Spinning a de-energized AC motor 4

[sry110]

Good Afternoon all,

I am working on a drivetrain application where we will have an 11kW, 400V/3-ph/50Hz, 4-pole motor that is 'dragged along' at 120 rpm for a short time period, not to exceed 20 seconds, at initial startup of the system. The motor would be connected to its 400V power supply during this period. After the <20 second period, the 11kW motor would be energized and would spin up to its full rated speed of approx. 1475 rpm.

A colleague of mine is concerned about "re-gen", or the motor acting as a generator while it is being pulled along at approx. 8% speed while being de-energized. I have not been able to find any resources citing that this would be detrimental to the motor or its control circuit, or pose a danger to the user, especially for this relatively brief operating period. That said, I am admittedly out of my depth in this particular topic so I would be open to any and all education on the matter.

 

[davidbeach]

How is it "connected to its 400V power supply" but needing later to "be energized and ... spin up to its full rated speed"?

 

[waross]

Consider a short power outage on a running motor.
An immediate reconnection, before the residual magnetism decays may be disasterous.
Often 8 to 10 seconds is adequate time for the residual to decay.
I don't see a problem.
Anecdote:
A machine was driven by two motors belted together at different speeds.
Admittedly small motors, about 4 to 8 kW.
The speed was changed by switching power from one machine to the other instantly.
Never had a problem, despite that one motor would be turning at a very high speed when the switch was made from high speed (much higher than synchronous speed) to low speed.

 

[edison123]

There is no 're-gen' in a motor with no power supply. Since the motor is already spinning via the load, it will have a shorter inrush current period than when it is started from zero speed. Just make sure the motor rotates in the same direction as it is being spun by the load. Otherwise, the shaft will likely shear off with much higher inrush currents.

 

[waross]

Otherwise, the shaft will likely shear off with much higher inrush currents.

We used to regularly plug (DOL reverse to stop) and instantly reverse NEMA "U" frame motors.
Not much more current than locked rotor, just a longer duration.
The newer "T" frame motors do not have the thermal capacity to reliably support instant reversing or plugging.

 

[sry110]

How is it "connected to its 400V power supply" but needing later to "be energized and ... spin up to its full rated speed"?

Allow me to clarify - The motor power leads are connected to the motor starter, but the motor starter (contact) is open such that no electrical power is available to the motor while it is being 'pulled along' at 120 rpm.

 

[sry110]

Consider a short power outage on a running motor.
An immediate reconnection, before the residual magnetism decays may be disasterous.
Often 8 to 10 seconds is adequate time for the residual to decay.
I don't see a problem.
Anecdote:
A machine was driven by two motors belted together at different speeds.
Admittedly small motors, about 4 to 8 kW.
The speed was changed by switching power from one machine to the other instantly.
Never had a problem, despite that one motor would be turning at a very high speed when the switch was made from high speed (much higher than synchronous speed) to low speed.

In your anecdotal example, are the 4kW and 8kW motors individually clutched in/out of engagement with the load, or is one motor always being pulled along de-energized while the other motor is in operation?

 

[sry110]

There is no 're-gen' in a motor with no power supply. Since the motor is already spinning via the load, it will have a shorter inrush current period than when it is started from zero speed. Just make sure the motor rotates in the same direction as it is being spun by the load. Otherwise, the shaft will likely shear off with much higher inrush currents.

This has been my assessment as well. Meanwhile, my colleague seems concerned that current induced in the de-energized motor windings due to the rotor being 'pulled along' at 120 rpm needs to be dissipated somehow.
Also to clarify my verbiage regarding the power supply, the motor would be wired to its motor starter, but the contact in the starter would be open such that no electrical power is available to the motor power leads while it is being pulled forward at 120 rpm.

 

[davidbeach]

Current requires a complete circuit, so no current. An induction machine, operating as a generator, only generates power, it does not generate voltage; so no voltage applied, nothing generated. When the voltage is applied there will be a back EMF from the machine, it will likely not be well aligned with the power system, but it will also have a very low magnitude and the resulting current into the machine from the power system will be very much like what it would be for a start from stand still.

 

[sry110]

Current requires a complete circuit, so no current. An induction machine, operating as a generator, only generates power, it does not generate voltage; so no voltage applied, nothing generated. When the voltage is applied there will be a back EMF from the machine, it will likely not be well aligned with the power system, but it will also have a very low magnitude and the resulting current into the machine from the power system will be very much like what it would be for a start from stand still.

Not to be argumentative but to make sure I am on the right track - my understanding is that back EMF is a voltage, albeit an induced voltage. If that is true, then would that not contradict your statement "An induction machine, operating as a generator, only generates power, it does not generate voltage"?

If the crux of the situation is that some relatively low magnitude of back EMF will be induced in the AC motor but that it does not pose a concern for safety of personnel or reliability of the motor, then I think we are on solid ground.

 

[EdStainless]

There would have to be one heck of a lot of residual magnetism in the rotor for your motor to create significant back EMF.

 

[wcaseyharman]

I would guess a de-energized motor could have some residual magnetism in the rotor; when the machine spins you could get low levels of voltage. Synchronous generators have this every time they start up, you can see a few hundred volts, certainly less than 1kV, on a 13.8kV machine, and synchronous machines, due to the steady DC magnet that is the field, would be more susceptible to residual magnetism - the magnetic field is always in the same direction on the rotor.
On a 400V induction machine I would guess it’d be pretty low, well less than 50V at rated speed. At 8% rated speed, I’d guess you’d be down in the single digits worst case.
I don’t think I’d worry about it. It’d likely be less severe than the coast down after the motor is turned off.

 

[waross]

are the 4kW and 8kW motors individually clutched in/out of engagement with the load

Belted together, one turns and both turn. At times one motor may be spun at several times its rated RPM.

 

[waross]

Synchronous generators have this every time they start up, you can see a few hundred volts,

That would be more universal if you said;
"You can see less than 7% of rated voltage."
Actually even less than 7% on low voltage generators, and it may be residual in the exciter in some cases. (The old brushed exciters)

 

[waross]

A spinning motor is normally safe to re-energize after 5 to 10 seconds, even though it may be spinning at 80% of rated speed.
If PF caps are connected to the motor leads, it is well to let the motor completely stop before re-energization.

A colleague of mine is concerned about "re-gen", or the motor acting as a generator while it is being pulled along at approx. 8% speed while being de-energized.

That is a valid question from a person who may know the basics but is unfamiliar with practical applications.
The answer?
No, there is nothing to worry about.

 

[jraef]

An AC squirrel cage induction motor will not become a generator just by spinning with no power supplied. To generate, there must be magnetic fields involved and there are no magnetic fields in the motor yet. The rotor magnetic fields are “induced” into it by the stator magnetic fields, which are created by the connection to the power source. No power source, no stator fields, no stator fields, no rotor fields, no rotor fields, no regeneration.

The rotor might have a tiny amount of residual magnetism in the steel core, but it’s not significant

“Induction generators” can only work by being connected to a grid supply. Other types of alternators use either external power sources to the rotor via slip rings, or permanent magnets embedded in the stator or rotor. Different animals.

.

 

[edison123]

I tested a 3000 RPM motor on no-load today. The starting inrush current was 60 A (with the final no load steady state current of 12 A). During the run-up starting from 500 RPM to 2500 RPM, I switched off and on with 1 sec, 2 sec, 3 sec, 5 sec and 10 sec delay between off and on. The inrush current stayed the same 60 A each time it was switched on. Cage rotor has jacksquat back emf / 're-gen' when the motor is not energized.

 

[jraef]

The peak magnitude will remain close to the same, the duration is what changes.