Prepared by Vahan Bagdasarian

How Variable Speed Drive For Pump MotorsImprove Groundwater Management Efficiency

International Center for Water Technology2014 Water Technology Conference

Variable Frequency Drives

What is a Variable Frequency Drive (VFD)?Variable Speed Drive (VSD)Its like the throttle in your car, the faster you want to go you apply more

gas.

Without it would be like a standard motor where you would have it going full speed and apply the breaks when you want to slow down without slowing down the motor. In your irrigation system you can use a throttling valve, pressure regulating valve or bypass valve. Not the most energy efficient method but it works.

VFD or VSD brings the motor speed down to match the actual demand needed by the application which can reduce energy consumption.

Variable Frequency Drives

Variable Frequency Drives (VFD)

Why are they used?

Reduce energy consumption

Lower Carbon emissions

Government, consumers, legislators demanding more energy efficient products

Rising energy prices

Variable Frequency Drives (VFD)

When should they be use to better your water management?

Various flow rates required in irrigation zones Depth of water level varies from start of irrigation

season to end of season Pressure to inlet of current pump varies Reduce flow to prevent over pumping on wells with

slow recharge and dynamic water drawdown during pumping.

Variable Frequency Drives (VFD)An example: (constant pressure) A fixed speed water supply pump has a Hnom = 45 psi at Qnom The flow has to be controlled by a throttle valve By reduced flow the pressure will increase to more than 75 psi

Pressure at Fixed speed pump solution =

75psi

System

Excesspressure

Reduced flow

H

Fixed speed

Required pressure = 45 psi

Pressure

Flow Q

Nominal flow Qnom

Variable Frequency Drives (VFD)

Pressure at Fixed speed pump solution =

75psi

System

Excesspressure

Reduced flow

H

Fixed speed

Required pressure = 45 psiPressure

Flow Q

Nominal flow Qnom

Pressure at Variable speed pump solution = 45 psi

Variable speed

Freq.conv.

Motor

An example: (constant pressure)A speed controlled pump automatically adapt

the pump performance to the actual need.The flow is reduced by lowering the speed of the pumpThe pressure is constant 45 psi

Sensor

Variable Frequency Drives (VFD)

The approximate savings which can be obtained100% Speed = 100% Load = No Savings 95% Speed = 85.7% Load = 14.3% Savings 90% Speed = 72.9% Load = 27.1% Savings 85% Speed = 61.4% Load = 38.6% Savings 80% Speed = 51.2% Load = 48.8% Savings 75% Speed = 42.2% Load = 57.8% Savings 70% Speed = 34.3% Load = 65.7% Savings 65% Speed = 27.5% Load = 72.5% Savings 60% Speed = 21.6% Load = 78.4% Savings 55% Speed = 16.6% Load = 83.4% Savings 50% Speed = 12.5% Load = 87.5% Savings 45% Speed = 9.1% Load = 90.9% Savings 40% Speed = 6.4% Load = 93.6% Savings 35% Speed = 4.3 % Load = 95.7% Savings 30% Speed = 2.7% Load = 97.3% Savings

50% Speed = 50% flow rate50% flow rate = 12.5% powerSavings 87 % on power

See Affinity laws the relationship between the flow, the rotational speed, the pressure head and the power.

Variable Frequency Drives (VFD)

The advantage of a frequency converter lays in controlling the speed of the pump in an intelligent way, such that the speed corresponds to the actual demand in the application.

The actual demand could be: Constant PressureConstant temperatureConstant differential pressureConstant flowEtc.

Example: Constant pressure

Pressure

Flow

Variable Frequency Drives (VFD)

The advantage of the frequency converter is that it can make a fixed speed pump into a variable speed pump with optimum performance in all duty points.

Example:QH curve, variable speed pump

Flow

Pressure

Variable Frequency Drives (VFD)

When they should not be used? When pumps have a constant flow and pressure

requirement

When you only require a different flow rate for short periods of time that will not save the required power for a return on your investment. Keep in mind that VFD units use 5% additional power so savings must be more than 5%. In these cases it may be better to use two pumps.

Variable Frequency Drives

Perfect control of flow and headAutomatically adapts to changes in system.

Time

P

r

e

s

s

u

r

e

with "soft-start" function

Protection of equipment (pipes,filters,valves)Less risk for water hammer

and cavitation.

Included electric protection

Integrated motor protection.

Faster installation/commissioningLess components that need to be adjusted to each other.

Direct input of control parameters, less calculations & manual settings.

Variable Frequency Drives (VFD)

Higher installation costs

VFD minimum speed: Start up to 30Hz should not be longer than 5 seconds. Below 30Hz is risk of vibrations in windings and heating of motor

Variable Frequency Drives (VFD)

Before After

Questions

WS Agriculture Irrigation0.1

Variable speed Affinity laws

16

Flow is proportional to pump speed :

Head is proportional to the square power of the pump speed

Power is proportional to the cube power of the pump speed

Q2 Rpm2=Q1 Rpm 1

h2 (Rpm2)2=h1 (Rpm1)2

P2 (Rpm2)3=P1 (Rpm1)3

NPSH is proportional to the square power of the pump speed

NPSH2 (Rpm2)2=NPSH1 (Rpm1)2

H2

P1P2

Q

Rpm 1

H

Rpm2

H1

Q

P

Q1Q 2