# DC motor conceptual question

S

#### Steve F.

Jan 1, 1970
0
All,

I have an application for a motor where it will assist the
acceleration of a vehicle, but it won't be used in steady state. In
the steady state it will be spinning (along with other components of
the drivetrain), but its back EMF will be larger the the driver
itself, therefore, at the *instant* the back EMF equals the driver
voltage, I'd like to switch the motor out of circuit so that there is
no load resulting from the motor. I guess the motor would be running
in "open circuit".

Is this bad? How else can I continue to spin the motor without it

Do standard electric car controllers do this?

Thanks

Steve

I

#### Ian Stirling

Jan 1, 1970
0
Steve F. said:
All,

I have an application for a motor where it will assist the
acceleration of a vehicle, but it won't be used in steady state. In
the steady state it will be spinning (along with other components of
the drivetrain), but its back EMF will be larger the the driver
itself, therefore, at the *instant* the back EMF equals the driver
voltage, I'd like to switch the motor out of circuit so that there is
no load resulting from the motor. I guess the motor would be running
in "open circuit".

Is this bad? How else can I continue to spin the motor without it

Ideal DC motor basics.
Torque = Current * k
RPM = voltage.
An ideal motors speed depends only on voltage.

Adding in resistance, you can model it as an ideal motor with a resistor
in series, so the current (torque) is equal to the differnence between
the applied voltage and the EMF generated.
This works in both directions, accelleration and decelleration.

If you disconnect an ideal motor, it will spin freely.
In practice, there will be various sorts of drag.
Simply connecting a motor across a voltage at 0 RPM may damage it if too
much current flows, you may need to ramp up the voltage.
Cutting it out of circuit when up to speed will work, but there will be
some drag.

S

#### Spehro Pefhany

Jan 1, 1970
0
All,

I have an application for a motor where it will assist the
acceleration of a vehicle, but it won't be used in steady state. In
the steady state it will be spinning (along with other components of
the drivetrain), but its back EMF will be larger the the driver
itself, therefore, at the *instant* the back EMF equals the driver
voltage, I'd like to switch the motor out of circuit so that there is
no load resulting from the motor. I guess the motor would be running
in "open circuit".

Not particularly, assuming it's a PM motor. If it has separate field
and armature you can de-energize the field winding.
How else can I continue to spin the motor without it

Mechanically uncouple the shaft, which will save the windage losses.

Best regards,
Spehro Pefhany

S

#### Steve F.

Jan 1, 1970
0

I should have been more clear, my current design does not allow a
mechanical de-coupling. If I did, well, that would fix everything

The motor is a permanent magnet motor. In the design, the motor is
directly coupled to the halfshaft of the vehicle (see at
http://www.fambro.com), that way it can be used in regen, however,
there are the parasitic -b*v losses associated with leaving the motor
mechanically connected....that is true.

What I'm most concerned about is spinning this motor, open circuit. I
don't
belive the back EMF on the terminals, with nowhere to go, will hurt
anything.....right?

R

Jan 1, 1970
0
All,

I have an application for a motor where it will assist the
acceleration of a vehicle, but it won't be used in steady state. In
the steady state it will be spinning (along with other components of
the drivetrain), but its back EMF will be larger the the driver
itself, therefore, at the *instant* the back EMF equals the driver
voltage, I'd like to switch the motor out of circuit so that there is
no load resulting from the motor. I guess the motor would be running
in "open circuit".

Is this bad? How else can I continue to spin the motor without it

Do standard electric car controllers do this?

Thanks

Steve
I have a couple of comments.

This sounds like you will only use the assist at low speeds and the drive
speed (and therefore Back EMF) can end up being substantially faster than
the electric motor would reach when driven. The first concern there is
that the electric motor may be driven faster than it is designed for.
Even if it is not the higher back EMF will mean that the motor will regen
back into the controller supply through the free-wheeling diodes even if
the drive is turned off. It would be necessary to disconnect the the
motor with a contactor (relay).

There are controllers that will limit the regen current from a PM motor
effectively following the regen curve down when coasting to a stop but I
don't know of any that either disconnect at speed or could be tricked
into doing so. Some of them go to max regen when told to stop.

All of the above assumes you are considering a PM motor. I think that's
probably the wrong motor type for what you are suggesting.

I'm assuming that you only want to drive this motor in one direction (any
direction changes are handled by a mechanical transmission). In that
case I would suggest using a series wound pump motor. This has the
advantage of never regening (the current simply keeps dropping as the
speed increases for any set voltage) and controllers are readily
available (SRE Controls, Curtis PMC and Sevcon all make
suitable controllers). The pump motors are also polarity insensitive,
they spin in the same direction no matter which terminal (they only have
2) is positive. Using this type of motor reduces the sensitivity of your
boost to the speed at which you disconnect, if the speed is higher than
what you would normally achieve at full voltage you just see the current
drop towards 0. When you reached the speed at which you would cut out
the boost you would simply cut the requested speed back to zero and the
controllers output would drop to zero and no current would flow through
the motor. I would still suggest that you may want to mechanically
decouple the motor if only to save wear and tear on the brushes.

If you want something really simple for testing you could even decide not
to cut out the motor electrically, if it's running fast the current
through it (and the torque generated), will be close to zero anyway.

Disclaimer: I used to work for SRE Controls

Hmmm, for those who may not know what a series wound pump motor is. A
series wound pump motor has the armature (rotor) windings and field
windings (stator) wired in series internally. This means that the
armature and field carry the same current and the torque will be roughly
proportional to the square of that current. These are commonly used in
material handling EVs (forklifts, reachtrucks etc..) to run the
hydraulic pump.

Robert

J

#### Jeff

Jan 1, 1970
0
Steve F. said:
All,

I have an application for a motor where it will assist the
acceleration of a vehicle, but it won't be used in steady state. In
the steady state it will be spinning (along with other components of
the drivetrain), but its back EMF will be larger the the driver
itself, therefore, at the *instant* the back EMF equals the driver
voltage, I'd like to switch the motor out of circuit so that there is
no load resulting from the motor. I guess the motor would be running
in "open circuit".

Is this bad? How else can I continue to spin the motor without it

It just becomes an open circuit DC generator. Voltage is proportional to
RPM. I'd worry about wearing out the brushes and commutator if you plan on
spinning the motor for long periods.
Do standard electric car controllers do this?

Most commercial electric motors are now 3 phase.

S

Jan 1, 1970
0
All,

I have an application for a motor where it will assist the
acceleration of a vehicle, but it won't be used in steady state. In
the steady state it will be spinning (along with other components of
the drivetrain), but its back EMF will be larger the the driver
itself, therefore, at the *instant* the back EMF equals the driver
voltage, I'd like to switch the motor out of circuit so that there is
no load resulting from the motor. I guess the motor would be running
in "open circuit".

You can simply put a diode that will stop power flow from motor to your supply.

T

#### Tony Williams

Jan 1, 1970
0
Steve F. said:
I have an application for a motor where it will assist the
acceleration of a vehicle, but it won't be used in steady state.
In the steady state it will be spinning (along with other
components of the drivetrain), but its back EMF will be larger
the the driver itself, therefore, at the *instant* the back EMF
equals the driver voltage, I'd like to switch the motor out of
circuit so that there is no load resulting from the motor. I
guess the motor would be running in "open circuit".

That feels something similar to the old starter motor
problem. Perhaps you could consider the same solutions,
maybe even using standard starter motor components, eg,
solenoid-operated meshing of the gears, etc.

G

#### Glen Walpert

Jan 1, 1970
0
That feels something similar to the old starter motor
problem. Perhaps you could consider the same solutions,
maybe even using standard starter motor components, eg,
solenoid-operated meshing of the gears, etc.

I think there would be some durability problems using old standard
starter motor components. The new standard 42 volt starter motors now
being developed for the next generation vehicles is quite different,
being integral with the flywheel and serving also as the alternator,
rated somewhere around 10 kW. Of course these are AC not DC, but
perhaps the OP could consider the same basic approach and have his
motor serve as generator also, eliminating the presumably existing
belt-drive alternator.

I

#### Ian Stirling

Jan 1, 1970
0
Steve F. said:

I should have been more clear, my current design does not allow a
mechanical de-coupling. If I did, well, that would fix everything

The motor is a permanent magnet motor. In the design, the motor is
directly coupled to the halfshaft of the vehicle (see at
http://www.fambro.com), that way it can be used in regen, however,
there are the parasitic -b*v losses associated with leaving the motor
mechanically connected....that is true.

What I'm most concerned about is spinning this motor, open circuit. I
don't
belive the back EMF on the terminals, with nowhere to go, will hurt
anything.....right?

<snip quoted message incorrectly placed at bottom>
If the insulation can handle it, and the various drags and the wear on
the brushes and bearings is not a problem, then it should work just
fine.

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