freewheeling diode


Thread Starter


I've heard the term freewheeling diode thrown around when someone is talking about an AC motor drive. Can you describe what a freewheeling diode is and its purpose?
A "freewheeling diode" is put into a circuit to protect the switching device from being damaged by the reverse current of an inductive load.
It is normally placed in a circuit so that it does not conduct when the current is being supplied to the inductive load.

When the current flow to an inductor (and an AC induction motor certainly is one) is suddenly interrupted, the inductor tries to maintain
the current by reversing polarity and increasing the voltage. Without the "freewheeling diode" the voltage can go high enough to damage the
switching device (IGBT, Thyristor, etc.). With it, the reverse current is allowed to flow through the diode and dissipate.

I don't think you would ever use a freewheeling diode on an _AC_ inductive load, since it only blocks conduction in one direction (and presents a near direct short in the other direction as the polarity reverses). Most commonly, the freewheeling diode is used when the load is inductive and the supply/control voltage is DC, as in 24VDC control of a relay or even a large contactor (such as might be used to turn a large AC induction motor on and off).

Your explanation appears to be accurate otherwise...

I believe the term freewheeling comes from the notion that current is freewheeling, for a short while, in the inductor/diode circuit after the supply voltage is switched off.
This diode is connected anti-parallel (opposite current flow) to every output power transistor stage. It allows a path for reactive/negative
(regen) power flow and protects the transistor by voltage clamping and returning the stored energy from the motor coils (inductance) when the output transistors are turned off. One by-product is that these diodes also serve as an output ground fault path even if transistors are off. Probably more than you want to know...

Donald Pittendrigh

Hi All

If you did use a free wheeling diode on AC, you would have no more problem, as the diode would not last long, and therefore wouldn't be there in the first place...... or was that the second place.

The original question referred to an AC drive???? does this really mean what I think it does, drive as in VSD, the previous respondant made issue
of the point that the AC motor is an inductor and that arc quenching was thus a valid function, I suppose in this case the freewheeling diode could
be switched by a contactor and such an application is therefore possible, has anyone heard of this before?

In 21 years of industrial control I have only seem these diodes on DC circuits such as contactors where they are used to protect the contacts in the "coil switching circuit" from arcing by providing a discharge path for the stored current in the coil. I would like to add that all to frequently these very important little devices are left out by younger design engineers who did not grow up with pure relay control. Be warned that PLC cards with relay contacts and sometimes those with solid state outputs as well can also experience significantly reduced operating times as a consequnce of this omission.

Donald Pittendrigh

Sorry, you are incorrect. An AC Drive converts ac to dc then back to ac via PWM. The output stage is on the dc bus. Lots of basic theory textbooks and info from manufacturers is available for further learning.
The original term "freewheeling" was related to device(s) that allowed power transmission shafts to rotate "freely" when its speed was greater
than that of the engine to which it was connected. In fact, I'm told, in the cars of the 20's and 30's a selector switch was installed on the steering wheel column allowing the driver to select the mode of operation, i.e., freewheel or not. When in the freewheel position "engine braking action" was disabled.

In electronics a diode is shunted across a coil. When the current thru the inductor is interrupted, a transient voltage appears across the coil. It's magnitude is determined by the relationship V=L(di/dt). The freewheeling diode slows down the di/dt transient by permitting the
current to continue to flow freely, or "freewheel", in the now isolated "loop" formed between the inductor and the diode, Thus, the current transient time to zero is increased, significantly reducing voltage magnitude.

Phil Corso, PE
(Boca Raton, FL)
Further to my earlier response... another "BITE" story:

I must be honest. I wasn't told about the car's selector switch. I actually worked with an elderly gentleman that owned such a car. Charlie, as he was called, was a Scotsman, and he was known to be quite frugal!

One day, Charlie asked why he had to replace brakes every six months. I told him it was because whenever he had the selector switch in the
"free" position there was no engine braking action, thus all braking effort was supplied by the wheel brakes. He proudly replied that "from
the time he bought the car he always put the switch in the 'free' position thinking that "free" meant that the car's gas-mileage would be higher"!

Phil Corso, PE
(Boca Raton, FL)
Good Job Phil...

Yea. I remember the old cars/trucks. Had a lever to move for free wheeling. Most of those were on the Overdrive models..

An AC motor variable speed drive uses a DC component (transistors, IGBT's, SCR's, etc) to switch the big juice during it's respective half wave. It needs a freewheeling diode to protect it just like Dan said. Another transistor, with it's own freewheeling diode handles the other half wave. Each phase has it's own set of these components, so there are generally 6 freewheeling diodes in a drive. Sometimes the freewheeling diode is built in to the IGBT, so it doesn't show up as a discrete component, but it's function is certainly present.

Bill Gausman
System One Control, St. Paul, MN
Sorry, but I didn't notice any reference to an AC DRIVE, only to an AC motor, and was therefore assuming a motor controlled by a contactor, in
which case the only place a freewheeling diode would/could be used would be on the coil of the contactor. So in effect I was not wrong, I was
apparently just answering the wrong question :p I certainly agree that in the case of a transistor inverter bridge, diodes are employed in this fashion...
Not quite. I mentioned it in a previous reply to this subject. An AC drive like almost all modern pwm drives converts ac to dc then back to ac via PWM. The PWM power stage that contains the freewheeling diodes works off a rock hard DC BUS. Nothing to do with half waves. In fact, you can think the input and output stages being decoupled... one is responsible to recharge the dc bus and the other draws power from it -
neglecting negative/regen power flow.

Al Pawlowski

Freewheeling diodes are used in the rectifier, and converter, sections of both ac and dc drives. They are used to provide paths for circulating currents between the phase sections, due to non-identical/ideal switching of the IGBT's, FET's, SCR's etc. They can also be used to improve power factor in some rectifiers.

And, although freewheeling could be used, my experience is that most electronics people refer to a diode (in reverse polarity to the supply)
placed across the terminals of an inductive load, such as a relay, as a "back", "protection" or "suppresion" diode.

I don't know where the term originated, but I would not be surprized to find it comes from dc traction motor work involving back emf protection,
or braking, when a (railroad or trolley) car is "freewheeling" i.e. moving while unpowered.

Al Pawlowski, PE, CCST
dba ALMONT Engineering
Baton Rouge, LA USA
I'm more a digital man than analogue although surprisingly enuf I work for a company that makes temperature and resistance measuring equipment. If memory serves me correctly,the first time I met the term freewheeling diode was in TV EHT circuits where one is used to boost the EHT. It is of course also mentioned in SMPS literature.
Why has no one mentioned this ?? - Ciao
We used to use the freewheeling diode in series with a resistor and capacitor (RCL circuit) to "time-out" the opening of a relay.
A freewheeling diode is a diode connected in reverse under normal operation but when the voltage across an inductor reverses it provides a short circuit hence a low voltage to keep the rate of change of current very low and this brake the motor or slow down the dropping of current in a relay.

When used with a switched iron or ferrite cored transformer with a single switch, the free wheeling diode will slow down the rate of change of the current fall and it will not transfer power to the secondary and most probably it will saturate the core to pass a heavy current when the inductor is switched back by the switching device. Better not to use a freeewheeling diode with a switched transformer only with a motor to brake it and waste power in the diode itself when needs a good heatsink.
>I've heard the term freewheeling diode thrown around when
>someone is talking about an AC motor drive. Can you describe
>what a freewheeling diode is and its purpose?

When a power drive circuit is used to drive an inductive load, a freewheeling diode is any diode which drains the stored energy during the non-driven part of the cycle.