Generator RPM/Voltage when driving resistive loads

[veggie]

A little bit out there but......

Given:
60hz is important when driving inductive loads or AC devices which are frequency sensitive.

Question:
But what about resistive loads like electric space heaters or insertion heating elements?
Should it not be possible to slow the engine (and generator) down to 3/4 or even 1/2 speed when powering only element heaters?
An example of this would be to slow the engine speed (hence the heat output) when the water in a hydronic system reaches adequate temp. allowing the elements to maintain the temperature without overheating the system.

Does anyone see a danger to the generator head in this case ?

veggie

[rcavictim]

Slowing the 3-phase gensets, either my VW powered or the Changfa 1115 powered, down to 45 or 40 Hz is routine here.  Incandescent lamps are less bright but still useable and the heaters still make heat, just less.  Engines are quieter and fuel use is significantly reduced.

[Thob]

I think it may depend on what type generator you have.  Some generators with AVRs will burn out the AVR attempting to maintain voltage as the engine is slowed down.  Others with AVRs will shut off the generator below some point.  Non-AVR generators should be fine.

[Ronmar]

I think it may depend on what type generator you have.  Some generators with AVRs will burn out the AVR attempting to maintain voltage as the engine is slowed down.  Others with AVRs will shut off the generator below some point.  Non-AVR generators should be fine.

Exactly!  If fused properly, an AVR, if still within it's frequency limit if so controlled, should blow the field fuse before self destructing...

[veggie]

In this case the generator is a Voltmaster brushless unit without an AVR.?

What about the stock ST5's also without AVR. ?

Veggie

[mike90045]

As voltage drops, the power delivered to the resistive element drops too.

So does the cooling fan in the generator.  At some point, you may pull more power, then the cooling fan can keep up with.   Not likely, but watch for it.

At some frequency, 40 hz ? the core can no longer function as a magnetic field shaper, and you loose most of the power.  It's like trying to put DC through a transformer, dropping the frequency down, at some frequency, it falls apart.

[rcavictim]

As voltage drops, the power delivered to the resistive element drops too.

So does the cooling fan in the generator.  At some point, you may pull more power, then the cooling fan can keep up with.   Not likely, but watch for it.

At some frequency, 40 hz ? the core can no longer function as a magnetic field shaper, and you loose most of the power.  It's like trying to put DC through a transformer, dropping the frequency down, at some frequency, it falls apart.

The mechanism Mike refers to is called 'core stauration'.  At a lower frequency you need more inductance to look like a resistance at that frequency.  It takes more iron and more windings to increase inductance.  That's why per watt, transformers for 25 Hz power are much larger than 60 Hz units and 400 Hz transformers in aircraft can be really small. When you no longer have enough inductance because of falling frequency the core saturates magnetically and the transformer or generator winding looks like a short circuit to the power rather than a healthy high impedance (resistance) that can handle it.