Fuse Rating on a Motor

[Brian Logan]

I've been asked if there is a formula for working out the fuse rating needed for
a single-phase pump that draws 4.3A with a startup current peak of 19.9A.
Apparently a 5A slow blow fuse is blowing. Not enough information to know
whether this is at startup or during operation. My initial thought was that the
fuse would stand the startup current providing it is less than the time-current
curve. I can't help thinking there's something I haven't thought of here. Any
suggestions?

 

[John Popelish]

I've been asked if there is a formula for working out the fuse rating needed for
a single-phase pump that draws 4.3A with a startup current peak of 19.9A.
Apparently a 5A slow blow fuse is blowing. Not enough information to know
whether this is at startup or during operation. My initial thought was that the
fuse would stand the startup current providing it is less than the time-current
curve. I can't help thinking there's something I haven't thought of here. Any
suggestions?

A 5 amp fuse for a 4.3 amp load is cutting it very close, even if
there were no startup surge. How did you determine the duration of
the surge under all conditions of motor load? Don't forget to take
line voltage tolerance into account.

 

[Frithiof Andreas Jensen]

curve. I can't help thinking there's something I haven't thought of here. Any
suggestions?

I cannot remember ever seing a fuse on motors above 1 HP - instead a 'motor
protection relay' (someone will know the correct term for it) is used.

It is a slow electromechanical 'resettable fuse' that can be adjusted to
match the thermal charateristics of the motor. One can buy different types
depending on the application.

 

[Brian Logan]

A 5 amp fuse for a 4.3 amp load is cutting it very close, even if
there were no startup surge. How did you determine the duration of
the surge under all conditions of motor load? Don't forget to take
line voltage tolerance into account.

Thanks for your response. I didn't determine the duration of the surge. You see
I was asked the question by a company I'm currently doing work for. It's the
first time they've used this pump. Presumably the startup surge current of 19.9A
came from the manufacturer's data. They've contacted the manufacturer's UK
office, but it doesn't seem to be anything more than an administrative office
and so must await a response from the Italian manufacturer.

What's the best way to work out an appropriate fuse, then? I haven't seen the
motor data yet, but presumably it has enough information to tell if the
time-current curve of the fuse covers the startup current. As for load, it's
pumping water is about all I can say about it. I suppose there could be a
blockage under extreme circumstances, but you'd want the fuse to blow then,
anyway.

As another poster said, maybe a circuit breaker would be more appropriate than a
fuse anyway. Any thoughts?

 

[John Popelish]

Thanks for your response. I didn't determine the duration of the surge. You see
I was asked the question by a company I'm currently doing work for. It's the
first time they've used this pump. Presumably the startup surge current of 19.9A
came from the manufacturer's data. They've contacted the manufacturer's UK
office, but it doesn't seem to be anything more than an administrative office
and so must await a response from the Italian manufacturer.

What's the best way to work out an appropriate fuse, then? I haven't seen the
motor data yet, but presumably it has enough information to tell if the
time-current curve of the fuse covers the startup current. As for load, it's
pumping water is about all I can say about it. I suppose there could be a
blockage under extreme circumstances, but you'd want the fuse to blow then,
anyway.

As another poster said, maybe a circuit breaker would be more appropriate than a
fuse anyway. Any thoughts?

Ordinary slow blow fuses do not have enough time delay for many motor
applications. Fuses designed for motor start applications often
include quite a large thermal mass that heat sinks the fuse element.
Regardless, if the locked rotor current is about 19 amps, you could
use a fuse with a higher ampacity than 5 amps and still protect the
motor in the event of a stall. I would guess that a 10 amp fuse is
about right. The only way to protect many motors from a slight
overload that eventually causes overheating and still run reliably
under all load and line tolerances is to include a thermal switch in
the motor.

 

[Brian Logan]

I cannot remember ever seing a fuse on motors above 1 HP - instead a 'motor
protection relay' (someone will know the correct term for it) is used.

It is a slow electromechanical 'resettable fuse' that can be adjusted to
match the thermal charateristics of the motor. One can buy different types
depending on the application.

Thanks for your response. I think I've seen the sort of thing you mean when
browsing through component catalogues. Not quite an ordinary circuit breaker as
I first thought you meant, then. I'll look more closely at them. Thanks for the
tip.

 

[John Woodgate]

I read in sci.electronics.design that Brian Logan <getsmedonospamwn@hnos

My
initial thought was that the fuse would stand the startup current
providing it is less than the time-current curve. I can't help thinking
there's something I haven't thought of here. Any suggestions?

I think you've got the right idea. You must, of course, use the
time/current curve for the *exact* fuse you are using.

 

[John Woodgate]

I read in sci.electronics.design that Brian Logan <getsmedonospamwn@hnos

What's the best way to work out an appropriate fuse, then? I haven't
seen the motor data yet, but presumably it has enough information to
tell if the time-current curve of the fuse covers the startup current.

Quite possibly it doesn't. You need to know how the starting current
tails off over several cycles when starting with the load that your pump
actually provides. You then need to plot this on the fuse time/current
graph and check that they don't cross.

 

[Brian Logan]

Ordinary slow blow fuses do not have enough time delay for many motor
applications. Fuses designed for motor start applications often
include quite a large thermal mass that heat sinks the fuse element.
Regardless, if the locked rotor current is about 19 amps, you could
use a fuse with a higher ampacity than 5 amps and still protect the
motor in the event of a stall. I would guess that a 10 amp fuse is
about right. The only way to protect many motors from a slight
overload that eventually causes overheating and still run reliably
under all load and line tolerances is to include a thermal switch in
the motor.

Thanks for the deeper insight. I think I've seen the type of fuses you mention.
I read (at http://www.airpaxppp.com/pppsite/copmp.html) that induction motors
are usually protected by a thermal device embedded in the motor. Is this the
type of device you refer to? I see that there are overload relays available for
three phase motors, but can't seem to find any for single phase. Is the idea
that the overload protection is generally built in so that there is no need for
external overload protection? If so, then this just means choosing a suitable
fuse. I see in the component catalogues so called "aM" fuses that are intended
for motor applications. Would these be of the type you refer to?

 

[Rene Tschaggelar]

I've been asked if there is a formula for working out the fuse rating needed for
a single-phase pump that draws 4.3A with a startup current peak of 19.9A.
Apparently a 5A slow blow fuse is blowing. Not enough information to know
whether this is at startup or during operation. My initial thought was that the
fuse would stand the startup current providing it is less than the time-current
curve. I can't help thinking there's something I haven't thought of here. Any
suggestions?

A pragmatic approach : the next bigger one?
perhaps as 'slow' fuse ?

Rene

 

[John Popelish]

Thanks for the deeper insight. I think I've seen the type of fuses you mention.
I read (at http://www.airpaxppp.com/pppsite/copmp.html) that induction motors
are usually protected by a thermal device embedded in the motor. Is this the
type of device you refer to? I see that there are overload relays available for
three phase motors, but can't seem to find any for single phase. Is the idea
that the overload protection is generally built in so that there is no need for
external overload protection? If so, then this just means choosing a suitable
fuse. I see in the component catalogues so called "aM" fuses that are intended
for motor applications. Would these be of the type you refer to?

Sorry, I am not familiar with an aM fuse. What general kind (size)
fuse are they using, now?

 

[Brian Logan]

Sorry, I am not familiar with an aM fuse. What general kind (size)
fuse are they using, now?

IEC cylindrical fuse. http://www.bussmann.co.uk/New CYLINDRICAL aM.pdf
At the moment they're using a standard 20x5mm glass cartridge fuse.

http://www.ferrazshawmut.com/resources/pdfs/electrical_handbook/FERRAZ_ELEC HNDBK52_63.pdf
has a table on page 59 (as printed on the page) that shows different fuse
ratings depending on whether an overload is used or not, and the load. This is
for American class RK5, though I suppose it's reasonable to expect the aM to
perform similarly. I can't check at the moment, but I suspect the motor has
internal overload protection, so the fuse rating could be increased if nuisance
tripping continued, I suppose.

 

[John Popelish]

IEC cylindrical fuse. http://www.bussmann.co.uk/New CYLINDRICAL aM.pdf
At the moment they're using a standard 20x5mm glass cartridge fuse.

http://www.ferrazshawmut.com/resources/pdfs/electrical_handbook/FERRAZ_ELEC HNDBK52_63.pdf
has a table on page 59 (as printed on the page) that shows different fuse
ratings depending on whether an overload is used or not, and the load. This is
for American class RK5, though I suppose it's reasonable to expect the aM to
perform similarly. I can't check at the moment, but I suspect the motor has
internal overload protection, so the fuse rating could be increased if nuisance
tripping continued, I suppose.

I would like to see the time versus current curves for those aM fuses,
but the Bussman is a terrible place to wonder through.

My gut feel for this is a 8 to 10 amp motor rated fuse. The 8mmX32mm
are a lot better than the 5mmX20mm size, especially with respect to
interruption current rating (what you get is the motor leads short
together) 20 kamps at 400 volts versus something like 63 at 250 volts.

 

[PDRUNEN]

Hi,


The thing I see most with DC motors is that the stall current starts dropping
very fast so that a 10 amp stall will drop to 8 amps in a matter of seconds and
can go even lower as the copper heats up.

For our application with a 9.5 Amp stall current, we ended up with a 4 amp fuse
which would blow in about 2 seconds after a locked rotor condition but would
not blow under normal operation. It is a slow blow fuse.

We also monitor the peak current in software (PIC 16F876) so that the software
will shut down the motor before the fuse blows. The motor power is control by
a relay and MOSFET driver for power.

Our choice with UL was either a fuse or a thermal device to shut down the
motor, the software did not count!

Paul

 

[John Woodgate]

(in <[email protected]>) about 'Fuse Rating

The thing I see most with DC motors is that the stall current starts
dropping very fast so that a 10 amp stall will drop to 8 amps in a
matter of seconds and can go even lower as the copper heats up.

For our application with a 9.5 Amp stall current, we ended up with a 4
amp fuse which would blow in about 2 seconds after a locked rotor
condition but would not blow under normal operation. It is a slow blow
fuse.

You find that because the inrush current normally lasts for only a few
hundred milliseconds, unless the motor is really struggling to
accelerate the load.

 

[Brian Logan]

I would like to see the time versus current curves for those aM fuses,
but the Bussman is a terrible place to wonder through.

Yeah, it's a bit half-hearted. The search facility reports an error message
about an index file not being found somewhere on drive D: !!!
Anyway, The only place I managed to find the time-current curves was via the RS
Components web site, which you need to go through a (free) registration to
access data sheets. If you go to http://rswww.com and select the search link and
enter 188-7044 for the RS part number, you will find a "Tech info" button to the
right of the list of fuses that is returned.

My gut feel for this is a 8 to 10 amp motor rated fuse. The 8mmX32mm
are a lot better than the 5mmX20mm size, especially with respect to
interruption current rating (what you get is the motor leads short
together) 20 kamps at 400 volts versus something like 63 at 250 volts.

Yes, it certainly is more appropriate. Fuses must be one of the most
underestimated components due to their apparent simplicity.

 

[N. Thornton]

Hi

With most water pumps the load decreases when the pipe is blocked,
rather than increase. As long as the motor isnt relying entirely on
the water flow to cool it, which it usually isnt, no protection is
needed against a block.

Regards, NT

 

[Brian Logan]

With most water pumps the load decreases when the pipe is blocked,
rather than increase. As long as the motor isnt relying entirely on
the water flow to cool it, which it usually isnt, no protection is
needed against a block.

I'm surprised a blockage decreases the load. I would have expected an increased
resistance to water flow and so an increased load.

It's a flameproof/explosion proof submersible pump, so relies on surrounding
water to cool it.

 

[John Woodgate]

I read in sci.electronics.design that Brian Logan <getsmedonospamwn@hnos

I'm surprised a blockage decreases the load. I would have expected an increased
resistance to water flow and so an increased load.

You never use a vacuum cleaner? The motor audibly accelerates when you
block the pipe, even partially. Less mass flow = less momentum
transferred = less energy output from the motor.

It's a flameproof/explosion proof submersible pump, so relies on surrounding
water to cool it.

Well, if the flow is blocked, the water remains in place to cool it,
doesn't it?

 

[N. Thornton]

On 27 Nov 2003 08:53:31 -0800, [email protected] (N. Thornton) wrote:

I'm surprised a blockage decreases the load. I would have expected an increased
resistance to water flow and so an increased load.

I think everyone is surprised when they first hear this, as its
counter intuitive. However if you look at the water path in the pump
when the in- or outlet is completely blocked, you'll notice a lot less
work is being done. All that happens is the water spins round in the
pump chamber freely with the impellor, with very little resistance to
its spinning.

The same happens with a cylinder vac: block the inlet and the motor
speeds up, not slows down. Cylinder vacs use a similar impellor type
to water pumps, ie centrifugal rather than traditional fan.

It's a flameproof/explosion proof submersible pump, so relies on surrounding
water to cool it.

Dont think I've seen many pumps explode I know its possible, but
not really a significant issue unlses you get into really large
equipment. Any description that hints that a competing product might
not be upto the same spec tends to attract buyers, so there are a lot
of non-issues eagerly described by sellers.

A pump bursting into flames would also indicate a substandard pump.
And be somewhat difficult to achieve underwater of course. I'm so
cynical.


Regards, NT