Ensuring motor/drive compatibility to increase HVAC system reliability

When the plant HVAC/R system shuts down because a motor fails, more than simple replacement may be needed to get the system up and running again. In a number of instances, plant engineers are finding that a motor fails prematurely because it was not designed for inverter duty.

Many of today’s HVAC/R systems are controlled by sophisticated drives and automated systems. However, the controls that let plants reap greater system efficiencies can also be the source of premature motor failure. The cost of HVAC downtime can be substantial and the cost of a new motor is typically the least expensive element. An uncomfortable workplace leads to lower productivity. If plant ventilation is required to ensure safety, the process may have to stop completely. For some operations, a halt in the process leads to product spoilage and major losses.

When a motor failure shuts down the HVAC/R system, a quick assessment of why the problem occurred must be made while the motor is replaced as rapidly as possible. Knowing the cause of the problem helps prevent future failures.

Avoiding those failures

Alternating-current (ac), squirrel-cage induction motors powered by adjustable-frequency drives are a popular option for varying the speed of HVAC/R and other industrial equipment. Technological advancements in power semiconductor devices, microprocessor controls, and software have reduced the size and cost of adjustable-frequency drives, while improving performance levels beyond those achieved by direct-current (dc) systems.

For years, ac drive systems have been used to provide variable speed power for fans and pumps. Now they are finding their way into applications once thought to be reserved for dc systems alone. AC drives are now used on condenser fans and air handlers, cranes, hoists, extruders, conveyors, elevators, machine tools, and other applications where speed and torque control are critical.

Pulse-width modulated drive

One dominant design used today is the pulse-width modulated (PWM) ac drive. PWM drives produce variable-frequency output by rectifying sinusoidal utility power, converting the drive’s ac input to dc, then switching it back into ac. A logic circuit and companion software control the switching to provide the variable voltage and frequency required to run an ac induction motor at variable speed.

Advancements in power electronic devices, in particular the insulated gate bipolar transistor (IGBT), have made these drives highly successful. These transistors provide high switching frequencies with low losses. The result is smaller, quieter drives with better torque and speed characteristics.

The output wave of a PWM drive is not the smooth sinusoidal waveform an ac induction motor would like to see. Instead, it is a series of constant height pulses (left). The height of each pulse is the drive’s dc bus voltage (level of the rectified input voltage or about 1.4 times the root mean square ac input voltage). The pulse width depends on the desired output voltage (wider the pulse, higher the average output voltage).

Pulses repeat at the drive’s internal carrier frequency, generally in the range of 2000 to 20,000 pulses/sec. Groups of positive pulses of various widths are strung together to form one-half of an output cycle. The drive logic then inverts the direction of the pulses and puts out a string of negative pulses to form the second half of the output cycle. This process, repeating again and again, determines the drive output frequency.

When a motor is connected to a drive, the motor and its connecting leads form an inductance-resistance-capacitance circuit. Rapid switching of the drive’s IGBTs induces a voltage overshoot which can reach 1500 V on a motor with 460-V drive input and as little as 50 ft of cable between the motor and drive (left).

The overshoot occurs at the edges of the drive’s rectangular output pulse In some applications, a continuous bombardment of these pulses can create motor/drive compatibility problems (right).

Handling compatibility issues

Continuously bombarding the motor winding with these pulses can create motor/drive compatibility problems in a small number of applications. Both the peak level of this overshoot and its rise time can create the difficulties. The National Electrical Manufacturers Association (NEMA) offers detailed technical information on application considerations for both general purpose motors used with inverters (MG 1-1993 Part 30) and definite purpose, inverter-fed motors (MG 1-1993 Part 31).

For motors rated 600 V or less, Part 30 suggests limiting the voltage overshoot to 1000 V peak with a rise time of 2 msec or longer when applying variable frequency drives to general purpose induction motors. Definite purpose inverter fed motors, covered by Part 31, must be capable of withstanding a 1600-V peak limit with a rise time of 0.1 msec or longer.

Drive carrier frequency also plays a part in the compatibility issue. The higher the drive carrier frequency, the more often the motor is hit with a voltage overshoot and the sooner it fails, unless the motor is specifically designed to eliminate or withstand the stresses caused by the overshoot. Maintenance can take steps to minimize a number of these problems. Some tips are outlined in the accompanying section, “Alleviating compatibility problems.”

— Edited by Jeanine Katzel, Senior Editor, 847-390-2701, j.katzel@cahners.com

Key concepts

AC, squirrel-cage induction motors powered by adjustable-frequency drives are an efficient way to vary the speed of HVAC/R equipment.

Pulse-width modulated ac drives equipped with insulated gate bipolar transistors are smaller and quieter and have better torque and speed characteristics than other types.

Specifying motors that handle voltage spikes caused by IGBT switching helps alleviate compatibility problems.

Alleviating compatibility problems

Maintenance personnel can help alleviate compatibility problems in several ways.

– Get involved in the motor/drive specification process.

– Insist that any motor installed on an inverter meets the requirements of NEMA MG-1 Part 31.40.4.2. Such motors are designed to handle voltage spikes caused by IGBT switching.

– Specify an energy-efficient motor. A motor lasts a long time; it may as well be saving money too.

– Determine if your motor is NEMA MG-1 Part 31 capable, especially if it is being installed with a drive in an existing application. If it is not Part 31 capable, the lead ength should be kept as short as possible. Line reactors may be needed at the drive output or motor input to reduce the voltage stress applied to the motor. Line reactors increase the rise time of the drive to an accept- able level. However, they are a more costly option and also reduce the voltage available to the motor. Caution is required because the additional impedance may reduce motor torque available at low frequency.

– Add a reactor at the drive output. This move conveniently solves many compatibility problems. However, when long leads are involved, the reactor may see the voltage spike from which it is protecting the motor. As a result, the reactor may become the weak link in the system.

– Install an output filter on the drive specifically designed to strip the high-frequency component from the drive’s output and increase the output rise time.

– When a new motor must be purchased, consider specifying one that minimizes or eliminates motor/drive compatibility problems.

What is corona?

The phenomenon known as “corona” has been around a long time. As variable frequency drives (VFDs) become smaller and more sophisticated, corona occurs more frequently and is receiving attention from the motor/drive industry.

VFDs create high voltage pulses at the motor, especially when the motor and drive are separated by long power leads. These spikes develop voltage potential between adjacent conductors in the motor winding.

When the electrical field generated in the air between the conductors is high enough, air molecules break down, creating what is known as corona . The discharge that is created forms ozone, a substance that causes the motor’s magnet wire insulation to disintegrate and fail prematurely.

Although corona occurs in only a small percentage of motors used with VFDs, the problem is occurring more frequently. Specifying equipment designed to resist or prevent corona helps avoid the cost of motor replacement and the downtime associated with the repair.

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