# DC motor graph



## Mitchman (Jan 14, 2008)

Hi!

I am starring in the graph of simple DC motor. I don't understand, why is motor speed ( n(RPM) ) falling, when current and power are growing? Talking about n curve on picture; n is speed of motor. I would say, that if curent and power are growing, speed will grow too.


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## major (Apr 4, 2008)

Mitchman said:


> I am starring in the graph of simple DC motor. I don't understand, why is motor speed ( n(RPM) ) falling, when current and power are growing? Talking about n curve on picture; n is speed of motor. I would say, that if curent and power are growing, speed will grow too.


Hi Mitch,

The graph is drawn at a constant voltage, 48V in this case. Load (which is torque) is the independent variable on the x-axis (horizontal). The other values are dependent on the load. As the load increases (moving to the right), the current increases. As you note, the RPM decrease as the load increases. Or stated another way, the motor slows down as it is loaded up.

Most electric motors behave this way. With DC motors the reason is that as the current increases, there is more voltage drop in the motor resistance which subtracts from the voltage to the armature causing rotation. In series wound DC motors there is a second reason. This is that the field current increases with load, which increases the flux causing a lower RPM for a given voltage.

As you noted, the power output increases with increasing load while the RPM decrease. This is because the power is the product of torque and RPM. And the torque increase is larger than the RPM decrease. This increasing power continues only to about midway between no load (zero torque) and zero RPM (stall torque). This curve only shows the low load part of the entire motor behavior. At about midway between zero and max torque, the power output will peak and then decrease as the motor continues to slow to stall. This peak is off to the right of the curve you show. 

I think the thing which confuses people about these motor curves is they compare it to stepping on the throttle in the car. The motor speeds up and produces more power as it speeds up. But remember, as you increase the motor speed in the car, you are increasing the motor voltage with the controller. These graphs show the motor output at a constant voltage. The motor factory makes the motor for the rated voltage and does not know what controller you will put on it. They give you the motor graph to tell you how the motor will perform at that rated voltage.

Hope that explained it for ya,

major


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## Sunking (Aug 10, 2009)

major said:


> These graphs show the motor output at a constant voltage. The motor factory makes the motor for the rated voltage and does not know what controller you will put on it. They give you the motor graph to tell you how the motor will perform at that rated voltage.


Major good reply. So how can one tell from the graph what current rating a controller needs to be from looking at the graph?


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## major (Apr 4, 2008)

Sunking said:


> Major good reply. So how can one tell from the graph what current rating a controller needs to be from looking at the graph?


Hi Sun,

This looks like an European motor maker's graph. They use a rating system labeled S2 & S3 as you see there. S3 is the duty cycle rating curve in percentage time on for the motor. The S2 curve is the "time on" curve or in other words how long the motor can run at that load until it reaches rated temperature starting at ambient. Both S2 and S3 become vertical at 31 Nm which indicate 100% duty cycle or continuous operation. At 31 Nm you can read that the power (P) is 7.3 kW and the current is 200 amperes.

So to take full advantage of the motor, your controller should be rated for 200 A, 48 V continuous, at a minimum. Higher controller ratings are recommended by some as safety factor.

Controllers also have current limit ratings. One cannot determine from this motor curve the appropriate current limit value. Often the controller rated at 200 A continuous will carry a 500 amp current limit. Maybe the current limit will have a time rating attached to it. It may vary between 10 seconds and 2 minutes. This 500 A current limit would be acceptable for this motor, IMO, and possibly even a much higher current limit. 

The short time base motor rating is found at the other end of the S curve. For instance, the S2 curve shows a 5 minute rating at 54 Nm. Which is 11 kW, 310 A. Sometimes you see a 2 minute rating on a controller. You could extend those motor curves out and guess that this motor might carry a 2 minute rating of 450 A, more or less.

Another note about this particular motor curve. It is nicely done. However it is done at a constant voltage, 48. So if one were to use this motor with a 48 volt battery, they would have to expect less RPM and power as the load increases than depicted by this curve. This is because battery voltage droops with increasing current. Current vs load is unaffected. So ratings are most often stated in terms of current for matching motors and controllers.

Regards,

major


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## Sunking (Aug 10, 2009)

Thanks Major you confirmed my thoughts. I am an EE but my area is power distribution, generation, protection, emergency/back up, lightning system, and grounding. But my hobby now days is modifying golf carts, and the OEM's offer little information on motors made for golf carts like GE D398 and D392, and it leaves me guessing trying to find the optimal controller. Voltage is no problem as most are good up to 72 to 96 volts, but there are a lot of current options and the price goes up with current as you know. All the folks I help, including myself are very budget conscious and I hate to use more controller than necessary.


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## Bowser330 (Jun 15, 2008)

Checkout the thread called "Future Netgain Offerings...." Post #96.
(Motor section, same section as this thread)

There are two files which show a different voltages and the kilowatts relative to RPM...


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## Mitchman (Jan 14, 2008)

Wooow, this is very nice explanation here. I had to read it at least three times.


And yes, this is european motor. And this, what you wrote about S2 and S3, is exactly the same as in the catalogue.


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