Emerson manual Rosemount 848L, Reference Manual, 00809-0100-4696,Rev AA September

Reference Manual

00809-0100-4696, Rev AA September 2004

Rosemount 848L

Restart Delay

A motor may be used in a condition where starting is difficult, causing the motor to heat rapidly until it gets running. That heat must be allowed to dissipate before the motor is started again. A simple time delay prevents the start button from working until a fixed delay time expires. Another example is the time required for compressor head pressure to bleed off after the compressor motor stops.

Maximum Restarts

Another way of handling difficult starts is to count the number of starts in a given time and lock out the start button if the count is exceeded. Locking it out means that the start button will not function until a latching relay has been manually reset by an operator who has verified the safety of the situation.

Winding temperature

The above restart limiters may not be necessary if the winding temperature can be measured and used as a permissive for starting. The winding temperature sensor may be a ten ohm length of copper wire that is wound into the motor along with the power windings.

Hand-Off-Auto

An operator may be required to perform some function near the motor, such as clean a pump strainer or jog the motor to get its load into the right position. The motor is normally controlled by the central system but must have a local station to allow the local operator to control the motor. The local station has buttons for Stop and Start, and a three position switch for Hand-Off-Auto (HOA) selection. The control room has control when the switch is in the Auto position. The motor will not run when the switch is in the Off position. The Hand position allows the local start and stop buttons to control the motor. The Off position is not as secure as the lockout procedure required when the equipment or the operator would be damaged if the motor started. This requires all concerned people to physically put a padlock on the Off position of the main circuit breaker for the motor. The motor may be started after the last person removes their lock.

Intermediate Stop

A reversible motor may be required to come to a complete stop before starting to run in the other direction. This may be done with a timer or a motion sensor on the motor (or driven load) shaft.

Redundant Motors

The process may require redundant motors for reliability. Usually this applies to pumps, so that there is no mechanical connection between the two motors. One pump may be shut off to replace seals (or the entire pump and motor) while the redundant pump maintains flow in the line. There are three ways to control redundant pumps:

Alternate Start

When the start button is pressed the pump that was not in use is started. Not in use refers to now or since the last time start was pressed.

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