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AssuredElectrical, Master Electrician
Category: Electrical
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Experience:  Contractor-42+ Years in the ElectricalTrade
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Im hooking up a DC motor and want to install an overload relay

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I'm hooking up a DC motor and want to install an overload relay for safety. The motor is a 1/4 hp motor rated for operation at 90 VDC; the current rating ("Arm A") appears to be 2.5 amps. How do I select the cutoff current for the overload relay? I assume it should be some amount higher than the rated 2.5 amps, but how much? Thanks.

AssuredElectrical :

Welcome. My name is XXXXX XXXXX would be glad to assist.

AssuredElectrical :

What type of overload are you using in the application? Many have a varied amperage range at the different setting. Like +/- 1/2 amp or so.

AssuredElectrical :

Alsom what is the service factor on the nameplate? Look for a section on the label for "SF"


Not sure I understand what you mean by type of overload. I was assuming a pretty standard overload relay, which seems to have a reset time around 20 seconds, but perhaps you are looking for something more?

AssuredElectrical :

How are you going to start and stop the motor?


Unfortunately, I don't have access to the motor right now so I can't see the nameplate. The spec sheet I have for the motor doesn't show a safety factor of SF anywhere.


The motor will be switched on/off each hour - running 15 minutes out of an hour. We'll use a timer that closes a relay - when that relay is closed, it will send 24 VDC to a contactor that closes and turns the motor on. I then want an overload relay in line with the motor for safety.

AssuredElectrical :

The service factor provides information on the insulation and how well the motor withstands heat etc. If it is rated at SF 1.0, then normal overload protection should be used right at nameplate amperage. If it is SF 1.15, then you can go 15% higher for a safety margin and actually run little over nameplate.

AssuredElectrical :

You do not run motors for extended periods over their nameplate amperage or else their life is decreased drastically.


That makes sense. So if I see SF 1.15 and the nameplate average is 2.5 A, I should be setting the overload circuit at about 2.88 A (right?). I'll have to get someone to look at the plate, which will take a day or so, but that should be enough.


Thanks. A quick follow up: does my basic hookup raise any concerns for you? I mean the timer closing a relay, relay feeds 24 VDC to contact to turn on/off, and overload relay in line with the contactor/motor for safety. Anything else I should be adding?

AssuredElectrical :

That is correct. The Service Factor being higher than 1, allows the motor to run hotter without damage

AssuredElectrical :

Guess you have a good constant power supply?


Yes - this is a solar system with backup batteries, so the DC should be very clean.

AssuredElectrical :

Using the timer and relay tfor motor starts and stops should work well.


Great. I think that covers it for me for today. Thanks for you help. Craig

AssuredElectrical :

You are quite welcome, glad to assist

AssuredElectrical and other Electrical Specialists are ready to help you
Customer: replied 3 years ago.



I have a follow on question from our conversation yesterday about a contactor and overload relay for a DC motor hookup.


It seems like the overload relays I can find are all designed for 3 phase motors, but I am using a DC motor. Can I still use these for a DC motor? If not, can you suggest where I can find an overload relay for a DC motor?


If I can use these for a DC motor, how it is connected (since the relays have 3 hookups for 3 phase)? And does the amperage rating translate 1:1 (i.e. if it is rated for 2.5 A on a 3 phase motor, does that mean it is rated for 2.5 A on a DC motor)?





I thought you had a DC control already in mind, sorry.

The AC current is measured differently than DC, so the AC starters do not work on the DC motors.

I suggest getting one of these CLICK HERE

Use the relay from the timer, but use 120 volts and connect to the switch in the controller.
That will allow power on the control at all times and then use the relay as the internal switch to start and stop the motor.

The control will give better performance and has the built in limits and fuses, just set per manual.

CLICK HERE and you can look at the Manual for connections and settings etc to be prepared beforehand.
Customer: replied 3 years ago.

I'll take a look at the device you suggest, thanks. I'm sure it will take me a little while to understand it.


When you say 120 V, I assume you mean AC. We have no AC power access here - it is remote installation running only from solar power (system goes up to 84 VDC to run the motor). Is your setup still possible w/o AC?



Not on that, it requires AC for operation.

Let me find something for your situation and will post back.
Customer: replied 3 years ago.

Great, thanks.

Ok, sorry for the delay.

This is what is below.
Contactor with 24vdc coil
The overload is a Thermal Overload which is rated on the DC because they use heat generated by the current so it doesn't matter if AC or DC.
It has the settings you need of 0.22A-0.32A

The overload will slide into the contactor terminals on the bottom.
Even though, there are 3 spots for power, you will only use 2.
We usually use the the outside 2 and leave the middle open when dealing with only single phase or DC.

Siemens Contactor-CLICK HERE

Siemens Thermal Overload for contactor above-CLICK HERE

AssuredElectrical and other Electrical Specialists are ready to help you
Customer: replied 3 years ago.

Great - thanks for your help. I'll check this out and let you know if I have other questions.


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