Overdriven motor calculations

[mic]

I am from an existing product. We have a 20Hp 50Hz gearmotor being driven by a VFD at 100Hz. This allows us to use a smaller motor while getting the same torque. Also note that the duty cycle is very short (10% max, 3% normal).

My problem is that the original load calculation was based on the 20Hp rating but increasing the frequency (or torque) should increase the current. Let me know if my assumption is crazy.

Does anyone know the formula for calculating the actual power of the motor based on frequency of the feed? Thanks.

c

 

[petersonra]

Re: Overdriven motor calculations

most VFDs do not increase the voltage once they get past 50 or 60 hz (whatever it is set for), so the amount of torque available flattens out at that point, as does the current used.

besides, you should be sizing your BC and OCPD for the VFD not the motor. The motor is than protected by the VFD for OC.

[ September 16, 2005, 05:13 PM: Message edited by: petersonra ]

 

[mkbuck]

Re: Overdriven motor calculations

In general, motor torque begins to drop above the rated frequency.

In addition that the volts per Hertz ratio of the motor design cannot be maintained, motor torque will drop above base speed because the motor current will decrease.

The current draw of the motor decreases because the increase in frequency increases the XL (inductive reactance) and the increase in speed, increases the counter EMF generated by the motor.

The combination of the two, force down the motor current, thus reducing the torque because the voltage does not increase in proportion to the increased impedance.

The manufactures product information should give you some guidance. The VFD may have torque boost, but you can't beat the laws of physical science.

MB

 

[mic]

Re: Overdriven motor calculations

In a normal application if you increase the source freqency then the motor speed increases and the torque decreases.

In this case however, the load is sufficiently large such that the motor speed does not increase but instead the slip increases. This results in increased torque and current. This is how we can use an undersized motor in the application (and save big $). For example, an application that would normally require a 30Hp motor could use a 20Hp motor if the line freq. was doubled and (1- slip) was halved. Also note that the duty cycle is critical to avoiding overheating.

In the end all I need is the relation between the rated power and the power under these circumstances. I fear that this is not as simple as I would like, I may have to start at the beginning and determine the torque requirements and compare them to the motor spec. Hopefully the mfg has the torque/speed graph based on 100Hz line freq.

Thanks for the help.

c

[ September 18, 2005, 02:55 AM: Message edited by: mic ]

 

[oliver100]

Re: Overdriven motor calculations

In the speed-torque curve of every asynchronous motor there is a point point where the motor riches its maximun torque. If the break torque (load) exceeds the maximum capabilities of the motor, the motor stalls (burns).

The frequency increase reduces the maximum motor torque. With the same load (break torque) your leveredge is a lot smaller than at 50 Hz. Therefore, any disturbance in the eltro-mechanical system (voltage change or mechanical malfunction) can make the motor stalling. Furthermore, if the motor protection is adjusted to allow this 3-10% duty cycle overloaded usage ( and I' am sure this is the case), the motor will be in flames in no time.

Economically - you save on the motor size, but you are paying for VFD. I do not know what is chipper - modifying the ratio of the gear box, or VFD, or the right size motor.

Please share with us your final results.

 

[mic]

Re: Overdriven motor calculations

Please keep in mind that the speed/torque curve is based on a fixed line frequency, if you change the line frequency the curve changes. The synchronous speed increase proportionally and the speed of max torque also increased. We are operating above the speed of max torque (ie in the stable region)based on 100Hz curve so we are not concerned about stalling.

With regards to overheating, the motor would burn if we tried to run it continuously, but we have a small duty cycle with few reps (approx 1 minute every half hour).

Two things I must note about cost; the VFD is required for ramping capabilities and the gearbox is required to reduce the speed in both scenarios. The cost of the VFD does not vary significantly with power rating. The motors on the other hand increase exponentially with power.

Over all I am not concerned with the design, the motor manufacturer signed off on the design and it has since been tested extensively. What I was trying to do was some reverse engineering to determine the actual power requirements.

c