Jraef said:
Sounds as though you have multiple issues going on here.
- As mentioned, motors run from VFDs will overheat much more easily if they are not designed specifically for that task. Most motors that are not designed for "inverter duty" (a non-defined term, but useful for the moment), are typically only going to be capable of a 2:1 turn down, meaning they can run at 1/2 speed effectively. Any lower than that and they no longer are able to keep themselves cool.
- When you turn the speed down with a VFD you are turning down the HP at the same rate, even though the torque remains the same (theoretically). If you do not reduce the load on the motor, you will overload it. Usually this just means the Overload Protection in the VFD will trip it off-line. But if not properly programmed, you can smoke the motor first. Check your drive programming.
- You mentioned "contactors" with your drive. That is rarely a good idea. can you explain how they are being used? Are you sure they are Drives and not Soft Starters? If they are Soft Starters, many of the smaller A-B products did NOT come with overloads, yet often times salesmen didn't know that and/or failed to mention that they needed to be externally provided. That would explain the burnouts.
- Dirt and contamination in a control cabinet is never good, but wouldn't likely lead to motor burnout unless it interferes with a contactor opening when tripped. But that would also prevent normal shut-downs as well.
Inverter rated motors differ very little from standard motors as far as thermal design.
The turndown ratio represents the full speed and the minimum speed ratio. The turndown ratio will be different for constant torque and variable torque applications for the same inverter rated motors. Inverter rated motors are commonly rated 20:1 variable and 10:1 constant torque application. Ex.:3600:180rpm or 3600:360rpm respectively. External fans can be added to maintain high airflow at low speed if you wish to operate outside the above parameters. This hardly addresses all the other considerations that need to be reviewed.
The equipment manufacturer of the conveyor belt needs to make sure that the maximum needed torque is calculated for the maximum amount of product to be carried. (It may be that the user tries to operate the conveyor beyond its design limit and conveys lead instead of oatmeal.)
The distance between the motor and the drive could be critical, especially if you are not using ASD rated motors. The type of cable and the carrier frequency could also be a source of trouble that can cause the winding to fail, not thermally due to overheating but the buildup of voltage spikes.
The control mode of the drive also need to suit the application.
If the drive is required to 'hold' the load, it can wreck havoc on the bearings. Shaft circulating currents can further aggravate this condition. Some motors do not take kindly to belt driven applications so if that is the case the Motor Manufacturer needs to know.
Make sure that the motor winding - in the case of multivolt application - is connected to the proper voltage.
