torque available from VFD controlled motors as function of speed

[GHELZER]

How big must a VFD controlled electric motor be to deliver a specified horsepower at a reduced speed.

If the driven equipment requires a torque of 4000 foot-pounds at 1200 RPM, then its required horsepower input is 914 HP.

If the equipment is driven by a 50 Hz electric motor with a nominal speed of 1500 RPM, and the VFD is used to reduce the motor speed to 1200 RPM, at which point it must deliver 4000 foot-pounds of torque to the driven equipment, what nominal (1500 RPM) horsepower motor is required?

What is the available torque as the speed of the motor is varied by means of a VFD. Does the torque decrease as speed decreases, stay constant as speed decreases, or increase as speed decreases.

The application is a reciprocating compressor where torque is essentially constant regardless of speed. As such, required power is directly proportional to speed.

 

[aolalde]

You need a motor 1142.5 HP at 1500 rpm.
The driver you need is a constant torque and the power will increase or decrease proportional to the speed.

HP = Torque (Lb-Ft) x rpm / 5252

 

[Marke]

Hello GHELZER

When you couple a VFD with and induction motor, there are constraints due to the iron circuit in the motor and the suplly voltage.

The major constrain with the induction motor, is that there is a limit to the flux that can be used within th iron circuit. If the flux is increased, the iron loss increases dramatically overheating the motor. Therefore, we must operate the induction motor at the maximum flux or less.
The motor is designed with maximum flux at rated frequency and voltage. As we reduce the frequency applied to the motor, we must also reduce the voltage applied to the motor to ensure the flux is not increased. The "constant" flux in the iron at speed less than rated speed limits the torque output to rated torque at full speed. This cuases the power rating of the motor to reduce directly with frequency (speed) reduction.
Above rated speed, the output voltage is limited to the supply voltage and so the flux falls with increasing frequency. This results in reducing torque but constant power rating.

If your motor is always going to operate at the lower speed, you must be careful of the motor cooling. The internal fan will be operating at a lower speed and this will cause an increaed fram temperature. The motor is capable of operating at rated torque at reduced speed, but continuous operation may require auxiliary cooling.

As an alternative to a 4 pole motor operating at reduced speed, you could consider a 6 pole motor operating overspeed. That way you can use a "914"HP motor and have no cooling issues.
The disadvantage of this is that the output waveform would be less sinuoidal and there would be an increase in motor losses due to the additional harmonics, but this is a common technique and does not prove to be an issue unless you are operating the motor at its full ratings continuously.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[DickDV]

This analysis needs to be done on the basis of torque, essentially ignoring horsepower or kw.

A standard induction motor on an inverter can develop its base speed rated torque at all speeds lower than base speed subject, of course, to speed limitations due to overheating. The slowest speed of thermal stability varies with the motor design and capacity so it needs to be evaluated carefully for each motor application, especially in the larger sizes such as we have here.

In the example given, the load requires 4000ft-lbs of torque (I assume this is maximum torque, also) at 1200rpm. If you are to use any motor with an equal or higher base speed, it will have to produce 4000ft-lbs at its base speed too.

Therefore, if the base speed is 1500rpm, the horsepower at 1500rpm is 4000 x 1500/5250 = 1143hp. If the base speed is 1800rpm, then the horsepower is 4000 x 1800/5250 = 1371hp. At 1200rpm, horsepower is 4000 x 1200/5250 = 914hp as mentioned. These are the motors that could be used as long as the thermal stability of the motor holds at the slower speed.