VARIABLE SPEED CONTROL OF A PUMP
How pump characteristics depend on a speed of rotation?
Formulas describing the dependence of curves of a centrifugal pump from speed are called Affinity laws.
- flow depends on speed proportionally
- pump's head depends on speed proportionally in the second degree
- shaft power depends on speed proportionally in the third degree
How it looks in a graph?
H-Q curves for different pump speeds
To receive values of Q and H for other speed, it's necessary to calculate H and Q values using affinity laws formulas.
The nominal speed of the pump is 1450 rpm.
We want to know H-Q curve for speed 1150 rpm.
For point #1- Q=Q1, H=H1, n=1450 rpm.
Using affinity formulas for n=1150 rpm we will receive Q=Q3, H=H3.
You can see that points #1 and #3 lies on the parabola.
For another point, we do the same calculations and receive Q-H values.
To receive a complete H-Q curve it's necessary to calculate H and Q values for several points for requered speed. Connecting these points we receive the complete Q-H curve.
Note that all points will belong to parabolas of similar modes. See in the graph.
How the pump efficiency depends on pump speed?
Pump efficiency depends very slightly on a pump speed and can be not taken into account.
For example reduction of speed in twice reduces the efficiency at approximately 2 %.
How shaft power depends on a pump speed?
Shaft power of centrifugal pumps depends on speed in a third degree. See affinity laws. Speed reduction has a great effect on a pump power reduction.
How a Variable Speed Drive (VSD) saves energy?
With a slowed down pump, the same rate of flow can be delivered without the large losses in the valve. Useful energy the same as before. Valve losses are eliminated.
COMPARISON OF EFFICIENCY CONTROL: VARIABLE ROTATION SPEED VS. THROTTLING
Why is the speed control of pumps used?
The main goal of pump speed control is the adaptation of the pump curve to the real pumping system curve.
This is important for pumping systems with great fluctuations of parameters
For example, in water supply systems day fluctuations of required flow and pressure.
Very often pumps users think that application of pump speed control automatically means saving energy and money.
But sometimes users say I have applied speed control but I didn't receive the reduction of the energy consumption and control system does not change the speed of rotation of the pump.
The common error which users make is the using affinity laws for calculation of operating point especially when a hydraulic system has a static head? See graph below.
Always consider pumping systems curve when selecting pump control method.
How does pump with speed control operate in pumping systems with mainly static head?
During the speed reduction of a pump, duty points of a pumping system with a mainly static head move to the left further from BEP toward the lower efficiency.
How does pump with speed control operate in a pumping system without static head and only friction losses?
During the reduction of a pump speed, a duty point leaves on the same affinity curve with the same efficiency.
The speed control of a pump is the best for pumping systems with friction losses.
On the contrary for pumping systems with a big static head, the speed control does not give a reduction of energy consumption.
When is it reasonable to use variable speed drives?
There is an overall recommendation - pumps operate for at least 2000 hours per year and process flow rate requirements vary by 30% or more over time.
Role of an Adjustable Speed Drive (ASD)
The true value of an ASD is the ability to precisely match motor and pump output to process requirements.
Potential benefits of precise process speed control:
•Improved product quality
•Improved process throughput
•Improved process control
Types of variable speed drives
- Mechanical variable-speed drives – hydraulic clutches, fluid couplings, and adjustable belts and pulleys.
- Electrical variable-speed drives – including eddy current clutches, wound-rotor motor controllers, and variable frequency drives (VFDs).
Variable speed drive benefits
- Controls speed variations
- Provides mechanical control
- Eliminates startup impacts causing system vibration
- Provides fault tolerance
- Supports soft starts
- Restarts spinning load
- Controls speed swings
- Enhances product quality
- Can conserve energy in some systems
- Improves power factor
Potential variable speed drive issues
- Harmonics could effect instrumentation
- Fault-out (equipment shut-down) when power quality varies
- Bearing currents
- Mechanical vibrations
- Increased noise (acoustical)
- Static head considerations
- May need to include a full voltage starter as a bypass control
How motors are affected by VSDs
- High switching frequencies: high switching frequencies with VSDs can create additional heating in the motor, the motor efficiency is lowered
- Vibration: evaluate the system so that natural or resonant frequencies are not excited anywhere in the system when the speed is reduced.
- Bearing problems: when using VSDs, voltage can build up on the motor shaft and cause a current to flow through the bearings which causes pitting of the races. This can be prevented by using insulated bearings or a shaft grounding device.
- Cooling air flow reduction: motor speed should not be reduced to the point of compromising fan cooling
- Conductor insulation breakdown: when the distance between a motor and VSD increases (usually over 30 ft), there is a voltage overshoot that can stress the motor insulation. Line filters can be used to minimize the effect of this.
- Service Factor: Many motor manufacturers will “de-rate” the motor service factor when used with VSDs
- Motor Torque: typically not an issue with centrifugal pump loads (variable torque), but when a constant torque is required, motors must be sized for the required torque at lower speeds.