DC Motor Overloads

[sgunsel]

I normally deal with ac motors, but customer wants to use a DC motor. Where can I get an external thermal overload for a 0.5 HP, 24 VDC, 20 FLA, SF:1, continuous duty, motor?

The motor does not have internal thermal protection. I don't know what the power source will be (solar, batteries, or ?).

Thanks.

 

[gar]

110428-1747 EDT

The way in which the motor will be used, and the startup characteristics will determine what overload protection you need.

Will the motor be fully loaded and for how long?
Is there any external current limiting for startup?
If not, and the motor will not be back driven, then startup current is the locked rotor current in the circuit.

You need more information.

.

 

[sgunsel]

The customer only says that they will provide 24 VDC power, presumably without current limiting. The motor will be fully loaded and run on a timer, from 1 to 24 hours.

I normally provide a contactor with thermal overload for ac motors, but the thermal overloads don't indicate DC ratings. The contactor dc ratings are very different from the ac ratings, when stated. Since most of the OLs sense phase loss as well, I'd like to know if they will work or not. Otherwise, it looks like I need to fuse at a maximum of 1.3 x FLA, which may not support starting current.

 

[broadgage]

I would use a standard thermal overload relay, just as used for AC motors.
Use this to control a contactor that switches the motor.
The contactor must of course have a coil and contacts suitable for 24 volts DC but I believe that the thermal overload relay could be a standard type.
These relays measure the heating effect of the current, they dont "know" what voltage is in the circuit, nor if this is AC or DC.

I would stress that I refer to the older thermal overload relays in which the motor current passes through a small heating element which causes a bi-metallic strip to bend when a certain current/temperature is reached.
The bending of this strip opens the coil circuit of the contactor.

 

[gar]

110429-0900 EDT

So we can assume the motor will be run at full load continuously. Then a major consideration is what happens during motor startup. If the load is an extremely high inertia type, taking many seconds to minutes to reach speed, then a special protective system would be required or a current limiter? For example a very large flywheel.

If the motor reaches full speed in a second or so, then a typical thermal type fuse or circuit breaker would work. If you used a fuse you probably would need to use a slow-blow type. Thermal circuit breakers or other thermal overload protectors do not care whether it is DC or AC because they are basically an inverse time RMS sensor.

Some electronic types of protector might not work.

For an initial approach I would see how long it takes for the motor to reach full speed with its maximum intended load. If the time to start was short I would try a slow-blow fuse at your 130% or the next larger.

For an interesting breaker series see:
http://www.mechprod.com/
The following has good technical data, but crashed on my computer under Internet Explorer:
http://www.mechprod.com/pdf/series-24.pdf

Note: the relatively fine increments of current that the Series-24 has available.

.

 

[sgunsel]

The mechproducts resource looks to be very useful. I notice that they rate 277 VAC circuit breakers for a maximum of 50 VDC, sometimes less.

 

[gar]

110429-1057 EDT

I believe you will find all mechanical contact devices carry a lower DC voltage rating than for AC. Harder to quench a DC arc than an AC arc. No zero crossings on the DC. Also more contact material transfer on DC than AC.

Note: there is a patent under Henry Ford's name with Emil Zoerlein as et al. For an ignition system breaker point mechanism that reversed the current thru the points every contact closure to alternate the current direction and thus reduce cumulative metal transfer from one contact to the other. This never went into production. Although a number of years later Ford Motor introduced a breaker point assembly with a hole in one contact. This reduced the buildup on the one contact to which metal was transferred. Zoerlein was head of electrical engineering and his office was immediately adjacent to Mr. Ford's with a door between the two offices. These two men worked very close together on ideas.
.

 

[Besoeker]

I normally deal with ac motors, but customer wants to use a DC motor. Where can I get an external thermal overload for a 0.5 HP, 24 VDC, 20 FLA, SF:1, continuous duty, motor?

The motor does not have internal thermal protection. I don't know what the power source will be (solar, batteries, or ?).

Thanks.

Broadgage made a good suggestion. A thermal type overload is a simple way to go and, being a heating element it will sense either AC or DC.
I'd just add one thing. If you are using a 3-phase overload, you might need to put all three element (heaters) in series. Most thermal overload units I've come across have differential protection for single phasing so need to have reasonably balanced currents.