Machinery imported into the United States is often rated at an operating frequency of 50 Hertz.Unless engineered for operation at 60 Hz., this can be problematic for electric motors. This is especially true when operating pump and fan loads.Too often the distributors and purchasers of this machinery assume that the Original Equipment Manufacturer has taken this into consideration.In the repair business we frequently realize this when motors are received for repair roasted out from overload.
AVariable Frequency Drive(VFD) can be used to properly address the issues associated when operating 50 Hz equipment at 60 Hz.
Motor speed is directly proportional to the operating frequency.Changing the operating frequency on a pump or fan increases the operating speed, and consequently increases the load on the motor.A pump or fan load is a variable torque load.A variable torque load varies by the cube of the speed.
A 50 Hz motor operating on 60 Hz will attempt to rotate at a 20% increase in speed.The load will become1.23 (1.2 x 1.2 x 1.2) or 1.73 times greater (173%) than on the original frequency.Redesigning a motor for that much of a horsepower increase is not possible.
One solution would be to modify the driven equipment to decrease the load.
This may include trimming the diameter of the fan wheel or impeller to provide the same performance at 60 Hz as the unit had at 50 Hz.This will require consultation with the equipment manufacture.Possibly the same manufacture who chose to install 50 Hz rated on a machine to operate on 60 Hz.(your confidence in them should already be suspect).There are other considerations associated with an increase in speed besides the increase in load.These include mechanical limitation, vibration limits, heat dissipation, and losses.
The best solution is to operate the motor at the speed for which it was designed.
If that is 50 Hz., then avariable frequency drivecan be installed.These drives will convert the 60 Hz line power to 50 Hz at the motor terminals.
There are numerous other benefits that will be realized with this solution.These benefits include:
- improved efficiency
- power regulation (often better than the utility will supply)
- motor over current protection
- better speed control
- programmable output to perform other tasks
- improved performance.
The same load formula above can be used when speed of a variable torque load is reduced using a Variable Speed Drive.Therefore a 20% decrease in speed will require 58% of the original Horsepower.