AC Motor Sizing Calculation 2

[davecu73]

I am trying to calculate the motor size for a variable duty cycle. I have four different sections within the cycle, each with their own required power. I want to use a squirrel cage induction motor and hence overload it during the short peak cycles. I was wondering, after taking the RMS power requirement of the cycle, what other motor factors do I consider. For example if it is 3ph, power factor, efficiency etc. and is there any website that would provide me with the standard equations to allow me to carry out the sizing task of the actual motor from only kowing the duty cycle power requirements

Thanks

 

[gcaudill]

What HP, speed, and voltage are you working with?

 

[davecu73]

The required duty cycle(this is the requirement at the pump)is as follows:
2.9HP for 32% of the cycle time
9.1HP for 18%
2.5HP for 32%
8.7HP for 18%

The speed is around 1500rpm

The voltage is typically 400V

 

[DickDV]

Since your only commercial choices are 5, 7.5, and 10hp, I would expect that a 7.5hp would tolerate this duty cycle. A 10hp would clearly be adequate and, for the small increase in cost, would likely be a good choice.

While the 5hp would be able to provide the torque required, I doubt that it would have the thermal capacity especially at 40 degrees C and for long periods of cycling.

 

[gcaudill]

Using a NEMA general purpose motor the HP ratings usually go 5, 7 1/2, 10, 15.... Standard RPMs are 1800 and 3600.

To use a 7 1/2 HP motor the service factor would have to be at least 1.25 to accomodate the 9.1HP peak load.

The Duty rating of the motor defines the amount of time (usually in minutes) for which the motor can safely handle the nameplate rated load. Most motors are rated for continuous loading.

If your process requires 1500RPM you will either need a variable speed drive or a gear box. If you are using a variable speed drive I would recommend you go with the 10HP motor and appropriately sized motor drive. In this case you will realize good operating efficiency and power factor for over your operating range.

 

[Servus]

Agree with the rest. Unless you're operating in a freezer, drop the extra $100-150 and get the 10HP. In fact, even if you WERE operating in a low ambient, I'd still recommend the 10HP. There's no need for RMS considerations.

 

[jraef]

Im curious, you are stating the load in HP, yet you seem to be basing it on IEC motor standards such as 1500RPM and 400V. is this a North American application or worldwide? If the latter, your HP should be converted to kW and motors sized from there.

"Venditori de oleum-vipera non vigere excordis populi"

 

[davecu73]

The power is indeed in kW but I converted to HP as asked.
I am based in the UK so applying european standards.

I understand why replies have given various sizes but I need to know the method of selection, formulae and factors to be considered, a sort of step by step procedure.
This is only one of many motors to be sized.

Hope this clarifies my confusion!! )

 

[aolalde]

My opinion is that engineering evaluation of cyclic loads should be based on calculation instead of guessing or gut feelings. There are lots of failed applications due to improper motor selection.

RMS average should consider the start stops per hour and load inertia as well as stand still time without cooling ventilation. The maximum load torque should not exceed 75% of the motor breakdown torque. Visit

http://www.mech.uwa.edu.au/DANotes/motors/unsteady/unsteady.html

for some help.

 

[rbulsara]

Refer to the follwing link. The duty cycle is discussed at the end.

It essetially says you need to contact the manufacturers because it is not a straight forward calculation item.

http://www.patchn.com/motorformula.htm

 

[artgodfrey]

I have seen and have in pdf file format a 1993 GE Selection and Application Guide that covers varying motor duty cycles. They suggest an RMS HP calculation. Of course you'd want to consider the worst case torque required to start.

 

[UKpete]

no1sun, some points to note:

1. you can only apply an RMS calculation if the actual time duration of each part of your duty cycle is short relative to the thermal time constant of your motor (e.g. if the total duration of the cycle is several hours, then you must rate the motor to the highest individual power requirement).

2. if using an induction motor you must satisfy the requirement mentioned by aolalde that the maximum load torque mustn't exceed about 75% of the maximum breakdown torque of the motor.

3. If your complete duty cycle is performed at constant speed, it is acceptable to use a motor rating calculated from the RMS rating of power. However, if each part of your duty cycle is at different speeds (say, you are using a VFD) you CANNOT do this - high power at low speed means a high motor current and high motor temperature. In this case, you should use the RMS motor current to determine the motor rating.

4. provided the above conditions are met, the RMS calculation can be performed using a standard RMS equation such as found in:

http://en.wikipedia.org/wiki/Root_mean_square

5. as you are dealing with IEC motors, forget references to service factor as we don't have this.

 

[mc5w]

Some other things to watch:

1. When operating on a duty cycle National Electrical Code Table 430.22(E) requires oversize conductors. The oversizing requirement should also be applied to the ability of the motor controller to operate a resistance heater. This is because repetitive peak currents can spot weld contacts turning a motor controller or circuit breaker into something that isn't. For a continuous duty motor in duty cycle operation the wire ampacity is 140% of motor nameplate rather than 125% for a normal application. You would need 10 HP controllers, circuit breakers, and wires anyways. For varying duty such as a roller coaster hoist chain motor the conductor requirement is 200% of motor nameplate rating.

2. Overloading a motor is hard on it's efficiency and life. Since MOST 3 phase motors get best efficiency at 2/3 to 3/4 of nameplate rating, a bigger motor doesn't hurt as long as it it not too big.

3. One of the US punch press manufacturers (Niagra) says that the peak current of a motor in a punch press application should NEVER exceed the nameplate rating. This is because the rotor cannot dissipate heat as easily as the stator even if the motor is open dripproof and cooling air is running through the rotor. Second Law of Thermodynamics. This also takes into consideration the rating of mechanical components as well. Punch presses generally run at 10 to thousands of mechanical cycles per minute.


By the way, punch presses use pneumatic pistons to counterbalance the weight of the ram and die. By adding a little extra air pressure the pneumatic counterbalances store energy when the ram is going down and then help the motor reaccelerate the system when the ram is going up. You can even get automatic controllers that will measure the cyclic motor current and automatically experiment with counterbalance pressure to minimize peak motor current.

4. You also did not state now much time would be spent during the heavy load periods. This is also a factor in sizing the motor.

Mike Cole, mc5w@earthlink.net