Baldor MN770 manual Appendix a, Load Weighing / Torque Feed Forward

Appendix A

Load Weighing / Torque Feed Forward

In many advanced elevator applications, the system is designed to weigh the elevator load to offset the counterweight of the car. We also refer to this as a torque feed forward application. Both the Baldor AC Vector (Series 18H and 22H) and Series 20H SCR Controls can be programmed to use the torque feed forward option (Level 1 Input block; Command Select parameter, 10V w/Torq FF).

In a typical installation, a car is balanced with a 40% load. In order to move the car in the up direction, the drive should receive a forward speed command. If the car is full, then it would have to generate positive torque in order to move the car forward or in the up direction.

A load cell is installed on the car to measure the loading of the car. Typically, 10 volts output from the load cell equals a full car and 0 volts equals an empty car. This means that 10 volts equals a value of positive torque and 0 volts equals a smaller absolute value of negative torque.

Since the output of the control requires the torque feed forward signal to be bipolar, then the load cell voltage needs to be scaled properly in order to generate the appropriate values of torque. A customer supplied signal processor board can be used for this purpose (the load cell could have this feature built in).

In a typical elevator application, the accelerating torque is significantly greater than the holding torque (or torque required at zero speed), which means that it is not necessary to use the full range (±10 volt) capability of the torque feed forward. This is shown in Figure A-1. A ±5 volt output of the signal processor board is probably adequate for most typical elevator applications.

Figure A-1 Typical Elevator Application

0-10V		±5V	JOG LOCAL PROG
			FWD DISP
Signal		Signal	STOP RESET
			REV SHIFT ENTER
	Signal
	Processor Board

Load

Cell

Baldor Series H

Vector or

DC SCR Control

Elevator Car

Counterweight

MN770 Appendix A-1