one soft start motor controller with 2 motors

[winslowfam]

Our client is designing a large conveyor system that requires 700HP to run. Because of limited physical space, they would like to run it with (2) 350HP motors, equally sharing the load. To save money, they are considering installing one 700HP soft start motor controller to run both motors. From what I've read, this is a common practice. In my review of the code, I know we must design for one (1) overload device per motor, and then it seems that if we follow the requirements of Art. 430.53(C) for it all, we should be fine. That Article specifies that the main short circuit protection rating for the motor controller must be sized based on Table 430.52 for the first motor (250% using inverse time breaker) plus add the FLA of the second motor...the conductors to each motor are protected by each overload device...and all components are to be listed, etc. My question is: for sizing the feeder breaker and conductors from the switchboard to the soft start, do I follow Art. 430.24 for the conductor sizing and then size my feeder breaker accordingly? or do I size the conductors as if it were one motor circuit...at 125% of FLA of 700HP?

Thanks!

 

[kwired]

I haven't done this, but would think you would basically install a feeder supplying two motor circuits as usual and install the soft starter in the feeder portion. Each individual motor still needs short circuit and ground fault protection as well as overload protection. You very well may extend controls from each individual motor to control the soft start/feeder if it is necessary to shut down the second motor if the other one shuts down for any reason but that is a design application and not a code requirement.

 

[winslowfam]

Yeah...I think so, too. But, I think I'm allowed to use one short circuit protection within the soft starter cabinet for the soft starter & both motors...sized per 430.53(C)(4). It would be the main breaker in the soft starter cabinet. Then, include an overload device for each motor in this same cabinet.

 

[Besoeker]

Our client is designing a large conveyor system that requires 700HP to run. Because of limited physical space, they would like to run it with (2) 350HP motors, equally sharing the load. To save money, they are considering installing one 700HP soft start motor controller to run both motors.
Thanks!

How would you ensure that they equally share the load?

 

[winslowfam]

I believe that our client will have them mechanically connected to share the load equally.

 

[eric9822]

How would you ensure that they equally share the load?

That is what I was thinking.

I believe that our client will have them mechanically connected to share the load equally.

I am not sure how that would work. My first thought is VFD's with a speed master and a torque follower. This may help. http://www.wmea.net/Technical Papers/Rockwell--Medium Voltage Drive on a Conveyor.pdf

 

[Besoeker]

That is what I was thinking.


I am not sure how that would work.

If the motors are identical and mechanically coupled, it would be reasonable to assume that they would run at the same slip and load.

 

[eric9822]

If the motors are identical and mechanically coupled, it would be reasonable to assume that they would run at the same slip and load.

True enough. I have just never tried having two motors drive the same load without a VFD and a load sharing control scheme. I guess I should get out more.

 

[kwired]

Yeah...I think so, too. But, I think I'm allowed to use one short circuit protection within the soft starter cabinet for the soft starter & both motors...sized per 430.53(C)(4). It would be the main breaker in the soft starter cabinet. Then, include an overload device for each motor in this same cabinet.

Read 430.53(C) carefully. Subpart 4 is not something that stands by itself you need to comply with other items as well. A part of 430.53(C) says:

shall be permitted
to be connected to one branch circuit where the motor
controller(s) and overload device(s) are (1) installed as a listed
factory assembly and the motor branch-circuit short-circuit
and ground-fault protective device either is provided as part of
the assembly or is specified by a marking on the assembly, or
(2) the motor branch-circuit short-circuit and ground-fault protective
device, the motor controller(s), and overload device(s)
are field-installed as separate assemblies listed for such use
and provided with manufacturers? instructions for use with
each other, and (3) all of the following conditions are complied
with:

sub part 4 is one of those mentioned following conditions

 

[Besoeker]

True enough. I have just never tried having two motors drive the same load without a VFD and a load sharing control scheme. I guess I should get out more.

I don't recall that it's something I've tried either and I've been in the motors and drives business for forty odd years. But if the two motors are directly and solidly coupled the proposed arrangement should work fine in my opinion.