8.5 Installing Peripheral Devices
■ General Precautions when Using Thermal Overload Relays
The following application precautions should be considered when using motor thermal overload relays on the output of
AC drives in order to prevent nuisance trips or overheat o f the motor at low speeds:
1. Low speed motor operation
2. Motor cable length
3. Nuisance tripping resulting from high AC drive carrier frequency
Low Speed Operation and Motor Thermal oL Relays
Generally, thermal relays are applied on general-purpose motors. When general-purpose motors are driven by AC drives,
the motor current is approximately 5% to 10% greater than if driven by a commercial power supply. In addition, the
cooling capacity o f a motor with a shaft-driven fan decreases when operating at low speeds. Even if the load current is
within the motor rated value, motor overheating may occur. A thermal relay cannot effectively protect the motor due to
the reduction of cooling at low speeds. For this reason, apply the UL recognized electronic thermal overload protection
function built into the drive whenever possible.
UL recognized electronic thermal overload function of the drive:
Speed-dependent heat characteristics are simulated
using data from standard motors and force-ventilated motors. The motor is protected from overload using this function.
Long Motor Cables
When a high carrier frequency and long motor cables are used, nuisance tripping of the thermal relay may occur due to
increased leakage current. To avoid this, reduce the carrier frequency or increase the tripping level o f the thermal
overload relay.
Nuisance Tripping Due to a High AC Drive Carrier Frequency
Current waveforms generated by high carrier frequency PWM drives tend to increase the temperature in overload relays.
It may be necessary to increase the trip level setting when encountering nuisance triggering of the relay.
W ARNING!
F ire Hazard. C onfirm an a c tu a l m o to r o v e rlo a d c o n d itio n is n o t p re s e n t p rio r to in c re a s in g the th e rm a l oL trip setting.
C h e c k lo c a l e le c tric a l co d e s b e fo re m a k in g a d ju s tm e n ts to m o to r th e rm a l o v e rlo a d settings.
T
YASKAWA ELECTRIC SIEP C710616 33D YASKAWA AC Drive L1000A Technical Manual
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