# Does the peak torque decrease at rated speed for BLDC motor?

Does the below BLDC motor have the same Peak Torque(short time) at the rated speed?

1. Rated_Voltage: 24V
2. Rated_Speed: 4000RPM
3. Back_EMF_Constant: 3.7Vp/KRPM
4. Peak_Current: 4.4A
5. Peak_Torque: 20oz-in
6. Resistance: 1.5Ohms
7. Inductance: 2mH

Here is my calculation:

• VOLTAGE = Rated_Speed * Back_EMF_Constant + Peak_Current * Resistance

If the above "VOLTAGE" exceeds Rated Voltage, then the motor can not exert Peak Torque(20oz-in) at the rated speed.

For the above motor:

• 21.4V = 4000RPM * 3.7Vp/KRPM + 4.4A * 1.5Ohms

Because 21.4v is less than Rated Voltage(24v), so it can exert the same Peak Torque at the rated speed.

Is my calculation correct ?

Thank you.

## 1 Answer

Peak torque for a BLDC motor is always at 0 RPM (unless there is a current limit, then it is the torque at whatever that current limit is). So VOLTAGE = Rated_Speed * Back_EMF_Constant + Peak_Current * Resistance will never be the case and should be rewritten as VOLTAGE = Rated_Speed * Back_EMF_Constant + Current * Resistance.

In your case, given the information you've provided, the stall current should be $\frac{24 V}{1.5 \Omega} = 16 A$. So the peak current of 4.4 A that you've listed must be something else. The rated current should be $24V = 4000 RPM * 3.7\frac{Vp}{kRPM} + I *1.5 \Omega => I = 6.1 A$.

All this is assuming constant velocity and constant current.

• Remember it's effectively AC drive and the motor has inductance which is neglected in this simplistic analysis. This will reduce the rated current. As we don't know how many poles the motor has, we don't know the actual driving frequency for 4000 rpm. However running a couple of scenarios, it appears that the specified inductance and resistance combine to give rated current I=(very close to 4.4A) for a 4 pole motor. – Brian Drummond Nov 6 '14 at 15:09
• @Brad: According to the datasheet(BL16B17-03 Lin), this motor has Rated Current of 2.2A and Peak Current of 4.4A, which are much less than your two calculated currents . Is it because of motor's thermal limitation? – Ron Mao Nov 6 '14 at 16:35
• The power loss from 4.4A going through 1.5 Ohms is 29.04W. However according its torque chart this motor is only about 20% efficient at 4.4A. That indicates a real power loss of ~85W! No wonder that it is only rated for 2.2A continuous and 29.5W (which is close to its maximum output power). – Bruce Abbott Jan 5 '15 at 19:02
• You can't measure efficiency like that. What about speed? What about voltage? – Gregory Kornblum Jun 7 '15 at 12:57