# Inductive Load Vs. Resistive Load



## Jayc316 (Jun 30, 2013)

Trying to find out the main difference between a 300A, 250V, contactor for a resistive (heating) load, and a contactor for a inductive (motor) load. :icon_confused:


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## chicken steve (Mar 22, 2011)

All i know is there are 3 distinctions, motor start, lighting & definite purpose Jay

each being _listed_ for the particulars they serve

~CS~


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## Meadow (Jan 14, 2011)

Inductive loads have inrush and voltage spikes. Those contactors tend to be tougher where resistives are less so. But like CS says they are listed for what application is best for them.


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## Tonedeaf (Nov 26, 2012)

Like meadow said, the inrush / lock rotor currents when a motor starts. The contacts are heavier duty.

For a motor with a full load amps of 10a, the starting current can be 100 amps.


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## Tsmil (Jul 17, 2011)

All of the above. Also, when looking at the contactor, it will have a HP rating if designed for motor start.


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## ablyss (Feb 8, 2014)

Load types will differ in inrush current and arching.

Inductive loads that use solenoids or transformers will have a higher arch when turned off/on.

Resistive loads like heaters and motor loads have similar arching, but motor loads will have an inrush of 6x the current.

Lamp loads fall into several categories: tungsten, fluorescent, electronic, HIDs, and more, but tungsten in general has an inrush like motors do. Usually 10-15x the current inrush.


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## chicken steve (Mar 22, 2011)

ablyss said:


> Lamp loads fall into several categories: tungsten, fluorescent, electronic, HIDs, and more, but tungsten in general has an inrush like motors do. Usually 10-15x the current inrush.


hmmm, i always tend to think the other way 'round on lights Ablyss.....:001_huh:~CS~


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## ablyss (Feb 8, 2014)

chicken steve said:


> hmmm, i always tend to think the other way 'round on lights Ablyss.....:001_huh:~CS~


Me too, but it can be counter-intuitive. A light can be anything from a single bulb, or an entire floor with many bulbs. In the latter the inrush is calculating the sum of all lights.


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## Big John (May 23, 2010)

While a cold resistive heating load will have a high inrush, once the temperature stabilizes it will have a relatively low interrupting current when the contactor attempts to open the load.

An inductive load will hit the contactor at both ends: There's a high initial inrush, and there's also a significant amount of energy that has to be dealt with when the circuit opens and the magnetic field collapses, so there's a good sized arc when turning the load off, too.

Just did a troubleshooting call for exactly this problem on an electromagnet: They were burning through manual disconnects that were in theory rated to handle the current, but they weren't designed to open an inductive coil that size and it was caused serious contact erosion.


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## Tsmil (Jul 17, 2011)

Big John said:


> While a cold resistive heating load will have a high inrush, once the temperature stabilizes it will have a relatively low interrupting current when the contactor attempts to open the load.


 Just a little confused by this statement. Other than incandescent lighting, what resistive heating loads have a high inrush?


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## Semi-Ret Electrician (Nov 10, 2011)

Tsmil said:


> Just a little confused by this statement. Other than incandescent lighting, what resistive heating loads have a high inrush?


I would think any load that has a very low cold resistance and then tapers off to a resistance in mid-range once it's up to normal temp. This resistance can only be calculated, not measured directly with a meter.

Similar to a motor starting to turn and one at normal RPM and developing counter-emf.


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## Tsmil (Jul 17, 2011)

Semi-Ret Electrician said:


> I would think any load that has a very low cold resistance and then tapers off to a resistance in mid-range once it's up to normal temp. This resistance can only be calculated, not measured directly with a meter. Similar to a motor starting to turn and one at normal RPM and developing counter-emf.


Like I asked! other than light bulbs, I can't think of any other resistive heating loads that fall into this category. I'm not saying that there is not, just it can't think of any.


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## JRaef (Mar 23, 2009)

Tsmil said:


> Just a little confused by this statement. Other than incandescent lighting, what resistive heating loads have a high inrush?


Most of them actually. It's called having a Positive Temperature Coefficient of Resistivity, meaning that as the temperature rises, the resistance rises as well. So when cold (off for a certain time), the resistance is low and current is high for a fraction of a second until the element heats up. NiChrome, the most common heating element material, has a relatively high PTC of Resistance, about 60x that of silver or copper. Tungsten is a lot lower than NiChrome, but still 4-5x that of silver or copper. So quartz and quartz halogen lamps, because the element inside is still tungsten, is high as well.

But Big John is right, the other side of the action, interrupting the arc when you open the contacts, is more of why there is a difference in the contact ratings. In those resistive loads, the arc extinguishes almost immediately. In an inductive load, a phenomenon called "inductive kickback" maintains the arc for a few milliseconds longer, and that makes a lot of difference in how much damage happens to the contact surface material. The temperature of an electrical arc is the same as the surface of the sun, so even a few milliseconds more has a lot more potential for damage. The larger the contact surface area, the more spread out that heat is and the more heat sink effect in the rest of the components. You don't need all that if it's a resistive load, so you can rate the contacts for a higher working current.


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## Tsmil (Jul 17, 2011)

JRaef said:


> Most of them actually. It's called having a Positive Temperature Coefficient of Resistivity, meaning that as the temperature rises, the resistance rises as well. So when cold (off for a certain time), the resistance is low and current is high for a fraction of a second until the element heats up. NiChrome, the most common heating element material, has a relatively high PTC of Resistance, about 60x that of silver or copper. Tungsten is a lot lower than NiChrome, but still 4-5x that of silver or copper. So quartz and quartz halogen lamps, because the element inside is still tungsten, is high as well.


Are all these heating loads for lighting? I guess I wasn't thinking about lighting as being a heating load.


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## Semi-Ret Electrician (Nov 10, 2011)

Incandescent lamps typically convert only 20% of their energy as visible light, 80% is given off as heat.

The high inrush current prompted the switch rating "T" for large tungsten loads.


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