Runaway Furnace Motor

[Train]

So our old farmhouse has an older type forced air furnace. By older type, I mean just a basic 1/2 horse furnace motor. No speed difference or anything else, just a motor driving the squirrel cage.
In the past during a long power outage, we ran it on a 4000 generator, wiring the gen directly to the furnace. Worked fine. Fast forward to this year, I set it up to go directly into the electrical panel using a 30 amp breaker, (max the gen is rated) so that we could run a few lights too.
So this year we needed to use it. The few lights we had on seemed fine, but when the blower for the furnace turned on, it was running way too fast, to the point we turned of the furnace breaker off. Power to the main was shut off, so we know that wasn't an issue. Any ideas?

 

[The Cobbler]

so it's just doing that when on generator power?
I'd be looking at the voltage & hertz output on the generator.
or if somehow it's getting 240 volts

 

[chinboys]

You might have a faulty blower motor resistor since you describe it as an older unit.

It ought to be located on the motor next to the start/run capacitor.

 

[Train]

The motor itself is only about 4 years old. Yes only on the generator power. When the main power came back on it ran fine and has been fine ever since. Can't see it getting 240 since the furnace is only on a 120 breaker. How do you check the hertz on the generator output?

 

[mike93lx]

The motor itself is only about 4 years old. Yes only on the generator power. When the main power came back on it ran fine and has been fine ever since. Can't see it getting 240 since the furnace is only on a 120 breaker. How do you check the hertz on the generator output?

A multimeter with a frequency setting

 

[fitter30]

Check voltage and cycles like others stated.

 

[Train]

I'll check that out. Unfortunately, I don't have a multimeter with that capability. Fortunately, I'll have to buy a multimeter with that capability.