Cub Cadet RZT-S Zero Electric - Manual (Page 67)

Electrical System
63
Electric motors
The electric motors used on the RZT-S Zero are 3
phase, brushless, DC motors. This type of motor allows for
fine control of the motor’s speed and torque. The construc-
tion of the hub and deck motors are very similar. The big-
gest difference between the two is the rotor shaft. The hub
motor is splined and the deck motor is keyed. The deck
motor also has extra bearings supporting the shaft. The
stator is similar to an alternator stator found under a fly-
wheel. The stator in these motors have 18 poles and is
mounted inside the motor. The rotor is similar to a fly-
wheel, it has 16 magnets and spins around the outside of
the stator. See Figure 5.14.
To understand how the motor works, let’s look at each
of its properties separately first.
• 3 Phase: Unlike most motors used on outdoor
power equipment, this motor has 3 separate
power circuits energizing the coils in the stator.
There are three groups of coils or phases. these phases will alternate between positive, off and negative
states. this creates the magnetic fields that will pull and repulse (one state pulls while the other repulses).
• Brushless: Since the rotor has the magnets instead of the coils, there is no need to send power to the
rotor. This does away with the brushes that ride on a commutator, greatly increasing the life of the motor
and minimizing the maintenance required. However, without commutators, the motor can not switch the
power from coil to coil by itself, to keep the motor spinning. The switching action is controlled by the con-
troller.
There are hall effect sensors in the stator that
will send a signal to the controller, letting it
know the location and speed of the rotor. This
feedback feature is what allows the motor to be
controlled with extreme accuracy. The control-
ler uses a Pulse Width Modulation (PWM) sig-
nal to control the motor. PWM means that a
pulsed signal is sent to the motor instead of a
constant steady voltage. The width of the pulse
is its duty cycle.
Duty cycle is the period of time a circuit is ener-
gized in a set period of time. The higher the
duty cycle, meaning the longer the circuit is
energized, the faster the motor will spin.
See Figure 5.15.
The frequency is the number of cycles per sec-
ond, measured in Hertz. The circuits frequency
is high enough that the pulses from the PWM can not be perceived by the human eye.
NOTE: One of the benefits of PWM is that the lower the duty cycle, the less power that is consumed by the
motor. If a motor draws a constant 48 volts and 10 amps, the motor will be pulling a constant 480 watts
(48V x 10A = 480W). that same motor with a PWM at 80% duty cycle will be an average of 384 watts
(480W x 80% = 384W) and with a 40% PWM signal the power consumption is an average of 192 Watts
(480W x 40% = 192W)
Green - U phase
Yellow - V phase
Blue - W phase
Red - north pole
Black - south pole
Figure 5.14
Rotor
Stator
Figure 5.15
Slow
Fast
40%
cycle
Duty
80%
Duty
cycle