Motor Rating Background Information

Introduction

It is not always easy to understand a manufacturer’s specification for a particular motor, not least because different manufactures vary as to how conservative they are in specifying their motors. This page is intended to clarify some of these points. Or course 4QD don’t make motors and we do not test out every motor, so you really need to confirm ratings with the motor manufacturer.


Current

Continuous current

The current a motor draws depends on the mechanical load. A motor with no load draws only a small current.

The current quoted on the motor’s name plate is the current that the motor may draw continuously without overheating.

For short periods, most motors will safely draw a lot more than their rated continuous current, typically 300% to even as much as 400% current overload for as long as a minute. Most applications only use high motor current only for short periods, so the name plate motor current rating is often not a very useful guide. Unfortunately most motor manufacturers are not good at specifying, for instance, a one minute current rating!

The actual current you require for a particular installation is a function of the mechanical loading on the motor: any lightly loaded motor will draw a far smaller current than will the same motor heavily loaded.

There is a Motor Current Calculator which enables you to plug-in different vehicle weights, speeds, hill gradients etc and will calculate the current the motor actually require for that mechanical loading.

So if a particular motor is adequate for a particular job and draws, for instance, 15 amps drawing a certain load up a particular gradient then fitting a larger motor will do nothing – the system will still draw about 15 amps with the same load up the same hill. However, with the larger motor the vehicle will be able to draw a larger load up the gradient – or draw the load up a steeper gradient. It will then draw more current.

Ascending the hill slowly does nothing either – if the hill requires 20 amps at full speed, then at half speed it will require half the power. However halving the speed means that the motor voltage has been halved. Halving the voltage and keeping the current the same will itself halve the motor’s output power. So you can see that the motor current depends only on the weight and the gradient of the hill and does not alter with speed.

Heating losses in the controller and motor vary with motor current, and are not affected by motor voltage, so going at half speed up the hill won’t change the energy required.

If the motor is too small for the required load, then it will simply overheat if the loading is too high. On short overloads you will loose top speed: if the motor has, for instance, a resistance of 0.24 Ohms and is operated off 24v, then, on stall, the motor may draw 24/0.25, or 96 amps (in practise it will be less because of extra resistance in wiring and controller). If you load the motor so it draws 60 amps, then the drop in the motor resistance (IR loss) will be 60*0.25, or 15v. With 24v supply, this only leaves 9v maximum for back EMF, so the motor cannot go faster than 9/24 full speed, i.e. slightly more than 1/3 full speed.

The larger the current rating of a particular motor, the lower its resistance, so the smaller the power loss in it and the faster it will be able to climb the gradient. Smaller resistance also means less heating, so a larger motor is usually more efficient! The exception is that larger motors usually have stronger springs on the brushes, so commutator friction is higher.

Stall current or Locked Rotor current

Many manufacturers quote a locked armature, or stall current, rating. This is the peak current that a switch or relay will have to handle if it quickly applies full battery voltage to the motor and is quoted so this can be properly rated.

When a speed controller is used, the picture gets more complicated. However if the speed controller has an internal current limit (as do all controllers sold by 4QD) then the stall current is not a useful figure as the controller will limit its output current and will never give more current than it is designed to give. If the motor ever stalls, the current limiting will protect the controller.

You should never allow the stall current to flow for more than a fraction of a second: it will damage most motors extremely quickly! So a controller with a current limit will also be helping to protect the motor if it is chosen properly.

Demagnetisation current

The effects of reducing the strength of the magnets are two-fold: firstly the motor will become less efficient and secondly the motor’s speed at a given voltage will increase.

The demagnetisation current for a particular motor is likely to be well below the stall current at its rated voltage, so demagnetisation is unlikely to happen unless you overvoltage the motor. However, such overvoltaging could occur if, for instance, a motor running at full speed in one direction is suddenly reversed by applying a full reverse voltage.

Such reversal cannot of course happen when a properly designed speed controller is used.


Speed

Rated speed

The rated sped is simply the speed (usually in revolutions per minute, rpm) that the motor will rotate when the voltage applied is the rated voltage and it is loaded so that it draws the rated current. Load the motor less than this – it will rotate a bit faster. Load it more, it will draw more current and will rotate slower.

Maximum speed

If you apply more voltage than the nameplate voltage, the motor will rotate faster. Speed (for an unloaded motor) is proportional to the applied voltage.

However – if you rotate a motor too fast – something bad will eventually happen. What is likely to go wrong first is something only the motor manufacturer will know, and may not tell you . However their rated speeds are usually very optimistic and you can apply over twice the rated voltage (double the speed) to many motors without damage.

Above the safe maximum voltage, one of two things is likely to happen:

  • Centrifugal breakdown. The armature can fly apart (it is this that limits maximum voltage on the Lynch/Agni motor) or wires can dislodge from the commutator or bulge out of the winding so they rub on the magnets/casing.
  • Plasma breakdown. The commutator is a switch, switching applied voltage from one armature winding to another. At some voltage the switching will fail and an arc will form. This can destroy the controller (but may not damage the motor). Failure voltage is dependent on atmospheric conditions (temperature, pressure, humidity) and is something the manufacturers may not even know.

Voltage and revs per volt

Motor operating voltage is a poorly understood rating. The voltage shown on its name plate is not the only voltage at which it will operate! It is simply the voltage at which it will rotate at the stated r.p.m. at the stated load! So dividing the nameplate speed by the nameplate voltage gives you an rpm per volt figure.

Rather than specify an operating voltage, understanding would be much easier if the speed was specified as, for instance, 100 rpm per volt. A motor with a speed of 10 rpm/volt would rotate at 1000 rpm with 10v applied and 3000 rpm with 30v applied. Varying the applied voltage is exactly the method a speed controller has to use to vary the speed.

As far as the author is aware, Lynch Motor Company is the only manufacturer to rate their motors in this way.

The name plate voltage, current and speed are related so that a motor loaded to draw the stated current, operating from the stated voltage, will run at the stated speed.

There is however, for any motor, a maximum safe rpm. Above this speed, centrifugal force may destroy the motor, or the electrical performance of the commutator may fail.


Brush Offset

For peak motor performance, the brushes need to be offset one direction for forward current, and in the other direction for reverse. If you try and reverse a motor with offset brushes, it will be very inefficient (and therefore slower speed) in reverse.

Many motors are designed with symmetrical brushes, giving equal performance in both directions. These will sacrifice a small amount of performance for the sake of symmetry. Most EMD motors are designed like this.

Be warned also that even a so-called symmetrical motor may have brushgear which is not perfectly centred, so speeds may be fractionally different in forward and in reverse. For this reason the Robot Wars Hints and Tips sheet suggests using a layshaft to reverse the rotation of one motor so that both motors always rotate in the same direction, rather than in counter directions. As far as we know the EMD motors are very good in this respect, the Bosch motors, not.

Other motors are specified by their manufacturer to have a preferred direction of rotation. These will have offset brushes. Reverse them with caution: check with the manufacturer about their reversing ability. They may have a reduced maximum permissible speed in reverse. The Bosch 750w motor is an example of one with brush offset.

The Lynch motor has user adjustable brushes which can be rotated to suit the proposed application. So for a motor which is not often reversed, and at slow speed, they can have some offset, but for a fully reversible motor, no offset.