Oil Film Bearing - See Non-Rolling Element Bearings
This discussion is limited to the following types of motors which are commonly used in pumping applications:
AC Induction motors, single and three phase, 208/230/480 volt 50/60 Hz., 100 hp or less, NEMA frame JP - JM - T and TS, TEFC and ODP enclosures, Insulation Class F, Continuous Duty, Service Factors 1.0, 1.15, and 1.25.
Definition of Overload - The electrical condition when a motor draws more current than it is rated to draw by the manufacturer, as indicated on the motor nameplate as FLA or FLC ("Full Load Amps" and "Full Load Current" respectively)? When a motor draws current greater than FLA continuously the motor windings may heat up beyond their temperature limits and consequently the winding insulation life expectancy may be shortened or even damaged quickly.
Disputed Definition of Overload
For motors with no Service Factor, (SF of 1.0), the concept and therefore definition of Overload is precise and commonly agreed upon as follows: A motor operating at a current level above FLA.
For motors with a Service Factor greater than 1.0 the definition of Overload is disputed as follows:
- Some claim that Overload is always defined as a motor drawing current above FLA, regardless of Service Factor.
- Others have two definitions of Overload as follows:
For Motors with SF of 1.0 - A motor drawing current above FLA.
For Motors with SF greater than 1.0 - A motor drawing current above FLA + Service Factor.
Related Subjects on this Website:
Glossary - Full Load Amps (FLA)
Non-Overloading Pumps and Motor Life Expectancy
See 2 Pump Graphs - Non-Overloading & Overloading
Learn How to Diagnose OVERLOAD problems
The Overload Relay is a device that predicts motor winding temperature based on current drawn by the motor, and then disconnect that motor if the predicted winding temperature rises above a set point.
Circuit breakers and fuses are Over-Current devices primarily protecting wiring from overheating and starting fires and causing electrocution. However, Circuit breakers and fuses are designed to prevent large overcurrent situations. What happens when something like a motor, draws more current than the motor's rated capacity, and yet insufficient over-current to open the fuse or circuit breaker? The motor would be able to heat up considerably, possibly starting fires, damaging wires, and potentially electrocute someone.
The Overload Relay also protects against over-current, similar to a circuit breaker or fuse, but with an important difference. Overload relays are capable of sensing smaller amounts of over-current than circuit breakers or fuses. However, this ability to detect small over-current situations could potentially cause nuisance shut-downs were it not for the Overload Relays inverse time delay feature.
To avoid nuisance shut offs, Overload Relays react to motor current draw with an inverse time delay period, allowing the motor to draw more current than the motor's rated current capacity, for certain periods of time. The time period allowed by the Overload Relay is inversely proportional to the amount of overload current. The larger the overload, the shorter time period that is allowed (inversely proportional).
Therefore, the Overload Relay is very sensitive to motor current draw, to prevent the motor from heating up and starting fires, while still allowing the motor to draw high current when needed, for short periods, such as when the motor starts, and also the motor reacts to minor periodic overload conditions.
The Overload Relay protects the motor windings from overheating which would destroy the windings, and yet, the primary intent of the electrical code is not so much to protect the motor, as to protect the immediate environment (including people) from an immediate and catastrophic situation leading to possible fire and electrocution. Common practice implements the Overload Relay to comply with NEC 430 and UL 508A, and yet the motor is not protected by that method from all damage, just serious and immediate damage that would cause an immediate fire or electrocution hazard.
The Overload Relay can however be implemented in such a manner as to not only comply with NEC 430 and UL 508A, but also to prevent all damage to the motor. Irrigation Craft is unique because we intentionally implement the Overload Relay in such a manner so as to protect the motor from any damage at all due to overheating, which is not required by NEC 430 or UL 508A.
Below - Siemens Overload Relay Used On Irrigation Craft Products.
One unique feature of this overload relay is the "Cage Clamp" type control wire connections. Cage Clamps are vastly superior to screw clamp terminals found on other brands. Cage Clamps apply a constant spring load to the wire, producing a vibration resistant, high pressure wire termination. NASA changed the Kennedy Space Center to Cage Clamp terminals because of their superiority.
And yet, the vibration resistance is not the main advantage, as important as that is. The main advantage is explained as follows. In any wire connection there are two main factors, surface area and pressure, with pressure being the most important part. Screw terminals rely on surface area, the less important factor, while cage clamp terminals rely on pressure. Cage Clamps apply a constant spring load pressure to the wire.
Before Irrigation Craft switched to Cage Clamp, we were forced to "age" the panels by building the control panel, tightening all the screw terminals, then we let the panel sit for two weeks (when we had time), and then we re-tightened all the terminations. This is not longer necessary with Cage Clamps. And the customer saves money also, because in the years to come, there will be less problems due to loose wire connections.
Related Subjects on this Website:
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