Frequency and Current!

[Eduardo Maun]

I'm working with motor pump and I found with my reading(s) if the frequency is reduce the current will decrease. Isn't current is proportional only to voltage? What is the reason behind this?

Thank you!

 

[iwire]

If the frequency is reduced the motor speed will decrease.

That results in less work being done by the motor and reduced current.

I believe all frequency drives also reduce the voltage along with the frequency

 

[winnie]

Ed,

I presume that you were using a VSD to adjust frequency. Were you measuring current at the input of the drive, or on the conductors between the drive and the motor?

You will find that current between the drive and the motor is approximately proportional to output torque. As you reduce frequency, the drive will reduce voltage to maintain proper magnetization. The motor slows down, but if the output torque were to remain constant, the current would remain constant.

However you specified a pump load. Pumps are known to require less torque at lower speed, so as you reduce the frequency and the speed goes down, the pump will demand less torque and the motor will require less current.

Mechanical power is the product of torque and speed. As you reduce the speed, the _power_ being delivered by the motor goes down. As iwire notes, as the power goes down, the current consumed by the VSD on its input will go down.

-Jon