A variable frequency drive was considered for use on a dough mixer. The advantages were that a reduced inrush current on start would lessen the frequency of motor burn out. The acceleration ramp of the drive would also eliminate the snapping of the feeder wires, which was quite audible, due to inrush current, thus increasing the expected life-time of the feeders.

Another advantage of the drive is that it would allow some reduction in speed which might prove to be beneficial with some products.

The Data

The nameplate of the two-speed motor read:
125 / 62.5 HP, 1780 / 890 RPM, 143 / 99 amps, 3 / 60 / 460

Measured amps under maximum load:
high speed: 160 amps
low speed: 111 amps

The nominal full load amp ratings taken from a motor catalog for a 1780 RPM motor at 460 volts:
For 125 HP: 144 amps
For 150 HP: 170 amps

Duty Cycle: 3 jogs and 1 start in a 15 second interval, followed by continuous run for ten minutes, cycle repeated continuously hour after hour.

The motor makes a loud whining sound in low speed jog.

If the amperage measurements as provided indicated a continuous full load amps then the measurement reveals an overloaded condition in both speeds.

However, if the mixer presented an overhauling load to the motor, then the maximum amps would indicate regenerative amps flowing from the motor and returning to the distribution system.

Preventing the Failed Application:

A Typical Motor Overload Condition

Operation of the existing motor with a variable frequency drive will be hindered by the following conditions:

a) RMS current to the motor will be increased by a nominal 5% due to the generation of harmonic currents by the drive. Thus, the maximum current to the motor will become 160 amps x 1.05 = 168 amps.

b) Reduction of the operating speed to 90% of maximum, as was requested, will lower the cooling effect of the motor fan. Thus, the motor will experience increased heating and a reduced HP capability to a nominal 95% of its rating.

The existing operation of the motor with the overloaded condition will cause motor slip to increase so that the motor may be operating at a speed less than the nameplated 1780 RPM. Adding an additional 5% of harmonic current will increase the internal motor losses, reduce torque output capability at the running speed and, therefore, cause a further reduction in speed. Thus it is possible that the addition of a variable frequency drive could cause the motor speed to drop to a point at which the motor torque required at that reduced speed will exceed its breakdown torque. At that point the motor will stall.

If the motor, however, was originally wound with a 150 HP winding that was “application derated” by the motor and equipment manufacturers to 125 HP, the motor will be able to operate within the nominal 150 HP rating of 170 amps with the addition of a variable frequency drive. It will then maintain a running speed no lower than 1780 RPM.

If the motor indeed has a 125 HP winding, the only option to resolve the inrush problem is to add a solid state reduced voltage starter that will accelerate the motor with a current ramp. Upon the motor reaching rated speed, a bypass contactor would be used to bring the motor directly across the line. The transition from the solid state control to the bypass contactor would be through a “closed transition” so that motor current would not be interrupted. Thus the motor will be subjected to harmonic currents generated by the SCRs only during start and jog.

The stressful full voltage motor starts will then be eliminated by the reduced voltage starter. The disadvantage is that the starter is not capable of varying the running speed of the motor.

In order to select either the variable frequency drive or the reduced voltage starter, the actual capability of the motor winding must first be determined. To do so, the motor must be loaded so that it is drawing its overloaded 160 amps. At that time a hand-held tachometer must be used to measure motor RPM. If the motor RPM is above 1780, then the motor has a 150 HP winding in it and a variable frequency drive may be added. If the motor RPM is below 1780 RPM, then the motor has a 125 HP winding and the reduced voltage starter should be used.

Preventing The Failed Application:

Motor Regeneration

If the motor was being driven into regeneration then the ammeter and tachometer would have revealed that condition. The ammeter would observe a cyclical fluctuation with each revolution of the mixing barrel. The amps would be varying between maximum amperage as given in the data and some value below the full load amp rating of the motor.

The tachometer would indicate a fluctuating speed. The RPM would be below rated base speed when the motor draws maximum amps and above base speed when the amp measurement was the lowest.

The actual full load amps would be measured at the point when the barrel is no longer driving the motor and the motor begins to drive the barrel.

If the full load amp reading then indicated a load of 125 HP or less, a VFD may be applied with dynamic braking resistors that will absorb the regenerative current and prevent the VFD from tripping due to the regeneration. With the high duty cycle of regeneration it may be economical to purchase a regenerative drive rather than utilizing dynamic braking resistors. The regenerative drive will pass the regenerative current from the motor to the distribution and maintain a lower power consumption within the facility.

Additional Considerations

The existing motor contactors could be used to switch the motor windings for operation on either speed prior to powering the VFD. Thus the existing motor could be used in either speed if it is determined that the maximum running load at 90% of each of the rated speeds was 95% of motor rating, as is indicated above.

If the existing contactors are eliminated and the motor is to be used on the 1780 RPM winding only, then the motor will operate at 95% HP output at the 90% speed point but it will develop a nominal 87% of its rating at the 890 RPM speed point and 85% at a point 10% lower than 890 RPM.

Thus if the horsepower requirements of the mixer exceeds the motor horsepower capability as noted above, then the motor will need to be replaced with an inverter duty motor that is cooled with a constant speed cooling fan or with a motor that is totally enclosed and non-ventilated (TENV).