TechTips

Categories

Literature

Technical Tips & FAQs

PCS Doesn’t Reach Full Speed At End Of Ramp?

Problem: If at the end of the PCS soft start (or current limit period) the motor does not reach running speed, what type of performance will be seen?

Solution: The person installing the PCS should attempt to configure it such that the time and torque (or current limit) switch settings allow the motor to reach full running speed before the time expires. Where the load is more dynamic, it is possible that at the end of the established starting time the motor may not reach running speed and is drawing current exceeding 100% of motor FLA (full load amp). In this case the PCS will transition to a quasi-full voltage, gating the SCR's to either 350% of the frame rating or 450% of the frame rating. This level depends on PCS frame ampacity (see table).

PCS Product	Ampacity at 480Vac 3phase	Full Voltage Capability after time expires*
PCS-003…037	3 amp to 37 amp	350% of frame rating amps
PCS-43…085	43 amp to 85 amp	450% of frame rating amps
PCS-108…480	108 amp to 480 amp	450% of frame rating amps
Note: * The PCS may trip on overload or another protection fault if the motor is not at running speed during this condition.

Example: Assume PCS-037 with the following settings

FLA rotary switch dialed to ~25amps

Dip Switch setting:

• sw 1- 8 = 10101000 : soft start for 5seconds at 35% starting torque, no stopping option
• sw 9-16 = 00100000 : class 10 overload set to manual reset

At the end of 5 seconds if the motor has not reached full running speed the PCS will transition to 350% of the frame rating. In this example it can provide up to 350% of 37amp or 129.5amps. This assumes the motor requires this amount of current or less to finish starting. This will occur for a few seconds before the PCS faults to protect the motor. Changing the overload class can extend this period and prevent the fault for a few additional seconds but you also risk motor damage if the motor is improperly sized for the application.