A question for the DC motor experts

[DTR]

Morning all,

I'm trying to get to the bottom of a problem at work. Apologies for being a bit vague, I might post up more details when I've got the time to put something coherent together!

We use 110V DC motors to drive a mechanism. A ballscrew is used to convert the rotary motion into linear, and there is a mechanical load on the linear part. The motors are only used in bursts of no more than 7 seconds. On some of the circuits we have current monitoring gear that records a trace on a graph. The graph generally shows an in-rush spike, then a low-current lull for around one second while lost motion is taken up. The current then increases to a steady 2A while the load is driven, until a switch removes the feed to the motor.

Here's the thing... on two particular mechanisms, the graph trace is showing the current dropping to zero for a fraction of a second, usually during the lull / lost motion period but not always. Our supplier has put this down to spring in the mechanism. He says that as the mechanism springs away from the drive, the motor is suddenly taken off-load so the current drops. But down to zero? I'm not sure I believe it.

Any opinions?
Thanks

 

[CHJ]

There isn't enough "spring/back pressure" that could in effect mechanically drive the motor in reverse for a few degrees by any chance.

Thinking in terms of motor behaving as a generator and the Back EMF cancelling the Incoming, result for a fraction of a second No current flow.

 

[woodfarmer]

I think maybe this is when the mechanism goes "over the hump", My diesel car when on the overrun does not inject any fuel at all so shows (temporarily) 999 MPG

 

[CHJ]

As a matter of interest are these old stile brushed universal motors or permanent magnet.

 

[DTR]

There isn't enough "spring/back pressure" that could in effect mechanically drive the motor in reverse for a few degrees by any chance.

Thinking in terms of motor behaving as a generator and the Back EMF cancelling the Incoming, result for a fraction of a second No current flow.

I wouldn't have thought so; if there is then there's probably a serious defect in the load! The motor is connected directly to a friction clutch, so any obstruction to the mechanism will cause the clutch to slip while the motor continues to drive. I don't think it would be possible to mechanically drive the motor backwards. If the motor suddenly comes off load and is freewheeling, could that cause a back EMF? Incidentally, the monitoring system will only measure a positive current, so if the back EMF outweighs the supply by any amount then it will register as zero.

The motor is indeed a brushed motor.

 

[DTR]

I think maybe this is when the mechanism goes "over the hump", My diesel car when on the overrun does not inject any fuel at all so shows (temporarily) 999 MPG

If you can figure out how to sustain it, you're onto a winner! Only drive down hill, perhaps?....

 

[CHJ]

...If the motor suddenly comes off load and is freewheeling, could that cause a back EMF? ...

Is the motor series wired, shunt wired or compound? there is a big difference in effect.

The Series wired will have high start torque but can in theory run to self destruct if no load (no back emf effect.)

Shunt wired should within reason self regulate speed (think back EMF cancelling field coil supply in simplistic terms and loses magnetic field)

Compound (both series and shunt fields) gets very theoretical but should give higher start torque and self regulate.

At least I think that's the theory, going back too many years.

 

[DTR]

I'm 99% certain it's series wired, but I can't confirm until I'm back in the office tomorrow. The machine's predecessor certainly was.

 

[woodfarmer]

It does not need to go backwards to generate electricity.

 

[DTR]

It does not need to go backwards to generate electricity.

No, but if it's spinning in the same direction that the supply is trying to drive it in, surely the generated current will be the same polarity?

 

[woodfarmer]

It does not need to go backwards to generate electricity.

No, but if it's spinning in the same direction that the supply is trying to drive it in, surely the generated current will be the same polarity?

The generated voltage will be the same polarity. and if you apply 12 + to a 12+ potential there is no differential to allow current to flow. just as if you connected two 12v batteries together in parallel.

Or look at it another way, if you applied power to a dc motor that was locked, nothing would move and it could not generate electricity because none of the fields would be changing, Under these steady state conditions the windings would only have their electrical resistance which is very low and excessive current would flow until something failed.
If the motor was free to spin with little mechanical resistance the flux changes would produce a little less power to the power being used to spin it. The windings now have high impedance due to the changing flux and generation by the motor. If you were to spin it even faster from some outside source eventually the power produced would be more than that which was used to keep it spinning. It would then be a generator and would try to feed power into the original source.

 

[SPSlick]

Reversing the polarity always works on TV

 

[DTR]

Or look at it another way, if you applied power to a dc motor that was locked, nothing would move and it could not generate electricity because none of the fields would be changing, Under these steady state conditions the windings would only have their electrical resistance which is very low and excessive current would flow until something failed.
If the motor was free to spin with little mechanical resistance the flux changes would produce a little less power to the power being used to spin it. The windings now have high impedance due to the changing flux and generation by the motor. If you were to spin it even faster from some outside source eventually the power produced would be more than that which was used to keep it spinning.

Ok I think I get it now, thanks