Varaible Speed Drives for Motor Driven Fire Pumps

Variable Speed Fire Pump Controllers

- by -

James S. Nasby

Columbia Engineering

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Topics to be CoveredVFD and Controller ElementsVFD CharacteristicsPrimary (Desired) Effects on MotorSecondary (Undesired) Effects On Motor On Power Grid (Mains)

System Characteristics & Effects Motor RequirementsApplicationsTesting

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ModelECV-250-46-*

250 Hp – Single Source

A.T.L Start in Bypass

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ModelECV-250-46-*

250 Hp – Single Source

A.T.L Start in Bypass

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Model ECVRTZ-100-46-*

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Inside View of Drive Bay

D.C. Link Reactor

Line Reactor

CoolerCPT & Fuses

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Vocabulary

VFD = Variable Frequency Drive AFD = Adjustable Frequency Drive VSD = Variable Speed Drive ASD = Adjustable Speed Drive,“Inverters”, “A.C. Drives”, "Drive"

Note: The above are all the same for our purposes.

IGBT = Insulated Gate Bipolar Transistor

“VFD” will refer to the Drive Unit Proper PWM = Pulse Width Modulation dV/dt = Voltage Rise Time (dU/dt in Europe)

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Vocabulary – cont’d

IGBT "Soft Switching" Reduces = dV/dt Reducing Motor Spikes (GE/Figi) PWM = Pulse Width Modulation (Used on All Modern VFDs) PID = "Proportional – Integral – Differential" Set Point Controller or Control Scheme

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Vocabulary – cont’dInverter Duty Motor Higher Winding Insulation Dielectric Strength

(Breakdown Voltage), Typically 1200 Vac Extra Thermal Capacity by way of:

1.15 Service Factor and/or Higher Insulation Class (Such as 155°C instead of

130°C)

Mitigate Bearing Currents by way of: Insulated Bearings Brushes to Drain (Bypass) Currents Around

Bearings

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Constant Torque Rating: For Mills and Take-up Reel Drives and Positive Displacement (Piston) Pumps Where Rated Torque is Needed at Zero (or

Near Zero) Speed.

Variable Torque Rating: For Fans, Blowers, Centrifugal Pumps & etc. Where the Load Torque Varies with Speed

(RPM). E.g.: Low Torque at Zero (and Near Zero) Speeds.

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Set-Point = Desired Outcome (e.g.: Desired Pressure)

System Response May Be One: Oscillatory Under Damped Critically Damped (with or w/o

Overshoot) Over Damped

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System Response Curves

Under Damped

Oscillatory

(With Overshoot)

Over Damped

Critically Damped

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Required Controller Elements

Controller Must be a Full Service Controller Plus a VFD Unit Plus VFD Fuses (at <299% of FLA) Plus Load Side Isolation Contactor (interlocked) Plus Line Side Isolation Contactor Plus By-Pass (Main) Contactor

All Fully Rated (Horsepower Rated)Line Reactor - 5% Minimum to:

Reduce Line Current Harmonics Reduce Line Transients to the VFD Reduce Capacitor Over-voltage Shutdown due to Line

Spikes and/or SurgesNote: All line spikes and surges get rectified regardless

of where they appear on the Waveform.

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Required Controller Elements - cont'd

Mechanical Interlock Between Bypass Contactor and VFD Output Contactor to

Prevent Back Feeding VFD on Controller Defect since this will cause the Main Fire Pump Circuit Breaker (C/B) to Trip. This causes the Fire Pump to be off line (dead).

Means to Lock VFD and Starting Parameters Typical VFD has over 150 settable parameters, many

of which will cause damage or no fire water if mis-set

PID to Control Pressure Must be able to Control Time Constants and Dynamic

Characteristics to Avoid Cycling or Hydraulic System Damage

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Power Circuit Schematic

* Was Optional

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VFD OCP Fuses and view of VFD Sans Cover

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Schematic Diagram

Single SourceA.T.L. (D.O.L.)Starting in Bypass ModeLow to ModerateHorsepower

Note Mechanical Interlock (Mech Intk) Between R1 and BP Contactors.

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Line Reactor and 2nd Cooler

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Two Coolers for 300 Hp VFD

Note: this unit sees regular service a pump company pump test facility.

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Must Include a CompleteFull Service Fire Pump Controller

Standard Full Service Circuitry and ComponentsStandard Mechanical Manual OperatorMode Switch for Full Speed Bypass Operation Easier Commissioning, Servicing and for Back-

up Automatic Fire Protection

Standard Pressure Switch/Transducer Bypass and Start use the Same Switch -or- Separate Swtiches used (some Mfrs)

Separate (2nd) Transducer for Recording (one Mfr.)

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Close-up of Pressure Switch and Both Pressure Transducers

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External Wiring Diagram

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Optional Controller Elements

Line Side EMI / RFI (EMC) FilterLoad Side (Motor) Output ReactorLoad Side (Motor) dV/dt FilterD.C. Link Reactor (Standard with Some)

Advanced Operator Display PanelsNetwork Connection

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VFD – Principles of Operation

Three Main Sections: Three Phase Rectifier Section (AC/DC) D.C. Filter Section - D.C. Bus Capacitors

(Bus Cap’s) Inverter Section (DC/AC): Pulse Width

Modulator (PWM) Modern VFD’s use IGBT’s for Inverter Inverter Typically Runs at 2 to 6

Kilohertz

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VFD = Motor Running Only (No Motor Starting Region)

Typical Motor Torque and Pump Torque Curves

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Lower Frequency Shortens Curve

This point Moves Left Relative to Frequency

100% = 60 Hz or 59 or 30 or any Frequency

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3 Phase

Line Freq.

AC / DC

SmoothingDC / AC

At “X” Khz

VFD – Principles of Operation

Modulation Frequeny Typically 2 to 10 Khz

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PWM Basic VFD Voltage WaveformPulse Mode Modulation at 2.0 to 10.0 Khertz

2 Khz

One Cycle

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PWM Motor Voltage Waveformwith No Filtering

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PWM Motor Voltage Waveformwith Load (Motor) Reactor

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PWM Motor Voltage Waveformwith Motor Filter

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Motor Electrical Measurements

Special Metering Required due to the High Voltage and Frequency

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Controller Operation1) Pressure Switch Trips2) Contactor R1 and R2 Close3) Signal VFD to Start4) Begin Timing for VFD Ready Signal (3-5 Sec.)5) Begin Pressure Switch Tripped

Timing (approx. 20 Seconds)6) VFD Sends “Drive Ready” Signal -or-

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Controller Operation – cont’d

7) Bypass Operation Begins after Restart Delay -- or --

8) VFD Begins Motor Acceleration9) VFD adjust motor speed to Achieve

Desired Pump Pressure Set-Point10) Pump Builds Up Sufficient Pressure to

Reset the Pressure Switch within 20 Seconds -or-

11) Begin Bypass Operation (Same as # 7 above)

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Controller Operation – cont’d

12) VFD Modulates Motor Speed Using its PID to Maintain System Pressure at Desires Set-Point Even with Flow and Suction Pressure Fluctuations -or-

13) Pressure Switch Re-Trips and Pressure Switch Tripped Timer Begins Timing Cycle (Again)

14) After 20 Seconds, By-Pass Operation Begins (same as #7 above)

15) By-Pass Operation is Manual Reset

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Bypass Starting Methods

Full Voltage (A-T-L)Primary ReactorSoft StarterSome Controllers also Provide Soft Stop

(extra circuitry needed for the Soft Stop Function)Some Units will Ramp Up to at Lease an Idle Speed and continue to increase speed until the pressure demand is met.

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VFD Starting Characteristics

All VFDs provide Soft Starting (Ramp-up)Max. Torque/Current can be set 150% Starting and 110% Running are Typical System Must be Suitable for Bypass Mode

Starting

Some Controllers also Provide Soft Stop (extra circuitry needed for the Soft Stop

Function)Some Units will Ramp Up to at Lease an Idle Speed and continue to increase speed until the pressure demand is met.

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Design Considerations

Capacitor Overvoltage Shutdown This is a Lockout Condition, Manual Reset is always

Required. 480 Vac x 1.414 = 678 + 10% = 747 Vac. Better VFD's

don't shut down until the capacitor voltage exceeds 800 Vac. Lesser ones shutdown at 750 Vac or less.

VFD Shutdown Temperature. Better VFDs don't shutdown until 70ºC and can be

restarted with no damage or degradation

VFD Pollution Degree Requirements must be met. Cooling should be Closed Loop (no Venting Pump Room

Air into Controller.

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Design Considerations - cont'd

VFD OCP Must be Fully Selectively Coordinated with the Main Fire Pump Circuit Breaker (C/B)

Said OCP will Most Likely be Fuses Coordination with the is Critical to Avoid Tripping which

Kills the Bypass Mode. Fuses (OCP) must trip before the 300% point Fuses must trip in less than 8 seconds at Locked Rotor

Current Fuses must still be large enough to hold with the Maximum

normal VFD input current including harmonic content. The Fuse curve must never touch the C/B curve to avoid

any risk of tripping. Fuse tolerance must also be considered. The plus

tolerance on the fuse curve can lead to tripping of the F.P. C/B.

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Fuses must be calculated for Each and Every controller Hp and Voltage (E.g.: Each nominal FLA).Controller cooling must keep the VFD within is maximum rated temperature. A 50ºC VFD will require the cooling to keep the

temperature rise to no more than 10ºC for a Controller Rated Maximum Temperature of 40ºC.

A VFD rated at only 40ºC will require de-rating unless refrigeration is allowed.

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Polution Degree Rating of the VFD must not be exceeded. This will typically require a non-vented NEMA 12 (U.L. Type 12) or better enclosure and cooling means. Internal Controller Wiring in the VFD input path must be sized for the harmonic content of the VFD.Successful Installations the Mfr. to ascertain and check the system hydraulic time constants and set the PID parameters accordingly.

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Application ConsiderationsAdditional Considerations:

* The controller is UL listed for fire pump applications.* The enclosure is rated for NEMA 12.* Maximum ambient must not exceed controller (marked) rating. * Mfr's vent air clearance spaces and service spaces must be

adheared to. * The motor must be suitable for use with a variable speed drive.* Motor current draw must not exceed 100% of FLA, even though the

motor may have a 1.15 or higher service factor* A gen-set must be suitable for use with a variable speed drive.* The power source must be capable of bypass mode Starting method.* The pump and motor must be rigidly coupled.* The pump and motor must be properly grouted.* A relief valve is required for emergency operation unless system

pressure can not be exceed at churn and full speed and max. suction pressure.

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Multi-Acre Multi-Building Campus…

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…and Back-up Distant Water Supply…

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Lead to MultipleSystem Time Constants

Note

253 Second

Chart Span

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Note 151 Sec.

Chart Span


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Note 86 Sec.

Chart Span


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Note 63 Sec.

Chart Span


Estimated 25s, 20s, 4s, 2s & ?? Hydraulic Time Constants

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Unit Testing

Small Unit after High and Low Temp. Testing:

Ta < 0ºC and

Ta > 60ºC

Thermocouple Switch Box on top of unit.

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Unit Testing

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Unit Testing - 350 Hp Unit

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Questions?

Technology

Varaible Speed Drives for Motor Driven Fire Pumps