20100325 bdc motor presentation
TRANSCRIPT
March 25, 2010 Slide 1
Brushless DC Motors
Basic Model and Selection
March 25, 2010 Slide 2
Prepared and Presented by Steven GarfinkelEllipsah LLC
Copyright © 2010 Steven Garfinkel
March 25, 2010 Slide 3
Motor Sample
March 25, 2010 Slide 4
Rotor
March 25, 2010 Slide 5
Stator with Rotor
March 25, 2010 Slide 6
Commutation Sensor
March 25, 2010 Slide 7
Custom Motor Prototype:Rotor
March 25, 2010 Slide 8
Lamination Stack
March 25, 2010 Slide 9
Phase Winding
March 25, 2010 Slide 10
Wound Stator
March 25, 2010 Slide 11
Lead Attach
March 25, 2010 Slide 12
Lead Out
March 25, 2010 Slide 13
Formed End Turns
March 25, 2010 Slide 14
Hall Effect PWA
March 25, 2010 Slide 15
Stator Assembly
March 25, 2010 Slide 16
The Pieces
• Laminations
• Magnets
• Insulation System
• Commutation System
March 25, 2010 Slide 17
Laminations
• Quality (M number) and thickness drive losses
• Lower M number has lower losses
• Typical: M19 0.014 thick
March 25, 2010 Slide 18
Magnets
• Rare Earth – Neodymium Iron and Samarium Cobalt.
• Critical characteristics for design Br, Hc, Energy Product, Operating Temperature.
• Maximum operating temperature typically defined as the temperature below which the magnet will not demagnetize when in a circuit with a permeance of 2.
• Maximum temperature can range from 85ºC for cheap NdFe magnets to 300ºC for SmCo.
March 25, 2010 Slide 19
Insulation System
• Critical characteristic: temperature rating
• Temperature rating typically defined as hot spot operating temperature that will give 10k hours life.
• Can be as high as 220ºC
March 25, 2010 Slide 20
Commutation System
• Hall Effects – limited by maximum operating range of hall effect sensor, typically 150ºC.
• Resolver – requires more complicated interface. Temperature range increased to insulation system of resolver.
March 25, 2010 Slide 21
The Goal
• Select the right size motor for a task
• The requirements:– Operating speed at load
– Supply Voltage
– Operating Dutycycle
– Thermal requirements
March 25, 2010 Slide 22
The Model
• Ignore commutation. Commutation is done in electronics and (for now) assumed correct.
• Start from the most basic model and add complexity
March 25, 2010 Slide 23
Model – Version 0
• Change in magnetic flux through coils produces back emf. A fixed voltage on the stator produces a fixed speed. Torque has no effect on the motor speed. The ratio of voltage to speed is the back emf constant Ke, which has units of V/kRPM or V/(radians per second)
• Quick test – attach hand drill to motor and turn motor while viewing commutation sensors and back emf on oscilloscope.
March 25, 2010 Slide 24
Diversion 1 – Sample Motor back emf
March 25, 2010 Slide 25
Diversion 1 – Prototype Motor back emf
March 25, 2010 Slide 26
Model – Version 1
• Torque load on the motor is matched by motor torque, producing current flow in the windings. The ratio of torque to current is called the torque constant Kt which has units of in-oz/amp or Nm/amp. Kt and Keare ratiometric, in fact:
• Ke[V/(rads/sec)] = Kt[Nm/amp]
March 25, 2010 Slide 27
Model – Version 1
• Model Equations
I = T/Kt
N = (V – I*Ra)/Ke
March 25, 2010 Slide 28
Model – Version 1Speed and Current Vs Torque
0
5
10
15
20
25
0 200 400 600 800 1000Torque
Speed [kRPM]
0
10
20
30
40
50
60
70
80
Current [A]
Speed
Current
March 25, 2010 Slide 29
Model – Version 1
• The slope of the speed – torque curve is sometimes called the speed regulation constant.
March 25, 2010 Slide 30
Model – Version 2
• The physical design has some inherent friction tf.
• The rotational magnetic field produces losses in the laminations which are proportional to speed. This constant is called Kd which has units of in-oz/kRPM.
March 25, 2010 Slide 31
Model – Version 2
• Model Equations
I0 = (T+ tf)/Kt
N0 = (V – I*Ra)/Ke
In = (T+ tf + Kd*Nn-1)/Kt
Nn = (V – I*Ra)/Ke
March 25, 2010 Slide 32
Model – Version 2Speed and Current Vs Torque
0
5
10
15
20
25
0 200 400 600 800 1000Torque
Speed [kRPM]
0
10
20
30
40
50
60
70
80
Current [A]
Speed
Current
March 25, 2010 Slide 33
Model – Version 3
• The stator windings have inductance. The windings are switched at the commutation frequency.
• The inductance impedes the flow of current at the leading edge. Effectively, some voltage is used to overcome the switched inductance.
• On the trailing edge, the stored energy in the winding continues the current flow, producing useful torque.
• The net effect of the leading and trailing edge produces a speed-torque slope higher then theoretical and a current-torque slope lower then theoretical.
• The resistance, when calculated based upon the decrease in speed versus torque, is typically from 2 to 4 times as high as the resistance of the stator winding and is speed dependent.
March 25, 2010 Slide 34
Model – Version 3
Speed and Current Vs Torque
0
5
10
15
20
25
0 200 400 600 800 1000Torque
Speed [kRPM]
0
10
20
30
40
50
60
70
80
Current [A]
Speed
Current
March 25, 2010 Slide 35
Winding Constants versus Size Constants
• Winding constants vary with the number of turns on the stator. Size constants are independent of turns.
• A small motor and a large motor can have the same Ke. The difference is that a large motor will have a lower resistance and therefore be more efficient than a small motor.
March 25, 2010 Slide 36
Winding Constants
• Kt, Ke, R, L are all winding constants.
• For a given motor, changing the number for turns by a factor of F gives new winding constants of:
Kt’=F* Kt
Ke’ = F * Ke
R’ = F2 * R
L’ = F2 * L
March 25, 2010 Slide 37
Size Constants
• Tf, Kd, Θ and Km are size constants.
• Θ is the thermal impedance from the motor hot spot to a reference point, which could be ambient or could be the temperature of a mounting surface.
• Km is called the motor constant. It has units of in-oz/√Watts.
March 25, 2010 Slide 38
Selecting a motor
• Since most of a motors losses are due to torque, to minimize the motor size, run it as fast as possible (high gear ratio) so that the torque is low at a given power output.
• Select from motors that can safely operate at the target speed. High speeds require a sleeved rotor.
• At the operating point, compare motors based upon Θ, Km, the maximum ambient temperatureand the insulation system maximum temperature of operation.
March 25, 2010 Slide 39
The data sheet doesn’t have Km!
Km = Kt [in oz/amp]/ √R
March 25, 2010 Slide 40
The data sheet doesn’t have Kd
• Test the motor at no load with different line voltages. Plot the motor current versus the speed. The slope of the line is Kd and the intercept is Tf.
• Since Kd is a size constant, estimate it based upon data provided for similarly sized motors.
March 25, 2010 Slide 41
Why don’t my Speed Torque Curves match my calculations?
• The test devices for motor inherently have high inertia.
• As the test proceeds, the winding heats up and the resistance increases, so a short test duration is desired.
March 25, 2010 Slide 42
Why don’t my Speed Torque Curves match my calculations?
• If the test starts at no load speed, the inertia of the test fixture causes the resulting test speed to be high and current to be low.
• The motor supplier may use similarly suspect data to obtain published Ke and Kt
values.
March 25, 2010 Slide 43
So I know what size motor I need, what’s next?
• Select a Ke so that the primary speed operating point is at 85 to 90 percent of the theoretical no load speed. If the speed requirement is firm, make sure you use the low line voltage and subtract any drop in the wiring and electronics. Efficiency at the operating point is approximately the ratio of the speed to theoretical no load speed.
• Obtain or calculate the remaining winding constants.
March 25, 2010 Slide 44
So I know what size motor I need, what’s next?
• Check to make sure the drive electronics will work with the derived motor inductance.
• Check to make sure the drive electronics can provide the desired current within its thermal envelope.
• Re-check the thermal analysis at maximum ambient temperature. Remember, the resistivity of copper increases with temperature.
March 25, 2010 Slide 45
Help!
• If you need help selecting a motor, a drive, or need a custom or semi-custom motor or drive call me:
Steve Garfinkel
973 432-7401