Motor Characteristics; Pulse Width Modulation Operation; Overload Protection Adjustment; Power Factor Correction - Toshiba W7 Installation And Operation Manual

Motor Characteristics

Listed below are some variable speed AC motor control concepts with which the user of the
W7 Adjustable Speed Drive should become familiar.

Pulse Width Modulation Operation

The W7 ASD uses a sinusoidal Pulse Width Modulation (PWM) control system. The output current
waveform generated by the ASD approaches that of a perfect sine wave; however, the output waveform
is slightly distorted. For this reason, the motor may produce more heat, noise, and vibration when
operated by an ASD, rather than directly from commercial power.

Overload Protection Adjustment

The W7 ASD software monitors the output current of the system and determines when an overload
condition occurs. The overload current level is a percentage of the rating of the motor. This function
protects the motor from overload.
The default setting for the overload detection circuit is set to the maximum rated current of the ASD at
the factory. This setting will have to be adjusted to match the rating of the motor with which the ASD is
to be used. To change the overload reference level, see

Power Factor Correction

DO NOT connect a power factor correction capacitor or surge absorber to the output of the ASD.
If the ASD is used with a motor that is equipped with a capacitor for power factor correction, remove
the capacitor from the motor.
Connecting either of these devices to the output of the ASD may cause the ASD to malfunction and trip,
or the output device may cause an over-current condition resulting in damage to the device or the ASD.

Light Load Conditions

When a motor is operated under a continuous light load (i.e., at a load of less than 50% of its rated
capacity) or it drives a load which produces a very small amount of inertia, it may become unstable and
produce abnormal vibration or trips because of an over-current condition. In such a case, the carrier
frequency may be lowered to compensate for this undesirable condition (see Program
Control
Note: For proper operation, the carrier frequency must be 2.2 kHz or above except when
operating in the Constant Torque or Variable Torque modes.

Load-produced Negative Torque

When the ASD is used with a load that produces negative torque (an overhauling load), the over-voltage
or over-current protective functions of the ASD may cause nuisance tripping.
To minimize the undesirable effects of negative torque the dynamic braking system may be used. The
dynamic braking system converts the regenerated energy into heat that is dissipated using a braking
W7 ASD Installation and Operation Manual
PWM Carrier Frequency).
Electronic Thermal Protection #1 on pg. 70.
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