Using Standard VFD’s with
Non Standard Motors - PMM
Miles Mahaffy
Product Marketing Manager, Vacon Inc.
1
Agenda
1. Why Permanent Magnet Motors
2. Permanent Magnet Motors Basics
3. PM Motor Efficiencies
4. Main differences to consider when using
PM motors with VFD’s
5. Not all PM’s motors are alike
6. Typical applications
2
Why Permanent Magnet Motors
Vacon
3
Why Permanent Magnet motors ?
● A general trend towards more energy efficient solutions
● The ”classic” induction motor is reaching a point where
improving its energy efficiency is not cost efficient
● PMSM motor technology maintains high efficiency over a
wide speed range compared to IM motors.
● PMSM technology can offer a solution for better efficiency not
just from the motor but from the complete mechanical
solution allowing for “Direct Drive”
● High power density can reduce the motor physical size for a
given application.
4
Permanent Magnet Motors Basics
Vacon
5
Permanent Magnet Motor Basics
● Stator with similar design as in
induction motor
● Rotor with permanent magnets
instead of squirrel cage
● Rotor following the rotating
magnetic flux -> synchronous
motor
Stator
Rotor
Voltage
Motor voltage
Back EMF
Speed
Permanent Magnet Motor Basics
There are different construction for the
rotors in PM motors
1. “Surface PM” Motors SPM have the magnets
mounted near the surface of the rotor.
2. Interior permanent Magnet motors (IPM’s).
Permanent Magnet Motor Basics
This geometric difference between IPM vs SPM machine
characteristics are as follows:
1) the rotor of the IPM is more robust permitting much higher speed
of operation (the magnets are physically more secure).
2) the effective air gap in the IPM is lower (higher magnetizing
inductance) , the armature reaction effect is dominant and
therefore control in the constant-torque as well as in the constantpower flux-weakening region is superior.
3) a saliency is introduced in the machine (Lq > Ld), and as a result
the torque is contributed by field as well as reluctance effect. This
also helps for detection of the magnetizing direction of the motor.
PM Motor Efficiencies
Vacon
9
Motor Efficiency
● Efficiency is typically higher than IE3
/ Premium Efficiency
● Remains high also in partial loads,
ideal for pump/fan applications
● In motors with high pole numbers
difference is bigger -> ideal for
gearless applications
● Enables reaching proposed IE4
efficiency class
IEC60034-30
Example: efficiency at partial loads
Nema
10
Proposed new IEC efficiency
classes
11
Main differences to consider when
using PM motors with VFD’s
Vacon
12
Main differences to consider when
using PM motors with VFD’s
● Different drive sizing principles for PM motors
● Drives are designed and sized for output current for
induction motors that have PF of around .85 at full load.
13
Main differences to consider when
using PM motors with VFD’s
● Due to lower PF with the IM, the drive input current will be
lower than the output due to the fact that the drive input PF
is close to 1.
● With the PM motor with higher PF, the input converter (diode
supply or AFE) may not be sized for the additional current.
Input
3
DC-Bus
Output
Motor
~
3
=
~
=
~1%
P = 3 × Urms × Irms
P = Udc × Idc
~1.5%
3~
5 ... 15%
P = 3 × Urms× Irms× cosφ
14
Main differences to consider when
using PM motors with VFD’s
● Different drive sizing principles for PM
motors
● PM motors are more demanding for the drive in low speed
applications due to lack of slip (lower output frequency)
and sometimes the motor is designed for low frequency
operation (nominal frequency can be as low as for example
5Hz).
● At low output frequencies, the IGBT junction temperature
tends to follows the instantaneous changes of the output
current. The peak junction temperature rise increases as
output frequency decreases.
● Verify any derating for low output frequency operation at
full loads with the VFD manufacturer
15
Main differences to consider when
using PM motors with VFD’s
● Brake resistor/AFE sizing due to higher system
efficiency, especially when changing to gearless
applications.
● Systems with IM and gearbox have lower
efficiency ratings which allow for smaller brake
choppers or AFE for regen applications
Input
DC-Bus
Output
Mechanical
Motor
3
~
=
=
~1%
P = 3 × Urms × Irms P = Udc × Idc
3
~
~1.5%
3~
5 ... 15%
P = 3 × Urms× Irms× cosφ
92 ... 82%
P=ω×T
16
Main differences to consider when
using PM motors with VFD’s
● Limited usability in field weakening area
(1.5x motor nominal realistic maximum)
● Maximum speed is where drive reaches
maximum voltage
● Over-speed control loss (runaway) ->
overvoltage due to back EMF
Voltage
Motor voltage
Back EMF
Speed
17
Main differences to consider when
using PM motors with VFD’s
● Rotor position needs to be known to
achieve high starting torque. Constant
torque applications ideally in closed loop
control
● Absolute encoder or sin/cos resolver is ideal
● Incremental encoder is often possible, it depends
on motor design (rotor position needs to be
identified at least every time the VFD is
powered-on)
18
Main differences to consider when
using PM motors with VFD’s
● Rotor position needs to be known to
achieve high starting torque. Constant
torque applications ideally in closed loop
control
● Open loop is possible (rotor position need to be
identified at every start) for variable torque
applications
19
Main differences to consider when
using PM motors with VFD’s
● Other Special situations to consider:
● Hanging/overhauling loads (hoists, lifts etc.)
● over-speed risk requires overvoltage protection
● Crowbar circuit and/or output contactors to protect
the drive
20
Main differences to consider when
using PM motors with VFD’s
● Other Special situations to consider:
● Use of Safe Torque Off (STO). IGBT failure can
rotate the motor some degree, this must be
taken into consideration.
● If the motor can be rotated by the load while the
drive is not running, the motor will produce
voltage at the terminals of the motor. A
contactor between the drive and the motor will
be required to prevent damage to the drive.
21
Not all PM’s motors are alike
Vacon
22
Not all PM’s motors are alike.
PM motor example data sheet:
Efficiency optimized
“induction IEC sizes”
Power optimized
“maximum power
per IEC size”
23
Not all PM’s motors are alike.
PM motor example data sheet
Motors for Direct Drive with Lower Rated Frequency and Speed
24
Typical applications
Vacon
25
Typical applications
Pump/fan, air handling units
• System efficiency
• Efficiency regulations
Wind mills
• System efficiency
• Reduction of gears
Elevators
• Gearless/machine-room less
• System efficiency
• Maintenance need
Compressors
• System efficiency
• Gearless turbo
Extruders
• System efficiency
• Gearless
26
Example of an application:
Active Heave Compensated Winch
27
Properties of an AHC Winch
●
High dynamic speed control
●
Full speed reverse in 3-6 seconds
●
Very low mechanical inertia in total
system – replacing hydraulics
●
Induction motors max 200 kW due
to inertia
●
PM motors in the 200-400 rpm
range is best : Direct drive no gear
box
28
Questions?
Vacon
29