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by
Dennis Clark
This month sees mostly questions about motors
and how to use them efficiently; some of them I can
even reasonably answer.
Q. I have two questions. First, could you explain how to implement dynamic braking using an H- bridge to return energy back to my robot batteries
to extend operation? I am very confused about this because
when the motor stops I know the current is able to flow
back through the protection diodes in parallel with the
transistors. Conversely, the motors back EMF is lower than
the supply voltage, therefore current couldn’t flow to the
higher potential without some sort of configuration to step
up that voltage, correct?
Also, could you explain why my motor is so hot when
I use locked anti-phase PWM compared to when I use sign
magnitude PWM? I did the calculations and the RMS
voltage is the same for both configurations except when
braked at 50% duty cycle while using locked anti-phase.
— Fran
A. Great questions! Unfortunately, I will have to disappoint you on your first question. Implementing regenerative braking is a very complex problem. You
have touched on some of the issues such as a mismatch in
the generated voltage as compared to the battery source
voltage, and how to line up the diodes in your charging
bridge. You will have to use a bridge rectifier here because
the voltage generated will be of the opposite polarity to
the voltage used to run the motor in the direction that it
is currently going.
Most papers that I have read on this topic prefer to
put a MOSFET switch in the circuit to prevent the bridge
from “sucking” power away from the motor when you
are driving the motor. Because you can’t just pump the
current being created straight back to the battery
(charging current can NOT be infinite or unbounded),
you also need to PWM the current to the battery to control
the charge rate.
14 SERVO 10.2009
In those (ahem) less expensive motors that we often
use for our robot drive trains, the back EMF is typically less
than what the drive H-bridge is supplying. In some cases,
this may not be so. Therefore, a way to reduce that voltage
may also be required. As you have noted, if the voltage is
lower than the battery voltage then we need to boost it up
so that current can flow back to the battery. Let’s look at
what we are proposing in our charging system:
•Rectification for current returned from the motor to
the battery (with losses).
•MOSFETs for isolation, switching, and PWM of
returned current (with losses).
•Boost or buck regulation of returned voltage
(with losses).
Every step along the way we have losses. Your bridge
will drop something in the region of 1.2V from your
generated voltage. Regulators will not be perfectly efficient;
if you are lucky, you could get 75% efficiency. Because you
are using PWM, you are literally throwing away some power
(and losing braking effectiveness) when you are shut off.
This does not sound ideal.
Every paper that I’ve read suggests that you need to
use high voltage systems (200+ volts) to get enough energy
back to make it worth the effort AND that your braking
needs to be slow and gradual to capture any significant
energy at all — either that or you need to be braking a lot
to get a lot of charge back to the battery. I’ve also read
the suggestion of the use of a brushless DC motor or a
synchronous AC motor for best effect. You don’t say as
much, but I don’t think that you were planning on
using either.
However, if you just wanted dynamic braking, this can
be done as simply as adding a MOSFET and a high power
resistor across the motor terminals. When you turn off
the power to the motor, you could then switch on the
MOSFET and short the windings of the motor together
which will cause braking proportional to the speed of the
