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Variable speed drive problems

VSD (variable speed drive) is widely used in the commercial and industrial industries like MUD Districts, HVAC, Potable and Waste Water, fans etc for constant pressure and/or flow under varying demands. They sometimes have problems, sometimes they cause other problems, sometimes both. People may already have problems with electrical noise on their service from other sources like high frequency arc heating or welding equipment

Power Factor Correction Capacitors , etc. Adding another large horsepower "noise maker" to the electrical service with an uncalculated upgrade won't help.

ENCLOSURES - Variable speed drives come in various NEMA enclosures such a 1, 12, 3R and 4. The heat sinks for the transistors may be internal or external. Those that are internal such as NEMA 1 or many smaller HP units (and some 3R) are usually vented and rely on airflow through the enclosures. Dusty locations can easily clog air filters, where included, or drag dust in on the microprocessor motherboard and power components, creating electronic problems and excess heat on the power components. Conductive dust can short out power and controls. Dust buildup on the fan blades and external heat sinks can cause excess heat and fan failures. Improper selection, application and/or maintenance can result in problems or premature OVERTEMP faults. Installing chassis mount or units inside improperly vented custom enclosures and/or outside locations exposed to sunlight aggravates this problem as well.

FEEDER SIZING - In the USA, The National Electrical Code requires a disconnecting means for a motor to be mounted within 50 feet AND in sight of the motor. Smaller variable speed drives require an external disconnect, the larger ones are built into the controller. The line and load wiring is required to be 125% MINIMUM of motor FLA. It's not uncommon to find wiring undersized, sometimes even less than 100% motor FLA. That is bad enough with a standard controller, it can be a potential disaster with variable speed drives without a line reactor for equipment operating at or near full load.

MOTOR FEEDER LENGTHS - Wires have capacitance and capacitors respond really well to high frequency like the harmonics generated on motor leads without load filters or reactors. The longer the leads, the worse the problem is, affecting both motor and variable speed drive performance. Follow the manufacturers' recommendations for motor feeder lengths. If these lengths need to be longer, check the instructions. They typically will have extended length requirements for load reactors and/or load filters. The filters offer the best resolution but are also more expensive.

LINE FEEDERS - If the pump has to operate at its max HP, the 125% may not be enough given the losses (excess current) required by the variable speed drive, especially with long feeders. In addition, noise from other high frequency devices like other variable speed drives, arc welders, etc can cause problems with the variable speed drive as well as the variable speed drive causing other problems on the electrical service. A line reactor or filter may be required there also. The lower the carrier frequency is, typically 2 KHz to 4 KHz, the noisier the motor but less losses and more efficient. The higher the carrier frequency, the quieter the motor operation but greater loses and more line current.

CONSTANT TORQUE LOAD OVERHEATING – When an application requires more current at a reduced speed, that means more heat and less cooling at the lower speeds. Most VSDs have limits to protect the VSD and consequently the motor. Too low a setting and there will be lot of trips. Too high a setting and there may be damage in high ambient temperatures.

Proper current limit settings will help eliminate both but may reduce mechanical performance. Auxiliary cooling fans may be required on the motors. Many larger inverter-duty motors have them as options. The VSD may need to be oversized for positive displacement pumps, cranes etc.

MOTOR COMPATIBILITY AND VARIABLE SPEED DRIVE LIMITS AND SETTINGS– Not all motors, especially older existing ones, may compatible with variable speed drives at full load without some type of load filtering due to excess heat and voltage spikes created by the harmonics if the motor is not INVERTER DUTY RATED. There may be additional arcing on the bearings with standard bearings. Motors are not designed to operate outside of their service factors without caution and properly sizing a variable speed drive. Typical problems and/or constant torque applications may require the next larger variable speed drive if the variable speed drive nameplate max amps are at the design point of a system. HP and especially amps are critical in sizing variable speed drives. Typical carrier frequency may be factory set to 4 KHz. Increasing it will quieten a noisy motor but can increase the input losses and output heat and any other frequency/noise related problems without proper filters and/or reactors on both line and load.

SHAFT CRITICAL SPEEDS, Failure to "lockout" these speeds, especially with a vertical turbine pump, can result in damage to the pump under prolonged operation due to excess vibration. most variable speed drives have three or so settings.

This part deals with some of the more common application and maintenance problems leading to typical fault codes associated with variable speed drive failures which can be found in the individual manufacturer's IOMs. This will help identify and possibly eliminate a problem before a unit shuts down in the middle of an operation.