Karl M. Hink, Executive Vice President Originally presented at the Power Quality 99 Conference ABSTRACT Motor protection filters are proven economical devices to improve inverter-fed motor reliability, increase motor life, and solve long lead over voltage problems. This paper demonstrates the benefits and applications of a number of different types of motor protection filters, including sine wave and single element low-pass filters. Output voltage waveforms are presented along with relative cost data. Contemporary pulse width modulated (PWM) inverters provide motors with a nearly sinusoidal current waveform, but their output voltage waveform can cause over voltage failures and fatigue failures in motor electrical insulation systems. Figure 1, for example, shows the typical motor voltage waveform for a drive operating from an inverter with a switching frequency of 6 KHz and a fundamental frequency of 60 Hz. The distance between the inverter and the motor is 750 ft. with 480V input voltage. Peak motor terminal voltage is 1460V. In this example, the conductors connecting the motor to the inverter act as a transmission line, and a voltage doubling is occurring at the motor because of a reflected wave. In addition to over voltage, notice that the motor is being subjected to fast dv/dt pulses. The number of these pulses the motor sees is determined by the switching frequency of the inverter. The faster the switching frequency, the more repetitive pulses the motor will see. It is these pulses which, accumulated over time, can lead to motor insulation system fatigue failures. 1. INTRODUCTION Figure 1. Typical Motor Voltage Waveform 2. MOTOR RATINGS >

Many inverters are applied to existing 460 volt NEMA Design BӔ motors. These motors are specified to operate at a peak voltage of 1000 volts and with wave front rise time not less than 2 microseconds, or a dv/dt less than 500 volts per microsecond. Experience has shown that these installations require the use of a motor protection filter to mitigate the high dv/dt of the inverter and promote long motor life. NEMA Inverter Duty 460V motors are specified to operate at a peak voltage not exceeding 1600 volts and with a wave front rise time not less than .1 microseconds, or a dv/dt of less than 16,000...

c Where Z > m is the motor impedance, and Z > c is the characteristic impedance of the line which is equal to ֢Ț L/C. The voltage at the motor is the sum of the incident and reflected waves. For motors smaller than about 25 HP, the reflection coefficient is 1.0. For long leads, voltage doubling occurs at the motor terminals. 4. CRITICAL CABLE LENGTH The critical cable length is the maximum cable length at which voltage amplification does not occur. It is the length at which the sum of the reflected and incident waves are equal to the peak value of the incident wave. If the propagation speed (the...

Table 2. Critical Lead Length for Various Rise Times Rise Time (microseconds) Critical Lead Length (meters) Critical Lead Length (feet) 2.00 100 328 1.0 50 164 0.50 25 82 0.10 5 16 0.05 2.5 8 5. LONG LEAD FILTER OPTIONS Four options are generally used for protecting inverter feed motors that operate from long leads. The first is known as a terminator. From transmission line theory, we know that if the load impedance matches the line impedance, then there is no reflected wave. The concept of the terminator is to place an impedance in parallel with the motor where the parallel combination is designed...

A third option is the use of a single element low-pass filter consisting of a 5% load reactor in series with the output of the inverter. The reactor is located at the inverter. Tests on 460 volt applications show that motor peak voltage is under 1000 volts for lead lengths up to 850 feet. The motor voltage waveform appears in Figure 6. Notice that with the addition of the output reactor, the sine wave begins to emerge from the PWM signal. There are no constraints on minimum switching frequency with this approach. Rise times are greater than 4 microseconds. 5% MOTOR850 ft @ 460 VAC max INVERTER...

Table 3. Long Lead Filters Performance and Cost Comparison Option Waveform Relative Cost (5HP/460V) Do Nothing 0 Snubber 1.48 5% Reactor 0.65 Sine Wave Filter 1.00 >