Retaining an Existing AC Motor The following are considerations when retaining an existing AC motor. Note that newer AC motor designs are more efficient and draw less current than older single or two-speed motors. When reusing an existing AC motor, drives may have to be oversized (extra cost) in order to meet motor current requirement. Accurate Nameplate Information. Verify motor horsepower, voltage, full load current and full load RPM. Actual Full Load Current. Actual full load current is very important in order to accurately determine drive size. Particularly with older motors, nameplate data is sometimes inaccurate, illegible or missing. It is recommended that you measure motor current and RPM, with a full load, in order to calculate motor slip (see chart) and properly size the drive.
Drive Too Small If the drive is not sized correctly, making a change in the field requires not only a drive change, but also changing the resistors in the dynamic braking circuit.
Drive Too Large While a drive that is larger than necessary will not typically create problems, there is no reason to buy a larger drive than you need.