Download Inverter Duty Motors VS General Purpose Motors

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Inverter Duty Motors VS General Purpose Motors
General purpose AC motors have been around for many
years. An inverter-duty motor is a much newer concept that
became necessary as motors began to be driven by
variable frequency drives. An inverter-duty motor can
withstand the higher voltage spikes producted by all VFDs
and can run at very slow speeds without overheating. This
performance comes at a cost: inverter-duty motors can be
much more expensive than general purpose motors.
General purpose motors can be run with drives in many
applications; however inverter-duty motors are designed to
handle much lower speeds without overheating and they
are capable of withstanding higher voltage spikes without
their insulation failing. With the increased performance
associated with inverter-duty motors comes an increase in
cost. This additional cost can be worth it if you need greater
performance. Ryan Chamberlin Inside Sales, Customer
[email protected]
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FAQ :: VFDs | Should I Use An Inverted Rated Motor On VFD
Applications?
Many electricians apply Variable Frequency Drives (VFDs) to AC Motors that are not inverter rated.
Many are not even aware that inverter rated motors exist. Inverter rated motors are not required for inverter
applications, however, when a VFD is applied to an inverter rated motor, the inverter rated motor has less
chance of premature winding failure. Inverter rated motors have unique windings designed for voltage spikes
up to approximately 1500 Volts. Variable Frequency Drives (VFDs) create artificial sine waves which allow
an AC Motor to function; normal AC Motor windings are designed for voltage up to around 600 volts. When
these spikes exceed the maximum, the winding insulation starts to break down and eventually will burn up.
Inverter rated motors are optimized with higher rated insulated (pulse-shielded) copper windings. These type
of windings can handle voltage spikes up to 1500 volts and higher before the insulation will begin to break
down. Inverter rated motors also have tighter bearing air gaps than normal AC Motors. These tighter air
gaps are designed to prevent voltage buildup in the proximity of bearing housings which can cause bearing
failure. The greater the distance the motor is from the drive the more profound the effects of inverter
generated voltage spikes. When the wire from an inverter to the motor is 50 ft or longer, an Inverter rated
motor should always be applied. Always chose an inverter rated motor on new applications. Operating a
Non-Inverter rated motor with an inverter will void the warranty. When the warranty claim is evaluated by a
qualified service shop and/or manufacturer, they will determine the cause of failure to be “Operating of a NonInverter rated motor with an Inverter.” Consult with a professional prior to making a decision on what type of
motor to use in your VFD application. Many customers tend to use an existing AC motor for the VFD
application if the motor is already available, and replace it with an inverter-rated motor when the original motor
fails. There is some risk in this practice should the failed motor damage your new inverter. The general
lifespan of an AC Motor is dependant on several factors such as:
Environment
Routine Maintenance
Using the correct type of motor and VFD for application.
Many electricians today apply Variable Frequency Drives (VFDs) for numerous reasons on AC Motor
applications. One advantage of using a VFD is to extend the lifespan of an AC Motor. VFDs allow soft-start
which in turn creates less wear and tear on a motor, however, if the motor being applied is non inverter rated,
some of these benefits may not apply. This does not mean you should only use a VFD on inverter rated motor
only, this just means you have overall greater benefits on inverter motors than basic AC motors.
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Variable Speed Drive Motors
Variable speed drive motors, also known as inverter-duty motors, are an asset in the industrial manufacturing
world. When using a variable speed drive, variable speed drive motors are less
likely to fail during production than general purpose motors. Variable speed drive
motors exceed general purpose motors in performance, because variable speed
drive motors are manufactured to operate at low speed (RPM) without overheating
during production.
Variable speed drives generate high voltage spikes in all electric motors. Variable
speed drive motors withstand the high voltage
spikes that are generated by variable speed drives, whereas general purpose
motors do not. General purpose motors do not have the insulation required to
withstand high voltage spikes, so the motor winding on a general purpose motor
used with a variable speed drive begins to breakdown, and ultimately, the winding will burn up.
Variable speed drives used on electric motors that are 100hp (or greater) induce
harmful voltage levels on the electric motor shaft. When harmful voltage levels
exceed the resistance of the motor bearing lubricant, those voltage levels dissipate
to the electric motor’s bearing housings. Harmful voltage levels sent to a motor’s
bearing housings eventually leads to bearing failure in the electric motor. Variable
speed drive motors use special bearings that are designed to withstand the
harmful voltage levels that are induced by variable speed drives.
General purpose electric motors can be driven by variable speed drives, but once
the general purpose motor fails, an inverter duty motor should replace the general
purpose motor. Precision Electric recommends maintenance technicians who use
general purpose motors on variable speed drive applications to keep a spare inverter-duty motor to minimize
downtime.
Variable speed drive motors are more expensive than general purpose motors, but the increase in cost is
worth preventing
downtime in variable speed drive applications when low
speed is present. Many variable speed drive applications do
not need to operate at low speeds, so variable speed drive
motors are cost-prohibited in applications where low speed
is not required.
Call Precision Electric for free quotes on variable speed
drives, variable speed drive motors, servo motors, and all
other industrial related equipment. Precision Electric offers
free repair quotes on all industrial electronics equipment
and industrial electronics field services.
Page 1 of 5
VFDs Can Control Multiple Motors
VFDs Can Control Multiple Motors
February 2011
Controlling Multiple Motors with One VFD Saves Money and Space
By Kay Dekker, Building Automation Marketing Specialist and Joe Koepke, Building Automation Application
Engineer, at the Drives & Motion Division of Yaskawa America
Many applications use one or more motors operating in parallel at the same desired speed. Using one
Variable frequency drive (VFD) to control these multiple motors provides a host of advantages as
summarized below.
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Saves money
Cuts cabinet size, complexity and design costs
Can reduce footprint of the motors and driven loads
Cuts maintenance time and cost
Reduces inventory stocking requirements
Reduces control system complexity.
Multiple motor fan system
Money is saved because one high horsepower rated VFD is less expensive than multiple low horsepower
VFDs. Each VFD requires its own circuit protection, so using one VFD reduces cost in this area as well.
The VFD enclosure can be smaller because one large VFD requires less cabinet space than multiple smaller
units. This saves space and money. Design costs are also cut because it’s easier to engineer an enclosure to
house one relatively large VFD as opposed to multiple smaller VFDs. One large VFD also produces less heat
than multiple smaller units, further simplifying enclosure design and saving energy.
In terms of the motors and connected loads, the footprint can also often be reduced. For example, it may
be possible to fit multiple small fans of a smaller size into a confined duct space as opposed to one large
fan.
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Maintenance time and costs are cut because only one VFD has to be serviced as opposed to multiple smaller
VFDs, often of varying sizes. This also reduces inventory stocking requirements. Each motor will be smaller
and usually available as on off-the-shelf standard product. Because many of the motors will be identical,
spares can be stocked and replaced quickly in a failure.
The overall control system also becomes much simpler. Instead of connecting many VFDs to the main
controller, usually a PLC, and synchronizing their operation; only one connection is required. When
programming the PLC, only one VFD speed control loop needs to be configured, instead of multiple
instances.
Taken in total, the above benefits may justify use of a VFD in applications where using one VFD per motor
is cost prohibitive. When this is the case, the application benefits from all of the advantages of operating
motors at controlled speeds including reduced energy costs, longer motor life and better operating
performance.
But even given these benefits, most VFD installations with multiple motors use one VFD per motor. Why is
that?
Design Constraints
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All motors must operate at same speed
Design must accommodate VFD as a single point of failure
VFD must be upsized unless all motors are started simultaneously
Multiple conditions must be met when applying one VFD for control of multiple motors. First, each motor
must have the same desired operating speed. With one VFD per motor, each motor can be controlled
separately and run at a different speed. This is not so when running multiple motors from one VFD.
Second, running multiple motors from one VFD creates a single point of failure. If the VFD fails, then all of
the motors connected to it are not usable. Various VFD bypass schemes can be used to overcome this
limitation, but these schemes all add cost and complexity to the system.
Although the VFD becomes a single point of failure, motor and connected load reliability actually improve
in many applications as there are now multiple smaller motors as opposed to one large motor. If one motor
fails, it’s often possible to continue operation with the remaining motors. In these cases, the remaining
motors run at a speed controlled by the VFD, and operate the overall system at reduced but often still
viable capacity.
Third, care must be taken when operating the motors. To minimize VFD size, all motors need to be started
up simultaneously. The VFD will ramp all the motors up to speed at a controlled rate, minimizing the inrush
current required by each motor at startup. If application requirements prevent all the motors from being
started up simultaneously, then the VFD must be upsized (see VFD Sizing section below).
What types of applications meet the three specific conditions detailed above? Applications that use dual
fans or pumps are good candidates. The VFD can ensure that the two units are operated at the desired
speed, and don’t end up fighting each other or having one unit carry more than its design load level. Air
handling systems, exhaust/supply fans, make-up air units, recovery wheels and fan arrays are also good
candidates for an installation where one VFD controls multiple motors.
Once it’s determined that the application falls within the required specific conditions, the next step is
detailed design, where care must be taken to select and apply the right VFD and associated components.
Design Details
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The VFD must be sized properly based on its connected motors. The first step is to sum total connected
motor horsepower or full load amps (FLA). Of the two, FLA is the better parameter to use, but it’s
sometimes not available. Once this sum is calculated, selection of the VFD based on total horsepower or
FLA can be made. The VFD should always be sized equal to or greater than this sum.
Normal VFD operation will enable all of the connected motors to maintain a constant speed, but only if the
correct type of motors are used. A standard induction motor tends to slip somewhat with respect to the
line frequency as its load varies, so speeds won’t be synchronous. The solution is to use three-phase,
inverter-duty synchronous induction motors as this ensures that the motor speeds will remain synchronous
with the line frequency.
Unlike with a single motor connected to a VFD, each motor must have its own overload and short circuit
protection. When controlling a single motor, a VFD with the right features can often provide short circuit
and overload protection to the motor, and may be able to sense an over current situation if the
circumstances are right. But a VFD only senses its total connected load, outputting as many amps as needed up to its current rating.
When controlling multiple motors, a single VFD can’t sense which motor is drawing high current, so it can’t
provide appropriate overload and over current protection to each individual motor. For instance, a 50hp/65amp VFD might be controlling four 10hp/14amp motors for a total connected load
of 56amps as shown in Figure 1. If one of the motors was overloaded and drawing 22 amps while the other
three motors continued to operate normally, it would be difficult to configure the VFD’s protection circuits
to sense the overload condition. This is why each individual motor must have its own short circuit and overload protection, installed at point
C in Figure 1. The VFD can sense an overcurrent situation if one is present in the wiring (B) from the VFD
output to each motor overcurrent protection device, because it’s rated for the combined total FLA. But,
each individual motor must have its own protection in the form of overcurrent and short circuit devices.
Figure 1. Each motor connected to the VFD must have its own short circuit and overload protection.
Overload protection is designed to disconnect the individual motor from the VFD if the motor draws current
greater than normal but less then eight times its FLA for a prolonged period of time. This protects the
motor and the motor conductors from excessive heating. The most common types of motor overload
protection technologies are bimetal and solid state.
Short circuit protection is designed to protect against short circuit and ground fault conditions where the
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overcurrent condition is greater than eight times the motor’s FLA. These types of conditions can be very
destructive, so the motor must be disconnected within a fraction of a second. The two main types of short
circuit protection devices are fuses and circuit breakers.
With individual motor protection, only the motor that faults is disconnected, and the remaining motors
continue to run. This is a must in applications that can’t afford to have the entire system shut down while a
single motor is repaired or replaced.
As previously mentioned, a disadvantage of controlling multiple motors is that the VFD becomes a single
point of failure for the entire system. This disadvantage can be eliminated by using a bypass circuit as
depicted in Figure 2.
Figure 2: This three-contactor bypass arrangement allows the connected motors to continue operation in
the event of VFD failure.
With this three-contactor bypass arrangement, the motors can still run if the VFD were to fail, although
only at rated speed rather than at a speed controlled by the VFD. The contactors upstream and
downstream of the VFD are opened, and the bypass contactor D is closed. This bypasses the VFD and
connects all of the motors directly to line power. This arrangement is very common in HVAC applications.
Engineers and companies are constantly looking for simple yet creative methods to optimize performance
and design of VFD, motor, and connected load systems. Using a VFD to control multiple motors fits the bill
as it saves money, reduces the footprint and simplifies maintenance. From simply using one VFD to control
two motors on a make-up air unit, to controlling a large 14 motor fan array on an air handler supply fan
with a single VFD—multiple motor configurations make sense in many different applications. VFD Sizing
When a single VFD is used to control multiple motors, VFD sizing and selection become more complex
unless all of the motors are started simultaneously. With multiple motors connected to one VFD, adding the
horsepower of each to obtain a total load and selecting the VFD accordingly may not be sufficient
depending on operating conditions.
One of more motors can’t be started up while one or more motors are already running unless the selected
drive is sufficiently oversized. To illustrate this point, consider the following example with three 460 VAC
motors connected to one VFD.
Two of the motors are rated at 5 HP with a full load amp (FLA) rating of 6.2 amps. The third motor is rated
at 10 HP with an FLA of 14 amps. If all motors are accelerated, decelerated and run in unison—the sum of
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the connected motor FLA allows use of a 20 HP drive. But if it were necessary to accelerate and run the 5
HP motors and then start the 10 HP, the sum of the FLA would have to be recalculated.
The FLA for each of the 5 HP motors would be used in the calculations, but the locked rotor amps (LRA) for
the 10 HP would have to be taken into account. The LRA is the amount of current drawn by a motor at
startup.
Because the 10 HP motor wouldn’t be accelerated from zero frequency and voltage to its running
condition, it would look at the drive as a fixed voltage/frequency line starter and would require its full LRA
rating to quickly accelerate to the drive’s output frequency.
Thus, the amp draw on the drive when the 10 HP drive is coming on line would be the FLA of each of the 5
HP motors, plus the LRA of 10 HP motor which is 86.5 amps. The total amp figure to be used for VFD sizing
is thus 6.2 FLA + 6.2 FLA + 86.5 LRA = 98.9 Amps. This amp load would require upsizing to a 75 HP VFD with
a minimum continuous output rating of 99 Amps, a 50% increase in VFD size.
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