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QSIMDO
12-22-2009, 12:27 PM
The wiring diagram for a DC speed control I bought shows fuses wired in between the main switch and the unit itself.

Would that not be redundant since there are breakers in the panel?
(230v)

Carld
12-22-2009, 12:40 PM
What they are probably showing or talking about is a fused disconnect. Are you hard wiring the speed control in or are you plugging it in to an outlet?

japcas
12-22-2009, 12:57 PM
QSIMDO, I'm not an electrician or a guru on electricity so I may not explain this technically correct so I'll try to keep it simple.

Often there will be an inline fuse to protect the equipment because the circuit you plug it into may be capable of handling a bigger load than the device your plugging in. So without an inline fuse your device could be overloaded and burn up without ever tripping the breaker. So the fuse is selected for the load that the device will see which should protect it in case of an overload on the device end.

mayfieldtm
12-22-2009, 01:04 PM
Often times, fuses are to protect the wiring, thus preventing fires.
I would use fuses at the controller for this reason.
Tom M.

QSIMDO
12-22-2009, 01:27 PM
What they are probably showing or talking about is a fused disconnect. Are you hard wiring the speed control in or are you plugging it in to an outlet?

To an outlet.

QSIMDO
12-22-2009, 01:30 PM
QSIMDO, I'm not an electrician or a guru on electricity so I may not explain this technically correct so I'll try to keep it simple.

Often there will be an inline fuse to protect the equipment because the circuit you plug it into may be capable of handling a bigger load than the device your plugging in. So without an inline fuse your device could be overloaded and burn up without ever tripping the breaker. So the fuse is selected for the load that the device will see which should protect it in case of an overload on the device end.

Well, that brings up another question as I can't remember seeing a spec for those fuses.
I'll have to go back & check.

QSIMDO
12-22-2009, 01:31 PM
Often times, fuses are to protect the wiring, thus preventing fires.
I would use fuses at the controller for this reason.
Tom M.

Makes perfect sense.

QSIMDO
12-22-2009, 01:44 PM
Looks like 8 amps.
Sounds right?

Carld
12-22-2009, 03:39 PM
If your plugging it into a wall receptacle I wouldn't use a fuse but if it makes you feel good then do it. Everybody likes to feel good;) .

japcas
12-22-2009, 04:19 PM
If your plugging it into a wall receptacle I wouldn't use a fuse but if it makes you feel good then do it. Everybody likes to feel good;) .

Carld, like I stated above I'm not an electrician but I don't understand why you wouldn't use a fuse. If he plugs this device in which he seems to think needs an 8 amp fuse into a 120 volt outlet that is protected by a 15 or 20 amp fuse then his device could burn up way before the breaker ever trips if it becomes overloaded or shorts. With a fuse built into the line it would blow hopefully before any major damage was done to the device. I may be overlooking something obvious but I don't understand why you would not put a fuse in the line.

SilveradoHauler
12-22-2009, 05:32 PM
I have several DC drives in the shop.

The wiring between the controller (DC Power supply) and the motor must be fused with a fuse at or less than the carrying capacity of the controller.

Otherwise you will burn out the SCR's in the controller when the DC motor is overloader or stalls. A DC motor will suck up current until something either moves or smokes!!! Example, a 2 hp controller will run a 5hp motor until the motor starts pulling a good load, then the fuses will blow (if they are installed) or the controller will fry (if no fuses are installed).

So, the simple answer is the fuses between the controller and the load (motor) are there to protect the controller. Does not make any difference if you have breakers in the line supply to the controller, you still gotta have fuses between the controller and the load.

Your controller instructions should give you a fuse size. Don't exceed it!

Carld
12-22-2009, 05:51 PM
It is a 230v controller and he is plugging it in. If it is on a separate circuit the breaker can be the size he needs to protect the speed controler. Since it is rated at 8A he could use a 10A breaker but that may be hard to find except at an electrical supply house.

If there are several things on the 230v circuit and the breaker is to high an amperage then he should use a fused disconnect.

It's real simple.

japcas, if it were a 120v outlet the unit would or should have a fuse in it, mine does.

SilveradoHauler, NO you don't have to have fuses IF the only thing on the 230v circuit is the speed controller, the breaker will do the job if it's the correct size. A correct size breaker and a fused disconnect would be redundant and not needed.

japcas
12-22-2009, 06:05 PM
Carld, when he said he was plugging it in to an outlet I assumed one that is already in the shop. What are the chances of it having the proper size breaker on the circuit. A fuse would be a simple way to ensure the controller is protected without having to change the breaker out.

Also, I have seen controllers that didn't have built in fuses. The wiring diagram shows one being added when wired between the switch and controller.

QSIMDO
12-22-2009, 07:08 PM
The wiring diagram shows one being added when wired between the switch and controller.


...which is precisely why I'd better do as the manufacturer indicated and put the fuses in.
It was $6 for 2 panel mounts and 2 fuses. Cheap!

Thanks very much gents!

jdunmyer
12-22-2009, 07:28 PM
There's another very good reason to use the proper fuse(s): A circuit breaker is very slow to "trip" when overloaded, unless the overload is quite large. Even then, a fuse is VERY much faster to open the circuit and protect the equipment. That's why nearly all motor starters (contactors) have fuses ahead of them, even though the starter, with proper overloads, supposedly protects the motor.

Carld
12-22-2009, 07:47 PM
A circuit breaker trips at approx. 80% of it's rated amperage and it's not slow to trip unless it is defective. It's not a slow blow as fuses can be but you will not exceed the rated amps of the breaker.

J Tiers
12-22-2009, 10:15 PM
A circuit breaker trips at approx. 80% of it's rated amperage and it's not slow to trip unless it is defective. It's not a slow blow as fuses can be but you will not exceed the rated amps of the breaker.

Not so.

it trips some where above its marked amperage, according to the current-time curve for that breaker and temperature. Nominally, it trips AT it's marked rating.... but that is "nominal".

You are not allowed to USE it above a load of 80% continuous load for a "molded case" breaker, which would be any you are going to use, generally.

It MAY have ANY characteristic, slow, fast, in-between, depending on how it is designed.

SilveradoHauler
12-22-2009, 10:31 PM
Carld said: "SilveradoHauler, NO you don't have to have fuses IF the only thing on the 230v circuit is the speed controller, the breaker will do the job if it's the correct size. A correct size breaker and a fused disconnect would be redundant and not needed."




Well, in theory, yes.

I am not talking about a fused disconnect, just a simple buss fuse holder mounted on the controller panel or inside the enclosure.

However, with a plugged in controller, you will find that most 230V circuits in domestic and small shop use have 30 amp breakers, some have 50 amp..

So, are we going to change out the breakers in the panels for our plugged in 8 amp controller, that may migrate from one 230 volt circuit to another? And then change the breaker out again when the 30 amp TIG welder is plugged in? Sort of a logistics problem.


You do not have to take my word for it, but in the world of small machines with DC drives, the best way to protect that expensive SCR controller is to fuse the armature circuit. I learned this the hard way by burning out one many years ago. All it takes is a Buss fuse holder on the panel with a twist out cap to change the fuse. Cheap.

Now, if we are talking about dedicated primary circuits to a SCR controller and a high HP drive, then yes, size the breakers properly.

Carld
12-22-2009, 10:50 PM
Well, I guess the web sites and book lied. You just can't trust the written word any more.

J Tiers
12-22-2009, 10:55 PM
Well, I guess the web sites and book lied. You just can't trust the written word any more.

How about the source, and not some bozo on the internet......... Square D trip time curve..........

http://static.schneider-electric.us/docs/Circuit%20Protection/Miniature%20Circuit%20Breakers/QO-QOB%20Circuit%20Breakers/730-11.pdf

Carld
12-22-2009, 11:00 PM
It don't matter, I'm wrong, your right. I'm not dragging this out like some like to do.

J Tiers
12-22-2009, 11:26 PM
It don't matter, I'm wrong, your right. I'm not dragging this out like some like to do.

Well, I'm only right IF I'm right, unlike some folks..... I'd rather YOU corrected ME if I'm actually wrong........ I was wrong at first in the ongoing shooting thread, and then I figured out what folks were really saying.

No matter.