Back EMF - Auto Restart on 300HP Fan

[jay pai1]

If our 300HP (2400V) fan trips offline or an accidental bump on stop button, operations has to wait till fan comes to a complete stop prior to restarting. This is written in a procedure, there is no PLC/DCS or relay (GE Multilin 469) that prevents an operator from restarting.

Is it really a concern to auto-restart this fan while it is spinning down due to back emf? Are there concerns over current imbalance, ground fault, transient voltages, etc.

 

[Jraef]

If our 300HP (2400V) fan trips offline or an accidental bump on stop button, operations has to wait till fan comes to a complete stop prior to restarting. This is written in a procedure, there is no PLC/DCS or relay (GE Multilin 469) that prevents an operator from restarting.

Is it really a concern to auto-restart this fan while it is spinning down due to back emf? Are there concerns over current imbalance, ground fault, transient voltages, etc.

The risk has to do with the motor magnetic fields being present when you reconnect, because since the motor immediately begins slowing down, they will be at a different frequency compared to the line, so it's like connecting two generators out of synch; BOOM!

Exactly how long the magnetic fields take to collapse when power is removed is variable by numerous factors such as permeability of the steel in the core, the way the windings are made, etc. etc, but as a gross general rule motors that size will rarely take more than 2-3 seconds. However that's not a guarantee, plus if you have capacitors connected for surge protection or power factor correction, it could be minutes because the caps will hold up the excitation, which holds up the caps, which holds up excitation, etc. etc. I've seen several situations where a timer was used based on their being no caps, then caps were added later and the time value not changed, so the boom happened. So usually a procedural rule such as what you describe is used because that exact time value is not known, or knowable, or someone is afraid that something else may change and nobody wants to stick their neck out.

 

[petersonra]

The risk has to do with the motor magnetic fields being present when you reconnect, because since the motor immediately begins slowing down, they will be at a different frequency compared to the line, so it's like connecting two generators out of synch; BOOM!

Exactly how long the magnetic fields take to collapse when power is removed is variable by numerous factors such as permeability of the steel in the core, the way the windings are made, etc. etc, but as a gross general rule motors that size will rarely take more than 2-3 seconds. However that's not a guarantee, plus if you have capacitors connected for surge protection or power factor correction, it could be minutes because the caps will hold up the excitation, which holds up the caps, which holds up excitation, etc. etc. I've seen several situations where a timer was used based on their being no caps, then caps were added later and the time value not changed, so the boom happened. So usually a procedural rule such as what you describe is used because that exact time value is not known, or knowable, or someone is afraid that something else may change and nobody wants to stick their neck out.

that is why is not uncommon to have some kind of braking to prevent the fan from moving when not energized.

Most VFDs these days have some kind of scheme that allows for a flying start.

 

[jay pai1]

Good info, appreciate the feedback. This is not a VFD application and there are no Caps in circuit.

So if I tell operations it is okay to re-start motor after 10 seconds...would you agree with that statement? Just looking for an opinion.

I did see 'block start' capabilities on Relay, might utilize that function.

 

[Besoeker]

The risk has to do with the motor magnetic fields being present when you reconnect, because since the motor immediately begins slowing down, they will be at a different frequency compared to the line, so it's like connecting two generators out of synch; BOOM!

I politely disagree. But what do I know...........

 

[junkhound]

Own experience is to take advantage of the remaining inerltia and hit the start button while still spinning.

No difference in magnetic field considerations than is seen during startup from a stop.

Whoever wrote the procedure likely had some safety criteria other than electrical in mind, like looking at impeller for damage or suck before restarting?

What else does the procedure say other than wait till it stops to restart - any inspection steps?

 

[Jraef]

Own experience is to take advantage of the remaining inerltia and hit the start button while still spinning.

No difference in magnetic field considerations than is seen during startup from a stop.

Whoever wrote the procedure likely had some safety criteria other than electrical in mind, like looking at impeller for damage or suck before restarting?

What else does the procedure say other than wait till it stops to restart - any inspection steps?

I don't mean to imply that you can't connect to a spinning motor. The issue is that you can't connect to it until AFTER the fields in the motor collapse.

 

[junkhound]

I don't mean to imply that you can't connect to a spinning motor. The issue is that you can't connect to it until AFTER the fields in the motor collapse.

PS: Am assuming OP motor is induction motor, otherwise all kinds of bad torques can get developed (tear mounting bolts, bend shafts ), even with just a fan as inertia. However, field for induction motor probably gone before one can reset a switch.
OP did not say what type motor is involved, my bad assumption if not.

 

[kwired]

I don't mean to imply that you can't connect to a spinning motor. The issue is that you can't connect to it until AFTER the fields in the motor collapse.

They must collapse pretty fast, especially in under 3 Hp motors. Seen several applications with small motors that stop and immediately reverse however fast the fwd and reverse contactor or even a drum switch can mechanically operate.

 

[Jraef]

They must collapse pretty fast, especially in under 3 Hp motors. Seen several applications with small motors that stop and immediately reverse however fast the fwd and reverse contactor or even a drum switch can mechanically operate.

Right. But his is 300HP. When testing large motors while on a design teamd for DC Injection Brakes, we had to determine the worst case scenario and found some in this size range that took 2 seconds or more.

 

[don_resqcapt19]

Is the same issue that you have when switching from generator power back to utility power? I have read that it is recommended to use a delay in the neutral position on transfer or a closed transition transfer switch to avoid potential damage to motors 50 hp and larger.