Guide to medium and high voltage motors

Despite low voltage motors having a wide range of potential uses in industry, there are still some cases when a higher voltage is needed to handle the job. In 2018, Hoyer will be expanding its product series with a range of medium voltage motors with voltages ranging from 3 kV to 11 kV, which are well-suited for heavy loads.

Hoyer’s technical manager, Bjarne Nør, provides an introduction to medium and high voltage motors.

Keeping the limits in check

There can be different ideas about what qualifies as a low, medium or high voltage motor.

“Some people consider anything above 1000 V as high voltage. The limits are clearly defined, however, in the IEC 60038 standard:  low voltage is up to 1000 V, medium voltage is from 1000 V to 35 kV, and high voltage is over 35 kV. The motors in our new product series are thus all placed in the medium voltage category,” says Bjarne Nør.

A higher voltage offers the potential for savings

Medium and high voltage is especially relevant for heavy applications, which require a motor with an output of 400 kW and upwards. Here, the higher voltage makes it possible to use smaller cables, thereby significant decreasing distribution cable costs.

“By increasing the voltage it is possible to lower the current. This means that smaller distribution cables can be used.  A higher voltage is also an obvious choice if cables have to be laid over long distances, for example in tunnels.”

These sectors should consider medium and high voltage

Medium and high voltage motors are especially well-suited for professionals that work with heavy loads.

“The product is ideal for the HVAC industry such as manufacturers of industrial fans and screw compressors but also the pump industry.”

Insulation system differs from low voltage

On a purely mechanical level, medium voltage motors do not differ significantly from a standard, asynchronous low voltage motor. We are still talking about a rib-cooled motor with relubricating bearings. There are however some important differences, explains Bjarne Nør:

“The stator on a medium voltage motor has an improved insulation system to rate it for medium voltage. This includes a vacuum impregnation system, whereby all recesses are filled with varnish together with corona protection materials to prevent electrical wear of the insulation material. In addition, there are greater creepage and air clearances from conductor to earth.”

As opposed to a low voltage motor, a medium voltage motor does not have a terminal connection board, but is instead connected using high voltage insulators.

Tested for both electric and thermal lifetimes

The higher the voltage, the greater the impact on the motor’s lifetime. In-depth requirements are thus placed on documentation and testing when it comes to quality assurance of the motors.

“While a low voltage motor will only be tested for thermal lifetime, our medium voltage motors are tested for both electric and thermal lifetimes. We have documentation on all our motors for a minimum lifetime of 20,000 hours. Naturally, we also have performance curves and the like, as well as the opportunity for classification on a project basis for the marine segment.”

If variable speeds are desired, it is possible to install the motors with current-insulated bearings, just like those used in low voltage motors, so the lifetime can be further increased due to the reduced bearing current.

Requires authorised installation and comprehensive advice

While most electricians can connect a low voltage motor, using higher voltages is more complicated. The use of medium voltage motors therefore requires comprehensive advice and close dialogue.

“You need to have special authorisation to install a medium voltage motor and establish the electrical connection.  Both our sellers and our technical department are ready to offer support, for example in choosing the correct motor type and establishing the electric interface, including clamps and cables,” finishes Bjarne Nør.

Facts about Hoyer’s selection of medium voltage motors

From the start of 2018, Hoyer will be offering a standard programme with medium voltage motors in the following voltage types:

• 3 kV / 3.3 kV
• 6 kV / 6.6 kV
• 10 kV / 11 kV

There is also an engineered programme with the possibility of customer-specific solutions.