AMP Draw varying with RPM

[frankszubos]

Hello, we are running a compressor with a 125hp 460v 1800rpm electric motor. We have a VFD varying the speed of the motor. My question is about the amp draw as the rpm varies. We are operating between 1400 and 1800 rpm. Assuming the load is constant should we see a reduction in amp draw as rpm decreases or should the amp draw be constant?

 

[Besoeker]

Hello, we are running a compressor with a 125hp 460v 1800rpm electric motor. We have a VFD varying the speed of the motor. My question is about the amp draw as the rpm varies. We are operating between 1400 and 1800 rpm. Assuming the load is constant should we see a reduction in amp draw as rpm decreases or should the amp draw be constant?

Depends on the type of compressor.

 

[Jraef]

Depends on the type of compressor.

What he said.

Basically if it is any type of Positive Displacement compressor, like a reciprocal, screw, lobed, scroll, diaphragm compressor etc., then the amps would stay relatively constant (there is some slight reduction as mechanical losses decrease a little with speed in some designs). These are what's called "Constant Torque" loads, meaning that regardless of speed, the torque required by the machine stays relatively constant, and torque = current for the most part.

If it is however a CENTRIFUGAL compressor, aka "Radial Compressor", then the amps would decrease with speed, and a rate greater than the speed change based on the Affinity Law for centrifugal loads wherein power to drive the load varies at the cube of the speed change. Note that I said "power", as in kW, not Amps, but Amps go down too.

 

[kwired]

From motor's perspective, less speed will ordinarily translate to less mechanical output power.

Normally the drive output voltage is still proportional (or close to proportional) to frequency, so if design full speed is 460 volts and 60 Hz, then running at 30 Hz will have output voltage of 230. To get same proportional output torque current will still be same as it is at 460V 60Hz. This voltage/current level still gives you around half the input power when output power is halved, mechanical differences at the two speeds will vary the actual values.

 

[drcampbell]

It also depends on which amp draw you're talking about.
The current into the VFD will drop with the motor speed.

p.s: The load isn't constant. It's roughly proportional to the motor speed.

 

[Besoeker]

p.s: The load isn't constant. It's roughly proportional to the motor speed.

As I said, it depends on the type of load. Centrifugal is usually cube law. So 90% speed would equate to about 70% load.

 

[Jraef]

It also depends on which amp draw you're talking about.
The current into the VFD will drop with the motor speed.
....

This is a good point nonetheless. "Amp draw" becomes a bit too vague when a VFD is involved, because the amp draw from the MOTOR will be different from the amp draw from the LINE. You need to specify which side of the VFD you are wanting to know about.

 

[frankszubos]

Thank you all for the responses. We are running a rotary screw compressor and measuring it on the output side of the VFD. I apologize for not giving full information! You're responses have definitely helped me.

 

[Jraef]

Thank you all for the responses. We are running a rotary screw compressor and measuring it on the output side of the VFD. I apologize for not giving full information! You're responses have definitely helped me.

So what are you seeing on the output side of the drive to the screw as you vary the speed?

 

[retirede]

Thank you all for the responses. We are running a rotary screw compressor and measuring it on the output side of the VFD. I apologize for not giving full information! You're responses have definitely helped me.

This gets more complicated. If the air demand is constant and the compressor was partially loaded at full speed, the throttling mechanism will try to increase the flow of the compressor as you slow it down. So the torque required by the compressor will rise accordingly. How much it rises depends on the type of control mechanism.