Switching inductive loads

Need to switch the following inductive loads in an installation:

1. Pool water feature pump around 500 watt
2. Garden light transformer 300 watt (Nelson
3. Floro lights approx 240 watts in a large garage

I have same spare relays on a 5512RVF unit already designed in to the job. These loads typical will only be switched once a day via a home automation system (garage could be more as manual switching employed not HA).

Will the 5512RVF relays cut the mustard for the task?

Cheers,

fleetz

 

Not a problem

Hey Fleetz, The L5512RVF will handle the loads without a problem.

 

If in doubt... switch it externally

My basic philosophy is that if i am unsure of the number of times a load is switching, or it is a higher current load (or it is a fair distance away, eg: Garden stuff) I will wire the c-bus relay output to an external relay & base. That way when the contacts wear out you are only going to have to throw out the external relay, and not burn out a relay on the c-bus output module. And as the c-bus relay module is only switching a very small current (the external relay coil) it will probably last a lot longer.

HTH

Grant

 

Thanks again Grant,

All good stuff and makes sense. Would you see once or twice a day as an issue with 5512RVF units? I must admit your solution is more longer term and a lot cheaper when the enevitable does happen and the contacts wear out.

What brand of external relay and base do you use? If I head down that path could you use a 30VDC/5A type relay to control the external relay or would you recommend still using a C-Bus relay contact? The reason I ask is I have a home automation system that could easily provide the 30VDC/5A type contact.

Cheers,

Fleetz

 

Depends

Fleetz,

Yes you could use xxVDC rated coils, as for the relay contacts this depends on the voltage of the load being switched.

I would generally use a 240 VAC relay coil, but this is only because the majority of my c-bus relay loads are 240VAC and keeping the voltages at each c-bus relay module the same potential is not a bad idea.

You can pick these up from most electrical wholesalers, brands differ depending on where you buy them from.

Grant

 

Hi Grant,

Thanks for that. I was in mixed minds about wether using low or high voltage relays to do the job. I have a home automation system (Ness M1) which have LV relays and of course C-bus relays. I think keeping it all in the 240V domain makes sense as you don't introduce LV power supplies and relays into a 240V application.

I have some spare space on DIN rails where the C-Bus gear is have you or others have any recommendations as to a relay brand that can be DIN mounted and a 240V relay coil?

Given the pool feature pump motor is 750W (thought it was 500W) is there a rule of thumb about the contactor rating when switching inductive loads? The larger the contactor the less wear? I was thinking around a 20Amp relay.

Appreciate you input to date,

Regards,

Fleetz

 

Hi Fleetz


These are the relays i tend to use when interface low voltage to 240v kit.......

http://www.newark.com/jsp/level5/module.jsp?moduleId=en_US/79371.xml

I source them from Lawrence + Hanson.....I m using 12v dc coils and they have led indication....

HTH
Frank

 

Thanks Frank

Regards,

Fleetz

 

The specs on the L5512RVF state the switch life on the relays to be a minimum of 60,000 switch operations. I can see what you are saying with another relay doing all the work reducing the load on the module relay, but for normal light/fan switching I would think >60,000 is enough to last a while

It would be nice for the module to keep track of the number of times it switched per output

Richie.

 

60,000 operation at max load is very impressive. Especially at the 10A inductive load.

An interesting spec on the installation manual which would be nice if a CIS member could clarify if possible. The contacts are rated at 10A inductive and 2A motor, isn't a motor an inductive load? Could we get a clearer definition of these ratings?

How dramatically would the 60,000 switch operations reduce if say the motor load was 3A instead of 2A. I realise the question is taking the unit outside of the quoted specification but this is a real world senario and in my case I have a pool feature pump motor that is rated at 750W (~3A) hence the question.

I am looking for an indication CIS members not any garantee. I want to access if I really need to go down a seperate contactor path for this circuit. If the contactor was say capable of 40,000 at the slightly higher 3A motor load and say it was switched a total of 4 times a day everday, then that set of contact could last 27 years! Even at 20,000 that's 13.5 years. The issue is the extra back EMF and wear on the contacts rather than if it capable of switching the load.

In the design of my C-Bus system, each relay unit has a minimum of 2 spare relay channels, indeed the relay unit with the garden transformers and the pool feature motor has 3 spare relays per unit so assuming the other electronics was still working swapping a failed set of contacts is not an issue.

I could also parallel up two contacts on the pool feature pump circuit, this would share the back EMF switch and would likely double or there abouts the number of contactor cycles. I still have two spare contacts should I chew the first two sets out.

So three senarios:

1. Use one 5512RVF relay and expect a derated number of switching cycles XX,000??
2. Parallel two 5512RVF relay contacts and possible double whatever (1) provided
3 . Have a separate contactor whose coil is driven by one of the 5512RVF relays and effectively remove the contactor issue to another location.

Which would you do?

Thanks and regards,

Fleetz

 

Fleetz,

you are quite correct motors are inductive loads however it is important to note that when a motor starts its instant on current is 6 - 12 times greater than its run current. ie if the run FLC (full load current) is 2amps then its start current can be anywhere from 12 - 24Amps. hence the reason why the relay is derated for motors.

That said if you only get 5000 operations life and are using a plug in base type relay the replacement cost of the relay is likely to be under $15 and the time it takes to replace would be about 10 seconds. This is an important conideration as well. If you opt for a DC coil then a snubber diode should be fitted across the coil as well.

BTW im a big fan of the Finder relay available through NHP which most electrical wholesalers will deal with.

Andrew

 

Thanks Andrew,

I will look athe Finder range not sure if NHP are here in Adelaide anyway will have a look on the net.

Assuming a 750watt motor what type of fuse/breaker would you suggest using and what current rating?

I am using Phoenix Contacts fused terminal blocks in the output which also allow for disconnect. Given it is fused (M205) what characteric (slow / delayed etc) would you suggest?

Appreciate you assistance on this one to date.

Regards,

Fleetz

 

Fleetz,

NHP are in all capital cities and Middies will be able to source them for you...

When sizeing circuit protection for motors the protection is divided into the following categories,

Circuit breakers are there to protect the cable and are sized with this in mind, they don't protect the equipment.

Cable protection is generally done with a circuit breaker that is of the appropriate rating for the cable suppling the motor. This is generally a 'D' curve breaker which permits a longer overload time to get the motor upto speed.

Motor protection is generally done with an overload relay these are far more sensitive to motor overloads and are generally adjustable to fine tune the allowed current to just above the FLC of the motor.

That all said you need to look at the price / performance issues here is it worth protecting a motor whose replacement cost would be less than the protection hardware - only you can make this decision.

To answer your direct question a slow blow fuse of sufficent capacity ie FLC of the motor will give you a degree of overload protection (but make sure you hav plenty of spares as they will wear out!?!?!)

Fleetz - The above goes with the writer that yes I am an electrician however i offer this advice to you to assist you in discussions with your REC as it is fixed wiring work. Only a registered electrical contractor can complete fixed wiring work of any type.

In fact i believe in SA you technically need a REC to change a light bulb?!?!? (you certainly used to... BTW i don't agree with this)

 

Thanks Andrew,

Appreciate the input.

Yes indeed I am working with an electrician and happy to do so on my project. I am organising all the C-Bus stuff and doing the donkey work and cobbling all the bits together for him to fit off. He is fitting off to the output units 3 X 5512RVF and 4 X 5508D1A and all the power, GPO's etc etc etc. I will be doing all the C-Bus cabling, input unit fit off and programming.

Thanks for clarifying the inductive load protection. Will purchase some relays and do the motor loads using them. All the advise all make good sense and will provide a better solution than otherwise might have been.

Regards,

Fleetz

 

Hi Frank,

The version you use is it suitable for a 750 watt / 3 Amp motor load?

Part no G2R-1-SN-DC12 (S) is rated at 7.5A inductive load. It not stating motor load rating. Do you think this is suitable?

Regards,

Fleetz