motor as a generator during a part of operating cycle 1

[medo79]

Dear All,
I want to ask if it is possible to use electric motor as a motor and generator in the same time according to the attached photo. On other word, When driving torque is required, the motor will work as an actuator. And, when the braking torque is required in braking mode, the motor will work as a generator. Could I use the motor in generator mode to charge a battery? If yes, what is the suitable circuit for that?

 

[Skogsgurra]

Yes. Can do.
Variations on that theme have been used from dawn of drives. DC motors are the easiest to use and understand, but induction motors can also be used.
Can not give detailed schematics here because do not know what motor and power range. Google "four quadrant" and "regen" for starters.


Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[medo79]

I am using geared DC motor which has 130N.m peak torque and 80N.m continous torque, and its speed is 165rpm.

 

[CJCPE]

The image posted is simply the diagram that defines four quadrant operation. Q1 is forward motoring, Q2 is forward braking, Q3 is reverse motoring and Q4 is reverse braking. As Gunnar said, four quadrant has been implemented with various types of motors mated with various types of control equipment. Most of the standard equipment available for sale is intended for mains AC power operation. Similar equipment, including circuitry for charging batteries with energy recovered during braking has been designed into electric vehicles. Extensive instructions in designing electric propulsion systems is beyond the scope of this forum.

CJC

http://www.vfdriveinfo.com

 

[Skogsgurra]

Something like 2 HP then, if we include losses in gear.

What voltage are you using? There is a big difference in technologies available for the 12 - 100 V range and the 200 - 400 V range. Which one are you using?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[medo79]

I am using 48 DC voltage

 

[TurbineGen]

Is this for an electric vehicle? This sounds a lot like an EV application. Are you expecting to only generate when braking or do you have another prime mover which will accelerate the motor beyond 165 rpm to use it as a generator?

Also, I'm assuming that it is permanent magnet motor, correct? Or is this a shunt motor?

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[waross]

For a shunt wound motor at 165 RPM, no load, the only current will be to supply the losses.
If you slow the motor down, it will act as a motor and the current will increase as the load increases.
If you over drive the machine it will act as a generator. The first few RPM over speed will be supplying the losses. As the speed is increased the current will drop to zero and them increase as energy is returned to the source. Some motors will transition from full motoring to full generating with as little as 5% change in speed.
Your motor is listed as 48V, 165 RPM.
A shunt motor with those ratings may have, as an example, the following characteristics:
1>Field current and rotor voltage fixed; Full load RPM, 165 RPM,/ Full generation, 173 RPM.
2> Field current and speed fixed: 49.2 Volts applied to the rotor, Full generation/46.8 Volts applied to the rotor, full motoring.
3> Voltage applied to the rotor and speed fixed: Field current -2.5%, full motoring/ field current +2.5%, full generation.
These figures are accurate for a shunt motor near rated speed. The accuracy of this model is compromised by losses, magnetic permeability and saturation, and in the case of compound and shunt motors, by interaction between rotor current and field strength.
Yes it can be done. The challenge is to design circuits that will adapt to smooth control over a wide speed range without excessive losses.
Field control is effective but has a limited range.
You may have success using four six volt batteries. You may be able to get efficient regeneration down to 10% to 12% of rated speed with field strengthening. Then transition to 12 Volts, 24 Volts and finally to 48 Volts.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[medo79]

The motor is permanent magnet motor, and my application is not related to electric vehicle application. My application is related to arm exoskeleton which the torque applied to it may be in the speed direction or oppsite. Therefore, I am thinking instead of damping this motion when torque and speed are oppsite and convert the dispatted energy to heat (when the motor will slow down the movement speed), I might use this energy and convert it to electricity to charge a battery.

 

[waross]

Well, with a PM motor, the back emf is proportional to speed over quite a wide range. I have used small PM motors off the shelf for tach generators with good results. You can plot a back EMF/speed curve for zero RPM to 165 RPM. If the applied voltage is below the curve the motor will be generating. If the applied EMF is above the curve the motor will be motoring. Burning the regenerated energy in a resistor is easy. Charging a battery is possible but difficult. Others here may comment on circuits to boost a lower voltage so as to charge a battery.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[medo79]

Waross,
What do you mean by "Charging a battery is possible but difficult"?

 

[waross]

Well as soon as you reduce the speed of a PM motor below rated speed the back EMF drops below the battery voltage. The back EMF may be dropped across a resistor to waste energy and dampen the oppose the driving force on the motor. But, if you intend to charge a 48 volt battery with a source of less than 48 volts, you need some type of DC to DC converter to boost the voltage above the battery voltage. Using such a converter to present a smooth controllable resistance to the driving force is probably possible but is beyond my area of expertise.
I may not be the right person to answer this question. (How to control a robotic arm smoothly and charge a battery efficiently when regenerating.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[TurbineGen]

Perhaps a boost regulator would do the trick. I have never seen a DC-DC converter used in this small a scale but a boost regulator *might* be possible. I'm not sure how you would adjust the charging rate of the battery and the damping rate of the motor.

The real problem is just as Bill explained. If you run a motor freely at 48 volts (no load) and disconnect the battery and measure the voltage at the instant the battery is disconnected, you will read a reduced voltage probably around 44 to 45 volts or so. You'll need around 52 or so to charge the battery. So even at no load you you need to step up the voltage to charge the battery.

Another option would be a higher voltage regenerative winding in the motor. Basically a motor/generator in one. If you can piggy back a higher voltage motor (say 90 volts at 165 rpm) to the end of the motor and connect that to your charging system is mechanically linked to the motor drive, but electrically separate. Thus when the motor runs at 165 rpm at 48 volts, the generator winding produces about 90 volts. When you need the regenerative braking, you begin to draw current from the generator winding. A brushless DC outrunner might be a good option for this.

Just some thoughts.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[cswilson]

If you are controlling this motor through a standard PWM "H-bridge" power stage, then the power stage itself will act as the boost converter. That is, when the motor acts as a generator, it can "charge up" the DC bus (48VDC in your case), even if the motor's own back EMF is lower than the bus voltage.

This happens in servo drives every time the motor is decelerated faster than losses would slow it. Usually servo drives have a pretty sizeable capacitor bank to absorb some of this, as the capacitor bank is needed to smooth out the rectified AC supply (not an issue when working from a battery). Many times, the capacitor bank can store enough energy to supply the next motoring cycle.

It also happens in hybrid/electricl vehicles every time the brake pedal is pressed. An issue in these vehicles is whether a significant capacitor bank is needed or not. Generally, with the NiMH batteries in the present generation, none is needed because the batteries can absorb the current fast enough. The newer Li-ion batteries cannot take in current so fast, and so may employ a bank of "super-capacitors" as a buffer.

Curt Wilson
Delta Tau Data Systems

 

[LionelHutz]

Basically, the ability to charge a battery or not depends on the power source being used (is it the same battery?) and the drive used to operate the motor. To even give a more general answer - it depends on the configuration of the whole system.

If this is a battery powered application then the PM motor drive should re-charge the battery. Curt basically already described this - if you are using battery as the power source then instead of a DC capacitor buss during regen you have a battery being charged.

Overall, you're still only providing sketchy details which won't help you get any specific answers. Try asking a more specific question besides "Could I use the motor in generator mode to charge a battery?" - the answer to this question is YES but YES doesn't help you actually know how at all.

 

[waross]

If a motor is being over driven it will charge just fine. However it is more of a challenge to charge the battery as the motor slows down and the back EMF drops below the battery voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cswilson]

Bill -- As I tried to make clear above, if this is an electronically controlled motor, where the motor is driven through a transistor power stage (as in a vector or servo drive), the potential problem you bring up is not a real issue, as the power stage handles the issue automatically.

When someone talks about "applying torque in the opposite direction of speed" and "4-quadrant operation", as medo79 does here, they are invariably talking about such an electronically controlled system. In this braking mode of operation, current flows "upward" through the flyback diodes to charge up the DC bus supplying the motor power stage. During part of the PWM cycle, the voltage of one lead of the motor is higher than the bus voltage due to the inductance of the motor, regardless of the back EMF at the time, permitting this "reverse" current flow to occur.

Curt Wilson
Delta Tau Data Systems

 

[sreid]

Another way to look at it is energy. If the motor is braking the Inertial Load, whatver enercy is not being consumed in the motor and amplifier has to be going somwhere. And the energy is either charging a capacitor or a battery

 

[waross]

Thanks Kurt.
It's always good to see you dropping in to share your knowledge.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[medo79]

Thanks to all. I would like if there is a simulation software to simulate the motor/generator with.