HowTo: High Speed AC Motor - Application Detail

Maximum Speeds of around 7000RPM to 9000RPM are available from small standard AC motors. A 73% increase in Speed and Power is available from most all small AC motors by using a 230V x 50Hz motor at 400V x 87Hz, when supplied by an Inverter Drive. This brings the base speed for a 2 pole 50Hz motor to around 4800RPM. The maximum speed is then around 2 x the new base which could be as high as 10,000RPM (in theory).

In practice the manufacturer will specify a small reduction in power to suit the extra losses at high frequency. The maximum mechanical speed is usually published in the motor data or technical catalogue. For the high speed mentioned above, the rotor will need special balancing to reduce the bearing side load at the very high speeds.

This is because a 400V Inverter controls Voltage and Frequency through 230V x 50Hz on its way to 400V x 87Hz. The motor remains correctly fluxed all the way through to 87Hz. Above 50Hz the motor will be capable of more power, in proportion to its speed above 50Hz. If in doubt as us.

The proceedure is; set the 400V Inverter 'Base Frequency' to 87Hz. This is near enough the same for 220V and 240V Motors. For 240V x 60Hz motors, set the base frequency to 104Hz. The parameter is known as 'Base Frequency' or 'Nominal Frequency' (depending on the Inverter manufacturer) in the motor settings of the Drive unit.

The Inverter provides protection from motor over-current (when correctly set). Most motor manufacturers will approve use of their 240V motors at 400V, as long as they are correctly fluxed. TEC, Marelli, Brooks and Leroy Somer have in the past.

Most small motors are arranged for 230/400V x 50Hz supply for Delta/Star connection, so can be run from an Inverter Drive at 400V x 87Hz with Delta connected windings. This is the standard arrangement for motors up to around 3kW, depending on the manufacturer.

Larger motors will need to be ordered at 240V or are available with special designs for high speed operation, please ask.

Size the drive to suit the motor full load current (not kW).