Motor Overload requirements

[sparky1966]

Hi,

I have been asked by the local church to find out about some overload protection for a motor which is powering the church organ. I am new to this and was hoping you could help. The motor is wired to a Modular Heating/Lighting Contactor (Sector IMO hc25-40 25A) which is connected to the Organ and then onto the CU. Am i correct in thinking that an RCBO would be sufficient at th CU or will they need something more. I hope you can help as they were getting a bit worried about not having overload protection.

Thanks for your time to read this post and I hope you can help.

 

[Marvo]

You wouldn't usually protact a motor against overload by using an RCBO. An RCBO is used to protect the user against shocks and to protect the cabling from the CU against overload. They're not sufficiently accurate to protect a motor from overload current damage, you would be lucky if it tripped even with fully locked rotor.

What type of motor is it?
Single or 3-phase?
What exactly does it drive?
How does it (or the contactor that supplies it) get turned on and off?

 

[sparky1966]

Hiya, Thanks for your reply.

From what i can see :-

it is a single phase motor. (cant see the make model etc as it is in a wooden box)
The motor drives the fan for the organ. (The organ is very old big.)
The contactor gets turned on by the on / off switch at the organ

 

[Marvo]

Usually you would use a contactor / thermal overload combination.

Contactor + Thermal Overload

​

If your contactor doesn't look like the one above then you can also use a manual motor starter for overload protection and keep your existing contactor.
​

Manual Motor Starter
​

​

 

[sparky1966]

Hi

This is the contactor it uses:-



Could you please advise on something which could replace or add to this item

Thanks for your help

 

[malcolmsanford]

That is just a contactor there, the top part which is out of shadow doesn't have any overload settings, unfortunately the bottom part which is in shadow is not visible but by the looks of things 90% sure that there is no overload protection there.

Quite often on smaller motors you will not have overload protection on the controller, there most likely be a thermal cutout incorporated within the motor or even a small overload relay, hard to tell without having the motor specs to hand or the motor itself.

How old is the motor? To be honest if this was installed by a good company then they would have designed it correctly, taking the overload into consideration. First off check to see if the motor itself as inbuilt overload protection. Again there are a couple of ways to do this, if its an overload realy it will be in the connection box of the motor, wired in series on the Line side of the connections. If it's an internal thermal type then it will inside on the windings, and you can't see that unfortunately. The second way is to contact the motor manufacter and ask them.

 

[Deleted member 9648]

You could easily replace that contactor for a contactor and overload combination. Check the full load current of your motor and choose the O/L range based on that.....ie,if your FLC is 10a choose something like a 8-12a range....you wholesaler should carry such items in stock.

 

[Marvo]

Problem with motor overload protection is with small single phase motors the contactor and thermal overload can end up costing the same as the motor itself. You could also look at using a klixon type device on the motor itself if you can find a place to mount it. Klixons will protect against over-current and also against motor overheating. It might also be worth hauling the motor out to see if it already has a thermistor on the windings for protection in which case you won't need to spend more money if it's wired correctly.

Klixon
Klixon 2
Thermistors

 

[kaven]

A bridge rectifier is a tool used to rework alternating present (AC) into direct current (DC). Moreover, a bridge rectifier is utilized in model railroads to supply the correct direct current to run the motors and other accessories. Placed within the decoder, the bridge rectifier has many functions. Let’s describe the features of a bridge rectifier:
* Present many N-channels which suggests that the facility inputs and the detrimental output of the rectifier are well connected.
* Present a multiplicity of P-channel which implies that there’s a connection between each energy inputs and the output of the rectifier.
* Facilitate third quadrant operation gate voltage management which implies that for mentioned multiplicity of stated N-channel MOSFET means and stated multiplicity of mentioned P-channel signifies that includes at the very least a component to assure fast MOSFET turn off.
With a purpose to operate the railroad, generate the minimal warmth required and get the highest present capacity, you want to configure the bridge rectifier. Via a diode, the mentioned part turns off the fast MOSFET.
There’s also the necessity for a certain part that can defend the gate terminals of the proposed multiplicity of the N-channel MOSFET means and said multiplicity of said P-channel MOSFET means from damaging voltages.
There’s additionally the necessity for inclusion of a voltage divide to determine optimum working voltages to the gate portals of stated multiplicity of said N-channel MOSFET means and mentioned multiplicity of mentioned P-channel means.
The method needs to also have a element to protect the gates of mentioned multiplicity of said N-channel MOSFET means and mentioned multiplicity of stated P-channel MOSFET means from false conduction due to some unauthentic noise impulse voltages channeled to mentioned fill A bridge rectifier.

 

[j_c]

The regs state that only motors over 0.75kw require overload protection. I'd recommend you fit the contactor and o/load shown in Marvos top pic as the bottom pic requires an auxiliary to be fitted etc. Fit the contactor in a start/stop enclosure where it will be operated from, it should come with a wiring diagram

 

[ThreePhased]

Sparky,
BS7671:2008 551.1.2 sez "Every electric motor having a rating exceeding 0.37kW shall be provided with control equipment incorporating means of protection against overload of the motor". Read all of 552, and see 537.4.
I would doubt that it would be correct to use anything other than a pukka motor starter of the correct design and rating for the motor, as part of a correctly designed installation, and use an MCB (usually type C for a motor inductive load) or HBC fuse of the correct ratings to protect the elements of the circuit. Bearing in mind that the motor characteristics are unknown, as is its heritage, it might be wise just to steer clear. If this matter must be progressed, i suggest a tong test analogue ammeter on the line, start the motor, use a moby phone video to record the readings, plot the time/current graph, and guesstimate the motor rating. One problem is you don't get a figure for the initial inrush current. Or you could take a crowbar to the wooden box (hope its ventilated!!???), get the plate details, do it quick while nobody is spectating. Also check there is a good earth to the motor.
Worth remembering "the road to hell is paved with good intention".

ThreePhased

 

[j_c]

Usually better to fit a d type mcb to a motor circuit. Motors can pull up to 10 times the running current for the first 3-4 seconds but this doesn't affect overload or mcb selection, they've been designed with this in mind. It does affect cable selection though, say a motors running current was 3 amps I'd use 2.5 cable to take the initial surge

 

[johnky1989]

contacter is an equipment prevent overload of motor