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Measuring Power Dissipation in Step Motors and Drives

by Jeff Kordik

Background In some applications, power dissipation in step motors and drives  is a critical aspect of system design. Too much heat from the drive can cause a cabinet to overheat. Too much power loss in the motor can cause the motor to overheat at high ambient temperatures. Heat can also transfer into attached equipment causing it to misbehave. One example is a digital ink pump, where excess heat from the motor can cause undesirable changes in the viscosity of the ink. A recent customer was modeling their enclosure and requested detailed power dissipation data. The exact customer and application are confidential, but we can say that their equipment needs to work around the clock in very hot places like West Texas and the Arabian Peninsula as well as very cold places like Prudhoe Bay, Alaska and Willits, North Dakota. The motor and drive system can be represented by this block diagram:


Test 1: Current, voltage and power into drive with motor under load For this measurement, we connected an Applied Motion Products STAC6 drive to an HT34-496 NEMA 34 high torque step motor with windings in parallel as recommended for 120 VAC operation. The motor was mounted to a Magtrol dynamometer set for 420 oz-in load. Motor current was set for 4.8 peak sine per phase. Motor was driven at 3.5 rev/sec, half of the peak speed of 7.0 rev/sec specified by the customer. The load was chosen by this calculation:


The equipment was connected as follows:


Test Results: Voltage into STAC6: 122.75 VAC Current into STAC6: 2.373 Arms Power into STAC6: 158.6W Test 2: Bridge rectifier current The customer was successful in modeling most of the internal drive components, but the model was predicting excess temperature on the bridge rectifier at high ambient temperatures. To characterize the current flowing through this component, we modified a STAC6 by connecting a #18 wire between the rectifier bridge and the capacitor bank so that we could record the current waveform flowing out of the bridge under load. Here's the setup:


Test Results: See plot below. Average voltage = 3.406 mV. Average current = 1.703A. Vertical scale: 10mV/div = 5A/div Horizontal scale: 5 msec/div


Test 3: Current, voltage and power out of STAC6 into motor under load The difference between the power entering the drive (Test 1) and the power leaving the drive and heading into the motor (Test 3 X2 because we only measured one motor phase) must be the power dissipated as heat by the STAC6. Here's the test set up:


Test Results (per motor phase): Voltage into motor: 107.9 VAC Current into motor: 3.65 Arms Power into motor: 65.8W Analysis: The power dissipated by the STAC6 drive under these conditions is: The average current through the bridge rectifier is 1.703A. Estimated rectifier power dissipation: (2)(0.75V)(1.703A) = 2.55W Estimated rectifier temp rise (at junction): (2.55W)(18 deg C/W) = 45.9 deg C Max KBU8G junction temp is 150 deg C. If ambient stays below 100 C this component should be fine.

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