Motors in Tandem


Thread Starter

D. C. Pittendrigh

Hi All

This discussion on motors in tandem, is quite opportune as I am currently battling with a conveyor belt application where a 4M wide 1200M long belt driven by 4 motors using scoop actuated
fluid couplings (which when fully engaged) uses almost twice the KW in 2 of the motors, as it does in the other 2. Can anyone direct me to a source of information on this subject, or perhaps offer an explanation as to why and how to control this.

My current project deals with control of the scoop actuators using a PI controller and an enertron KW transducer, the idea is to bring in
the scoops in such a way as to share the load evenly between the 4 motors, allowing each motor to catch up to the average of the 4 motor feedbacks, before the next setpoint step is issued. This however seems a little futile, if the motors will never do the same work during fully laden operation.

Donald Pittendrigh

This looks like a standard load sharing problem. I would suggest using cascaded controller with a master speed control setting the load current references for the individual motors.

This is typical in long conveyor applications. There will always be one drive set that "hogs" the load (mechanical reasons). Do no try to "control" it with the scoop couplings by operating one set with the scoops partially
engaged. These are normally not sized to handle the load (thermally).

When you say the fluid couplings are fully engaged do you mean "locked-up?" If so, and if the 4 drives have identical characteristics, and they are fed from the same power source, then the overload of two could be caused by the phenomena I addressed earlier, ie, their respective rotors are improperly aligned when locked-up! Thus there is "give" as was the case that Kim Ground described (in "Re: APPS: Two motors assisting each other"). There is no "slip", excuse the pun, between drives. The problem is exacerbated if the motor speeds vary between sub- and hyper-synchronous speed.

Phil Corso, PE
Trip-A-Larm Corp
(Deerfield Beach, FL)
Off-liners have asked me to explain, in lay terms, why the "CAUTION" flag for tandem-coupled motor applications.

All electrical machinery have a "load-angle!" It doesn't matter that they are DC or AC, synchronous or induction, squirrel-cage or
slip-ring, operated sub- or hyper-synchronously, motor or generator, positive or negative-slip, and even transformers. Well, you get the point.

Q) Why then, is the phenomena not discussed? Or understood?

A) Because 99% of the applications involve just one motor. Very few applications involve a physical connection between machines. However, it does give meaning to the existence of slip-ring motors.

An example. What happens if like-motors are connected in tandem and their stators connected for the same rotation? Right... they turn the load in the same direction. (How well? That's the reason for this paper.) Now, connect them so their rotations are opposite. Right again... they won't turn at all.

Now, the simple explanation. The force, F, on a current-carrying conductor of rotor length, L, in a magnetic-flux field, B, and the current magnitude is I, is equal to BxLxI. Motors have two
elements, one the stator, the other, the rotor. Let the axis of Phase A's rotating flux-field have angle and magnitude (call it spatial vector, Fs). It interacts with the rotor, inducing current. Force is then exerted on the rotor conductors. Additionally, a rotor flux, hence spatial vector (call it Fr) is produced. Fs and Fr are separated by an angle a, called the "load angle."

The difficult part. The rotor's spatial flux vector is separated into a tangential and radial elements, called FrSina and FrCosa, respectively. The former gives rise to tangential force or torque. Fortunately, the latter causes equal but opposite forces on the rotor which cancel out. If a were equal to 0°, then there would be no
tangential force, hence no output. The tangential force affects each rotor conductor in a timed sequence, i.e., first one, then the next one, etc. Hence the expression, "squirrel-cage."

Now, the easy part. If rotors of tandem-coupled motors are in angular alignment, that is, their load-angles are the same, then their torques are additive. However, if displaced, their torques are
not additive. Mathematical analysis shows that for motors wired for the same rotation, but displaced by 120° (electrical) the net power output is only 1 per unit or equal to one motor. If displaced 180° (reverse rotation), then their torques are diametrically opposed. I asked list
members to provide details on cited conditions, but alas no feedback.

Q.E.D, perhaps Caveat Lector!

Phil Corso, PE
Trip-A-Larm Corp
(Deerfield Beach, FL)

Robert Miller

I work with 3 phase asynchronous motors often in dual or triple tandem. sometimes 5 motors (series connected) powering down hole submersible pumps in the oil field. HP can range from 50 to 500 hp per motor. Motor diameters range from 3.75" to 5.5" depends primarily on the installed casing size. Recently we have suffered broken shaft couplings between the motors. The investigating team was proposing that the motor phases were labelled incorrectly. From your discussion I understand that a dual motor system would not turn. What about a triple tandem system if one of the 3 motors was inadvertently connected for reverse rotation? How would wye or delta connection make a difference?