Commissioning: Getting it Right: Motor Current Sensors
Chain of EventsI discovered some motor current sensors that weren’t indicating as expected because particular events did not occur for the equipment I was testing. The specific steps in my test procedures required equipment to respond per the design sequence of operations. However, in two separate instances, the expected event did not occur in accordance with the sequence of operations. While testing an air-handling unit with variable-speed supply and return fans, I noted that the heating valve did not open as required.
The heating valve controller was commanding the heating valve to open because the supply air temperature was below the supply air temperature setpoint.
Investigation revealed that the supply fan motor current sensor indicated the supply fan to be off because the indicated current was less than the motor current proof-off setpoint. The supply fan motor was actually operating. The design sequence of operations required the heating valve to be closed if the motor current sensor indicated the supply fan motor to be off. The control software implemented the design sequence of operations correctly.
A similar situation occurred while testing an air-handling unit with evaporative cooling in addition to variable-speed supply and return fans. I noted that the evaporative cooling water pump did not start as required. The evaporative cooling controller in the control software was commanding the water pump to operate (cooling required) because the supply air temperature was above the supply air temperature setpoint.
Investigation revealed that the return fan motor current sensor indicated the return fan to be off because the indicated current was less than the motor current proof-off setpoint. The return fan motor was actually operating. The design sequence of operation required the cooling pump to be off if the return fan motor current sensor indicated the return fan motor to be off. Again, the control software implemented the design sequence of operations correctly.
Cause and EffectWhy did the current sensors indicate the motors to be off when the motors were actually operating? A common problem for both events was that the variable-speed drives were operating the motor at a relatively low speed (50% or lower). Also, the current sensors range of operation did not extend to zero current.
Depending on the current sensor model, the range of operation extended down to 1A or 2A. Because of the current sensors range, the proof-off setpoints were essentially at the bottom end of the range (proof-off at 1.1A or 2.1A). The motors operating at low speed were drawing fewer amps than the proof-off setpoints, thus, a motor off condition was detected. The current sensors range did not match the current range of the operating motors.
Predicting the current range of operating motors is not necessarily straightforward because a fan or pump motor operated by a variable-speed drive has a relatively wide range of current draw. The current varies as an exponent of three of the speed. A motor operating at 50% speed draws about 12.5% of the current draw at 100% speed. A motor operating at 25% speed draws about 1.5% of the current draw at 100% speed.
For the particular problems described, either the motor speeds were lower than expected by design or the specified current sensors range simply did not match the expected current (speed) range. The motor speeds for the air-handling units’ fans could have been lower than expected if the motors and fans were oversized for the final as-built air system configuration.
In addition to a possible oversizing problem, the sequence of operations may dictate a relatively low motor speed. For the air-handling unit with evaporative cooling, the return fan speed was controlled based on the number of kitchen exhaust fans that were operating. With three exhaust fans operating the return fan speed was only 20% and the motor amp draw was very low.
I decided that I would always include details of how to commission current sensors within the test procedures that I write. I plan to manually lower the speed (at the variable-speed drive control panel) of motors and document the speed and current at which the proof-off is indicated. If the motor speed is less than the documented speed during normal operations, then the proof-off indication will not be valid and correction action will be appropriate.