Motor selection

[yurilevi]

Hello,

May be somebody who has experience can point me generally what type of motor I need for my application.
There are the requirements:

vertical mounting
rotor speed: 5000[rpm]
rotor mass: 600[kg]
rotor moment of inertion: 60[kg*m^2]
acceleration time: 10-15[min]

The motor is for testing system and will operate without load, i.e. only rotate the rotor at 5000[rpm].

Thank you

 

[bgilbert]

What you want to know first is the amount of torque you need to accelerate your inertia. You want to accelerate very slowly your inertia(within 10-15 min), but the inertia is very large:

Torque = Inertia x Speed / Time

You have
Speed = 524 rad/s (5000 rpm)
Time = 600 s (10 min)

Therefore:
Torque = 52.4 Nm
This torque would be accelerate continuously the inertia.

The other limitation will be the max motor speed. You will need a motor that can sustain over 5000rpm without overheating. You need to know that the motor will not be able to provide a continuous torque when it reaches its maximum continuous speed. Therefore you may want a motor that can provide higher torque (say 100Nm) while its maximum speed is about 6000rpm. With such a configuration you will accelerate faster at the beginning (using 100Nm) then accelerate slower when the speed increases. This is an example only, I did not do the maths.

Hope this helps to start.

 

[yurilevi]

Thank you, bgilbert. Your information was very helpfull.
According to the torque and the speed as you calculated the motor minimal power has to be 52.4*524=27.5[kW].
Because there will not be any load during operation, I am thinking that standard AC 30-40[kW] motor will made the job.

 

[ScottyUK]

A standard motor fed from a conventional starter will not be able to provide acceleration over such a long period without tripping the overload protection. It would be an ideal application for a variable speed drive which can deliver constant torque from standstill up to rated speed. Your drive motor will possibly require an auxiliary cooling blower because of the prolonged period when it is operating at full torque but low speed where the shaft-driven fan will contribute virtually nothing to the cooling of the motor.

Out of curiousity, what is the load - a large centrifuge?


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If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

Looks to me like 5000 rpm requires a VFD to supply 83.3 Hz or a universal motor.

 

[yurilevi]

For the tests we are using steel disk in order to simulate centrifugal compressor mass and inertion, therefore there will not be load once the final speed (5000rpm) is reached.

 

[ScottyUK]

David - assuming direct drive. Of course, everything else is assuming direct drive too. Don'tcha hate having to assume...?

Yuri - Dvaid makes an excellent point: synchronous and induction motors are limited to base speeds of 3000 rpm in 50Hz nations or 3600 rpm in 60Hz nations. You can also have fractions of these speeds, e.g. 1/2, 1/3, 1/4, etc. You could use a DC drive or an SR motor to reach 5000 rpm, but these would be very expensive when you come to build the compressor and need a larger motor. You could consider a belt drive to increase the motor speed in the ratio 5:3. There is also a possibility to reach 83Hz using a standard AC motor and VSD but these require an unusual configuration of the motor and motor, essentially connecting the motor windings in LV delta and running at the HV star voltage but at a frequency of [√]3x nominal.


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If we learn from our mistakes I'm getting a great education!

 

[yurilevi]

Now we are using for compressor (with the dimesion (mass and inertion) of the testing disk) 160[kWatt], 87[Hz], 400[V] motor. I'm not sure, but I'm thinking that the motor has independent cooling fan.
ScottyUK - I can't use belt. Only direct driving. You wrote that I can use standard AC motor and VSD to reach 83[Hz], but these require an unusual configuration of the motor. I'm sorry but I'm not familiar with electricity. The required changes are diffucult? At the motor manufactory can do this configuration?

 

[ScottyUK]

Well, it's certainly not difficult to reconfigure the motor from delta to star, but at 160kW you are likely to need either a standard 690V drive and a standard 400V/50Hz motor to use with it, or you will need a custom-built 230V/50Hz motor and use it with a 400V drive. Basically you are allowing the motor to run at a higher voltage than nominal by using it at a higher frequency. There is a linear relationship between voltage and frequency, up to the point where you get in trouble with insulation. If you maintain the V/Hz relationship then the available torque will remain constant.

More likely is that you will reach the mechanical limit of the motor - at 5000 rpm you will need to talk to the motor manufacturer and see whether they will warrant their machine for operation at such a high speed. Both the bearings and the mechanical integrity of the the rotor itself need to be assessed. Otherwise you are in the market for an expensive custom-built motor - 5000 rpm is well within the mechanical range for a synchronous machine, and likely achieveable with a purpose-built induction machine.


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If we learn from our mistakes I'm getting a great education!