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Loads driven by Twin motors - What are the motors' requirements ?

Loads driven by Twin motors - What are the motors' requirements ?

(OP)
I have seen that in the loads (cement mills, sugar crane crushers etc.) requiring two motors (connected at the both ends of the driven load shaft), both the motors are from the same manufacturer and are theoretically identical ones.

Questions

1. What happens if one of these motors is replaced by one from another manufacturer (with same KW & speed etc.) ?

2. Is it possible to have two motors with different power ratings (say 1200 KW and 800 KW) whose total KW will match the required load KW (2000 KW) ? If yes, how will be the load sharing between these motors ?

3. Do the slip/torque curves play a role in these twin motors application ?

Thanks in advance.


 

Muthu
www.edison.co.in

RE: Loads driven by Twin motors - What are the motors' requirements ?

Are these motors speed controlled in any way? With separate controllers?

If not, they will share load but the sharing is entirely dependent on the speed/torque characteristic. To find the approximate load sharing, do this:

1. Draw speed/torque curves for the two motors in one diagram. Single straight lines from synch speed to 1/1 load speed will do.
2. Draw a vertical line from 1/2 torque (as if both motors shared equal).
3. The vertical line cuts the two speed/torque curves at different heights.
4. Draw a horizontal line through the mid-point (between where the line cuts the speed/torque curves).
5. Read respective loads from where the horizontal line cuts the two motor speed/torque curves.
 

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Loads driven by Twin motors - What are the motors' requirements ?

2
Yes, the torque vs slip curves determine load sharing since of course the 2 motors operate at the same speed.

s1np = Full Load Slip of motor 1
T1np = Full Load torque of motor 1
s2np = Full Load Slip of motor 2
T2np = Full Load Torque of motor 2

Tnp/snp is the slope on the torque vs slip curve. So we can see that for a given slip, the ratio of torque (and power) carried by motor 1 compared to motor 2 must be:
Ratio  = (T1np/s1np)/(T2np/s2np) = (T1np / T2np)*(s2np/s1np)

=====================================
(2B)+(2B)'  ?

RE: Loads driven by Twin motors - What are the motors' requirements ?

Simple answer;
If the full load speed of the motors is the same they will share full load in proportion to their HP rating.
Caveat; If the actual full load speed is not quite the same as the rated full load speed, the sharing won't be exactly proportional.
If the smaller motor has greater RPM slip than the larger motor, it will assist but will not develop full power.
If the smaller motor has less slip than the larger motor, it will reach full load before the larger motor and may limit the torque available to less than the large motor acting by itself.
If sharing seems to be not as calculated, check the actual RPMs against rated RPMs.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Loads driven by Twin motors - What are the motors' requirements ?

(OP)
Thanks guys.

I am looking at torques of two different motors with two different KW ratings at the same speed. Won't the torques be different ? So won't the bigger torque motor be driving the other motor (and the load) rather than both of them driving the load ?

Have you seen such a combination of two different KW motors driving the same load ?

Muthu
www.edison.co.in

RE: Loads driven by Twin motors - What are the motors' requirements ?

Give us the data. Than we can answer.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Loads driven by Twin motors - What are the motors' requirements ?

Quote:

two different motors with two different KW ratings at the same speed
If you're saying nameplate speed is exactly the same, then the motors should share load in proportion to their steady state kw rating, which is a good thing.  For example
motor 1 = 100kw at 1490rpm
motor 2 = 200kw at 1490 rpm.

As we increase load of the driven machine we might expect to see the following progression of load sharing:
motor 1  motor 2
10kw   20 kw
20kw   40 kw
50kw   100 kw
100kw  200 kw

That assumes the nameplate speeds are accurate, which is not always a great assumption. NEMA requires them accurate to within 5rpm, which can be a pretty big difference in this situation.
 

=====================================
(2B)+(2B)'  ?

RE: Loads driven by Twin motors - What are the motors' requirements ?

(OP)
Gunnar - It's a generic question. So no data.

pete - My question was about the different torques assuming both the motors have identical speeds.

Muthu
www.edison.co.in

RE: Loads driven by Twin motors - What are the motors' requirements ?

Clarification in bold:
For example
motor 1 = 100kw at 1490rpm
motor 2 = 200kw at 1490 rpm.

should have been
For example nameplate data is:
motor 1 = 100kw at 1490rpm
motor 2 = 200kw at 1490 rpm.

Quote:

My question was about the different torques assuming both the motors have identical speeds.
What's wrong with that? Motor 2 will deliver twice as much torque as motor 1 during steady state, which again is what we want.

During starting, the load sharing would not be as predictable.   

=====================================
(2B)+(2B)'  ?

RE: Loads driven by Twin motors - What are the motors' requirements ?

Nice! Pete.
 

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Loads driven by Twin motors - What are the motors' requirements ?

Thanks Pete.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Loads driven by Twin motors - What are the motors' requirements ?

(OP)
Thanks Lyle. That was quite useful.  

Muthu
www.edison.co.in

RE: Loads driven by Twin motors - What are the motors' requirements ?

Thanks Lyle.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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