Exceptional Opportunities in Process Control – VSD Dynamics and Rangeability

The figures in the attached VariableSpeedDriveRangeability.pdf and the following discussion is an excerpt from the ISA book The Essentials of Modern Measurements and Final Elements – A Guide to Design, Configuration, Installation, and Maintenance.

The 4 main practical reasons that variable speed drives (VSD) drives are not used as extensively as one might think for pump control are as follows [35].

1. Drives are generally not built just for pumps. They handle conveyors, extruders, etc. There are a lot of VSD menu choices and options not pertinent to pumping applications.

2. Users don’t like the complexity of the VSD. The user must address setup, maintenance, and design issues. Special practices are needed to prevent EMI in instrument signals and from getting harmonics back into the power supply.

3. Someone needs to do the right calculations on dollars saved. Typically calculations don’t take into account the drop in drive efficiency at low speeds. The duty cycle (amount of time speed is really turned down) is not known in advance. If there is a high static head, the energy savings of a drive disappear.

4. It is rare to compare a VSD and valve. There are generally no decision points in the project for this comparison.

Is a Valve or VSD Faster?

Exceptionally fast loops (e.g. furnace pressure, liquid pressure, and surge control) can ramp off-scale in milliseconds. These loops have essentially a zero process deadtime and may have a high process gain due to a narrow control range (e.g. fractional inches of water column for furnace pressures). These loops require DCS scan times of 0.05 to 0.1 seconds. Special fast scan rate digital controllers or analog controllers are needed. DCS scan time requirements of 0.2 seconds or less signify a VSD opportunity. A properly designed VSD has no measureable dead time while control valves and dampers take anywhere from 0.2 to 2.0 seconds to start to move. For example, an incinerator pressure and polymer pressure loop that could get into trouble in less than 0.1 second required a VSD and analog controller to stay within the desired control band [20][23][35].

The VSD has a negligible time delay unless a deadband or dead zone is introduced in the drive electronics to reduce reaction to process measurement noise or a low resolution input card is used. A control valve or damper has a deadtime that is proportional to the resolution limit (sticktion) or deadband (backlash) divided by the rate of change of the process controller output. For large or fast changes in signal this deadtime disappears.

A pneumatic actuator has a pre-stroke deadtime that is the time it takes for the actuator pressure to change enough to move the actuator shaft. For large actuators, the pre-stroke deadtime can be several seconds unless a booster is added.

The inertial time constant of liquid flow response is inversely proportional to flow. Consequently, the process lag at low flow rates and at the initial start of flow can be quite slow (e.g. 5 seconds) compared to the process lag at normal flows (e.g. 0.5 seconds). The comparison between VSD and control valve response should be at normal flows.

In a published comparison of the dynamic response of a control valve and a pump for flow control for a system with negligible static head, the integral times were about the same for the VSD and valve loops. However, the controller gain could be increased by over a factor of 6 for the VSD loop. As a result, the set point response was faster [38]. In this test the valve deadband was about 8% and there was no static head. In unpublished lab test results of control valves with low sticktion, low backlash, and a digital positioner and a VSD with a volts/hertz PWM drive for liquid flow control, the speed of response of the valve and VSD were similar.

Variable speed drives, control valves, and dampers have a velocity limited exponential response. The velocity limiting in a drive depends upon the available motor torque and the inertia of the motor rotor, the pump shaft, and the pump impeller. The exponential term is generally much smaller for a VSD than for a control valve or damper. On the other hand, the velocity limiting is slower for a VSD unless the actuator size is large and boosters are not used. Consequently, for small changes in signal, a well designed VSD is faster. Conversely, for large changes in signal, a small control valve is faster (see section on dynamics). This leads to the conflicting statements about whether a VSD or control valve is faster. Which final element is faster often depends upon the size of the change in signal.

VSD Best Practices

To summarize, a VSD is most likely to offer energy savings or better loop performance as a final element for the following types of applications:

• Loops that require 0.2 seconds or faster scan time

• Valves and dampers with 0.5% or more sticktion or backlash

• Large utility flows

• Integrating and runaway processes without a secondary flow loop

• Low static head processes requiring frequent turndown

A tachometer or inferential speed feedback signal should be sent to the process controller in the DCS that is sending the signal to the drive. The speed feedback should be used in a similar way to the position feedback from a digital positioner to prevent the process controller output from changing faster than the final element can respond. The use of the dynamic reset limit option for the loops in the DCS can automatically prevent the process controller from outrunning the final element response (see section on dynamics).

For best performance users should consider the following during the specification and implementation of variable speed drive systems:

• High resolution input cards

• Pump head well above static head

• On-off valves for isolation

• Design B TEFC motors with class F insulation and 1.15 service factor

• Larger motor frame size

• XPLE jacketed foil/braided or armored shielded cables

• Separate trays for instrumentation and VFD cables

• Inverter chokes and isolation transformers

• Ceramic bearing insulation

• Pulse width modulated inverters

• Properly set deadband and velocity limiting in the drive electronics

• Drive control strategy to meet rangeability and regulation requirements

• Dynamic reset limiting using inferential speed or tachometer feedback

VSD Response

The response of variable speed drives more closely resembles a pure ramp with no rounding or time delay provided a filter or deadband has not been added in the drive electronics to attenuate process noise in the process controller output signal. The ramp time in the VSD depends upon the size of the load compared to the available torque from the motor. In general, the ramp time of a VSD is longer than the stroking time of a control valve but is shorter than the stroking time of a large damper. Longer than necessary VSD ramp times may inadvertently be imposed in the drive electronics.

There is essentially no sticktion or backlash in variable speed drives for axial and centrifugal blowers, fans, and pumps but this does not necessarily mean there is no resolution limit or deadband in the VSD response.

Controller outputs invariably have fluctuations that originate from process or sensor noise and transmitter resolution limits. These fluctuations are not representative of the actual value of the process variable and are best ignored. These fluctuations are particularly large and fast for flow and pressure loops. A deadband is sometimes introduced in the VSD electronics to prevent changing the speed. The effect may be a true deadband where the desired speed does not change upon a change in direction until the change in signal is larger than the deadband setting. The effect here is similar to backlash in a control valve. In other cases, it may be a deadzone setting, in that the desired speed does not change until the accumulated change in signal since the last change in speed is larger than the deadzone setting. Here the effect is similar to a resolution limit.

If there is no deadzone setting, the resolution limit in a VSD is largely determined by the input card. Assuming there is no sign bit, the VSD resolution limit is simply 100% divided by 2 raised to the number of bits (n) of the input card. Unfortunately, VSD manufacturers did not understand the limit cycle that would result from the resolution limit and offered an 8 bit input card (0.4%) as the standard card. Higher resolution input cards (e.g. 12 bit and 16 bit) should be specified to make the VSD I/O resolution comparable to the DCS I/O resolution.

VSD Installed Gain

In a variable speed drive for liquid flow, the pump characteristic curve shifts with pump speed. Since there is no control valve, there is no valve drop and the flow is at the intersection of the pump curve and the system frictional loss curve.

For a negligible static head and an idealized pump, motor, and VSD, the change in flow with speed is linear. If the static head is negligible, the loss in pump efficiency and the increase in slip at low speed, cause a decrease in gain (sensitivity) at low speed. This loss of sensitivity is seen as a flattening at low speed in the plot of flow versus speed.

If we ignore the loss in pump efficiency and increase in slip, a pump curve that approaches the static head will show a sharp bend downward to zero flow at low speed. The plummet of the speed at low speed causes a significant increase in gain and a nosier flow at low speeds [46].

A flat pump curve will cause almost a quick open type of flow characteristic. The high gain (sensitivity) at low speed can cause cycling [46]. Operation on a relatively flat pump curve can occur from improper pump selection or over-sizing.

VSD Rangeability

For variable speed drives, estimating rangeability gets tricky. The decrease in process gain from speed slip offsets the increase in process gain as the pump discharge head approaches the static head. If there are no overheating or cogging problems as suggested is the case for a pump and valve system with a well designed open loop (volts/hertz) PWM drive, high resolution input card, and negligible static head, the rangeability is normally 40:1. When the pump head is operating near the static head, the minimum controllable flow is set by rapid changes in the static head and frictional loss. These rapid changes could be due to noise and sudden or large disturbances. The speed can not be turned down below the amplitude of these fast fluctuations.

The rangeability of a VSD could drop to 4:1 for the following systems:

(1) Older VSD technologies such as 6-step voltage (excessive slip at low speed)

(2) Systems with a high static head (flow plummets to zero at a low speed)

(3) Operation on the flat portion of the prime mover curve (cycling at low speed)

(4) Hot gases (motor overheats at a low speed)