Question

A pump is being powered by a 3-phase asynchronous AC motor. Without using a variable speed drive, explain two ways that you can adjust the flow rate in the system. How does a VSD control the flow rate?

Answer 1

Without using a Variable Speed Drive, we can control flow rate of a system using below methods.

1.Throttling the discharge

In the case of a pump, there is a performance curve and system resistance curve. Pump is operating on the intersection of these curves.By closing a valve at the discharge will throttle the discharge and change system resistance curve. This again leads to change in point of intersection with performance curve and results in change of flow. This method of controlling the flow will waste the energy of the system.

Here flow rate can be find out using the formula,

Where Q is the flow rate in litre/sec,

H is the total head in meters

SG is the specific gravity, E is the efficiency and P is the power

2.Bypassing

Here we use a manual or automatic control valve. Some of the discharge flow will be allowed to return to the suction tank using a new line installed with a control valve. When we open the valve, discharge reduces while increasing the flow to suction tank.This method of controlling the flow rate also results in wastage of energy.

A Variable speed drive(or variable frequency drive in case of  DC) is also called adjust speed drive used to control the flow rate. In contrary to what mentioned above, it can reduce wastage of energy. It can be programmed for a soft start and soft stop. It can be seen with a PID controller also.

A variable speed drive controls frequency and voltage to the motor. Variable speed drive has a convertor which can convert AC to DC and an invertor which functions the opposite. It also has a DC Bus. Phase of the motor gets connected to Positive DC bus when we close the invertor top switches and voltage on the phase becomes positive.

If we close convertor's bottom switches then phase gets connected to negative DC bus. Thus we can generate required frequency.The same system regulate the voltage also.

Through this motor speed gets controlled which in turn controls the flow rate.