too hot.

 

While I have no idea why it was tripping a breaker I do not see removing a breaker, likely installed for protection, as a good idea. Next yes, you could likely affix a thermal sensor to the motor which would shut the motor down for excessive temperature rise. You mention 11.3 amps and 24 volts which is not a 180 watt motor. Anyway, you could also add a current shunt to monitor current and if the motor exceeds a current limit shut the motor down. Do you have any name plate data for this motor?

Also in the video it does not appear they are looking at temperature. but rather current. I see no temperature sensor but see a few what look to be low resistance high wattage resistors likely acting as a current shunt. They measure the voltage drop across the shunt which is proportional to the current. When they load the motor the current increases until they stall the motor and the motor is shut down.

Ron

 

What you need to obtain is the continuous torque rating of the motor, then fit a suitable thermal re-settable overload, based on the continuous torque rating (current), the time delay feature of the O/L allows the motor to enter the peak torque range for brief periods such as start up and intermittent high loading without burning out the motor.
If the continuous rating current rating is observed then overheating should not occur.
If the motor is rated at 180w then you should be able to use a continuous current rating of up to 15a, or if the plate rating states 11.2, then this should be the value selected for the continuous O/L on a 12v supply.
Max.

 

Thanks for the reply guys. Well the motor is from a Chinese manufacturer so I'm not quite sure how accurate the specs are. Yes computing for the (W=AxV) would not be accurate. The only thing I know is it is 24v ... A and W are unknown any way to test and figure this out?

Why would a 180 watt motor get too hot?

We use this for a dodgem car. When the car bumps a wall or another car the customers keep pressing the pedal which would cause the motor to heat up.

While I have no idea why it was tripping a breaker I do not see removing a breaker, likely installed for protection, as a good idea. Next yes, you could likely affix a thermal sensor to the motor which would shut the motor down for excessive temperature rise. You mention 11.3 amps and 24 volts which is not a 180 watt motor. Anyway, you could also add a current shunt to monitor current and if the motor exceeds a current limit shut the motor down. Do you have any name plate data for this motor?

For simplicity we removed the breaker as it was annoying to keep removing the dodgem seats to reset the breaker. I figured since it was always tripping there must be a way to keep the current at a certain level. And as a protection if it reaches a current limit the power to the motor would be cut.

Also in the video it does not appear they are looking at temperature. but rather current. I see no temperature sensor but see a few what look to be low resistance high wattage resistors likely acting as a current shunt. They measure the voltage drop across the shunt which is proportional to the current. When they load the motor the current increases until they stall the motor and the motor is shut down.

Yes I'm looking at cutting the power to the motor when the current increases just like the video. What kind of resistors do we need? How many ohms should we start at? A friend told me to add a capacitor but I'm not sure how many farads to start too?

What you need to obtain is the continuous torque rating of the motor, then fit a suitable thermal re-settable overload, based on the continuous torque rating (current), the time delay feature of the O/L allows the motor to enter the peak torque range for brief periods such as start up and intermittent high loading without burning out the motor.
If the continuous rating current rating is observed then overheating should not occur.
If the motor is rated at 180w then you should be able to use a continuous current rating of up to 15a, or if the plate rating states 11.2, then this should be the value selected for the continuous O/L on a 12v supply.
Max.

The specifications on the motor says it is 24v, 11.3a, 180w, rotating speed 1100r/min, torque 1.7Nm. What components should we use to complete this overload circuit?

 

I've seen a video on youtube providing constant current supply but for an LED circuit. How do we go about it for our use?
https://youtu.be/iuMngik0GR8

 

Yes I'm looking at cutting the power to the motor when the current increases just like the video. What kind of resistors do we need? How many ohms should we start at? A friend told me to add a capacitor but I'm not sure how many farads to start too?

Before I forget a capacitor is frequently placed across the motor power lines as close to the motor as possible for noise reduction. Typical value is around 0.1 uF. Keep in mind this is just to reduce noise generated by the motor, it has nothing to do with protecting the motor.

Looking at the video it appears they use a latching circuit to start / stop the motor. Notice how they touch a terminal briefly with a wire and click the motor runs and when the current gets too high again a click and the motor stops.The resistor used to sense current is usually a very low resistance high wattage resistance. Commonly called a current shunt, keeping in mind any voltage developed across the shunt is not applied to the motor. Current shunts less than one ohm are typical and most under a tenth of an ohm. The shunt voltage is amplified and can be applied to a comparator so when the voltage which is proportional to motor current exceeds a preset level the relay powering the motor drops out.

DC current shunts from China can be had pretty inexpensive, for example this 50 Amp 75 mV shunt cost less that $5 USD. A Google of "overcurrent protection circuit" will bring up dozens of circuits which could be used. Additionally a Google of "overcurrent protection module" will get you plenty of turn key plug and play solutions. You can build or buy and sometimes it is easier and less costly to buy a ready solution.

Ron

 

You just need a reed switch. Wind 2 - 3 turns around the switch. When the current is HIGH, the switch will close. This will charge a cap and eventually turn ON a Darlington transistor connected to a HORN relay and turn OFF the motor. The cap will gradually discharge and allow the motor to turn ON again.

 

I use the P&B W57 series of circuit breaker, they are selected using the time/current chart in the app note.
Digikey sell them, among others a 20a -25a should allow for 1 sec overload..
Max.

 

Hello,

It all depends on where the breaker is mounted.
When it is close to the motor, you will have to go there.
But when it is close to the control panel, it will be more easy to reset.

Bertus

 

use current sensor and mcu... then you can stop it at precise value and keep output off for desired period to get motor chance to cool off
if you don't like to program temperature curve, just add temperature sensor...

http://www.allegromicro.com/en/Prod...ifty-Amp-Integrated-Conductor-Sensor-ICs.aspx

 

I have already shown the cheap and simple solution I provided to a set of dodge-em-cars over 20 years where you could easily adjust the current and timing. No amusement owner is going buy a microcontroller and fiddle around with parts that could easily be destroyed in the rough handling of these cars get as they move from location to location.

 

@guterkerl Personally I would be adverse to using a automatic O/L reset for a motor, I have seen too many burn out this way, when a manual reset type is used, it is an opportunity to check the temperature of the motor, the issue with a DC brushed motor is, the heat produced from overload originates in the armature, this takes some time for this effect to radiate to the outer frame, when the outer is hot to the touch, the armature is usually approaching the destruction point.
Max.

 

"Personally I would be adverse to using a automatic O/L reset for a motor,

That is exactly what I have provided.

 

Thanks for all your reply guys. Been busy researching and trying to understand all the things you suggested ... I'll also try to research more on the recommended solutions and post here again for future reference to others having a similar problem.