Still Learning -Whats this for connected to contactor and overload.

[Ironbones]

Hi All

Could someone explain this to me please. Connected to the contactor, over load are two coils (are they ferrite chokes) please see photos. Also why only two and not three one for each phase, marked with a 65 in the photos.

The motor is used to control a blower unit, that moves a bean crust/shell through a plastic pipe to a collection bag.

Any help you can give me, really would appreciate it.

Cheers All, thanks for reading.

 

[marconi]

I think the two devices are current transformers which are measuring the currents in two of the three lines feeding the motor. One can see thin red and blue wires coming out of the device as well as the two thick black conductors. Since the motor is a 3 wire balanced load the two wattmeter method (Blondel's Theorem) can be used to measure the instantaneous power being drawn by the motor. Elsewhere line voltages are measured (or assumed).

These two lines' currents and their corresponding line voltages with respect to the common unmetered line are input to a wattmeter to indicate power. See Figure 4 of :

Three Phase Power Measurement in Three Phase Circuits - Electrical Academia - http://electricalacademia.com/electric-power/three-phase-power-measurement-three-phase-circuits/

See also Blondel's Theorem:

https://wikivisually.com/wiki/Blondel's_theorem

 

[westward10]

I believe they are saturable reactors on the overload relays.

 

[marconi]

Re: westward10's #3:

And the reason why there are only two saturable reactors is because the thermal overload module is a two heater elements type? ie: it only has heaters in two lines, the third line being a solid connetion between in and out. The two saturable reactors are wired in parallel with these heaters.

And the red and blue wires I thought entered the SRs do not - I need to go to Specsavers.

 

[FatAlan]

I reckognise a din rail in the background

 

[Deleted member 26818]

It’s hard to tel, but do those black cables loop through twice or just once?

 

[Pete999]

Hi All

Could someone explain this to me please. Connected to the contactor, over load are two coils (are they ferrite chokes) please see photos. Also why only two and not three one for each phase, marked with a 65 in the photos.

The motor is used to control a blower unit, that moves a bean crust/shell through a plastic pipe to a collection bag.

Any help you can give me, really would appreciate it.View attachment 48190 View attachment 48191 View attachment 48192 View attachment 48192 View attachment 48193

Cheers All, thanks for reading.

CTs for metering purposes maybe

 

[marconi]

I found this image of a Mitsubishi thermal motor overload relay with saturable reactors:

MITSUBISHI ELECTRIC Global website - http://www.mitsubishielectric.com/fa/id_en/products/lv_distri/lv-switchgears/overload-relays/index.html#pageUnit03

Such a blower as mentioned by the the OP perhaps has a long run up period which requires this type of O/L protection device to avoid nuisance trips during start up.

 

[Ironbones]

Hi All, thanks for all your help and links I will read on this area to increase my under standing.

 

[Baddegg]

I reckognise a din rail in the background

I’m with you Alan ha haha

 

[marconi]

Re #9 Ironbones: You know a motor draws more current on start up than when running normally. You also know a thermal overload relay detects overload by the Ohmic heating (I2R) of bimetallic heater elements. For a motor which starts 'quickly' the rise in temperature is not enough to trip. For a motor which starts 'slowly' there is a prolonged period of Ohmic heating of the heater elements which could cause the relay to trip which would be a nuisance. When the motor is running normally one wants the overload trips to ignore short term overcurrents but respond to sustained overcurrents. So how to ignore the initial surge current? One answer is a saturable reactor(inductor) which shunts the heater element. A saturable reactor's inductance and hence reactance depends on the current flowing through it - the higher the current the lower the inductance and hence its reactance. So, during the start up when the surge current is high the saturable reactance shunts the current to flow through it more than through the heater element.

Simply put at high currents, the magnetic dipoles in the metal of reactor/inductor's former are all aligned which means any further increase in magnetising force (ie:Amp turns) (due to an increase in motor current) causes no further (or much reduced) increase in magnetic flux - saturation. The effective inductance and hence reactance thus reduces to a small value and the current through it 'only' has to contend with the resistance of the winding - which is low. Careful selection of materials for the former and number of turns around it ensures that the onset of this reduction in reactance occurs for currents beyond the upper end of the operating current range of the thermal part of the relay.

A saturable reactor is an example of a non-linear passive component; its electrical properties, in this case inductance does change with the current through it. All chokes, transformers, motors can/do saturate if the current is high enough.

Others can polish up this explanation.

 

[Ironbones]

I found this image of a Mitsubishi thermal motor overload relay with saturable reactors:

MITSUBISHI ELECTRIC Global website - http://www.mitsubishielectric.com/fa/id_en/products/lv_distri/lv-switchgears/overload-relays/index.html#pageUnit03

Such a blower as mentioned by the the OP perhaps has a long run up period which requires this type of O/L protection device to avoid nuisance trips during start up.

Marconi, thanks for your explanation its greatly appreciated.

 

[Lucien Nunes]

The purpose of a thermal overload relay is to detect long, low level overcurrents that would overheat a motor winding. In a perfect world, the relay would track the temperature of the winding closely (as more sophisticated electronic protection does) responding only when the winding is at risk. Although more sluggish than a fuse, the heater box still has a lower thermal time-constant than that of the motor, due to a lower mass. When presented with a short, higher overload situation such as a start against a high-inertia load, the adiabatic surge of heat input may cause the overload relay to trip while the motor is still nearly cold.

Under these conditions, the voltage across the heaters is sufficient to drive enough current through the reactor to saturate it, diverting some current away from it and reducing the relay's short-term sensitivity. The time / current curve of the overload relay is therefore modified by the non-linear response of the reactor. The heaters will still be heated to tripping point, but not by a heat input proportional to the square of the line current as would otherwise occur.

An alternative approach used in older gear was the hydraulically damped magnetic overload, which had a non-linear response. Allen-West were very partial to a hydraulic dashpot or three, as anyone who has ever had a case of A-W starters overturn in the back of the van will know.

 

[westward10]

Somewhere amongst my junk I have one of the old glass bottles which came with the Allen-West starters still with its oil and cork stopper. The dash pots had a disc in them with varying size holes which was spun around to adjust the dampening.

 

[FatAlan]

I reckognise a din rail in the background

Just recognised my bad spelling

 

[marconi]

Re FatAlan #15 - I wish the only bad thing I had to confess was a rong spelling - you must be very saintly.