Efficiency: putting motors through their paces

MOTORS ARE tested for different reasons: verifying data from software simulations or the motor’s nameplate, independent verification of compliance for notified bodies, or to check if a motor or drive system meets a customer’s specifications. Testing can also be used for troubleshooting and parameterisation. And for motor manufacturers, testing is generally the most appropriate way to push the envelope and discover what is possible. 

Testbed operation allows load and no-load runs in a safe setting. Drive systems can be tested with extreme overloads, voltage drops or other non-standard situations. Load testing creates a broad database about the tested equipment that be used to identify faults later, when equipment is in the field.

Is your data of good quality?
Conventional tests with a direct coupled load mainly show that the motor works properly on load. Efficiency values from such a setup are guesswork. Menzel’s slightly more sophisticated setup allows calibrated measurements and gives information on the actual efficiency ratios. The input current is measured with current and voltage transformers. A torque measurement shaft installed between motor and load machine measures the generated torque. Losses and efficiencies are calculated from the difference between the (electrical) input power and the (mechanical) output power. With the tested motor and the load generator operated in a closed loop, this setup only consumes power for startup and any energy loss. Therefore, supply lines do not have to be over-dimensioned, and energy-saving tests can be performed over hours and hours. This setup also allows for precise simulation of the customer’s application load profile to determine the motor’s performance at individual load points.

Heat run – coupled or uncoupled
During normal operation, the motor’s thermal behaviour is continuously monitored with probes in the windings and on the bearings. Winding resistance is measured in a de-energised state after the heat run to determine final temperature. Another viable alternative, if direct coupled testing is impossible or uneconomic, is dual frequency testing per IEEE/IEC 60034-29. This once-common procedure is still used in the US and Asia but is rare in Europe today. The test specimen is operated at no load; two supply voltages with different frequencies are induced. These frequencies are adjusted so the motor’s rotational speed oscillates. The current input is varied to give a heat run at rated voltage, rated current and rated speed. The temperature rise of the motor is within safe margins, at maximum 6% higher than regular load. Because dual frequency testing doesn’t require coupling to a load machine, vertical machines, and machines with special shaft ends, can be tested without modifications. This method is efficient because only the energy losses must be covered as active power. However, it only determines thermal characteristics, not efficiency. But test locations, without the facilities to test high-output machines at load, can use dual frequency testing if they have the necessary know-how. Menzel Elektromotoren, runs dual frequency tests on motors up to 5 MW, considerably extending its scope of temperature rise testing.

System tests with added value
New problems arise when components, tested and proven individually, are combined into a drive system. Comprehensive system tests are used to detect and resolve interface problems between components before commissioning – simplifying planning and reducing time. There is high demand for testing configurations with a transformer, VFD, and motor. Load testing helps verify system efficiency at different load points but is also used to set parameters on the VFD – so all settings and start-up are completed before shipping – enabling the plug-and-play solutions customers demand.
Performing a system check requires more than just test facilities: there must also be sufficient space to securely place and operate all components, flexible measuring equipment and, most importantly, personnel with the necessary experience to ensure a safe setup that will provide accurate data.
Mathis Menzel is CEO of Menzel Elektromotoren
www.menzel-motors.com
Tel 01664 500844