motor serial connection

[rbarbali]

Hi.
I found two single phase 110 Vac asynchronous motors (fans) connected in series to a 220 Vac line, something like this:

live 220 Vac ----------- 110 Vac motor -------------- 110 Vac motor ---------------------neutral 220 Vac

Is there any problems with this connection? the motors are working.
Someone can explain to me what happens if one of the motors try to get more current than the other?
Or if there´s an unbalance of any type?

rbarbali

 

[Lucien Nunes]

In the most general case of a motor (say with a permanent-magnet DC motor) current is proportional to torque, while voltage is proportional to speed. Series connection makes the two currents equal, therefore tends to make the two torques equal but allows the speeds to differ. However, with induction motors the speed is approximately proportional to frequency, so if the load torque/speed curves are similar, both the speeds and torques will be reasonably similar.

Series connection for identical pairs of small axial fans works particularly well because the torque/speed curve of the loads is similar, and the current doesn't vary very much across all working conditions of the motors because of the high leakage reactance. The voltage divides fairly equally even with differing airflows and the speeds are similar.

Most other types of applications cannot use this method of connection because varying load causes the voltage to divide unequally. Some configurations would be completely uncontrollable and unstable. An exception is where separately-excited motors are mechanically coupled together to run at the same speed. The series connection then tends to cause all the motors to deliver the same torque and hence share the load.

 

[rbarbali]

Thank you Lucien, you're very clear.
In the case of mechanically coupled motors of different power capacities, both motors deliver same power? Let's say we have 2 motors of 10 hp and 15 hp, what happens if the required load is, say 25 hp? the smaller motor can't deliver 12.5 hp, the bigger motor delivers 15 hp? or the smaller motor slows down the entire power, acting like a brake?

 

[Lucien Nunes]

If the motors are not identical then how the power divides will depend on the torque constants, not on the maximum ratings. If one motor delivers twice as many newton-metres per amp as the other, then it will take 2/3 of the torque load, regardless of whether it is rated to do so. There is no guarantee that one motor would not be overloaded unless designed for this duty.

If they are connected correctly, neither motor will act as a brake because the torques must always be in the same direction (because the currents are). You could get one motor acting as a brake to the other if they were connected in parallel, and had differing speed constants.

 

[rbarbali]

If the motors are not identical then how the power divides will depend on the torque constants, not on the maximum ratings. If one motor delivers twice as many newton-metres per amp as the other, then it will take 2/3 of the torque load, regardless of whether it is rated to do so. There is no guarantee that one motor would not be overloaded unless designed for this duty.

If they are connected correctly, neither motor will act as a brake because the torques must always be in the same direction (because the currents are). You could get one motor acting as a brake to the other if they were connected in parallel, and had differing speed constants.

Thanks a lot, Lucien!

 

[Lucien Nunes]

I should add that this is all a simplification based on motors with ideal characteristics. With AC motors, the magnetising and power currents are in quadrature; when connected in series only the vector sum of these is equal, not the individual components of the current. Therefore dissimilar characteristics can cause a phase difference between the motor voltages, further complicating the behaviour. For the two small identical fans things are relatively tame because of the high resistance and this is why it is one of the few situations where such a connection would be considered.