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I understand that Series Wound DC motors are used in traction applications like golf carts . and that these motor suffer an effect called runaway ( if no load is applied the motor increase its speed until something breaks)

I have a scenario in my head that i cannot answer : if the golf cart is descending down a hill and no brake is applied , would the motor be in the danger zone of being unloaded and over-accelerate if power is applied to it ?

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  • \$\begingroup\$ If it is free-wheeling down the hill, no power is going into the motor so it will not over-accelerate. If someone has their foot on the accelerator, then the fail safes in the cart will limit the speed of the motor. Ignoring fail safes, then the question is about the air resistance and all the other factors that slow down forward motion, meaning that the motor won't be unloaded completely. But in theory (infinite, frictionless vacuum with no safety measures), yes. \$\endgroup\$ – Puffafish Dec 23 '16 at 9:50
  • \$\begingroup\$ @Puffafish , So Practically the speed will be limited even if the load is just the gears and friction .. and what are the failsafes in a cart ? are you talking about the controller ?? i have been surfing series controller datasheets and non provide a function or feature regarding protection against runaway , assuming this property can be measured and prevented in a halfbridge controller . Sorry for the newbie questions , but i am new to this field :) \$\endgroup\$ – ElectronS Dec 23 '16 at 9:59
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In your scenario, there is a load. The motor is connected to the drive train, so there is always some kind of load. It is doubtful that anything would cause the motor itself to be in danger of damage. Running into a tree or something else at the bottom of the hill will certainly damage the cart, and probably the driver, but doubtfully the motor.

Pure series wound motors do suffer from potentially destructive speeds with no load on the output shaft, but that is due to the fact that with no load, as the motor accelerates, the current through the windings, both armature and field will drop. As the field current drops, so does the field magnetic strength, which could possibly allow the motor to accelerate to dangerous speeds.

The advantage of series wound motors is that when there is a very high load, the field strength increases, which allows the motor to generate more torque, which is necessary on traction motors to get things going.

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  • \$\begingroup\$ @R Drast : the way i see it , series motors acts like a motor and gearbox . providing torque at low speeds and high speed when no load is required for the motor (or by field weakening).. assuming you stay within safety limits. So it is way better for electric vehicle application than PMDC motors ,right ?? \$\endgroup\$ – ElectronS Dec 24 '16 at 9:56
  • \$\begingroup\$ It is best to use a motor that targets the application. Traction motors do just that for applications that require high starting torques... from golf carts to Locomotives, and they are very simple to control (read: inexpensive). The same can now be done with three phase motors and vector drives, but at a much higher cost for the control. \$\endgroup\$ – R Drast Dec 27 '16 at 11:21
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Assuming there is always somebody on the controls, if you are going too fast, then you back off on the throttle. This applies whether going uphill, downhill, on the flat.

If you are asking whether a series motor can provide braking, then that's a different matter.

Consider an ideal un-energised series motor. Connect a resistor to the terminals and spin it. As there's no current flowing, there's no field, so it generates no voltage, which means no current flows, so it generates no voltage, which means ... This is the self-excitation problem of a series machine.

In the real world, the motor stator iron will have some residual field. This means that it will generate some voltage. If the external resistance is low enough, then sufficient extra current will flow to enhance the field for it to generate more voltage.

A series machine that's designed to be a generator will typically use a hard rather than soft iron stator, to retain a large field, or may even have some permanent magnet material in the stator to provide a starting field.

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  • \$\begingroup\$ @R Drast : the way i see it , series motors acts like a motor and gearbox . providing torque at low speeds and high speed when no load is required for the motor (or by field weakening).. assuming you stay within safety limits. So it is way better for electric vehicle application than PMDC motors ,right ?? \$\endgroup\$ – ElectronS Dec 24 '16 at 9:57

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