3 phase motor rotation change the weird way

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Ole electric

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Ok, thought I saw everything...
A production supervisor came to me the other day and said his operator keeps changing the 1 HP 3 phase bench grinder direction on him by spinning the shaft in the opposite direction by hand 1st then flipping the switch on. I said no-way!
I tried it and what do you know, it works.
Ok you engineers, tell me about inducing magnetic flux possibly??
They want me to make not possible. Not sure how to go about it.
 
You can do that with many 1ph motors, but a 3ph motor's torque should overcome any inertia.
 
drcampbell said:
Are you sure it's a three-phase motor?
A bench grinder doesn't require a lot of power, or starting torque.
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That's what I was thinking. Just because it's connected to a three phase feeder doesn't mean it's a three phase motor. I'll bet it is a single phase to neutral or phase to phase connection.
 
Yup. The motor is (or is operating as) a single phase induction motor with no capacitor to generate starting torque.

If you get it spinning then it will keep going in whatever direction it started at.

Jon
 
winnie said:
Yup. The motor is (or is operating as) a single phase induction motor with no capacitor to generate starting torque.

If you get it spinning then it will keep going in whatever direction it started at.
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I remember a home-brew project in Popular Mechanics decades ago, where a guy pullied two motors together to a power tool which I can't remember. He would select its operating direction by choosing which motor to start first.
 
My brother had a 1978 rd400 set up for racing and had a pvl ignition on it. It would sometimes start in reverse... Release the clutch and it would go backwards! Wierdest thing i ever did see...
 
rambojoe said:
My brother had a 1978 rd400 set up for racing and had a pvl ignition on it. It would sometimes start in reverse... Release the clutch and it would go backwards! Weirdest thing i ever did see...
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2-stroke Diesel engines often do this intentionally. Large ships usually don't have a reverse gear, they just stop the engine and start it up again the other way.

Some antique 4-stroke Diesel engines (with a mechanical injection pump) are capable of doing this, too. They suck air in through the tailpipe and blow exhaust out through the air cleaner. It most often happens when trying to haul a heavy load up a hill -- there's suddenly not enough breakaway torque to sustain the hill climb, the engine stops, the truck begins to roll backwards downhill and the engine starts running in reverse, accelerating the descent.
 
If you run a reversing 3 phase motor, stop, and then suddenly reverse it is running in negative torque. This means since Power = torque x speed, the power will flow in the other direction…it becomes a generator. In fact there is no difference between a “motor” and a “generator” other than terminology.

So as a generator we also get HP = torque x RPM and so mechanical power is reversed…it is braking instead of motoring. It will slow down quickly.

Once it fully stops the torque doesn’t change…we are still applying torque at the same direction. Speed has gone to zero though so temporarily power is zero. We have high current though since it is at stall with power factor = 0. At this point it will accelerate up to speed just like it would if it started from a dead stop.

This is normal reversing. A second possibility that you can do with contractors is reverse quickly, too quickly. Right after you open the contactor the magnetic field in the motor starts to collapse. If you reverse before it collapses completely you plug the motor. Current can be up to twice LRC until the field reverses. This is why cranes for instance often have very short delay timers (0.1 seconds).
 
Ole electric said:
Ok, thought I saw everything...
A production supervisor came to me the other day and said his operator keeps changing the 1 HP 3 phase bench grinder direction on him by spinning the shaft in the opposite direction by hand 1st then flipping the switch on. I said no-way!
I tried it and what do you know, it works.
Ok you engineers, tell me about inducing magnetic flux possibly??
They want me to make not possible. Not sure how to go about it.
Click to expand...
What does it do if you start it from a standstill?

As mentioned probably lost an input lead or have an open winding. If you get it to turn either direction it can still run on something like this that doesn't require a lot of torque.
 
paulengr said:
If you run a reversing 3 phase motor, stop, and then suddenly reverse it is running in negative torque. This means since Power = torque x speed, the power will flow in the other direction…it becomes a generator.
Click to expand...

This is not quite correct. If you reverse a standard across the line 3 phase motor as you describe, you do get negative torque and negative shaft power, but the motor does not operate as a generator. All the absorbed kinetic energy goes toward heating up the rotor.

The motor is 'motoring' in the reverse direction. You have positive (motoring) slip but slip > 1.

paulengr said:
In fact there is no difference between a “motor” and a “generator” other than terminology.

So as a generator we also get HP = torque x RPM and so mechanical power is reversed…it is braking instead of motoring. It will slow down quickly.
Click to expand...

Quite correct, however to operate an induction motor as a generator you want the drive to be in the direction of rotation but at reduced frequency, so that rotor speed is greater than synchronous speed and you get negative slip.

With negative slip you get true generator operation.

Jon
 
Assuming it really is a 3 phase grinder and it is being fed with 3 phase, this would indicate a lost phase, either a bad connection, a blown fuse, a burned contact in the starter, an open winding etc. it it is a 3 phase motor but someone hooked it up to a single phase source, it would act exactly the same. This needs attention one way or another.
 
Ole electric said:
Ok, thought I saw everything...
A production supervisor came to me the other day and said his operator keeps changing the 1 HP 3 phase bench grinder direction on him by spinning the shaft in the opposite direction by hand 1st then flipping the switch on. I said no-way!
I tried it and what do you know, it works.
Ok you engineers, tell me about inducing magnetic flux possibly??
They want me to make not possible. Not sure how to go about it.
Click to expand...
It's not possible with a motor connected to all three phases.
 
Tulsa Electrician said:
Lock out the circuit, tag it out of service, remove and replace. Repair any wiring defects.
Then train employees that doing this can cause serious injury.
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YES! I'm a little disappointed (in myself, along with everybody else here) that this didn't appear until the seventeenth post.

ggunn said:
... Just because it's connected to a three phase feeder doesn't mean it's a three phase motor. I'll bet it is a single phase to neutral or phase to phase connection.
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I've never ever (honest!) seen a 230-volt single-phase motor on a 277-volt circuit. You?
 
drcampbell said:
YES! I'm a little disappointed (in myself, along with everybody else here) that this didn't appear until the seventeenth post.


I've never ever (honest!) seen a 230-volt single-phase motor on a 277-volt circuit. You?
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You probably can get away with doing that to some extent on something that doesn't run continuously though.
 
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