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1.4 Notes on the application of inverters
1.4.1 Notes on motors combined with inverters
Keep in mind the following notes when using the VF-A7 in combination with a motor.
Comparison with commercial power operation
The VF-A7 inverter uses a sinusoidal PWM control system. However, the waveforms of
electric currents passing through the main and control circuits are not perfectly sinusoidal but
slightly distorted though they are very close to perfect sine waves. For this reason, a motor
produces more heat, larger noise and larger vibration when operated by means of the inverter
than when operated directly by commercial power.
Operation in low speed ranges
Operating a general-purpose motor by means of the inverter causes a decrease in the cooling
efficiency of the motor. So, reduce the motor's output below the rated load when operating it
in a low speed range.
If you wish to operate a motor continuously at the rated torque, then use a Toshiba VF motor
designed specially for use in conjunction with an inverter. When the inverter is combined with
a VF motor, its overload protection level needs to be changed to "VF motor" ( setting).
Adjustment of overload protection level
The VF-A7 inverter has an overload detection circuit (electronic thermal detection) to protect
the motor from overload. The reference current for the electronic thermal detection is set to
the rated current of the inverter at the factory, and it needs to be adjusted to the rated current
of the general-purpose motor combined with it.
High-speed operation at a frequency of 60 Hz or over
When a motor is operated at a frequency of 60 Hz or over, it produces larger noise and larger
vibration, which can exceed a limit that the motor or its bearings can withstand. Contact the
motor maker if you wish to operate the motor at such a high frequency.
Load of an oil lubrication type
When a speed reducer or a gear motor of an oil lubrication type is operated by the inverter, its
oil lubrication efficiency decreases in low speed ranges. Inquire of the speed reducer maker
about the allowable speed reduction range.
Extremely light load or load producing a very small moment of inertia
When a motor is operated under an extremely light load (e.g., at a load factor of less than
50%) or it drives a load which produces a very small moment of inertia, it sometimes becomes
unstable, for example, it produces abnormal vibration or trips because of an over-current. In
such a case, lower the carrier frequency to cope with this problem.
Unstable operation
When the inverter is used in combination with one of the following motors or loads, it
sometimes makes the operation of the motor or load unstable.
A motor with a rated capacity that exceeds the motor capacity recommended for the inverter
A special type of motor, for example, an explosion-proof motor
When using the inverter for such motors, lower the inverter's carrier frequency to stabilize
the operation. (In vector control mode, do not lower it below 2.2 kHz.)
A motor with a large backlash, which is coupled with a load
In this case, use the S-pattern acceleration/deceleration function, or in vector control
mode, adjust the response time (setting of moment of inertia) or switch to V/f control
mode to stabilize the operation.
Use an inverter which matches the input power rating of your three-phase induction motor.
The use of an inverter unsuitable for your motor can cause it to rotate in a wrong direction,
and thus lead to serious accidents, including its burning due to overheating.
Warning
Mandatory