PM DC Motor Characteristics

[FredRosse]

I am a ME, and would like to know about DC PM Motor External Characteristics.

For the motor in question (Briggs & Stratton ETEK) I have 48 VDC performance data (RPM vs Torque, Amps vs Torque). The outputs here are far beyond my needs.

I would like to operate on 12 and/or 24 Volts DC (Lead-acid battery), how does the Amperage vs torque and speed vs. torque change for these conditions? Specifically, the explainations I have seen show very linear relationships for DC PM motors, but is this just a "typical" explaination, while real motors are not so linear? Is there a breakdown of these relationships at the low voltage, ranging down to 25% of rated voltage? What percentage of rated continuous torque with reduced voltage is allowed?

I can adjust the drive belt ratio at will, so I can keep motor loads within allowable limits, but how do I determine these limits?

Thanks in advance

Fred Rosse

 

[Skogsgurra]

Fred,

The curves are really quite linear. You should not have any problem running your motor off 24 V DC. The possible output power will be a little less than half the name-plate data. You will still get rated torque but speed will be one half. This is the first aproximation.

If you want to be more detailed, there are two things to think about: you will probably have less cooling at lower speeds and the internal voltage drop in a 48 V armature is higher than it would be in a 24 V armature.

The reduced cooling means that you shall be careful with rated torque. Make sure that the motor does not get too hot. It is usually not a problem.

The higher internal voltage drop means that the motor speed is more load dependent than a dedicated 24 V motor would be. If the dedicated 48 V motor has 5 percent speed drop from idle to rated load, you will have the same absolute speed drop at 24 V - but it will be twice as much in percent of your actual speed.

There is one advantage running off reduced voltage, the brushes and the commutator live much longer.

 

[aolalde]

The performance will be as Skogsgurra described.

RPM~ V/(k*phi)
Ia = (V - k*phi*rpm)/Ra ~ constant
P = V*Ia*eff

phi = flux per pole due to the permanent magnets.
k = constant to adjust for units used.