Hi guys, I’m creating a set up for a 208vac make up air unit that requires the unit to toggle on off on a preset time. I’m using a standard 120v digital timer switch and wiring it to energize a contactor. The contactor will switch on the unit when energized. My timer switch does not have an inductive load rating but it does how ever have a HP rating of 1hp so I’ve deduced that at 120vac the timer can handle a 6 amp inductive load. The contactors I’ve been looking at do not say what the current draw on the coil is and if I had to guess it’s probably way lower than 6amps but out of curiosity how would I go about determining the amp draw on the coil of the contactor with out an ammeter. If I had the coil resistance and the voltage what formula would I be using?

 

how would I go about determining the amp draw on the coil of the contactor with out an ammeter.

You can measure the resistance of the coil and use Ohm's law to calculate a coil current from the applied voltage, but the inductance of the coil will make the current significantly less than that for an AC voltage.
It's quite likely to be less than the 6A rating you have determined for the timer.

 

Hi guys, I’m creating a set up for a 208vac make up air unit that requires the unit to toggle on off on a preset time. I’m using a standard 120v digital timer switch and wiring it to energize a contactor. The contactor will switch on the unit when energized. My timer switch does not have an inductive load rating but it does how ever have a HP rating of 1hp so I’ve deduced that at 120vac the timer can handle a 6 amp inductive load. The contactors I’ve been looking at do not say what the current draw on the coil is and if I had to guess it’s probably way lower than 6amps but out of curiosity how would I go about determining the amp draw on the coil of the contactor with out an ammeter. If I had the coil resistance and the voltage what formula would I be using?

Measure the current through your contactor. Without an AC ammeter, place a small known resistance in line with the contactor; something like 1Ω. Then power up the contactor and measure the AC voltage across the resistor. Divide by the resistance and that's the AC current through the contactor.

 

You can measure the resistance of the coil and use Ohm's law to calculate a coil current from the applied voltage, but the inductance of the coil will make the current significantly less than that.
It's quite likely to be less than the 6A rating you have determined for the timer.

Yeah that would give the current of the coil of wire with out considering the inductive reactance of the coil which adds impedance, lowering the amount of current draw significantly, so I guess you could do that and then as long as that number falls under 6amps you know your in the clear but is there a way of calculating the actual current draw of the coil?

 

Measure the current through your contactor. Without an AC ammeter, place a small known resistance in line with the contactor; something like 1Ω. Then power up the contactor and measure the AC voltage across the resistor. Divide by the resistance and that's the AC current through the contactor.

That Makes sense your just using ohms law to determine the drop on a resister in series, but does that mean there is no way to find the current draw on a coil simply knowing wire resistance of the coil and applied voltage? Would you need to know the reactance or inductance of the coil? Just curious how you would go about in theory.

 

Would you need to know the reactance or inductance of the coil? Just curious how you would go about in theory.

Yes, you would need to know the inductance of the coil to accurately calculate the current.

But here's a way to do an approximation.
The values below are from a Grayfurnanceman YouTube video on the resistance and coil current of two otherwise identical relays.
From that, the DC resistance of the DC coil is about 6.5 times the DC resistance of the AC coil, but their operating currents are very similar (as you would expect), due to the inductance of the AC coil when powered from AC.

I think it's safe to assume that relation should approximately hold for your contactor, so:

• Measure its coil resistance
• Multiply that by 6.5
• Use that value in Ohm's law (V/R) to get current
• Multiply that by (0.3 / 0.26 = 1.15)

That should be a reasonable approximation of the AC coil current.

Make sense?

 

Yes, you would need to know the inductance of the coil to accurately calculate the current.

But here's a way to do an approximation.
The values below are from a Grayfurnanceman YouTube video on the resistance and coil current of two otherwise identical relays.
From that, the DC resistance of the DC coil is about 6.5 times the DC resistance of the AC coil, but their operating currents are very similar (as you would expect), due to the inductance of the AC coil when powered from AC.

I think it's safe to assume that relation should approximately hold for your contactor, so:

• Measure its coil resistance
• Multiply that by 6.5
• Use that value in Ohm's law (V/R) to get current
• Multiply that by (0.3 / 0.26 = 1.15)

That should be a reasonable approximation of the AC coil current.

Make sense?

View attachment 259870

That makes complete sense since an ac coil will have a much lower resistance then a dc coil given that an ac coil has inductive reactance and a dc coil only really has reactance when the coil is turned on or off. This seems like a very good method to get an approximate current rating. The last step confused me a bit though, why exactly do you multiply by 0.3/0.26=1.15? Is that just an example?

 

why exactly do you multiply by 0.3/0.26=1.15?

Because that's the ratio between the AC and DC measured currents in the posted values.

 

The contactor coil will take far less than 6A. To preserve the timer contacts, make sure you connect a VDR (MOV varistor) in parallel with the coil. Something like this. The actual specification depends on the coil voltage and the amount of energy it will have to dissipate. Bigger (size) is better!
Some contactor manufacturers make add-on devices especially designed to fit the contactor.
This will protect the timer contacts from sparking and will extend the life of the contacts.

 

The contactor coil will take far less than 6A. To preserve the timer contacts, make sure you connect a VDR (MOV varistor) in parallel with the coil. Something like this. The actual specification depends on the coil voltage and the amount of energy it will have to dissipate. Bigger (size) is better!
Some contactor manufacturers make add-on devices especially designed to fit the contactor.
This will protect the timer contacts from sparking and will extend the life of the contacts.

Is a metal oxide varistor preferred over fly back diodes in this type of scenario?

 

Is a metal oxide varistor preferred over fly back diodes in this type of scenario?

Fly back diodes only work for inductive spike suppression with DC, not AC.

 

An alternative would be to use a TVS diode in parallel with the contacts, rated slightly higher than the peak applied voltage. In 120VAC RMS, peak voltage is about 170 Volts. So using a 190V or 200V TVS would be reasonable. I've been using that type of diodes for years with excellent results.

 

An alternative would be to use a TVS diode in parallel with the contacts, rated slightly higher than the peak applied voltage. In 120VAC RMS, peak voltage is about 170 Volts. So using a 190V or 200V TVS would be reasonable. I've been using that type of diodes for years with excellent results.

That’s a good idea too, thanks. How do you feel about my estimation on the switches inductive ampacity limit of 6 amps given that it says it’s rated for 1HP at 120vac? I did 746watts/120v equals 6amps. A motor gives off counter emf just as a contactors coil would, correct? The type of switch I’m using is an intermac ST01k astronomical timer switch. I figure it should probably do just fine with out any counter emf protection

 

An alternative would be to use a TVS diode in parallel with the contacts, rated slightly higher than the peak applied voltage. In 120VAC RMS, peak voltage is about 170 Volts. So using a 190V or 200V TVS would be reasonable. I've been using that type of diodes for years with excellent results.

Also I should note the timer switch has a rated voltage of 120-277vac and 2HP at 240vac. So it seems a voltage spike caused by emf off a 120v contactor coil should be pretty harmless right?

 

Also I should note the timer switch has a rated voltage of 120-277vac and 2HP at 240vac. So it seems a voltage spike caused by emf off a 120v contactor coil should be pretty harmless right?

Not harmless, inductive voltage spikes can easily reach 6 to 10 times a circuit's working voltage. Again, a TVS diode is in order for this sort of application.

 

The contactors I’ve been looking at do not say what the current draw on the coil is

And you don't mention what contactors you have been looking at so we have even less information than you. contactors tend to be used for large loads. something that is mere 2HP or so is very small load in this case. AC coils of such devices only need little energy (couple of W). this means at 120VAC current will be rather low. AllenBradley bulletin 100-C09 shows that AC coils consume only about 5VA or 2W.

 

And you don't mention what contactors you have been looking at so we have even less information than you. contactors tend to be used for large loads. something that is mere 2HP or so is very small load in this case. AC coils of such devices only need little energy (couple of W). this means at 120VAC current will be rather low. AllenBradley bulletin 100-C09 shows that AC coils consume only about 5VA or 2W.

It’s a generic 30amp 3 poll contactor with 120vac coil

 

that is what i expected so it should match the info provided by AB.

 

that is what i expected so it should match the info provided by AB.

Ok so I ended up putting and MOV in parallel with the contactors coil and now I’m just wondering out of curiosity if that MOV goes bad what happens? Will I have a dead short from A1 to A2 or will it just Burn up and allow the contactors coil to function as normal operation except now with out CEMF protection?

 

ask manufacturer of the MOV. in my experience MOVs tend to fail short which is good. that is why they are used in surge protection.