Motor torque/power

[cs.dk]

Hi,
I've got a theoretic question.

If we have 2 almost identical motors, lets say @3kW. One is labeled 230V/400V, the other 400V/690V - If we hook'em up to 3x400V, first motor in star-configuration, the second in delta-configuration. Lets assume that they have same ammount of poles, will they perform the same torque/power? What if we slow them down or speeding them up with a VFD - Will they still behave like 2 identical motors, though they are different??

Thanks in advance.

 

[Besoeker]

Hi,
I've got a theoretic question.

If we have 2 almost identical motors, lets say @3kW. One is labeled 230V/400V, the other 400V/690V - If we hook'em up to 3x400V, first motor in star-configuration, the second in delta-configuration. Lets assume that they have same ammount of poles, will they perform the same torque/power? What if we slow them down or speeding them up with a VFD - Will they still behave like 2 identical motors, though they are different??

Thanks in advance.

Yes, pretty much unless there are differences in efficiency.

And welcome to the forum!

 

[m sleem]

Hi,
I've got a theoretic question.

If we have 2 almost identical motors, lets say @3kW. One is labeled 230V/400V, the other 400V/690V - If we hook'em up to 3x400V, first motor in star-configuration, the second in delta-configuration. Lets assume that they have same ammount of poles, will they perform the same torque/power? What if we slow them down or speeding them up with a VFD - Will they still behave like 2 identical motors, though they are different??

Thanks in advance.

Torque is relative to ampere, and ampere is not the same between star and delta connection.

 

[Besoeker]

Torque is relative to ampere, and ampere is not the same between star and delta connection.

Power = torque times speed.
P = Tw (w is omega)
Same P, same w, same T.

?

 

[winnie]

To expand a bit on besoeker's answers:

There are a huge number of ways that motors with the same nominal ratings (voltage,horsepower, speed) can be different. But you can eliminate these differences from the discussion by assuming the same motor frame simply connected differently for different voltages.

Consider a motor with _lots_ of connections brought out, enough that you could connect it in 400V wye or 400V delta. (Okay, theoretically this is impossible, because sqrt(3) is not a rational number...but you can get close.)

In any of the possible connections, when connected for proper voltage, the voltage across each coil will be correct, and the torque/current curve for each coil will be the same (as for any other possible connection with its corresponding correct voltage).

When the over all connection is for 400V, no matter that the internal connection is wye or delta, the torque/current characteristic will be essentially the same.

Perhaps a deeper question: given two motors with identical frame (stator steel, rotor, copper slot fill, etc), one motor connected for 400V wye, the other wound and connected for 400V delta, both placed under the same load and provided with the same supply, what _differences_ in operation could be expected? IMHO to a good approximation the operation would be the same, but I expect second order effects that could be different.

-Jon

 

[mike_kilroy]

Perhaps a deeper question: given two motors with identical frame (stator steel, rotor, copper slot fill, etc), one motor connected for 400V wye, the other wound and connected for 400V delta, both placed under the same load and provided with the same supply, what _differences_ in operation could be expected? IMHO to a good approximation the operation would be the same, but I expect second order effects that could be different.

-Jon

No Jon, they will both operate the same. Period. Same answer as for original post.

 

[cs.dk]

Yes, pretty much unless there are differences in efficiency.

And welcome to the forum!

Thanks for the replys, and the welcome:thumbsup:

 

[winnie]

No Jon, they will both operate the same. Period. Same answer as for original post.

As I said, I believe that they will operate the same to a good approximation, but I suspect that second order differences in operation are possible.

For example, I suspect that the response to internally generated third harmonic currents will be different. 3rd harmonic current can't circulate in a wye connected machine, but can circulate in a delta connected machine.

I'd expect slightly different response to unbalanced supply, and possibly other 'second order' effects.

By 'second order' effects I am using shorthand for effects that vanish in idealized circumstances, and only show up as small percentages of the non-ideal aspects of real machines.

-Jon

 

[m sleem]

Where we are using the star-delta connection to soften the starting current, would that mean the motor's performance will be different in either cases?

 

[Besoeker]

Where we are using the star-delta connection to soften the starting current, would that mean the motor's performance will be different in either cases?

If you apply the same voltage in either case, then yes.
In star, the torque will be down to about one third.

 

[m sleem]

If you apply the same voltage in either case, then yes.
In star, the torque will be down to about one third.

That's what i meant, the OP says two identical motors, one is star and the other is delta connection, where the connection is different at the same voltage level, then the performance will be different accordingly due to the torque difference, Excuse me if my explanation is not comprehensive. I almost forgot the basic formulas.

 

[cs.dk]

That's what i meant, the OP says two identical motors, one is star and the other is delta connection, where the connection is different at the same voltage level, then the performance will be different accordingly due to the torque difference, Excuse me if my explanation is not comprehensive. I almost forgot the basic formulas.

I wrote 2 different motors (different windings), all other specs are equal.
The motor in star config can handle 230V pr. winding. The motor in delta config can handle 400V pr. winding. - Both will be connected to 3x400V.

If I couple motor 1 in delta, the windings will burn out.
I'm aware that motor 2 coupled in star will have reduced torque. But thats out of the question.

 

[Besoeker]

That's what i meant, the OP says two identical motors, one is star and the other is delta connection, where the connection is different at the same voltage level, then the performance will be different accordingly due to the torque difference,.

Not identical. Almost identical.
The first is 230/400 so the windings are 230V and need to be connected in star to accommodate a 400V supply.
The second is 400/690 so the windings are 400V and need to be connected in delta to accommodate a 400V supply.

The motors are not identical.

 

[CIECO]

Same voltage to the coils

Same voltage to the coils

The motors will act the same as long both motors are of the same design. The winding only operates on one voltage it the connection that connects the winding in series or parallel.

 

[Besoeker]

The motors will act the same as long both motors are of the same design. The winding only operates on one voltage it the connection that connects the winding in series or parallel.

Not sure what you're asking.
Same speed and same power gives the same torque provided rated voltage is applied to the windings.