FLOMATCHER/MARATHON

SPECIALISTS IN VARIABLE SPEED MOTOR CONTROL SYSTEMS

Prior customers and other sources have asked Flomatcher, Inc. if it is possible to upgrade their old systems with something more modern for a variable speed drive. The inference here of course is to use a variable frequency drive (VFD). Flomatcher has investigated the situation by consulting a number of motor and VFD manufacturers. The consensus of these discussions is that although it is possible to use a VFD with a wound rotor motor, it is strongly not recommended.

The crucial point is that wound rotor motors were simply not designed to operate with VFDs. The insulation in these motors will not withstand the type of operating current that a VFD provides to run the motor. This is not to say that a wound rotor motor will not function with a VFD but that there is no guarantee as to the life of the motor. The probability of damage to the motor is very likely and in that instance the VFD could also be harmed.

Another problem with retrofitting wound rotor motors with VFDs is that the motor itself needs to be modified. The rotor windings of the motor need to be shorted out. This is not a simple matter as the rotor is shorted out by bussing the slip rings. This should only be done in a motor shop that is thoroughly familiar with the procedure. After the motor is altered, there is a possibility that it will not know which way to turn. In this eventuality, a starting capacitor network may have to be added to solve this situation.

There are other appurtenances that need to be considered in retrofitting a wound rotor motor with a VFD. An isolation transformer should be considered for each drive or be sized so that it will handle all the drives. The preferred configuration would be an isolation transformer for each drive. Another thing to think about is that some VFDs will need some kind of line reactor. Depending on the VFD it may need a line reactor on the line side, on the load side, or possibly both.

It is Flomatcher’s position that a wound rotor motor should not be used with a VFD. If an upgrade of a system is desired, other options should be considered. One such option if a VFD is favored is that the wound rotor motor should be replaced with a squirrel cage motor meant to be used with VFDs. Another option is that the wound rotor motor be controlled by an electronic drive designed to operate the motor by acting on the rotor windings as was originally intended. The obvious advantages of using a drive designed to operate on the rotor windings is that the motor does not have to be replaced and voltages on the rotor can be considerably lower than on the stator (some stator voltages can be 4160 vac or higher). High stator voltages can require different conductor insulation (generally equating to more expensive), larger conductor spacing (requiring more room), and other items that greatly increase the cost and difficulty of the installation of a system that controls the speed of the motor on the stator side.

Another thing to keep in mind about upgrading systems is whether the old controls will integrate with the new drives. This can sometimes be very costly and the end result is that the new system may not operate as well as the old system. Upgrading a system is a very serious business and should only be undertaken by someone that is fully aware of all of the ramifications. There are any number of suppliers in the world that will hang a VFD on the wall, patch in the controls and leave, never to be seen again. They do not have to operate the equipment and neither will they service the equipment or furnish any customer assistance. The watchword here is to only consider a system upgrade by a company that will responsive and accountable to the customer.

Flomatcher has been supplying variable speed drives and system controls for over four decades. Although we will discourage the use of VFD’s to control the speed of a wound rotor motor, we have the experience and background to offer other possibilities for system upgrades. We welcome the opportunity to investigate any needs for system upgrades and new systems.

When considering a system upgrade the following questions should be taken into account: