Application Guide

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During starting, mechanical stresses increase rapidly with increases in motor sizes. Mechanical failure through metal fatigue is
common on large machines that are started too frequently. When a VFD is applied to a motor, surge current is almost
eliminated depending on the inertia of the load and the time required for the motor to reach full or required speed. The result is
that there is virtually no limit to number of motor starts per hour, regardless of size.
Ambient Temperature
As indicated earlier, general purpose motors are normally designed to run at full load with nameplate conditions and an
ambient temperature of 104°F.
Motors are often installed in areas where there is little or no ventilation. The heat losses from the motor may cause the
surrounding air temperature to exceed the original design conditions of the motor. The application of a VFD in this case has
little effect on life expectancy (see above).
Altitude
The altitude, height above sea level, has a bearing on motor temperature since the air density, and thus its ability to absorb
heat, reduces with altitude. Generally 3280 feet is the height at which derating should commence.
Terminal Voltage
VFD control of the motor terminal voltage is direct. The VFD overall control strategy includes design to provide this function. As
motor terminal voltage has a major impact on motor performance, great effort has been made by manufacturers to optimize
voltage control under all conditions.
Power Supply Frequency
The motor design frequency is usually 50 or 60 cycles. When a VFD drives a motor, it is usually to control the motor speed. It
therefore follows that the VFD has design functions embedded within its control strategy to ensure that the motor is not
adversely affected by changes in the frequency supplied by the VFD.
Current Waveform
Manufacturers have taken major steps over the last few years to improve the quality of the modern VFD current waveform.
Motors are machines designed to operate with a sine wave input. A few years ago, the VFD produced current waveforms that
were coarse and anything but sinusoidal. The result of this was rough running and on some occasions overheating of the
motors. A modern VFD produces current waveforms that show little distortion from the sine wave and thus motor and drive
performance has dramatically improved. Motor losses and temperatures are both reduced.
Further Protection
Under all operating conditions the VFD monitors the speed and load imposed on the motor. A model of these conditions is
continuously updated and checked against standard acceptable limits. In the event that these limits are exceeded, the motor is
in an overloaded condition. Unless this overload is removed, the VFD takes the decision to trip the drive to safeguard the
motor. If this motor model does not to provide the desired protection level, the VFD can take direct measurement of motor
temperature via internal thermistors.
Differences Between Star Delta and Delta Star
Low power three-phase motors, 5 HP and below, are generally wound so the motor can operate on 200 or 400 Vac. To operate
on 200 Vac, connect the motor in delta. To operate on 400 Vac, connect the motor in star. Select the correct VFD based on
power supply voltage and phase number. Connect the motor to match the power supply voltage.
Motors above this capacity generally are wound to be operated in star mode for starting, to reduce surge current, and delta
mode when the load is up to speed, to provide nameplate power output. Multiple contactors and various other devices are used
to perform this change over function.
Under normal circumstances when a VFD is applied to a motor, connect the motor in Delta mode. The VFD will start the motor
smoothly and without current surge. No changeover contactors are required.