Motor experts please.

A strange one today.
I have an old machine to go to and it has 2 motors on the same common shaft.

The shaft, through gear box's turns squeeze rollers.
It was someones idea a long while ago to put 2 different speed motors on the same shaft to have a fast and slow mode.
But while one motor is running - it turns the other one via pulley's and belts.

I have to connect the star delta slow motor today.
The fast motor is a 5KW 1440 rpm and the slow is an 11KW 875? rpm.

Can this motor act as a small generator when the other is running?
I will insist the machine is stopped while I do it but all the same, I don't really know the answer.

Do you?

Interresting question.

I'm guesing, as it's a star delta starter, that we are talking about a quirrel cage induction motor. I have some vague recollections from college about flemings left hand rule for motors and right hand rule for generators but as there is no magnettic field to cut through then how would there be any generation? However, if there were some residual magnetism then maybe you would generate?

Nick

The motor that is being driven by the other motor will be a generator.
but if it is not connected, i.e. its contactor is off, then the generated voltage wont do anything.

There is a risk of electrocuting, if you disconnect one motor (for maintenance ?) and the other motor is allowed to start and drive both motors on the common shaft.
I would have a service disconnect that disconnects both motors.
Alternatively (but not a satisfactorily safe design) is to warn about the risk by signage.

I am certainly not a motor expert, but from my limited experience with micro hydro... unless the spinning generator motor is connected to mains supply and running at faster than synchronous speed, it will not generate any power.

To generate power without mains supply, the motor requires sufficient external capacitance across two phases, and some residual magnetism, to begin generating power.

I am assuming it is a typical asyncronous AC motor.
If that is actually the case, when it is driven on its shaft by another motor it will definitely generate a voltage on its terminals.

No, it will not be a generator if these are 3 phase motors (in most normal cases).

The way you work a generator (like a car's alternator), is you have to have something with a magnetic field already in it while turning in order for it to generate electricity.


If you spin a 3 phase motor you don't have any resistance but the air gap. When people run a generator with a DC motor, that motor has permanant magnets, or in the case of an alternator a wound rotor with voltage passing through (magnet).

like other stated, you could generate some voltage by spinning faster than synchronous speed, but the case of a 3 phase induction motor generating voltage simply by being spun with no external forces on it........ I don't know if i've ever seen that be a problem for anything. The magnetic field in a 3 phase motor that has no voltage going to it already, is too weak to do much of anything, even above synchronous speed.

I would be cautious.
https://sciencing.com/rewire-motor-generate-ac-current-8016485.html

You can use just about any motor to generate electric current, if it is wired correctly and you follow specific rules for its use. Modern AC induction motors are quite simple to wire as alternating current generators, and most will begin generating electricity the first time you use them. These motors don't use magnets and the generator relies on residual magnetism to generate current. Because of this, some induction motors used as generators might need a little boost from a battery to start generating current.

Click to expand...

The article do say that some motors need a battery boost. Emphasis on the 'some'.

JesperMP said:

I would be cautious.
https://sciencing.com/rewire-motor-generate-ac-current-8016485.html
The article do say that some motors need a battery boost. Emphasis on the 'some'.

Click to expand...

a boost from a battery is the equivalent to building a rotary phase converter using a single phase 220v motor, and generating the third leg off of a 3 phase motor connected together with it. The third leg is generated by the 3 phase motor connected to the single phase motor and capacitors attached to the third leg.

As long as it's disconnected from the system as the other is running, it won't generate much of anything.

The last thing I remember reading on this would be that your motor is only turning in the first place because the magnetic fields are interacting with the squirrel cage rotor and pulling it into position as it rotates, you only ever have power if you are lagging. It shouldn't reach perfect synchronous speed because then there would be no torque to drive anything, it should always lag.

and then, if you were drive it beyond that, you get the opposite effect. by leading with the rotor, you can use the same magnetic fields generated by the incoming electricity to then generate voltage.

If I were to spin an alternator without any kind of magnetic field in it, we all know that's how you get a dead battery, the inability to generate any kind of energy due to not having the 3 things necessary. a magnetic force, conductors, and movement of the two past each other.



This is one of those things that I would love to set up a bench test for and actually measure what kind of voltage I can get out of a motor with, and without the leads hooked up a supply voltage.

But I think the best example for people who work in the industry is using a VFD as an example. if you get an OverVoltage fault, it's generally caused in most running applications, by the motor and load spinning down faster than the drive is attempting to operate it, so the induced voltage by the motor spinning faster than the supply is too much for it and it generates more voltage than expected.

but if the VFD cuts out and there is no longer a magnetic field for it to rotate faster than, you don't keep generating voltage back through the VFD, and a brake resistor doesn't work on a motor without voltage already traveling through it.






(I'm not an expert, but I feel like I've encountered a lot of different scenarios working in the field. These are just my observations)

If it is a squirrel cage non permanent magnet motor, then there will be nothing to generate the magnetic field if it is not connected to mains. Therefore I don't believe it will create any voltage. Does anyone have a motor on their desk and a DMM?

I couldn't try it today as there were mechanical faults being repaired at the same time. Just a quick on off direction test.
They are both standard squirrel cage motors just different poles.
They are interlocked so only one can run at a time.
I shall put my meter on the none running contactor out-going terminals tomorrow

If there is not current in AC motor's rotor the motor will not be generator, but if the AC motor is not fully stopped, and there is some external force that rotates the shaft, there will be some current "left" in rotor, so rotor will be magnetized and it will become generator.

I think as was mentioned before the residual magnetism could cause the offline motor to generate a voltage it were the slower motor I think it could actually generate a voltage higher than the line volage creating a dangerous condition.
I have seen interment's that can read speed of an unpowered motor by measuring the magnetic field they generate
A better solution would be used only the faster motor and power it with a vfd then you could set the speed that you want and eliminate the potential problems with the second motor

i would be worried about the slower motor running at 144 rpm. can these bearings take the speed increase?
james

Thas a good question about the bearings of both motors actually
I have on more that few times ordered a new 1800 rpm 4 pole motor from a good motor shop and had them replace the origenal bearing with precision bearing and balance and tune the motor to run at over 5000 rpm i also have them VPI it while it's apart.
the extra cost pays for itself as I have never had a motor failure when i do that.
I have noticed that the newer vector rated motors are rated from the factory for 5000 rpm