Understanding Relay Ratings for Controlling Motors: A Comprehensive Guide

[srinaath]

hi am planning to buy a electromechanical relay with following specification:
16A
256VAC 60HZ,
3PDT

this relay is used for controlling a heater. heater's specs are
2000W
256VAC

I have found one matching for this spec from TE.. its specs are
16A
256VAC
switching capacity: 6000VA

what does this switching capacity means?? does this specify power rating of relay?

 

[davenn]

VA = Watts, so it's saying that the absolute maximum switching capability is 6 kW

you should really look at SSR's ( solid state relays) or other contactor relays maybe

 

[srinaath]

VA = Watts, so it's saying that the absolute maximum switching capability is 6 kW

you should really look at SSR's ( solid state relays) or other contactor relays maybe

but in the outer cover of TE relay...it is mentioned like this
COIL:24VDC
16A 260VAC 50/60HZ
3/4 HP 120VAC 50/60HZ
1HP 208-600 VAC 50/60HZ

I am confused with this 1 HP rating. what's the difference between this 1 HP and switching capacity of 6000VA...Please clarify me this rating.

 

[davenn]

I am confused with this 1 HP rating. what's the difference between this 1 HP and switching capacity of 6000VA...Please clarify me this rating.

not sure

@jim hardy can you help please ? Dave

 

[jim hardy]

I am confused with this 1 HP rating. what's the difference between this 1 HP and switching capacity of 6000VA...Please clarify me this rating.

Contacts have a rating for how much continuous current they can carry,
and another for how much current they can interrupt,
and that ability to interrupt is different for inductive and non inductive loads
and it differs greatly for AC and DC supply.
So you might _edit: should find several ratings for the same contact.

Since a typical small motor can draw 9 or 10 times running current while it's starting, it would be a mistake to control it with a switch rated for just the full load amps listed on the motor's nameplate. To help us along they often tell us what size motor the switch should be able to comfortably handle .

They also often give current ratings for both a non-inductive (resistive) load and inductive around 0.75 power factor. Inductive will usually be slightly lower.
Your heater will be a resistive load.
Incandescent lamps are resistive but have high inrush, like motors do, so you may find a tungsten lamp load rating specified.

Here's an example from a GE relay catalog page
http://www.cesco.com/b2c/product/GE-Lighting-Controls-RR7-Low-Voltage-Relay/8606

Lamp Load: 125 VAC 20 AMP Tungsten Filament, 277 VAC 20 AMP Ballast, Resistive Load: 277 VAC 20 AMP, Motor Load: 1/2 HP At 110 - 125 VAC, 1-1/2 HP At 220 - 277 VAC

note that 20 amps at 120 volts is 2400 Watts = 3.2 hp, about 6X the ½hp motor rating. That's because the motor is inductive and it has inrush.

Here's part of an information brief from Carling Switch folks that's intended to familiarize users...
https://www.carlingtech.com/amp-hp-volts

The voltage rating is a function of a switch's ability to suppress the internal arc that occurs when a switch's contacts open. The voltage rating specified on Carling Technologies' switches represents the maximum voltage allowable for the switch to function properly at the rated current. The amp rating of a Carling switch is the maximum current in amperes the switch will carry continuously. So, in the example below the maximum amp rating for this switch at 250 volts AC (VAC) is 10 amps; the maximum amp rating at 125 volts AC for the same switch is 15 amps.

Switches that are going to be subjected to high-inrush inductive loads, such as an AC motor, will often be rated in horsepower in addition to volts and amps. This rating reflects the amount of current the switch contacts can handle at the moment the device is turned on. An AC motor will draw up to eight times its running current when first turned on or when held stationary while energized (stalled rotor). The switch in the example below would be rated for use with a 3/4HP motor at 125 through 250 volts AC.

Typical Carling Technologies' switch rating:
10A 250VAC
15A 125VAC
3/4HP 125-250VACAC/DC
Carling offers both AC (alternating current) and DC (direct current) switch voltage ratings. AC or alternating current is an electric current or voltage that reverses its direction of flow at regular intervals and has alternately positive and negative values, the average value of which over a period of time is zero. The number of times this value changes (or cycles) per second is it's frequency. Frequency is measured in Hertz (Hz). The more cycles per second, the higher the frequency. The electrical "grid" in North America is based on a very stable 60Hz frequency. Most European countries are based on a 50Hz frequency. All of Carling Technologies' AC voltage ratings are listed at 50/60Hz, and all Carling Technologies' agency approved switches will list specific AC voltage ratings.

DC or Direct Current is an electric current or voltage which may have pulsating characteristics, but which does not reverse direction. It's potential is always the same relative to ground, and it's polarity is either positive or negative. A battery is one example of a source of direct current.

A Carling AC rating is followed by "VAC", for example 125VAC is 125 volts AC. Carling AC/DC ratings are followed by "V" only, without the letters AC and DC following. For example a 125V rating would be read as 125 volts AC and 125 volts DC.DC Rule of Thumb
For those switches that list an AC voltage rating only, the "DC Rule of Thumb" can be applied for determining the switch's maximum DC current rating. This "rule" states the highest amperage on the switch should perform satisfactorily up to 30 volts DC. For example, a switch which is rated at 10A 250VAC; 15A 125VAC; 3/4HP 125-250VAC, will be likely to perform satisfactorily at 15 amps up to 30 volts DC (VDC).Types of Loads
An electric load is the amount of electric power delivered or required at any specific point or points on a system. The requirement originates at the energy consuming equipment of the consumers. More simply put, a load is the piece of equipment you turn on and off.

Resistive loads primarily offer resistance to the flow of current. Examples of resistive loads include electric heaters, ranges, ovens, toasters, and irons. If the device is supposed to get hot and doesn't move, it's most likely a resistive load.

Inductive loads are usually devices that move and normally include electric magnets, like an electric motor. Examples of inductive loads include such things as power drills, electric mixers, fans, sewing machines, and vacuum cleaners. Transformers also produce inductive loads.

High Inrush loads draw a higher amount of current or amperage when first turned on, compared to the amount of current required to continue running. An example of a high inrush load is a light bulb, which may draw 20 or more times its normal operating current when first turned on. This is often referred to as lamp load. Other examples of loads that have high inrush are switching power supplies (capacitive load) and motors (inductive load).UL/CSA Ratings
Typical UL/CSA amperage rating is a single value which represents inductive/resistive loads. If a horsepower rating is listed, it indicates the switch is appropriate for use on motor loads that are rated at the given horsepower. If there is no horsepower rating listed, switches are tested to an inductive/non-horsepower load at 75% of the power factor.

A typical example of a UL/CSA Rating is listed below:
10A 250VAC
15A 125VAC
3/4HP 125-250VACEuropean Ratings
The typical European rating will distinguish between resistive and inductive load ratings. Below is an example of a typical European rating:
16(4)A 250V ~ T85 µ

In this example the 16 = resistive load amperage; (4) = inductive load amperage; A= amperage; 250V= voltage; ~ = AC; T85= Maximum operating temperature in centigrade; µ = micro-gap (<3mm) approved.

If there is less than 3mm of air space between a switch's contacts in the open position, a micro-gap approval (µ) may be granted. This mark indicates that the switch has general application approval with a qualifier that another device, such as a cord and plug, must provide an alternate means of disconnection from the main power source.L & T Ratings
An "L" rating denotes the ability of a switch to handle the initial high inrush characteristics of a Tungsten Filament Lamp on AC voltage only. A "T" rating is the equivalent lamp load for DC.H Rating
An "H" rating denotes a non-inductive resistive rating. Ratings listed in Carling Technologies' product information may appear with the symbol "H" or with the words "non-inductive" or "resistive". "H" ratings are typically required for switches used in commercial oven applications.

I've not yet located the IEC and NEMA standards.

But that's why you see so many different numbers for the same contact..

Here's another brief from a switch manufacturer

http://76.12.210.115/pdf/electricalratings.pdf

 

[davenn]

Contacts have a rating for how much continuous current they can carry,
and another for how much current they can interrupt,
and that ability to interrupt is different for inductive and non inductive loads
and it differs greatly for AC and DC supply.
So you might find several ratings for the same contact.

Since a typical small motor can draw 9 or 10 times running current while it's starting, it would be a mistake to control it with a switch rated for just the full load amps listed on the motor's nameplate. To help us along they often tell us what size motor the switch should be able to comfortably handle .

They also often give current ratings for both a non-inductive (resistive) load and inductive around 0.75 power factor. Inductive will usually be slightly lower.
Your heater will be a resistive load.
Incandescent lamps are resistive but have high inrush, like motors do, so you may find a tungsten lamp load rating specified.

Thanks Jim

 

[Averagesupernova]

https://www.physicsforums.com/threads/spark-in-relay-contacts.880861/#post-5535189

I am link to the above thread since it had a pretty good youtube vid that very clearly shows the difference between switching AC and DC.

 

[srinaath]

Contacts have a rating for how much continuous current they can carry,
and another for how much current they can interrupt,
and that ability to interrupt is different for inductive and non inductive loads
and it differs greatly for AC and DC supply.
So you might _edit: should find several ratings for the same contact.

Since a typical small motor can draw 9 or 10 times running current while it's starting, it would be a mistake to control it with a switch rated for just the full load amps listed on the motor's nameplate. To help us along they often tell us what size motor the switch should be able to comfortably handle .

They also often give current ratings for both a non-inductive (resistive) load and inductive around 0.75 power factor. Inductive will usually be slightly lower.
Your heater will be a resistive load.
Incandescent lamps are resistive but have high inrush, like motors do, so you may find a tungsten lamp load rating specified.

Here's an example from a GE relay catalog page
http://www.cesco.com/b2c/product/GE-Lighting-Controls-RR7-Low-Voltage-Relay/8606

note that 20 amps at 120 volts is 2400 Watts = 3.2 hp, about 6X the ½hp motor rating. That's because the motor is inductive and it has inrush.

Here's part of an information brief from Carling Switch folks that's intended to familiarize users...
https://www.carlingtech.com/amp-hp-voltsI've not yet located the IEC and NEMA standards.

But that's why you see so many different numbers for the same contact..

Here's another brief from a switch manufacturer

http://76.12.210.115/pdf/electricalratings.pdf

View attachment 104986

thanks jim and davenn ...for your valuable support

 

[srinaath]

I have doubt here...
my relay spec sheet for the above one says something like this...
NEMA pilot duty rating

i did some little research in internet and found that make current means pilot duty inrush current and break current is steady state current..
A represents 1 HP and B represents 1/2 HP...
600 means voltage rating...
hence A600 means 600 V 1 HP...i got this from below table...

but my doubt is
1) i couldn't understand the meaning of pilot duty...i mean is it the one which denotes ON/OFF relays...
2)what does that NEMA table implies?...i mean does it says that particular relay has passed this condition mentioned in the table?
3)say am connecting a motor with 1 HP,460V
LRA is 10.3 A
FLA is 1.3 A
so according to table my relay has A600 rating...
here should i need to check that the LRA and FLA are within make and break currents respectively?
4)what does the maximum Volt amps indicate...i mean i know my load is 1 HP...but according to A600 should my relay withstand 7200VA?

@jim hardy and @davenn can u please add some comments to these questions?

 

[jim hardy]

I am not versed in those industry standards.

When starting, i go to manufacturers datasheets to learn the terminology. So, here we go.

1) i couldn't understand the meaning of pilot duty...i mean is it the one which denotes ON/OFF relays...

Here's one company's explanation of the term "Pilot Duty"
http://www.macromatic.com/blog/relays/what-are-pilot-duty-ratings-and-which-macromatic-products-have-them/

Issue:
What are Pilot Duty Ratings and which Macromatic products have them?

Solution/Resolution:
A pilot duty rating is a contact rating intended for contacts that control the coil of another relay or contactor. These ratings are applicable when controlling loads like relay coils, contactor coils, solenoids, and other similar inductive loads. Contacts with a pilot duty rating have passed standardized testing to prove they can reliably control a pilot duty load. The highly inductive nature of pilot duty loads is hard on contacts; controlling pilot duty loads using contacts not rated accordingly can cause premature failure and improper contact operation. For more detailed information on applying pilot duty ratings in your application, see http://www.macromatic.com/blog/relays/how-to-apply-pilot-duty-ratings.

Note they describe interposing relays, relays that drive relays not motors.

2)what does that NEMA table implies?...i mean does it says that particular relay has passed this condition mentioned in the table?

I don't know anything about that table or where it's from.
If i don't answer that question i might be thought ignorant , but if i do answer it'll probably remove all doubt.
So here's best i can do for you:
This continuation of Macromatic's tutorial
http://www.macromatic.com/blog/relays/how-to-apply-pilot-duty-ratings/
says

Note 4 – Product qualification tests are performed to obtain pilot duty ratings. The tests are performed at the voltage and current values provided in the table (i.e. 125 vdc, make 0.22 A, break 0.22 A). Special rules must be considered if an application is operating at different voltage levels than specified in the table.

and i think that is industry practice. Does your source say they tested to those conditions ? If so, probably that particular type of relay contact has passed such a test.

A represents 1 HP and B represents 1/2 HP...

Hmmm. Where did that table come from ?
According to Macromatic's tutorial, " A " means 10 amp thermal current (continuous) and B means 5 amp. No mention of horsepower.
But note these are for Pilot Duty relays, one ought really use a contactor for a motor. That's because it might be asked to break LRA .

3)say am connecting a motor with 1 HP,460V
LRA is 10.3 A
FLA is 1.3 A
so according to table my relay has A600 rating...
here should i need to check that the LRA and FLA are within make and break currents respectively?

Yes, going strictly by the rules.

I would prefer for a design with my name on it, that LRA be within thermal current because the motor could stall .
FLA should certainly be within break current.
But as i said above, i think i wouldn't use a pilot relay for powering a motor of significant size. Should the motor stall the relay might be asked to interrupt LRA.

4)what does the maximum Volt amps indicate...i mean i know my load is 1 HP...but according to A600 should my relay withstand 7200VA?

Answer: Briefly it should withstand it. 7200 is its maximum 'make' rating, observe that amps is always 7200 / voltage. That's because on "make" a contact bounces and arcs and might weld. At higher voltage they reduce the allowed current to discourage contact welding...
.....
Aha! I finally see it ... your table tells us an A600 contact should be equivalent to a switch rated under other standards for 1hp motor or 720 VA coil . But 7200 VA exceeds the thermal rating of the contact. It can take that for only a short interval. See Note 3 at Macromotion tutorial

Note 3 – The make current is applied for a very short duration during the product qualification test and is used to simulate inrush current of an inductive load. For Codes A, B, and C, the pilot-duty inrush current (make) is ten times the steady state current value (break).

Pilot Duty relay should control the motor starter(contactor)'s coil.
I'd have no fear of using one for a smaller motor, though, one whose LRA is within relay BREAK current.

See recent thread Snubber Circuit. Control relays driving a motor were burning up.

Well i learned something about those relay ratings today . Thank you !

here's a snip from an Allen-Bradley catalog . I hope to find and read the NEMA standard.

Corrections are welcome.

old jim

 

[srinaath]

I am not versed in those industry standards.

When starting, i go to manufacturers datasheets to learn the terminology. So, here we go.Here's one company's explanation of the term "Pilot Duty"
http://www.macromatic.com/blog/relays/what-are-pilot-duty-ratings-and-which-macromatic-products-have-them/Note they describe interposing relays, relays that drive relays not motors.I don't know anything about that table or where it's from.
If i don't answer that question i might be thought ignorant , but if i do answer it'll probably remove all doubt.
So here's best i can do for you:
This continuation of Macromatic's tutorial
http://www.macromatic.com/blog/relays/how-to-apply-pilot-duty-ratings/
says

and i think that is industry practice. Does your source say they tested to those conditions ? If so, probably that particular type of relay contact has passed such a test.Hmmm. Where did that table come from ?
According to Macromatic's tutorial, " A " means 10 amp thermal current (continuous) and B means 5 amp. No mention of horsepower.
But note these are for Pilot Duty relays, one ought really use a contactor for a motor. That's because it might be asked to break LRA .
Yes, going strictly by the rules.

I would prefer for a design with my name on it, that LRA be within thermal current because the motor could stall .
FLA should certainly be within break current.
But as i said above, i think i wouldn't use a pilot relay for powering a motor of significant size. Should the motor stall the relay might be asked to interrupt LRA.Answer: Briefly it should withstand it. 7200 is its maximum 'make' rating, observe that amps is always 7200 / voltage. That's because on "make" a contact bounces and arcs and might weld. At higher voltage they reduce the allowed current to discourage contact welding...
.....
Aha! I finally see it ... your table tells us an A600 contact should be equivalent to a switch rated under other standards for 1hp motor or 720 VA coil . But 7200 VA exceeds the thermal rating of the contact. It can take that for only a short interval. See Note 3 at Macromotion tutorialPilot Duty relay should control the motor starter(contactor)'s coil.
I'd have no fear of using one for a smaller motor, though, one whose LRA is within relay BREAK current.

See recent thread Snubber Circuit. Control relays driving a motor were burning up.

Well i learned something about those relay ratings today . Thank you !

here's a snip from an Allen-Bradley catalog . I hope to find and read the NEMA standard.View attachment 105360
Corrections are welcome.

old jim

I am not versed in those industry standards.

When starting, i go to manufacturers datasheets to learn the terminology. So, here we go.Here's one company's explanation of the term "Pilot Duty"
http://www.macromatic.com/blog/relays/what-are-pilot-duty-ratings-and-which-macromatic-products-have-them/Note they describe interposing relays, relays that drive relays not motors.I don't know anything about that table or where it's from.
If i don't answer that question i might be thought ignorant , but if i do answer it'll probably remove all doubt.
So here's best i can do for you:
This continuation of Macromatic's tutorial
http://www.macromatic.com/blog/relays/how-to-apply-pilot-duty-ratings/
says

and i think that is industry practice. Does your source say they tested to those conditions ? If so, probably that particular type of relay contact has passed such a test.Hmmm. Where did that table come from ?
According to Macromatic's tutorial, " A " means 10 amp thermal current (continuous) and B means 5 amp. No mention of horsepower.
But note these are for Pilot Duty relays, one ought really use a contactor for a motor. That's because it might be asked to break LRA .
Yes, going strictly by the rules.

I would prefer for a design with my name on it, that LRA be within thermal current because the motor could stall .
FLA should certainly be within break current.
But as i said above, i think i wouldn't use a pilot relay for powering a motor of significant size. Should the motor stall the relay might be asked to interrupt LRA.Answer: Briefly it should withstand it. 7200 is its maximum 'make' rating, observe that amps is always 7200 / voltage. That's because on "make" a contact bounces and arcs and might weld. At higher voltage they reduce the allowed current to discourage contact welding...
.....
Aha! I finally see it ... your table tells us an A600 contact should be equivalent to a switch rated under other standards for 1hp motor or 720 VA coil . But 7200 VA exceeds the thermal rating of the contact. It can take that for only a short interval. See Note 3 at Macromotion tutorialPilot Duty relay should control the motor starter(contactor)'s coil.
I'd have no fear of using one for a smaller motor, though, one whose LRA is within relay BREAK current.

See recent thread Snubber Circuit. Control relays driving a motor were burning up.

Well i learned something about those relay ratings today . Thank you !

here's a snip from an Allen-Bradley catalog . I hope to find and read the NEMA standard.View attachment 105360
Corrections are welcome.

old jim

wow...that was a superb and lucid explanation...thank you @jim hardy .