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An Overloaded Dough Mixer
Copyright © 2001 Francis J. Martino

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).

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Middlebury, CT  06762
USA
Phone: (203) 217-2353
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