DC Motor Overspeed: Cause & Effect

[brent1369]

i don't get how a dc motor will overspeed when you don't give the field windings power
cause you need flux to make the motor move so how will taking away the flux make the motor go super fast

 

[berkeman]

i don't get how a dc motor will overspeed when you don't give the field windings power
cause you need flux to make the motor move so how will taking away the flux make the motor go super fast

Welcome to the PF.

What makes you think it will over-rev if there is no electrical or mechanical power input? Can you post a link to where you are reading this?

 

[brent1369]

Welcome to the PF.

What makes you think it will over-rev if there is no electrical or mechanical power input? Can you post a link to where you are reading this?

i find it everywhere, and there is electrical input on the rotor winding but none on the stator aka field winding

 

[cnh1995]

i find it everywhere

For a dc motor,
speed is proportional to back emf / field flux. If you reduce the field-flux of an already running motor, its speed increases. If you reduce it to a very low value, the speed becomes dangerously high. Note that you can't actually reduce the flux to 0 as it will simply stop the motor because there will be no torque production (and you get the 0/0 indeterminate form).

Similarly, if you "start" the motor at a lower value of flux than before, it will run at a higher steady-state speed than before (assuming same load torque in both cases).

 

[Averagesupernova]

It's a well known fact that losing the field current on a shunt wound motor will cause the motor to over rev.

 

[brent1369]

now i still don't get it if you lose field current you will have no flux right ? so how can the motor overspeed then

 

[brent1369]

cause F= I armature x B
so if no induction there is no force
and also decreasing the voltage of field will lower induction so also u lower Er so the current would increase and also the speed.
but wouldn't there be a limit to where decreasing the voltage of field will decrease the rotor speed, cause the flux and Er would be so low?

 

[Averagesupernova]

https://www.google.com/search?q=los...droid-verizon&sourceid=chrome-mobile&ie=UTF-8

 

[anorlunda]

now i still don't get it if you lose field current you will have no flux right ? so how can the motor overspeed then

You are ignoring a key phrase in the answers, Shunt Wound. Not all DC motors are alike.

See this article that explains different types.

https://en.wikipedia.org/wiki/DC_motor#Wound_stators

 

[Merlin3189]

Perhaps you also need to consider hysteresis in the iron core of the field windings. If you remove the current, a remanant field will remain.

 

[Swamp Thing]

Perhaps you also need to consider hysteresis in the iron core of the field windings. If you remove the current, a remanant field will remain.

I think this is the only convincing way to explain how the motor could turn at all when the field circuit is actually open.

Edit : Come to think of it, would it turn if the field is open? It's possible that you need to short the field winding in order to see this effect. If this is true, then it could be something to do with an induction (armature reaction) effect of the turning rotor acting on the field poles, somewhat akin to induction motors.

 

[Averagesupernova]

Keep in mind no one has said it will over speed to dangerous speeds with a heavy load. Most likely result will be a sudden increase in current and blown fuse/breaker or damage to the motor.

 

[essenmein]

Less field = more speed is a counter intuitive effect.

I kind of didn't believe it till I got to play with an externally excited synchronous machine, reduce field, unloaded speed goes up, increase field, it slows down.

Another counter intuitive thing is if you want more torque, even if in field weakening region, you need more field and more field weakening.

 

[essenmein]

Then there are two ways of generating torque, magnetic torque and reluctance torque. Most machines will have a combination of both, so you can easily have significant torque generated with zero field current (note this is synchronous driven by inverter).

 

[anorlunda]

I kind of didn't believe it till I got to play with an externally excited synchronous machine, reduce field, unloaded speed goes up, increase field, it slows down.

Huh? If a synchronous machine is syncrhonized to the grid, you can't change its speed. By unloaded, do you mean open circuit?

 

[essenmein]

Huh? If a synchronous machine is syncrhonized to the grid, you can't change its speed. By unloaded, do you mean open circuit?

No grid here! I control the frequency.

So this is a case where you command 100% iq and 0% id, since there is no load, machine speed increases rapidly until you have reached maximum voltage. Since still controlling id to 0, there is no field weakening.

Once in this state, changing the DC link voltage (and therefore the maximum voltage available to the machine) will change the speed (increase voltage, increase speed), and changing the field current changes the speed, reduce field, increases speed.

 

[essenmein]

Huh? If a synchronous machine is syncrhonized to the grid, you can't change its speed. By unloaded, do you mean open circuit?

Also the acceleration in this 100% iq condition is crazy, 100%iq was about 105Nm torque. This is enough to accelerate the machine shaft and the attached dyno inertia to about 3000rpm in less than 360deg mechanical.

 

[anorlunda]

No grid here! I control the frequency.

I don't understand. Are you running with a resistive load? Is there a speed governor? How are you controlling the frequency?

 

[essenmein]

I don't understand. Are you running with a resistive load? Is there a speed governor? How are you controlling the frequency?

All of the machines I've worked with are inverter driven... Traction drives/hybrid etc.

 

[essenmein]

More or less this general topology:

 

[anorlunda]

OK, but in that case:

I kind of didn't believe it till I got to play with an externally excited synchronous machine, reduce field, unloaded speed goes up, increase field, it slows down.

that statement is entirely or partly function of your test setup, not a statement about all synchronous machines.

I think the only universal statement you can make about the speed is that the rate of change of speed is proportional to the mechanical - electric power mismatch. Both the mechanical and electric powers can be complex functions of things external to the synchonous machine, not necessarily a function of the machine itself.

 

[essenmein]

Sorry, but the effect is entirely analogous to a DC machine. Max speed is determined by available voltage and the induced BEMF, forget frequency, the inverter and position sensors take care of that. You described the thing perfectly with the cowboy riding the rotor in the other thread, the controller cares naught about frequency, it doesn't even know frequency, you are regulating from the view of the cowboy.

So at full field, BEMF reaches maximum voltage at a lower rotating speed than it does if you reduce the field current.

Parasitic load does obviously affect this speed, but fundamentally its the reducing BEMF per rpm as you reduce field current that increases the machine speed.

 

[Averagesupernova]

I think the only universal statement you can make about the speed is that the rate of change of speed is proportional to the mechanical - electric power mismatch.

Not so. One property of a shunt wound motor is constant speed regulation with a varying mechanical load. It's not perfect of course but many times better than a series wound motor.

 

[anorlunda]

One property of a shunt wound motor is constant speed regulation with a varying mechanical load.

Sorry, that post was responding to comments about AC synchronous motors, not DC motors of any type including shunt wound.

It is very easy to get lost in threads because there are many post commenting on other comments rather than the OP subject. If it becomes extreme, the mentors will take actions, but even in the best threads we can't restrict comments to those which reply only to the OP.

 

[Averagesupernova]

Sorry, that post was responding to comments about AC synchronous motors, not DC motors of any type including shunt wound.

It is very easy to get lost in threads because there are many post commenting on other comments rather than the OP subject. If it becomes extreme, the mentors will take actions, but even in the best threads we can't restrict comments to those which reply only to the OP.

I couldn't agree more. Sometimes a lot more things are learned from an offshoot of the original question. After I posted I wondered if I had gotten my threads mixed up. Would have felt pretty silly then.