Selecting a fuse for a PWM'd DC motor

Thread Starter

cmartinez

Joined Jan 17, 2007
6,663
The motor in question is this one.
  • 1/10 hp 1750 RPM, 1.5 amps, 90 Volts DC
I've been reading this document, but I'm still not sure what to choose. I'm under the impression that this is an area in which experience plays an important role.

The motor is pwm'd at between 7% and 95% of it's capacity. It's not subject to sudden changes in load, and power is always applied incrementally from 0.5% to whatever setting it's currently working on. That is, the motor is gradually accelerated and not jerked by the sudden inrush of full power.

What rating and type of fuse would be best for this application?
 

shortbus

Joined Sep 30, 2009
7,334
C, it's weird you bring this up, because I'm in the middle of doing a build using a 90VDC motor and speed controller. While mine isn't PWM as such but an off the shelf SCR type controller, the fuse isn't between the motor and the controller but is between the line and the controller. You want to protect the controller and by doing that the motor takes care of itself. The motor is able to take much more voltage and current than a controller.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
6,663
The motor is able to take much more voltage and current than a controller.
Oh yeah, I'm aware of that. And yes, it's the controller I'm trying to protect. I've already placed a current-limiter thermistor, and it's working quite well because it also reduces noise. But I already had an incident in which the thermistor blew up and caught fire... the thing acted as a fuse of sorts. So now I'm going to place a real fuse right before it.

It seems that @mvas has got it about right. The first thing I wanted to do was to determine the working range of the fuse. And now I'll be buying several fuse sizes and see which one works best.
 
Last edited:

shortbus

Joined Sep 30, 2009
7,334
The one my controller uses is just a standard glass type fuse. Didn't know there were DC only fuses, mine is powered from 120VAC so it came with a 240VAC fuse. And for 1/10HP it calls for a 2Amp.

Is a thermistor the same as a MOV? I went and got my controller manual an is shows a MOV in the input line.
 

MaxHeadRoom

Joined Jul 18, 2013
19,681
A MOV and a thermistor are usually two different components, the MOV works on quenching over-voltage by means of a voltage dependent resistor, the thermistor operates by changing the resistance with temperature rise.
Max.
 

mvas

Joined Jun 19, 2017
538
The one my controller uses is just a standard glass type fuse. Didn't know there were DC only fuses, mine is powered from 120VAC so it came with a 240VAC fuse. And for 1/10HP it calls for a 2Amp.

Is a thermistor the same as a MOV? I went and got my controller manual an is shows a MOV in the input line.
I did not say, the fuse was for "DC only".
I said, the fuse MUST be rated for a DC Voltage.

The DC Voltage rating for a fuse is DIFFERENT than the AC voltage rating
Some fuses do not even have a DC Rating and therefore they are not applicable for circuits with DC Voltages.
 

MaxHeadRoom

Joined Jul 18, 2013
19,681
The motor in question is this one.
  • 1/10 hp 1750 RPM, 1.5 amps, 90 Volts DC
What rating and type of fuse would be best for this application?
Motors such as the one you show often have a manufacturers torque curve specifications, these show the torque rating for continuous current and the torque curve/current for peak/momentary operation.
The momentary obviously being the maximum current for the motor and any fuse should be based on this and the time interval allowed in the peak period.
Max.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
6,663
Motors such as the one you show often have a manufacturers torque curve specifications, these show the torque rating for continuous current and the torque curve/current for peak/momentary operation.
The momentary obviously being the maximum current for the motor and any fuse should be based on this and the time interval allowed in the peak period.
Max.
I couldn't find the data you suggested, but I did find its general specs at its official website. It says that its rated at 1.1 amps at continuous duty. So maybe a 2 amp fuse sounds about right.
 

MisterBill2

Joined Jan 23, 2018
4,846
The circuit protection thermistoers are totally opposite from a MOV, both in operation and in purpose. The mov is intended to switch on when transients hit so as to short circuit them, while the thermistors intended to protect from overloads are intended to increase their resistance in the event of an overload, thus reducing the current. A fuse should be rated for less current than the semiconductor doing the switching in the drive circuit, while the thermistor should be rated to pass the rated current of the item protected.Quite different indeed.
 

shortbus

Joined Sep 30, 2009
7,334
I did not say, the fuse was for "DC only".
I said, the fuse MUST be rated for a DC Voltage.

The DC Voltage rating for a fuse is DIFFERENT than the AC voltage rating
Some fuses do not even have a DC Rating and therefore they are not applicable for circuits with DC Voltages.
Again I never heard of that. Back in the day of glass fuses in cars, the parts store fuses were always stamped with AC 220V. and the cars were of course 12V DC. The voltage having to do only with the thickness of the glass in the fuse.
 

MisterBill2

Joined Jan 23, 2018
4,846
Again I never heard of that. Back in the day of glass fuses in cars, the parts store fuses were always stamped with AC 220V. and the cars were of course 12V DC. The voltage having to do only with the thickness of the glass in the fuse.
The voltage rating of a fuse has to do with the highest voltage it can interrupt when it fails. At some voltage a fuse will arc and keep right on conducting. And DC has a greater tendency to do that than AC has.
 

shortbus

Joined Sep 30, 2009
7,334
All I do know is the type fuse used in these drives is the common AG type glass fuse, of differing numbers, like AG3. The AG in that number stands for "Automotive Glass", and guess what kind of voltage an automobile uses? So AG type fuses are good for both AC and DC at the rated voltage. The original AG was ~5/8" long. and wasn't long enough to stop the 'arc over' at higher AC voltages, so the next popular fuse that cars then almost universally used was the AG3 which is 1 1/4" long and could withstand the higher AC voltages. I still have a couple of the small boxes of NAPA car fuses and guess what they say on the side of the end cap? AG3 250V

If C is using a normal speed controller for the project the fuse is in the AC input any way so the AG3 is the type of fuse it uses.
 
Top