We at Gemco engineering are getting sporadic calls from users that have had our blenders for years and all of a sudden they now blow fuses and trip on motor overloads when they are started. We experienced the phenomena here as well when we built a duplicate unit for a customer. The original machine was built in about 1990; the duplicate was 2003. During factory testing, we blew the fuses. The original machine was running with 10 amp fuses for years. The new one needed 25 amp fuses so it would start without blowing the fuses.

We called the customer to confirm that they were still using the same fuses and they were. We replaced the motor with another one with no change in amp draw. We then replaced the motor again with a different make but again the results were the same. We then called in the factory engineers from both motor manufacturers for an explanation.

After much probing, we found the cause of the change in amp draw and blender operation. The laws in the mid-1990’s that mandated high efficiency motors subtly changed the basic motor design. The new motors are more efficient as required. As a trade off however, the starting torque is significantly less. That lower starting torque creates higher amp spikes during start-up of a blender and the problems with fuses and overloads.

The analogy of a motor on a Gemco blender is essentially a flywheel application. There is a large, relatively balanced mass that must be accelerated to speed. The faster it gets to speed, the higher power requirement is required during that acceleration period. Once the unit is at speed, the power required to maintain rotation is simply a function of the RPM and the moment arm from the center of rotation to the center of mass of the powder in the blender.

The remedies for high starting amps are pretty straight forward. Bigger, slow acting fuses/breakers and overloads are the easy remedy. The motor manufacturers did not care that we were drawing six to eight times full load amps during start-up. They said it was normal and the motors are built to take it.

A second remedy is to use a soft starter or VFD rather than a direct-on-line starter. Basically this approach extends the acceleration time for the blender which lowers the power requirement and therefore peak amp draw during start-up acceleration. Start-up takes longer to come to speed but the peak amp draw is lower.

The third remedy is to buy a larger motor or a special high starting torque motor. Unfortunately high starting torque motors are not common in the smaller size motors (below 25 HP) that make up the vast majority of Gemco blending applications. A larger motor is fine as long as it can be found in the same frame size to simplify mounting. Sometimes a larger motor in the same frame size is only available with an intermittent duty rating. That is fine for Gemco blender operations. The higher HP rating is only used for the start-up acceleration period. It just allows larger fuses/breakers and overloads to be used to handle the higher starting amps. While running it will still run at the original low amp draw of the smaller motor.

Gemco has made a design change in its blenders as a result of this new motor reality. All direct drive and portable units now come with a VFD rather than a direct-on-line starter. The operation of the VFD is transparent to the user; the same start and stop buttons are all that are available to control the unit. The user can not set or change the operating speed. What the VFD provides is a soft-start/soft-stop capability by ramping the speed up and down over a few seconds. The new design means that the units do not jump to start or jerk to stop. And the normal starting amp spike is essentially eliminated.

Contact Gemco for more details on motor requirements and our new soft-start direct-drive blenders.